117 solved out of 419 shown (show only solved or open)
OPEN - $500 If$A\subseteq \{1,\ldots,N\}$with$\lvert A\rvert=n$is such that the subset sums$\sum_{a\in S}a$are distinct for all$S\subseteq A$then $N \gg 2^{n}.$ Erdős called this 'perhaps my first serious problem'. The powers of$2$show that$2^n$would be best possible here. The trivial lower bound is$N \gg 2^{n}/n$, since all$2^n$distinct subset sums must lie in$[0,Nn)$. Erdős and Moser [Er56] proved $N\geq (\tfrac{1}{4}-o(1))\frac{2^n}{\sqrt{n}}.$ (In [Er85c] Erdős offered \$100 for any improvement of the constant $1/4$ here.)

A number of improvements of the constant have been given (see [St23] for a history), with the current record $\sqrt{2/\pi}$ first proved in unpublished work of Elkies and Gleason. Two proofs achieving this constant are provided by Dubroff, Fox, and Xu [DFX21], who in fact prove the exact bound $N\geq \binom{n}{\lfloor n/2\rfloor}$.

In [Er73] and [ErGr80] the generalisation where $A\subseteq (0,N]$ is a set of real numbers such that the subset sums all differ by at least $1$ is proposed, with the same conjectured bound. (The second proof of [DFX21] applies also to this generalisation.)

This problem appears in Erdős' book with Spencer [ErSp74] in the final chapter titled 'The kitchen sink'. As Ruzsa writes in [Ru99] "it is a rich kitchen where such things go to the sink".

The sequence of minimal $N$ for a given $n$ is A276661 in the OEIS.

Additional thanks to: Zach Hunter and Ralf Stephan
SOLVED - $1000 Can the smallest modulus of a covering system be arbitrarily large? Described by Erdős as 'perhaps my favourite problem'. Hough [Ho15], building on work of Filaseta, Ford, Konyagin, Pomerance, and Yu [FFKPY07], has shown (contrary to Erdős' expectations) that the answer is no: the smallest modulus must be at most$10^{18}$. An alternative, simpler, proof was given by Balister, Bollobás, Morris, Sahasrabudhe, and Tiba [BBMST22], who improved the bound on the smallest modulus to$616000$. OPEN -$5000
If $A\subseteq \mathbb{N}$ has $\sum_{n\in A}\frac{1}{n}=\infty$ then must $A$ contain arbitrarily long arithmetic progressions?
This is essentially asking for good bounds on $r_k(N)$, the size of the largest subset of $\{1,\ldots,N\}$ without a non-trivial $k$-term arithmetic progression. For example, a bound like $r_k(N) \ll_k \frac{N}{(\log N)(\log\log N)^2}$ would be sufficient.

Even the case $k=3$ is non-trivial, but was proved by Bloom and Sisask [BlSi20]. Much better bounds for $r_3(N)$ were subsequently proved by Kelley and Meka [KeMe23]. Green and Tao [GrTa17] proved $r_4(N)\ll N/(\log N)^{c}$ for some small constant $c>0$. Gowers [Go01] proved $r_k(N) \ll \frac{N}{(\log\log N)^{c_k}},$ where $c_k>0$ is a small constant depending on $k$. The current best bounds for general $k$ are due to Leng, Sah, and Sawhney [LSS24], who show that $r_k(N) \ll \frac{N}{\exp((\log\log N)^{c_k})}$ for some constant $c_k>0$ depending on $k$.

Curiously, Erdős [Er83c] thought this conjecture was the 'only way to approach' the conjecture that there are arbitrarily long arithmetic progressions of prime numbers, now a theorem due to Green and Tao [GrTa08] (see [219]).

In [Er81] Erdős makes the stronger conjecture that $r_k(N) \ll_C\frac{N}{(\log N)^C}$ for every $C>0$ (now known for $k=3$ due to Kelley and Meka [KeMe23]) - see [140].

SOLVED - $10000 For any$C>0$are there infinitely many$n$such that $p_{n+1}-p_n> C\frac{\log\log n\log\log\log\log n}{(\log\log \log n)^2}\log n?$ The peculiar quantitative form of Erdős' question was motivated by an old result of Rankin [Ra38], who proved there exists some constant$C>0$such that the claim holds. Solved by Maynard [Ma16] and Ford, Green, Konyagin, and Tao [FGKT16]. The best bound available, due to all five authors [FGKMT18], is that there are infinitely many$n$such that $p_{n+1}-p_n\gg \frac{\log\log n\log\log\log\log n}{\log\log \log n}\log n.$ The likely truth is a lower bound like$\gg(\log n)^2$. In [Er97c] Erdős revised the value of this problem to \$5000 and reserved the \$10000 for a lower bound of$>(\log n)^{1+c}$for some$c>0$. See also [687]. OPEN Let$C\geq 0$. Is there an infinite sequence of$n_i$such that $\lim_{i\to \infty}\frac{p_{n_i+1}-p_{n_i}}{\log n_i}=C?$ Let$S$be the set of limit points of$(p_{n+1}-p_n)/\log n$. This problem asks whether$S=[0,\infty]$. Although this conjecture remains unproven, a lot is known about$S$. Some highlights: •$\infty\in S$by Westzynthius' result [We31] on large prime gaps, •$0\in S$by the work of Goldston, Pintz, and Yildirim [GPY09] on small prime gaps, • Erdős [Er55] and Ricci [Ri56] independently showed that$S$has positive Lebesgue measure, • Hildebrand and Maier [HiMa88] showed that$S$contains arbitrarily large (finite) numbers, • Pintz [Pi16] showed that there exists some small constant$c>0$such that$[0,c]\subset S$, • Banks, Freiberg, and Maynard [BFM16] showed that at least$12.5\%$of$[0,\infty)$belongs to$S$, • Merikoski [Me20] showed that at least$1/3$of$[0,\infty)$belongs to$S$, and that$S$has bounded gaps. In [Er85c] and [Er97c] Erdős asks whether$S$is everywhere dense. SOLVED -$100
Let $d_n=p_{n+1}-p_n$. Are there infinitely many $n$ such that $d_n<d_{n+1}<d_{n+2}$?
Conjectured by Erdős and Turán [ErTu48]. Shockingly Erdős offered \$25000 for a disproof of this, but as he comments, it 'is certainly true'. (In [Er85c] he goes further and offers 'all the money I can earn, beg, borrow or steal for [a disproof]'.) Indeed, the answer is yes, as proved by Banks, Freiberg, and Turnage-Butterbaugh [BFT15] with an application of the Maynard-Tao machinery concerning bounded gaps between primes [Ma15]. They in fact prove that, for any$m\geq 1$, there are infinitely many$n$such that $d_n<d_{n+1}<\cdots <d_{n+m}$ and infinitely many$n$such that $d_n> d_{n+1}>\cdots >d_{n+m}.$ Additional thanks to: Mehtaab Sawhney OPEN Is there a covering system all of whose moduli are odd? Asked by Erdős and Selfridge (sometimes also with Schinzel). They also asked whether there can be a covering system such that all the moduli are odd and squarefree. The answer to this stronger question is no, proved by Balister, Bollobás, Morris, Sahasrabudhe, and Tiba [BBMST22]. Hough and Nielsen [HoNi19] proved that at least one modulus must be divisible by either$2$or$3$. A simpler proof of this fact was provided by Balister, Bollobás, Morris, Sahasrabudhe, and Tiba [BBMST22]. Selfridge has shown (as reported in [Sc67]) that such a covering system exists if a covering system exists with moduli$n_1,\ldots,n_k$such that no$n_i$divides any other$n_j$(but the latter has been shown not to exist, see [586]). Additional thanks to: Antonio Girao SOLVED For any finite colouring of the integers is there a covering system all of whose moduli are monochromatic? Conjectured by Erdős and Graham, who also ask about a density-type version: for example, is $\sum_{\substack{a\in A\\ a>N}}\frac{1}{a}\gg \log N$ a sufficient condition for$A$to contain the moduli of a covering system? The answer (to both colouring and density versions) is no, due to the result of Hough [Ho15] on the minimum size of a modulus in a covering system - in particular one could colour all integers$<10^{18}$different colours and all other integers a new colour. OPEN Let$A$be the set of all integers not of the form$p+2^{k}+2^l$(where$k,l\geq 0$and$p$is prime). Is the upper density of$A$positive? Crocker [Cr71] has proved there are are$\gg\log\log N$such integers in$\{1,\ldots,N\}$. Pan [Pa11] improved this to$\gg_\epsilon N^{1-\epsilon}$for any$\epsilon>0$. Erdős believed this cannot be proved by covering systems, i.e. integers of the form$p+2^k+2^l$exist in every infinite arithmetic progression. The sequence of such numbers is A006286 in the OEIS. See also [10], [11], and [16]. Additional thanks to: Ralf Stephan OPEN Is there some$k$such that every integer is the sum of a prime and at most$k$powers of 2? Erdős described this as 'probably unattackable'. In [ErGr80] Erdős and Graham suggest that no such$k$exists. Gallagher [Ga75] has shown that for any$\epsilon>0$there exists$k(\epsilon)$such that the set of integers which are the sum of a prime and at most$k(\epsilon)$many powers of 2 has lower density at least$1-\epsilon$. Granville and Soundararajan [GrSo98] have conjectured that at most$3$powers of 2 suffice for all odd integers, and hence at most$4$powers of$2$suffice for all even integers. (The restriction to odd integers is important here - for example, Bogdan Grechuk has observed that$1117175146$is not the sum of a prime and at most$3$powers of$2$, and pointed out that parity considerations, coupled with the fact that there are many integers not the sum of a prime and$2$powers of$2$(see [9]) suggest that there exist infinitely many even integers which are not the sum of a prime and at most$3$powers of$2$). See also [9], [11], and [16]. Additional thanks to: Bogdan Grechuk OPEN Is every odd$n$the sum of a squarefree number and a power of 2? Odlyzko has checked this up to$10^7$. Granville and Soundararajan [GrSo98] have proved that this is very related to the problem of finding primes$p$for which$2^p\equiv 2\pmod{p^2}$(for example this conjecture implies there are infinitely many such$p$). This is equivalent to asking whether every$n$not divisible by$4$is the sum of a squarefree number and a power of two. Erdős thought that proving this with two powers of 2 is perhaps easy, and could prove that it is true (with a single power of two) for almost all$n$. See also [9], [10], and [16]. Additional thanks to: Milos OPEN Let$A$be an infinite set such that there are no distinct$a,b,c\in A$such that$a\mid (b+c)$and$b,c>a$. Is there such an$A$with $\liminf \frac{\lvert A\cap\{1,\ldots,N\}\rvert}{N^{1/2}}>0?$ Does there exist some absolute constant$c>0$such that there are always infinitely many$N$with $\lvert A\cap\{1,\ldots,N\}\rvert<N^{1-c}?$ Is it true that $\sum_{n\in A}\frac{1}{n}<\infty?$ Asked by Erdős and Sárközy [ErSa70], who proved that$A$must have density$0$. They also prove that this is essentially best possible, in that given any function$f(x)\to \infty$as$x\to \infty$there exists a set$A$with this property and infinitely many$N$such that $\lvert A\cap\{1,\ldots,N\}\rvert>\frac{N}{f(N)}.$ (Their example is given by all integers in$(y_i,\frac{3}{2}y_i)$congruent to$1$modulo$(2y_{i-1})!$, where$y_i$is some sufficiently quickly growing sequence.) An example of an$A$with this property where $\liminf \frac{\lvert A\cap\{1,\ldots,N\}\rvert}{N^{1/2}}\log N>0$ is given by the set of$p^2$, where$p\equiv 3\pmod{4}$is prime. For the finite version see [13]. SOLVED -$100
Let $A\subseteq \{1,\ldots,N\}$ be such that there are no $a,b,c\in A$ such that $a\mid(b+c)$ and $a<\min(b,c)$. Is it true that $\lvert A\rvert\leq N/3+O(1)$?
Asked by Erdős and Sárközy, who observed that $(2N/3,N]\cap \mathbb{N}$ is such a set. The answer is yes, as proved by Bedert [Be23].

For the infinite version see [12].

OPEN
Let $A\subseteq \mathbb{N}$. Let $B\subseteq \mathbb{N}$ be the set of integers which are representable in exactly one way as the sum of two elements from $A$. Is it true that for all $\epsilon>0$ and large $N$ $\lvert \{1,\ldots,N\}\backslash B\rvert \gg_\epsilon N^{1/2-\epsilon}.$
Asked by Erdős, Sárközy, and Szemerédi, who constructed an $A$ such that for all $\epsilon>0$ and all large $N$ $\lvert \{1,\ldots,N\}\backslash B\rvert \ll_\epsilon N^{1/2+\epsilon},$ and yet there for all $\epsilon>0$ there exist infinitely many $N$ where $\lvert \{1,\ldots,N\}\backslash B\rvert \gg_\epsilon N^{1/3-\epsilon}.$

Erdös and Freud investigated the finite analogue in 'a recent Hungarian paper', proving that there exists $A\subseteq \{1,\ldots,N\}$ such that the number of integers not representable in exactly one way as the sum of two elements from $A$ is $<2^{3/2}N^{1/2}$, and suggest the constant $2^{3/2}$ is perhaps best possible.

OPEN
Is it true that $\sum_{n=1}^\infty(-1)^n\frac{n}{p_n}$ converges, where $p_n$ is the sequence of primes?
Erdős suggested that a computer could be used to explore this, and did not see any other method to attack this.

Tao [Ta23] has proved that this series does converge assuming a strong form of the Hardy-Littlewood prime tuples conjecture.

SOLVED
Is the set of odd integers not of the form $2^k+p$ the union of an infinite arithmetic progression and a set of density $0$?
Erdős called this conjecture 'rather silly'. Using covering congruences Erdős [Er50] proved that the set of such odd integers contains an infinite arithmetic progression.

Chen [Ch23] has proved the answer is no.

OPEN
Are there infinitely many primes $p$ such that every even number $n\leq p-3$ can be written as a difference of primes $n=q_1-q_2$ where $q_1,q_2\leq p$?
The first prime without this property is $97$. The sequence of such primes is A038133 in the OEIS. These are called cluster primes.

Blecksmith, Erdős, and Selfridge [BES99] proved that the number of such primes is $\ll_A \frac{x}{(\log x)^A}$ for every $A>0$, and Elsholtz [El03] improved this to $\ll x\exp(-c(\log\log x)^2)$ for every $c<1/8$.

Additional thanks to: Ralf Stephan and Terence Tao
OPEN
We call $m$ practical if every integer $n<m$ is the sum of distinct divisors of $m$. If $m$ is practical then let $h(m)$ be such that $h(m)$ many divisors always suffice.

Are there infinitely many practical $m$ such that $h(m) < (\log\log m)^{O(1)}?$ Is it true that $h(n!)<n^{o(1)}$? Or perhaps even $h(n!)<(\log n)^{O(1)}$?

It is easy to see that almost all numbers are not practical. Erdős originally showed that $h(n!) <n$. Vose [Vo85] proved the existence of infinitely many practical $m$ such that $h(m)\ll (\log m)^{1/2}$.

The sequence of practical numbers is A005153 in the OEIS.

OPEN
Let $n_1<n_2<\cdots$ be an arbitrary sequence of integers, each with an associated residue class $a_i\pmod{n_i}$. Let $A$ be the set of integers $n$ such that for every $i$ either $n<n_i$ or $n\not\equiv a_i\pmod{n_i}$. Must the logarithmic density of $A$ exist?
SOLVED
Let $A\subset\mathbb{N}$ be infinite. Must there exist some $k\geq 1$ such that almost all integers have a divisor of the form $a+k$ for some $a\in A$?
Asked by Erdős and Tenenbaum. Ruzsa gave the following simple counterexample: let $A=\{n_1<n_2<\cdots \}$ where $n_l \equiv -(k-1)\pmod{p_k}$ for all $k\leq l$, where $p_k$ denotes the $k$th prime.

Tenenbaum asked the weaker variant (still open) where for every $\epsilon>0$ there is some $k=k(\epsilon)$ such that at least $1-\epsilon$ density of all integers have a divisor of the form $a+k$ for some $a\in A$.

SOLVED - $100 An$\epsilon$-almost covering system is a set of congruences$a_i\pmod{n_i}$for distinct moduli$n_1<\ldots<n_k$such that the density of those integers which satisfy none of them is$\leq \epsilon$. Is there a constant$C>1$such that for every$\epsilon>0$and$N\geq 1$there is an$\epsilon$-almost covering system with$N\leq n_1<\cdots <n_k\leq CN$? By a simple averaging argument the set of moduli$[m_1,m_2]\cap \mathbb{N}$has a choice of residue classes which form an$\epsilon(m_1,m_2)$-almost covering system with $\epsilon(m_1,m_2)=\prod_{m_1\leq m\leq m_2}(1-1/m).$ A$0$-covering system is just a covering system, and so by Hough [Ho15] these only exist for$n_1<10^{18}$. The answer is no, as proved by Filaseta, Ford, Konyagin, Pomerance, and Yu [FFKPY07], who (among other results) prove that if $1< C \leq N^{\frac{\log\log\log N}{4\log\log N}}$ then, for any$N\leq n_1<\cdots< n_k\leq CN$, the density of integers not covered for any fixed choice of residue classes is at least $\prod_{i}(1-1/n_i)$ (and this density is achieved for some choice of residue classes as above). Additional thanks to: Mehtaab Sawhney OPEN -$500
If $A\subseteq \mathbb{N}$ is such that $A+A$ contains all but finitely many integers then $\limsup 1_A\ast 1_A(n)=\infty$.
Conjectured by Erdős and Turán. They also suggest the stronger conjecture that $\limsup 1_A\ast 1_A(n)/\log n>0$.

Another stronger conjecture would be that the hypothesis $\lvert A\cap [1,N]\rvert \gg N^{1/2}$ for all large $N$ suffices.

Erdős and Sárközy conjectured the stronger version that if $A=\{a_1<a_2<\cdots\}$ and $B=\{b_1<b_2<\cdots\}$ with $a_n/b_n\to 1$ are such that $A+B=\mathbb{N}$ then $\limsup 1_A\ast 1_B(n)=\infty$.

SOLVED - $100 Is there an explicit construction of a set$A\subseteq \mathbb{N}$such that$A+A=\mathbb{N}$but$1_A\ast 1_A(n)=o(n^\epsilon)$for every$\epsilon>0$? The existence of such a set was asked by Sidon to Erdős in 1932. Erdős (eventually) proved the existence of such a set using probabilistic methods. This problem asks for a constructive solution. An explicit construction was given by Jain, Pham, Sawhney, and Zakharov [JPSZ24]. OPEN -$1000
Let $h(N)$ be the maximum size of a Sidon set in $\{1,\ldots,N\}$. Is it true that, for every $\epsilon>0$, $h(N) = N^{1/2}+O_\epsilon(N^\epsilon)?$
A problem of Erdős and Turán. It may even be true that $h(N)=N^{1/2}+O(1)$, but Erdős remarks this is perhaps too optimistic. Erdős and Turán [ErTu41] proved an upper bound of $N^{1/2}+O(N^{1/4})$, with an alternative proof by Lindström [Li69]. Both proofs in fact give $h(N) \leq N^{1/2}+N^{1/4}+1.$ Balogh, Füredi, and Roy [BFR21] improved the bound in the error term to $0.998N^{1/4}$, which has been further optimised by O'Bryant [OB22] to yield $h(N)\leq N^{1/2}+0.99703N^{1/4}$ for sufficiently large $N$.

SOLVED
Given any infinite set $A\subset \mathbb{N}$ there is a set $B$ of density $0$ such that $A+B$ contains all except finitely many integers.
Conjectured by Erdős and Straus. Proved by Lorentz [Lo54].
OPEN
Is there a set $A\subset\mathbb{N}$ such that $\lvert A\cap\{1,\ldots,N\}\rvert = o((\log N)^2)$ and such that every large integer can be written as $p+a$ for some prime $p$ and $a\in A$?

Can the bound $O(\log N)$ be achieved? Must such an $A$ satisfy $\liminf \frac{\lvert A\cap\{1,\ldots,N\}\rvert}{\log N}> 1?$

Such a set is called an additive complement to the primes.

Erdős [Er54] proved that such a set $A$ exists with $\lvert A\cap\{1,\ldots,N\}\rvert\ll (\log N)^2$ (improving a previous result of Lorentz [Lo54] who achieved $\ll (\log N)^3$). Wolke [Wo96] has shown that such a bound is almost true, in that we can achieve $\ll (\log N)^{1+o(1)}$ if we only ask for almost all integers to be representable.

The answer to the third question is yes: Ruzsa [Ru98c] has shown that we must have $\liminf \frac{\lvert A\cap\{1,\ldots,N\}\rvert}{\log N}\geq e^\gamma\approx 1.781.$

OPEN
Let $A\subset\mathbb{N}$ be such that every large integer can be written as $n^2+a$ for some $a\in A$ and $n\geq 0$. What is the smallest possible value of $\limsup \frac{\lvert A\cap\{1,\ldots,N\}\rvert}{N^{1/2}}?$
Erdős observed that this value is finite and $>1$.
SOLVED
For any permutation $\pi\in S_n$ of $\{1,\ldots,n\}$ let $S(\pi)$ count the number of distinct consecutive sums, that is, sums of the shape $\sum_{u\leq i\leq v}\pi(i)$. Is it true that $S(\pi) = o(n^2)$ for all $\pi\in S_n$?
It is easy to see that $S(\iota)=o(n^2)$ if $\iota$ denotes the identity permutation, as studied by Erdős and Harzheim [Er77]. Motivated by this, Erdős asked if this remains true for all permutations.

This is extremely false, as shown by Konieczny [Ko15], who both constructs an explicit permutation with $S(\pi) \geq n^2/4$, and also shows that for a random permutation we have $S(\pi)\sim \frac{1+e^{-2}}{4}n^2.$

SOLVED
Let $B\subseteq\mathbb{N}$ be an additive basis of order $k$ with $0\in B$. Is it true that for every $A\subseteq\mathbb{N}$ we have $d_s(A+B)\geq \alpha+\frac{\alpha(1-\alpha)}{k},$ where $\alpha=d_s(A)$ and $d_s(A) = \inf \frac{\lvert A\cap\{1,\ldots,N\}\rvert}{N}$ is the Schnirelmann density?
Erdős [Er36c] proved this is true with $k$ replaced by $2k$ in the denominator (in a stronger form that only considers $A\cup (A+b)$ for some $b\in B$, see [38]).

Ruzsa has observed that this follows immediately from the stronger fact proved by Plünnecke [Pl70] that (under the same assumptions) $d_S(A+B)\geq \alpha^{1-1/k}.$

OPEN
Find the optimal constant $c>0$ such that the following holds.

For all sufficiently large $N$, if $A\sqcup B=\{1,\ldots,2N\}$ is a partition into two equal parts, so that $\lvert A\rvert=\lvert B\rvert=N$, then there is some $x$ such that the number of solutions to $a-b=x$ with $a\in A$ and $b\in B$ is at least $cN$.

The minimum overlap problem. The example (with $N$ even) $A=\{N/2+1,\ldots,3N/2\}$ shows that $c\leq 1/2$ (indeed, Erdős initially conjectured that $c=1/2$). The lower bound of $c\geq 1/4$ is trivial, and Scherk improved this to $1-1/\sqrt{2}=0.29\cdots$. The current records are $0.379005 < c < 0.380926\cdots,$ the lower bound due to White [Wh22] and the upper bound due to Haugland [Ha16].
SOLVED
We say that $A\subset \mathbb{N}$ is an essential component if $d_s(A+B)>d_s(B)$ for every $B\subset \mathbb{N}$ with $0<d_s(B)<1$ where $d_s$ is the Schnirelmann density.

Can a lacunary set $A\subset\mathbb{N}$ be an essential component?

The answer is no by Ruzsa [Ru87], who proved that if $A$ is an essential component then there exists some constant $c>0$ such that $\lvert A\cap \{1,\ldots,N\}\rvert \geq (\log N)^{1+c}$ for all large $N$.
OPEN
Does there exist $B\subset\mathbb{N}$ which is not an additive basis, but is such that for every set $A\subseteq\mathbb{N}$ of Schnirelmann density $\alpha$ and every $N$ there exists $b\in B$ such that $\lvert (A\cup (A+b))\cap \{1,\ldots,N\}\rvert\geq (\alpha+f(\alpha))N$ where $f(\alpha)>0$ for $0<\alpha <1$?

The Schnirelmann density is defined by $d_s(A) = \inf_{N\geq 1}\frac{\lvert A\cap\{1,\ldots,N\}\rvert}{N}.$

Erdős [Er36c] proved that if $B$ is an additive basis of order $k$ then, for any set $A$ of Schnirelmann density $\alpha$, for every $N$ there exists some integer $b\in B$ such that $\lvert (A\cup (A+b))\cap \{1,\ldots,N\}\rvert\geq \left(\alpha+\frac{\alpha(1-\alpha)}{2k}\right)N.$ It seems an interesting question (not one that Erdős appears to have asked directly, although see [35]) to improve the lower bound here, even in the case $B=\mathbb{N}$. Erdős observed that a random set of density $\alpha$ shows that the factor of $\frac{\alpha(1-\alpha)}{2}$ in this case cannot be improved past $\alpha(1-\alpha)$.

This is a stronger propery than $B$ being an essential component (see [37]). Linnik [Li42] gave the first construction of an essential component which is not an additive basis.

OPEN - $500 Is there an infinite Sidon set$A\subset \mathbb{N}$such that $\lvert A\cap \{1\ldots,N\}\rvert \gg_\epsilon N^{1/2-\epsilon}$ for all$\epsilon>0$? The trivial greedy construction achieves$\gg N^{1/3}$. The current best bound of$\gg N^{\sqrt{2}-1+o(1)}$is due to Ruzsa [Ru98]. (Erdős [Er73] had offered \$25 for any construction which achieves $N^{c}$ for some $c>1/3$.) Erdős proved that for every infinite Sidon set $A$ we have $\liminf \frac{\lvert A\cap \{1,\ldots,N\}\rvert}{N^{1/2}}=0,$ and also that there is a set $A\subset \mathbb{N}$ with $\lvert A\cap \{1\ldots,N\}\rvert \gg_\epsilon N^{1/2-\epsilon}$ such that $1_A\ast 1_A(n)=O(1)$.

Erdős and Rényi have constructed, for any $\epsilon>0$, a set $A$ such that $\lvert A\cap \{1\ldots,N\}\rvert \gg_\epsilon N^{1/2-\epsilon}$ for all large $N$ and $1_A\ast 1_A(n)\ll_\epsilon 1$ for all $n$.

OPEN - $500 For what functions$g(N)\to \infty$is it true that $\lvert A\cap \{1,\ldots,N\}\rvert \gg N^{1/2}g(N)$ implies$\limsup 1_A\ast 1_A(n)=\infty$? It is possible that this is true even with$g(N)=O(1)$, from which the Erdős-Turán conjecture [28] would follow. OPEN -$500
Let $A\subset\mathbb{N}$ be an infinite set such that the triple sums $a+b+c$ are all distinct for $a,b,c\in A$ (aside from the trivial coincidences). Is it true that $\liminf \frac{\lvert A\cap \{1,\ldots,N\}\rvert}{N^{1/3}}=0?$
Erdős proved that if the pairwise sums $a+b$ are all distinct aside from the trivial coincidences then $\liminf \frac{\lvert A\cap \{1,\ldots,N\}\rvert}{N^{1/2}}=0.$
OPEN
Let $M\geq 1$ and $N$ be sufficiently large in terms of $M$. Is it true that for every maximal Sidon set $A\subset \{1,\ldots,N\}$ there is another Sidon set $B\subset \{1,\ldots,N\}$ of size $M$ such that $(A-A)\cap(B-B)=\{0\}$?
OPEN - $100 If$A,B\subset \{1,\ldots,N\}$are two Sidon sets such that$(A-A)\cap(B-B)=\{0\}$then is it true that $\binom{\lvert A\rvert}{2}+\binom{\lvert B\rvert}{2}\leq\binom{f(N)}{2}+O(1),$ where$f(N)$is the maximum possible size of a Sidon set in$\{1,\ldots,N\}$? If$\lvert A\rvert=\lvert B\rvert$then can this bound be improved to $\binom{\lvert A\rvert}{2}+\binom{\lvert B\rvert}{2}\leq (1-c)\binom{f(N)}{2}$ for some constant$c>0$? OPEN Let$N\geq 1$and$A\subset \{1,\ldots,N\}$be a Sidon set. Is it true that, for any$\epsilon>0$, there exist$M=M(\epsilon)$and$B\subset \{N+1,\ldots,M\}$such that$A\cup B\subset \{1,\ldots,M\}$is a Sidon set of size at least$(1-\epsilon)M^{1/2}$? See also [707]. SOLVED Let$k\geq 2$. Is there an integer$n_k$such that, if$D=\{ 1<d<n_k : d\mid n_k\}$, then for any$k$-colouring of$D$there is a monochromatic subset$D'\subseteq D$such that$\sum_{d\in D'}\frac{1}{d}=1$? This follows from the colouring result of Croot [Cr03]. Croot's result allows for$n_k \leq e^{C^k}$for some constant$C>1$(simply taking$n_k$to be the lowest common multiple of some interval$[1,C^k]$). Sawhney has observed that there is also a doubly exponential lower bound, and hence this bound is essentially sharp. Indeed, we must trivially have$\sum_{d|n_k}1/d \geq k$, or else there is a greedy colouring as a counterexample. Since$\prod_{p}(1+1/p^2)$is finite we must have$\prod_{p|n_k}(1+1/p)\gg k$. To achieve the minimal$\prod_{p|n_k}p$we take the product of primes up to$T$where$\prod_{p\leq T}(1+1/p)\gg k$; by Mertens theorems this implies$T\geq C^{k}$for some constant$C>1$, and hence$n_k\geq \prod_{p\mid n_k}p\geq \exp(cC^k)$for some$c>0$. Additional thanks to: Mehtaab Sawhney SOLVED Does every finite colouring of the integers have a monochromatic solution to$1=\sum \frac{1}{n_i}$with$2\leq n_1<\cdots <n_k$? The answer is yes, as proved by Croot [Cr03]. See also [298]. SOLVED -$100
If $\delta>0$ and $N$ is sufficiently large in terms of $\delta$, and $A\subseteq\{1,\ldots,N\}$ is such that $\sum_{a\in A}\frac{1}{a}>\delta \log N$ then must there exist $S\subseteq A$ such that $\sum_{n\in S}\frac{1}{n}=1$?
Solved by Bloom [Bl21], who showed that the quantitative threshold $\sum_{n\in A}\frac{1}{n}\gg \frac{\log\log\log N}{\log\log N}\log N$ is sufficient. This was improved by Liu and Sawhney [LiSa24] to $\sum_{n\in A}\frac{1}{n}\gg (\log N)^{4/5+o(1)}.$ Erdős speculated that perhaps even $\gg (\log\log N)^2$ might be sufficient. (A construction of Pomerance, as discussed in the appendix of [Bl21], shows that this would be best possible.)

SOLVED
Are there infinitely many integers $n,m$ such that $\phi(n)=\sigma(m)$?
This would follow immediately from the twin prime conjecture. The answer is yes, proved by Ford, Luca, and Pomerance [FLP10].
SOLVED
Let $A=\{a_1<\cdots<a_t\}\subseteq \{1,\ldots,N\}$ be such that $\phi(a_1)<\cdots<\phi(a_t)$. The primes are such an example. Are they the largest possible? Can one show that $\lvert A\rvert<(1+o(1))\pi(N)$ or even $\lvert A\rvert=o(N)$?
Erdős remarks that the last conjecture is probably easy, and that similar questions can be asked about $\sigma(n)$.

Solved by Tao [Ta23b], who proved that $\lvert A\rvert \leq \left(1+O\left(\frac{(\log\log x)^5}{\log x}\right)\right)\pi(x).$

In [Er95c] Erdős further asks about the situation when $\phi(a_1)\leq \cdots \leq \phi(a_t)$.

OPEN - $250 Schoenberg proved that for every$c\in [0,1]$the density of $\{ n\in \mathbb{N} : \phi(n)<cn\}$ exists. Let this density be denoted by$f(c)$. Is it true that there are no$x$such that$f'(x)$exists and is positive? OPEN Is there an infinite set$A\subset \mathbb{N}$such that for every$a\in A$there is an integer$n$such that$\phi(n)=a$, and yet if$n_a$is the smallest such integer then$n_a/a\to \infty$as$a\to\infty$? Carmichael has asked whether there is an integer$t$for which$\phi(n)=t$has exactly one solution. Erdős has proved that if such a$t$exists then there must be infinitely many such$t$. See also [694]. OPEN -$250
Let $A$ be a finite set of integers. Is it true that for every $\epsilon>0$ $\max( \lvert A+A\rvert,\lvert AA\rvert)\gg_\epsilon \lvert A\rvert^{2-\epsilon}?$
The sum-product problem. Erdős and Szemerédi [ErSz83] proved a lower bound of $\lvert A\rvert^{1+c}$ for some constant $c>0$, and an upper bound of $\lvert A\rvert^2 \exp\left(-c\frac{\log\lvert A\rvert}{\log\log \lvert A\rvert}\right)$ for some constant $c>0$. The lower bound has been improved a number of times. The current record is $\max( \lvert A+A\rvert,\lvert AA\rvert)\gg\lvert A\rvert^{\frac{1558}{1167}-o(1)}$ due to Rudnev and Stevens [RuSt22] (note $1558/1167=1.33504\cdots$).

There is likely nothing special about the integers in this question, and indeed Erdős and Szemerédi also ask a similar question about finite sets of real or complex numbers. The current best bound for sets of reals is the same bound of Rudnev and Stevens above. The best bound for complex numbers is $\max( \lvert A+A\rvert,\lvert AA\rvert)\gg\lvert A\rvert^{\frac{5}{4}},$ due to Solymosi [So05].

One can in general ask this question in any setting where addition and multiplication are defined (once one avoids any trivial obstructions such as zero divisors or finite subfields). For example, it makes sense for subsets of finite fields. The current record is that if $A\subseteq \mathbb{F}_p$ with $\lvert A\rvert <p^{5/8}$ then $\max( \lvert A+A\rvert,\lvert AA\rvert)\gg\lvert A\rvert^{\frac{11}{9}+o(1)},$ due to Rudnev, Shakan, and Shkredov [RSS20].

There is also a natural generalisation to higher-fold sum and product sets. For example, in [ErSz83] (and in [Er91]) Erdős and Szemerédi also conjecture that for any $m\geq 2$ and finite set of integers $A$ $\max( \lvert mA\rvert,\lvert A^m\rvert)\gg \lvert A\rvert^{m-o(1)}.$ See [53] for more on this generalisation and [808] for a stronger form of the original conjecture. See also [818] for a special case.

SOLVED
Let $A$ be a finite set of integers. Is it true that, for every $k$, if $\lvert A\rvert$ is sufficiently large depending on $k$, then there are least $\lvert A\rvert^k$ many integers which are either the sum or product of distinct elements of $A$?
Asked by Erdős and Szemerédi [ErSz83]. Solved in this form by Chang [Ch03].

Erdős and Szemerédi proved that there exist arbitrarily large sets $A$ such that the integers which are the sum or product of distinct elements of $A$ is at most $\exp\left(c (\log \lvert A\rvert)^2\log\log\lvert A\rvert\right)$ for some constant $c>0$.

SOLVED - $100 A set of integers$A$is Ramsey$2$-complete if, whenever$A$is$2$-coloured, all sufficiently large integers can be written as a monochromatic sum of elements of$A$. Burr and Erdős [BuEr85] showed that there exists a constant$c>0$such that it cannot be true that $\lvert A\cap \{1,\ldots,N\}\rvert \leq c(\log N)^2$ for all large$N$and that there exists a Ramsey$2$-complete$A$such that for all large$N$$\lvert A\cap \{1,\ldots,N\}\rvert < (2\log_2N)^3.$ Improve either of these bounds. The stated bounds are due to Burr and Erdős [BuEr85]. Resolved by Conlon, Fox, and Pham [CFP21], who constructed a Ramsey$2$-complete$A$such that $\lvert A\cap \{1,\ldots,N\}\rvert \ll (\log N)^2$ for all large$N$. See also [55] and [843]. SOLVED -$250
A set of integers $A$ is Ramsey $r$-complete if, whenever $A$ is $r$-coloured, all sufficiently large integers can be written as a monochromatic sum of elements of $A$. Prove any non-trivial bounds about the growth rate of such an $A$ for $r>2$.
A paper of Burr and Erdős [BuEr85] proves both upper and lower bounds for $r=2$, showing that there exists some $c>0$ such that it cannot be true that $\lvert A\cap \{1,\ldots,N\}\rvert \leq c(\log N)^2$ for all large $N$, and also constructing a Ramsey $2$-complete $A$ such that for all large $N$ $\lvert A\cap \{1,\ldots,N\}\rvert \ll (\log N)^3.$ Burr has shown that the sequence of $k$th powers is Ramsey $r$-complete for every $r,k\geq 1$.

Solved by Conlon, Fox, and Pham [CFP21], who constructed for every $r\geq 2$ an $r$-Ramsey complete $A$ such that for all large $N$ $\lvert A\cap \{1,\ldots,N\}\rvert \ll r(\log N)^2,$ and showed that this is best possible, in that there exists some constant $c>0$ such that if $A\subset \mathbb{N}$ satisfies $\lvert A\cap \{1,\ldots,N\}\rvert \leq cr(\log N)^2$ for all large $N$ then $A$ cannot be $r$-Ramsey complete.

SOLVED
Suppose $A\subseteq \{1,\ldots,N\}$ is such that there are no $k+1$ elements of $A$ which are relatively prime. An example is the set of all multiples of the first $k$ primes. Is this the largest such set?
This was disproved for $k=212$ by Ahlswede and Khachatrian [AhKh94], who suggest that their methods can disprove this for arbitrarily large $k$.

Erdős later asked ([Er92b] and [Er95]) if the conjecture remains true provided $N\geq (1+o(1))p_k^2$ (or, in a weaker form, whether it is true for $N$ sufficiently large depending on $k$).

OPEN - $500 Is there$A\subseteq \mathbb{N}$such that $\lim_{n\to \infty}\frac{1_A\ast 1_A(n)}{\log n}$ exists and is$\neq 0$? A suitably constructed random set has this property if we are allowed to ignore an exceptional set of density zero. The challenge is obtaining this with no exceptional set. Erdős believed the answer should be no. Erdős and Sárkzözy proved that $\frac{\lvert 1_A\ast 1_A(n)-\log n\rvert}{\sqrt{\log n}}\to 0$ is impossible. Erdős suggests it may even be true that the$\liminf$and$\limsup$of$1_A\ast 1_A(n)/\log n$are always separated by some absolute constant. OPEN Is $\sum_{n\geq 2}\frac{1}{n!-1}$ irrational? The decimal expansion is A331373 in the OEIS. OPEN Is $\sum_{n\geq 2}\frac{\omega(n)}{2^n}$ irrational? (Here$\omega(n)$counts the number of distinct prime divisors of$n$.) Erdős [Er48] proved that$\sum_n \frac{d(n)}{2^n}$is irrational, where$d(n)$is the divisor function. SOLVED Let$F_{k}(N)$be the size of the largest$A\subseteq \{1,\ldots,N\}$such that the product of no$k$many distinct elements of$A$is a square. Is$F_5(N)=(1-o(1))N$? More generally, is$F_{2k+1}(N)=(1-o(1))N$? Conjectured by Erdős, Sós, and Sárkzözy [ESS95], who proved $F_2(N)=\left(\frac{6}{\pi^2}+o(1)\right)N,$ $F_3(N) = (1-o(1))N,$ and also established asymptotics for$F_k(N)$for all even$k\geq 4$(in particular$F_k(N)\asymp N/\log N$for all even$k\geq 4$). Erdős [Er38] earlier proved that$F_4(N)=o(N)$- indeed, if$\lvert A\rvert \gg N$and$A\subseteq \{1,\ldots,N\}$then there is a non-trivial solution to$ab=cd$with$a,b,c,d\in A$. Erdős (and independently Hall [Ha96] and Montgomery) also asked about$F(N)$, the size of the largest$A\subseteq\{1,\ldots,N\}$such that the product of no odd number of$a\in A$is a square. Ruzsa [Ru77] observed that$1/2<\lim F(N)/N <1$. Granville and Soundararajan [GrSo01] proved an asymptotic $F(N)=(1-c+o(1))N$ where$c=0.1715\ldots$is an explicit constant. This problem was answered in the negative by Tao [Ta24], who proved that for any$k\geq 4$there is some constant$c_k>0$such that$F_k(N) \leq (1-c_k+o(1))N$. OPEN Let$f(n)$be a number theoretic function which grows slowly (e.g. slower than$(\log n)^{1-c}$) and$F(n)$be such that for almost all$n$we have$f(n)/F(n)\to 0$. When are there infinitely many$x$such that $\frac{\#\{ n\in \mathbb{N} : n+f(n)\in (x,x+F(x))\}}{F(x)}\to \infty?$ Conjectured by Erdős, Pomerance, and Sárközy [ErPoSa97] who prove this when$f$is the divisor function or the number of distinct prime divisors of$n$, but Erdős believed it is false when$f(n)=\phi(n)$or$\sigma(n)$. OPEN -$250
Let $a,b,c$ be three integers which are pairwise coprime. Is every large integer the sum of distinct integers of the form $a^kb^lc^m$ ($k,l,m\geq 0$), none of which divide any other?
Conjectured by Erdős and Lewin [ErLe96], who (among other related results) prove this when $a=3$, $b=5$, and $c=7$.

In [Er92b] Erdős wrote 'last year I made the following silly conjecture': every integer $n$ can be written as the sum of distinct integers of the form $2^k3^l$, none of which divide any other. 'I mistakenly thought that this was a nice and difficult conjecture but Jansen and several others found a simple proof by induction.'

In [Er92b] Erdős makes the stronger conjecture (for $a=2$, $b=3$, and $c=5$) that, for any $\epsilon>0$, all large integers $n$ can be written as the sum of distinct integers $b_1<\cdots <b_t$ of the form $2^k3^l5^m$ where $b_t<(1+\epsilon)b_1$.

OPEN
Let $3\leq d_1<d_2<\cdots <d_k$ be integers such that $\sum_{1\leq i\leq k}\frac{1}{d_i-1}\geq 1.$ Can all sufficiently large integers be written as a sum of the shape $\sum_i c_ia_i$ where $c_i\in \{0,1\}$ and $a_i$ has only the digits $0,1$ when written in base $d_i$?
Conjectured by Burr, Erdős, Graham, and Li [BEGL96]. Pomerance observed that the condition $\sum 1/(d_i-1)\geq 1$ is necessary. In [BEGL96] they prove the property holds for $\{3,4,7\}$.

Additional thanks to: Boris Alexeev and Dustin Mixon
OPEN
Let $A = \{ \sum\epsilon_k3^k : \epsilon_k\in \{0,1\}\}$ be the set of integers which have only the digits $0,1$ when written base $3$, and $B=\{ \sum\epsilon_k4^k : \epsilon_k\in \{0,1\}\}$ be the set of integers which have only the digits $0,1$ when written base $4$.

Does $A+B$ have positive density?

A problem of Burr, Erdős, Graham, and Li [BEGL96]. More generally, if $n_1<\cdots<n_k$ have $\sum_{i=1}^k\log_{n_k}(2)>1$ and $A_i$ is the set of integers with only the digits $0,1$ in base $n_i$ then does $A_1+\cdots+A_k$ have positive density?

OPEN - $250 Let$f(n)$be maximal such that if$A\subseteq\mathbb{N}$has$\lvert A\rvert=n$then$\prod_{a\neq b\in A}(a+b)$has at least$f(n)$distinct prime factors. Is it true that$f(n)/\log n\to\infty$? Investigated by Erdős and Turán [ErTu34] (prompted by a question of Lázár and Grünwald) in their first joint paper, where they proved that $\log n \ll f(n) \ll n/\log n$ (the upper bound is trivial, taking$A=\{1,\ldots,n\}$). Erdős says that$f(n)=o(n/\log n)$has never been proved, but perhaps never seriously attacked. OPEN Let$\epsilon>0$and$N$be sufficiently large depending on$\epsilon$. Is there$A\subseteq\{1,\ldots,N\}$such that no$a\in A$divides the sum of any distinct elements of$A\backslash\{a\}$and$\lvert A\rvert\gg N^{1/2-\epsilon}$? It is easy to see that we must have$\lvert A\rvert \ll N^{1/2}$. Csaba has constructed such an$A$with$\lvert A\rvert \gg N^{1/5}$. OPEN Let$k\geq 3$. Can the product of any$k$consecutive integers$N$ever be powerful? That is, must there always exist a prime$p\mid N$such that$p^2\nmid N$? Conjectured by Erdős and Selfridge. There are infinitely many$n$such that$n(n+1)$is powerful (see [364]). Erdős and Selfridge proved that$N$can never be a perfect power. Erdős remarked that this 'seems hopeless at present'. See also [364]. Additional thanks to: Julius Schmerling SOLVED -$250
The density of integers which have two divisors $d_1,d_2$ such that $d_1<d_2<2d_1$ exists and is equal to $1$.
In [Er79] asks the stronger version with $2$ replaced by any constant $c>1$.

The answer is yes (also to this stronger version), proved by Maier and Tenenbaum [MaTe84]. (Tenenbaum has told me that they received \$650 for their solution.) See also [449]. OPEN Let$s_1<s_2<\cdots$be the sequence of squarefree numbers. Is it true that, for any$\alpha \geq 0$, $\lim_{x\to \infty}\frac{1}{x}\sum_{s_n\leq x}(s_{n+1}-s_n)^\alpha$ exists? Erdős [Er51] proved this for all$0\leq \alpha \leq 2$, and Hooley [Ho73] extended this to all$\alpha \leq 3$. See also [208]. OPEN Let$F(k)$be the number of solutions to $1= \frac{1}{n_1}+\cdots+\frac{1}{n_k},$ where$1\leq n_1<\cdots<n_k$are distinct integers. Find good estimates for$F(k)$. The current best bounds known are $2^{c^{\frac{k}{\log k}}}\leq F(k) \leq c_0^{(\frac{1}{5}+o(1))2^k},$ where$c>0$is some absolute constant and$c_0=1.26408\cdots$is the 'Vardi constant'. The lower bound is due to Konyagin [Ko14] and the upper bound to Elsholtz and Planitzer [ElPl21]. SOLVED A set$A\subset \mathbb{N}$is primitive if no member of$A$divides another. Is the sum $\sum_{n\in A}\frac{1}{n\log n}$ maximised over all primitive sets when$A$is the set of primes? Erdős [Er35] proved that this sum always converges for a primitive set. Lichtman [Li23] proved that the answer is yes. SOLVED Show that, for any$n\geq 5$, the binomial coefficient$\binom{2n}{n}$is not squarefree. It is easy to see that$4\mid \binom{2n}{n}$except when$n=2^k$, and hence it suffices to prove this when$n$is a power of$2$. Proved by Sárkzözy [Sa85] for all sufficiently large$n$, and by Granville and Ramaré [GrRa96] for all$n\geq 5$. More generally, if$f(n)$is the largest integer such that, for some prime$p$, we have$p^{f(n)}$dividing$\binom{2n}{n}$, then$f(n)$should tend to infinity with$n$. Can one even disprove that$f(n)\gg \log n$? OPEN Is it true that all sufficiently large$n$can be written as$2^k+m$for some$k\geq 0$, where$\Omega(m)<\log\log m$? (Here$\Omega(m)$is the number of prime divisors of$m$counted with multiplicity.) What about$<\epsilon \log\log m$? Or some more slowly growing function? It is easy to see by probabilistic methods that this holds for almost all integers. Romanoff [Ro34] showed that a positive density set of integers are representable as the sum of$2^k+p$for prime$p$, and Erdős used covering systems to show that there is a positive density set of integers which cannot be so represented. See also [851]. SOLVED Let$x>0$be a real number. For any$n\geq 1$let $R_n(x) = \sum_{i=1}^n\frac{1}{m_i}<x$ be the maximal sum of$n$distinct unit fractions which is$<x$. Is it true that, for almost all$x$, for sufficiently large$n$, we have $R_{n+1}(x)=R_n(x)+\frac{1}{m},$ where$m$is minimal such that$m$does not appear in$R_n(x)$and the right-hand side is$<x$? (That is, are the best underapproximations eventually always constructed in a 'greedy' fashion?) Erdős and Graham write it is 'not difficult' to construct irrational$x$such that this fails (although give no proof or reference, and it seems to still be an open problem to actually construct some such irrational$x$). Curtiss [Cu22] showed that this is true for$x=1$and Erdős [Er50b] showed it is true for all$x=1/m$with$m\geq 1$. Nathanson [Na23] has shown it is true for$x=a/b$when$a\mid b+1$and Chu [Ch23b] has shown it is true for a larger class of rationals; it is still unknown whether this is true for all rational$x>0$. Without the 'eventually' condition this can fail for some rational$x$(although Erdős [Er50b] showed it holds without the eventually for rationals of the form$1/m$). For example $R_1(\tfrac{11}{24})=\frac{1}{3}$ but $R_2(\tfrac{11}{24})=\frac{1}{4}+\frac{1}{5}.$ Kovač [Ko24b] has proved that this is false - in fact as false as possible: the set of$x\in (0,\infty)$for which the best underapproximations are eventually 'greedy' has Lebesgue measure zero. (It remains an open problem to give any explicit example of a number which is not eventually greedy, despite the fact that almost all numbers have this property.) Additional thanks to: Vjekoslav Kovac OPEN Let$s_1<s_2<\cdots$be the sequence of squarefree numbers. Is it true that, for any$\epsilon>0$and large$n$, $s_{n+1}-s_n \ll_\epsilon s_n^{\epsilon}?$ Is it true that $s_{n+1}-s_n \leq (1+o(1))\frac{\pi^2}{6}\frac{\log s_n}{\log\log s_n}?$ Erdős [Er51] showed that there are infinitely many$n$such that $s_{n+1}-s_n > (1+o(1))\frac{\pi^2}{6}\frac{\log s_n}{\log\log s_n},$ so this bound would be the best possible. In [Er79] Erdős says perhaps$s_{n+1}-s_n \ll \log s_n$, but he is 'very doubtful'. Filaseta and Trifonov [FiTr92] proved an upper bound of$s_n^{1/5}$. Pandey [Pa24] has improved this exponent to$1/5-c$for some constant$c>0$. See also [489] and [145]. Additional thanks to: Zachary Chase OPEN Let$d_n=p_{n+1}-p_n$. The set of$n$such that$d_{n+1}\geq d_n$has density$1/2$, and similarly for$d_{n+1}\leq d_n$. Furthermore, there are infinitely many$n$such that$d_{n+1}=d_n$. In [Er85c] Erdős also conjectures that$d_n=d_{n+1}=\cdots=d_{n+k}$is solvable for every$k$. SOLVED Are there arbitrarily long arithmetic progressions of primes? The answer is yes, proved by Green and Tao [GrTa08]. The stronger claim that there are arbitrarily long arithmetic progressions of consecutive primes is still open. See also [3] and [141]. SOLVED -$500
Let $n\geq 1$ and $A=\{a_1<\cdots <a_{\phi(n)}\}=\{ 1\leq m<n : (m,n)=1\}.$ Is it true that $\sum_{1\leq k<\phi(n)}(a_{k+1}-a_k)^2 \ll \frac{n^2}{\phi(n)}?$
The answer is yes, as proved by Montgomery and Vaughan [MoVa86], who in fact found the correct order of magnitude with the power $2$ replaced by any $\gamma\geq 1$ (which was also asked by Erdős in [Er73]).
SOLVED
Is there a set $A\subset\mathbb{N}$ such that, for all large $N$, $\lvert A\cap\{1,\ldots,N\}\rvert \ll N/\log N$ and such that every large integer can be written as $2^k+a$ for some $k\geq 0$ and $a\in A$?
Lorentz [Lo54] proved there is such a set with, for all large $N$, $\lvert A\cap\{1,\ldots,N\}\rvert \ll \frac{\log\log N}{\log N}N$ The answer is yes, proved by Ruzsa [Ru72]. Ruzsa's construction is ingeniously simple: $A = \{ 5^nm : m\geq 1\textrm{ and }5^n\geq C\log m\}+\{0,1\}$ for some large absolute constant $C>0$. That every large integer is of the form $2^k+a$ for some $a\in A$ is a consequence of the fact that $2$ is a primitive root of $5^n$ for all $n\geq 1$.

In [Ru01] Ruzsa constructs an asymptotically best possible answer to this question (a so-called 'exact additive complement'); that is, there is such a set $A$ with $\lvert A\cap\{1,\ldots,N\}\rvert \sim \frac{N}{\log_2N}$ as $N\to \infty$.

OPEN
Let $n_1<n_2<\cdots$ be the sequence of integers which are the sum of two squares. Explore the behaviour of (i.e. find good upper and lower bounds for) the consecutive differences $n_{k+1}-n_k$.
Erdős [Er51] proved that, for infinitely many $k$, $n_{k+1}-n_k \gg \frac{\log n_k}{\sqrt{\log\log n_k}}.$ Richards [Ri82] improved this to $\limsup_{k\to \infty} \frac{n_{k+1}-n_k}{\log n_k} \geq 1/4.$ The constant $1/4$ here has been improved, most lately to $0.868\cdots$ by Dietmann, Elsholtz, Kalmynin, Konyagin, and Maynard [DEKKM22]. The best known upper bound is due to Bambah and Chowla [BaCh47], who proved that $n_{k+1}-n_k \ll n_k^{1/4}.$

The differences are listed at A256435 on the OEIS.

OPEN
Let $d_n=p_{n+1}-p_n$. Prove that $\sum_{1\leq n\leq N}d_n^2 \ll N(\log N)^2.$
Cramer proved an upper bound of $O(N(\log N)^4)$ conditional on the Riemann hypothesis. The prime number theorem immediately implies a lower bound of $\gg N(\log N)^2$.

The values of the sum are listed at A074741 on the OEIS.

OPEN
For every $c\geq 0$ the density $f(c)$ of integers for which $\frac{p_{n+1}-p_n}{\log n}< c$ exists and is a continuous function of $c$.
OPEN
Let $N_k=2\cdot 3\cdots p_k$ and $\{a_1<a_2<\cdots <a_{\phi(N_k)}\}$ be the integers $<N_k$ which are relatively prime to $N_k$. Then, for any $c\geq 0$, the limit $\frac{\#\{ a_i-a_{i-1}\leq c \frac{N_k}{\phi(N_k)} : 2\leq i\leq \phi(N_k)\}}{\phi(N_k)}$ exists and is a continuous function of $c$.
OPEN
Let $f(n)$ count the number of solutions to $n=p+2^k$ for prime $p$ and $k\geq 0$. Is it true that $f(n)=o(\log n)$?
Erdős [Er50] proved that there are infinitely many $n$ such that $f(n)\gg \log\log n$. Erdős could not even prove that there do not exist infinitely many integers $n$ such that for all $2^k<n$ the number $n-2^k$ is prime (probably $n=105$ is the largest such integer).

The sequence of values of $f(n)$ is A109925 on the OEIS.

SOLVED
Let $A\subseteq \mathbb{N}$ be a set such that $\lvert A\cap \{1,\ldots,N\}\rvert \gg \log N$ for all large $N$. Let $f(n)$ count the number of solutions to $n=p+a$ for $p$ prime and $a\in A$. Is it true that $\limsup f(n)=\infty$?
Erdős [Er50] proved this when $A=\{2^k : k\geq 0\}$. Solved by Chen and Ding [ChDi22].

OPEN
Let $c_1,c_2>0$. Is it true that, for any sufficiently large $x$, there exist more than $c_1\log x$ many consecutive primes $\leq x$ such that the difference between any two is $>c_2$?
This is well-known if $c_1$ is sufficiently small.
SOLVED
Let $f:\mathbb{N}\to \{-1,1\}$ be a multiplicative function. Is it true that $\lim_{N\to \infty}\frac{1}{N}\sum_{n\leq N}f(n)$ always exists?
Wintner observed that if $f$ can take complex values on the unit circle then the limit need not exist. The answer is yes, as proved by Wirsing [Wi67], and generalised by Halász [Ha68].
OPEN
Is there an infinite set of primes $P$ such that if $\{a_1<a_2<\cdots\}$ is the set of integers divisible only by primes in $P$ then $\lim a_{i+1}-a_i=\infty$?
Originally asked to Erdős by Wintner. The limit is infinite for a finite set of primes, which follows from a theorem of Pólya.
Additional thanks to: Boris Alexeev and Dustin Mixon
OPEN
For every $n\geq 2$ there exist distinct integers $1\leq x<y<z$ such that $\frac{4}{n} = \frac{1}{x}+\frac{1}{y}+\frac{1}{z}.$
The Erdős-Straus conjecture. The existence of a representation of $4/n$ as the sum of at most four distinct unit fractions follows trivially from a greedy algorithm.

Schinzel conjectured the generalisation that, for any fixed $a$, if $n$ is sufficiently large in terms of $a$ then there exist distinct integers $1\leq x<y<z$ such that $\frac{a}{n} = \frac{1}{x}+\frac{1}{y}+\frac{1}{z}.$

OPEN
Let $a_1<a_2<\cdots$ be a sequence of integers such that $\lim_{n\to \infty}\frac{a_n}{a_{n-1}^2}=1$ and $\sum\frac{1}{a_n}\in \mathbb{Q}$. Then, for all sufficiently large $n\geq 1$, $a_n = a_{n-1}^2-a_{n-1}+1.$
A sequence defined in such a fashion is known as Sylvester's sequence.
OPEN
Let $C>1$. Does the set of integers of the form $p+\lfloor C^k\rfloor$, for some prime $p$ and $k\geq 0$, have density $>0$?
Originally asked to Erdős by Kalmár. Erdős believed the answer is yes. Romanoff [Ro34] proved that the answer is yes if $C$ is an integer.
SOLVED
Let $(a,b)=1$. Every large integer is the sum of distinct integers of the form $a^kb^l$ with $k,l\geq 0$.
Proved by Birch [Bi59].
OPEN
Let $n_1<n_2<\cdots$ be a sequence of integers such that $\limsup \frac{n_k}{k}=\infty.$ Is $\sum_{k=1}^\infty \frac{1}{2^{n_k}}$ transcendental?
Erdős [Er75c] proved the answer is yes under the stronger condition that $\limsup n_k/k^t=\infty$ for all $t\geq 1$.
OPEN
Are there infinitely many $n$ such that, for all $k\geq 1$, $\omega(n+k) \ll k?$ (Here $\omega(n)$ is the number of distinct prime divisors of $n$.)
Related to [69]. Erdős and Graham [ErGr80] write 'we just know too little about sieves to be able to handle such a question ("we" here means not just us but the collective wisdom (?) of our poor struggling human race).'

OPEN
Is $\sum_n \frac{\phi(n)}{2^n}$ irrational? Here $\phi$ is the Euler totient function.
The decimal expansion of this sum is A256936 on the OEIS.
SOLVED
Is $\sum \frac{\sigma(n)}{2^n}$ irrational? (Here $\sigma(n)$ is the sum of divisors function.)
The answer is yes, as shown by Nesterenko [Ne96].
OPEN
Is $\sum \frac{p_n}{2^n}$ irrational? (Here $p_n$ is the $n$th prime.)
The decimal expansion of this sum is A098990 on the OEIS.
OPEN
Let $k\geq 1$ and $\sigma_k(n)=\sum_{d\mid n}d^k$. Is $\sum \frac{\sigma_k(n)}{n!}$ irrational?
This is known now for $1\leq k\leq 4$. The cases $k=1,2$ are reasonably straightforward, as observed by Erdős [Er52]. The case $k=3$ was proved independently by Schlage-Puchta [ScPu06] and Friedlander, Luca, and Stoiciu [FLC07]. The case $k=4$ was proved by Pratt [Pr22].
SOLVED
Let $a_1<a_2<\cdots$ be an infinite sequence of integers such that $a_{i+1}/a_i\to 1$. If every arithmetic progression contains infinitely many integers which are the sum of distinct $a_i$ then every sufficiently large integer is the sum of distinct $a_i$.
This was disproved by Cassels [Ca60].
OPEN
Let $A\subseteq \mathbb{N}$ be such that $\lvert A\cap [1,2x]\rvert -\lvert A\cap [1,x]\rvert \to \infty\textrm{ as }x\to \infty$ and $\sum_{n\in A} \{ \theta n\}=\infty$ for every $\theta\in (0,1)$, where $\{x\}$ is the distance of $x$ from the nearest integer. Then every sufficiently large integer is the sum of distinct elements of $A$.
Cassels [Ca60] proved this under the alternative hypotheses $\lvert A\cap [1,2x]\rvert -\lvert A\cap [1,x]\rvert\gg \log\log x$ and $\sum_{n\in A} \{ \theta n\}^2=\infty$ for every $\theta\in (0,1)$.
OPEN
Are there infinitely many $n$ such that there exists some $t\geq 2$ and $a_1,\ldots,a_t\geq 1$ such that $\frac{n}{2^n}=\sum_{1\leq k\leq t}\frac{a_k}{2^{a_k}}?$ Is this true for all $n$? Is there a rational $x$ such that $x = \sum_{k=1}^\infty \frac{a_k}{2^{a_k}}$ has at least two solutions?
Related to [260].
SOLVED
Let $X\subseteq \mathbb{R}^3$ be the set of all points of the shape $\left( \sum_{n\in A} \frac{1}{n},\sum_{n\in A}\frac{1}{n+1},\sum_{n\in A} \frac{1}{n+2}\right)$ as $A\subseteq\mathbb{N}$ ranges over all infinite sets with $\sum_{n\in A}\frac{1}{n}<\infty$. Does $X$ contain an open set?
Erdős and Straus proved the answer is yes for the 2-dimensional version, where $X\subseteq \mathbb{R}^2$ is the set of $\left( \sum_{n\in A} \frac{1}{n},\sum_{n\in A}\frac{1}{n+1}\right)$ as $A\subseteq\mathbb{N}$ ranges over all infinite sets with $\sum_{n\in A}\frac{1}{n}<\infty$.

The answer is yes, proved by Kovač [Ko24], who constructs an explicit open ball inside the set. The analogous question for higher dimensions remains open.

OPEN
Is there a covering system all of whose moduli are of the form $p-1$ for some primes $p\geq 5$?
Selfridge has found an example using divisors of $360$ if $p=3$ is allowed.
SOLVED
If a finite system of $r$ congruences $\{ a_i\pmod{n_i} : 1\leq i\leq r\}$ covers $2^r$ consecutive integers then it covers all integers.
This is best possible as the system $2^{i-1}\pmod{2^i}$ shows. This was proved indepedently by Selfridge and Crittenden and Vanden Eynden [CrVE70].
OPEN
Is there an infinite Lucas sequence $a_0,a_1,\ldots,$ where $(a_0,a_1)=1$ and $a_{n+2}=a_{n+1}+a_n$ for $n\geq 0$ such that all $a_k$ are composite, and yet no integer has a common factor with every term of the sequence?
Whether such a composite Lucas sequence even exists was open for a while, but using covering systems Graham [Gr64] showed that $a_0 = 1786772701928802632268715130455793$ $a_1 = 1059683225053915111058165141686995$ generate such a sequence. This problem asks whether one can have a composite Lucas sequence without 'an underlying system of covering congruences responsible'.
OPEN
For which $n$ is there a covering system $\{ a_d\pmod{d} : d\mid n\}$ such that if $x\equiv a_{d_1}\pmod{d_1}$ and $x\equiv a_{d_2}\pmod{d_2}$ then $(d_1,d_2)=1$?
The density of such $n$ is zero.
OPEN
Let $A=\{n_1<\cdots<n_r\}$ be a finite set of integers. What is the minimum value of the density of integers not hit by a suitable choice of congruences $a_i\pmod{n_i}$? Is the minimum achieved when all the $a_i$ are equal?
OPEN
Let $k\geq 3$. Is there a choice of congruence classes $a_p\pmod{p}$ for every prime $p$ such that all except finitely many integers can be written as $a_p+tp$ for some prime $p$ and integer $t\geq k$?
Even the case $k=3$ seems difficult. This may be true with the primes replaced by any set $A\subseteq \mathbb{N}$ such that $\lvert A\cap [1,N]\rvert \gg N/\log N$ and $\sum_{\substack{n\in A\\ n\leq N}}\frac{1}{n} -\log\log N\to \infty$ as $N\to \infty$.

For $k=1$ or $k=2$ any set $A$ such that $\sum_{n\in A}\frac{1}{n}=\infty$ has this property.

OPEN
Let $n_1<n_2<\cdots$ be an infinite sequence of integers with associated $a_i\pmod{n_i}$, such that for some $\epsilon>0$ we have $n_k>(1+\epsilon)k\log k$ for all $k$. Then $\#\{ m<n_k : m\not\equiv a_i\pmod{n_i} \textrm{ for }1\leq i\leq k\}\neq o(k).$
Erdős and Graham [ErGr80] suggest this 'may have to await improvements in current sieve methods' (of which there have been several since 1980).

Note that since the $k$th prime is $\sim k\log k$ the lower bound $n_k>(1+\epsilon)k\log k$ is best possible here.

OPEN
Let $n_1<n_2<\cdots$ be an infinite sequence with associated congruence classes $a_i\pmod{n_i}$ such that the set of integers not satisfying any of the congruences $a_i\pmod{n_i}$ has density $0$.

Is it true that for every $\epsilon>0$ there exists some $k$ such that the density of integers not satisfying any of the congruences $a_i\pmod{n_i}$ for $1\leq i\leq k$ is less than $\epsilon$?

The latter condition is clearly sufficient, the problem is if it's also necessary. The assumption implies $\sum \frac{1}{n_i}=\infty$.
OPEN
Let $A\subseteq \mathbb{N}$ be an infinite set and consider the following greedy algorithm for a rational $x\in (0,1)$: choose the minimal $n\in A$ such that $n\geq 1/x$ and repeat with $x$ replaced by $x-\frac{1}{n}$. If this terminates after finitely many steps then this produces a representation of $x$ as the sum of distinct unit fractions with denominators from $A$.

Does this process always terminate if $x$ has odd denominator and $A$ is the set of odd numbers? More generally, for which pairs $x$ and $A$ does this process terminate?

In 1202 Fibonacci observed that this process terminates for any $x$ when $A=\mathbb{N}$. The problem when $A$ is the set of odd numbers is due to Stein.

Graham [Gr64b] has shown that $\frac{m}{n}$ is the sum of distinct unit fractions with denominators $\equiv a\pmod{d}$ if and only if $\left(\frac{n}{(n,(a,d))},\frac{d}{(a,d)}\right)=1.$ Does the greedy algorithm always terminate in such cases?

Graham [Gr64c] has also shown that $x$ is the sum of distinct unit fractions with square denominators if and only if $x\in [0,\pi^2/6-1)\cup [1,\pi^2/6)$. Does the greedy algorithm for this always terminate? Erdős and Graham believe not - indeed, perhaps it fails to terminate almost always.

OPEN
Let $p:\mathbb{Z}\to \mathbb{Z}$ be a polynomial whose leading coefficient is positive and such that there exists no $d\geq 2$ with $d\mid p(n)$ for all $n\geq 1$. Is it true that, for all sufficiently large $m$, there exist integers $1\leq n_1<\cdots <n_k$ such that $1=\frac{1}{n_1}+\cdots+\frac{1}{n_k}$ and $m=p(n_1)+\cdots+p(n_k)?$
Graham [Gr63] has proved this when $p(x)=x$. Cassels [Ca60] has proved that these conditions on the polynomial imply every sufficiently large integer is the sum of $p(n_i)$ with distinct $n_i$. Burr has proved this if $p(x)=x^k$ with $k\geq 1$ and if we allow $n_i=n_j$.
SOLVED
Let $f(k)$ be the maximal value of $n_1$ such that there exist $n_1<n_2<\cdots <n_k$ with $1=\frac{1}{n_1}+\cdots+\frac{1}{n_k}.$ Is it true that $f(k)=(1+o(1))\frac{k}{e-1}?$
The upper bound $f(k) \leq (1+o(1))\frac{k}{e-1}$ is trivial since for any $u\geq 1$ we have $\sum_{u\leq n\leq eu}\frac{1}{n}=1+o(1),$ and hence if $f(k)=u$ then we must have $k\geq (e-1-o(1))u$. Essentially solved by Croot [Cr01], who showed that for any $N>1$ there exists some $k\geq 1$ and $N<n_1<\cdots <n_k \leq (e+o(1))N$ with $1=\sum \frac{1}{n_i}$.
SOLVED
Let $f(k)$ be the minimal value of $n_k$ such that there exist $n_1<n_2<\cdots <n_k$ with $1=\frac{1}{n_1}+\cdots+\frac{1}{n_k}.$ Is it true that $f(k)=(1+o(1))\frac{e}{e-1}k?$
It is trivial that $f(k)\geq (1+o(1))\frac{e}{e-1}k$, since for any $u\geq 1$ $\sum_{e\leq n\leq eu}\frac{1}{n}= 1+o(1),$ and so if $eu\approx f(k)$ then $k\leq \frac{e-1}{e}f(k)$. Proved by Martin [Ma00].
SOLVED
Let $k\geq 2$. Is it true that there exists an interval $I$ of width $(e-1+o(1))k$ and integers $n_1<\cdots<n_k\in I$ such that $1=\frac{1}{n_1}+\cdots+\frac{1}{n_k}?$
The answer is yes, proved by Croot [Cr01].
OPEN
Let $k\geq 2$. Is it true that, for any distinct integers $n_1<\cdots <n_k$ such that $1=\frac{1}{n_1}+\cdots+\frac{1}{n_k}$ we must have $\max(n_{i+1}-n_i)\geq 3$?
The example $1=\frac{1}{2}+\frac{1}{3}+\frac{1}{6}$ shows that $3$ would be best possible here. The lower bound of $\geq 2$ is equivalent to saying that $1$ is not the sum of reciprocals of consecutive integers, proved by Erdős [Er32].

This conjecture would follow for all but at most finitely many exceptions if it were known that, for all large $N$, there exists a prime $p\in [N,2N]$ such that $\frac{p+1}{2}$ is also prime.

OPEN
Is it true that there are only finitely many pairs of intervals $I_1,I_2$ such that $\sum_{n_1\in I_1}\frac{1}{n_1}+\sum_{n_2\in I_2}\frac{1}{n_2}\in \mathbb{N}?$
For example, $\frac{1}{3}+\frac{1}{4}+\frac{1}{5}+\frac{1}{6}+\frac{1}{20}=1.$ This is still open even if $\lvert I_2\rvert=1$. It is perhaps true with two intervals replaced by any $k$ intervals.
OPEN
Is it true that, for all sufficiently large $k$, there exist finite intervals $I_1,\ldots,I_k\subset \mathbb{N}$ with $\lvert I_i\rvert \geq 2$ for $1\leq i\leq k$ such that $1=\sum_{i=1}^k \sum_{n\in I_i}\frac{1}{n}?$
OPEN
Let $a\geq 1$. Must there exist some $b>a$ such that $\sum_{a\leq n\leq b}\frac{1}{n}=\frac{r_1}{s_1}\textrm{ and }\sum_{a\leq n\leq b+1}\frac{1}{n}=\frac{r_2}{s_2},$ with $(r_i,s_i)=1$ and $s_2<s_1$? If so, how does this $b(a)$ grow with $a$?
For example, $\sum_{3\leq n\leq 5}\frac{1}{n} = \frac{47}{60}\textrm{ and }\sum_{3\leq n\leq 6}\frac{1}{n}=\frac{19}{20}.$

The smallest $b$ for each $a$ are listed at A375081 at the OEIS.

OPEN
Let $n\geq 1$ and define $L_n$ to be the least common multiple of $\{1,\ldots,n\}$ and $a_n$ by $\sum_{1\leq k\leq n}\frac{1}{k}=\frac{a_n}{L_n}.$ Is it true that $(a_n,L_n)=1$ and $(a_n,L_n)>1$ both occur for infinitely many $n$?
Steinerberger has observed that the answer to the second question is trivially yes: for example, any $n$ which begins with a $2$ in base $3$ has $3\mid (a_n,L_n)$.

More generally, if the leading digit of $n$ in base $p$ is $p-1$ then $p\mid (a_n,L_n)$. There is in fact a necessary and sufficient condition: a prime $p\leq n$ divides $(a_n,L_n)$ if and only if $p$ divides the numerator of $1+\cdots+\frac{1}{k}$, where $k$ is the leading digit of $n$ in base $p$. This can be seen by writing $a_n = \frac{L_n}{1}+\cdots+\frac{L_n}{n}$ and observing that the right-hand side is congruent to $1+\cdots+1/k$ modulo $p$. (The previous claim about $p-1$ follows immediately from Wolstenholme's theorem.)

This leads to a heuristic prediction (see for example a preprint of Shiu [Sh16]) of $\asymp\frac{x}{\log x}$ for the number of $n\in [1,x]$ such that $(a_n,L_n)=1$. In particular, there should be infinitely many $n$, but the set of such $n$ should have density zero. Unfortunately this heuristic is difficult to turn into a proof.

OPEN
Let $A$ be the set of $n\in \mathbb{N}$ such that there exist $1\leq m_1<\cdots <m_k=n$ with $\sum\tfrac{1}{m_i}=1$. Explore $A$. In particular,
• Does $A$ have density $1$?
• What are those $n\in A$ not divisible by any $d\in A$ with $1<d<n$?
Straus observed that $A$ is closed under multiplication. Furthermore, it is easy to see that $A$ does not contain any prime power.
OPEN
Let $k\geq 1$ and let $v(k)$ be the minimal integer which does not appear as some $n_i$ in a solution to $1=\frac{1}{n_1}+\cdots+\frac{1}{n_k}$ with $1\leq n_1<\cdots <n_k$. Estimate the growth of $v(k)$.
Results of Bleicher and Erdős [BlEr75] imply $v(k) \gg k!$. It may be that $v(k)$ grows doubly exponentially in $\sqrt{k}$ or even $k$.

An elementary inductive argument shows that $n_k\leq ku_k$ where $u_1=1$ and $u_{i+1}=u_i(u_i+1)$, and hence $v(k) \leq kc_0^{2^k},$ where $c_0=\lim_n u_n^{1/2^n}=1.26408\cdots$ is the 'Vardi constant'.

SOLVED
Let $N\geq 1$ and let $t(N)$ be the least integer $t$ such that there is no solution to $1=\frac{1}{n_1}+\cdots+\frac{1}{n_k}$ with $t=n_1<\cdots <n_k\leq N$. Estimate $t(N)$.
Erdős and Graham [ErGr80] could show $t(N)\ll\frac{N}{\log N},$ but had no idea of the true value of $t(N)$.

Solved by Liu and Sawhney [LiSa24] (up to $(\log\log N)^{O(1)}$), who proved that $\frac{N}{(\log N)(\log\log N)^3(\log\log\log N)^{O(1)}}\ll t(N) \ll \frac{N}{\log N}.$

OPEN
Let $N\geq 1$ and let $k(N)$ denote the smallest $k$ such that there exist $N\leq n_1<\cdots <n_k$ with $1=\frac{1}{n_1}+\cdots+\frac{1}{n_k}.$ Is it true that $\lim_{N\to \infty} k(N)-(e-1)N=\infty?$
Erdős and Straus [ErSt71b] have proved the existence of some constant $c>0$ such that $-c < k(N)-(e-1)N \ll \frac{N}{\log N}.$
SOLVED
Let $N\geq 1$ and let $k(N)$ be maximal such that there are $k$ disjoint $A_1,\ldots,A_k\subseteq \{1,\ldots,N\}$ with $\sum_{n\in A_i}\frac{1}{n}=1$ for all $i$. Estimate $k(N)$. Is it true that $k(N)=o(\log N)$?
More generally, how many disjoint $A_i$ can be found in $\{1,\ldots,N\}$ such that the sums $\sum_{n\in A_i}\frac{1}{n}$ are all equal? Using sunflowers it can be shown that there are at least $N\exp(-O(\sqrt{\log N}))$ such sets.

Hunter and Sawhney have observed that Theorem 3 of Bloom [Bl23] (coupled with the trivial greedy approach) implies that $k(N)=(1-o(1))\log N$.

Additional thanks to: Zachary Hunter and Mehtaab Sawhney
SOLVED
Let $N\geq 1$. How many $A\subseteq \{1,\ldots,N\}$ are there such that $\sum_{n\in A}\frac{1}{n}=1$?
It was not even known for a long time whether this is $2^{cN}$ for some $c<1$ or $2^{(1+o(1))N}$. In fact the former is true, and the correct value of $c$ is now known.
• Steinerberger [St24] proved the relevant count is at most $2^{0.93N}$;
• Independently, Liu and Sawhney [LiSa24] gave both upper and lower bounds, proving that the count is $2^{(0.91\cdots+o(1))N},$ where $0.91\cdots$ is an explicit number defined as the solution to a certain integral equation;
• Again independently this same asymptotic was proved (with a different proof) by Conlon, Fox, He, Mubayi, Pham, Suk, and Verstraëte [CFHMPSV24], who prove more generally, for any $x\in \mathbb{Q}_{>0}$, a similar expression for the number of $A\subseteq \{1,\ldots,N\}$ such that $\sum_{n\in A}\frac{1}{n}=x$;
• The above papers all appeared within weeks of each other in 2024; in 2017 a similar question (with $\leq 1$ rather than $=1$) was asked on MathOverflow by Mikhail Tikhomirov and proofs of the correct asymptotic were sketched by Lucia, RaphaelB4, and js21.
SOLVED
Does every set $A\subseteq \mathbb{N}$ of positive density contain some finite $S\subset A$ such that $\sum_{n\in S}\frac{1}{n}=1$?
The answer is yes, proved by Bloom [Bl21].

SOLVED
Is there an infinite sequence $a_1<a_2<\cdots$ such that $a_{i+1}-a_i=O(1)$ and no finite sum of $\frac{1}{a_i}$ is equal to $1$?
There does not exist such a sequence, which follows from the positive solution to [298] by Bloom [Bl21].
SOLVED
Let $A(N)$ denote the maximal cardinality of $A\subseteq \{1,\ldots,N\}$ such that $\sum_{n\in S}\frac{1}{n}\neq 1$ for all $S\subseteq A$. Estimate $A(N)$.
Erdős and Graham believe the answer is $A(N)=(1+o(1))N$. Croot [Cr03] disproved this, showing the existence of some constant $c<1$ such that $A(N)<cN$ for all large $N$. It is trivial that $A(N)\geq (1-\frac{1}{e}+o(1))N$.

Liu and Sawhney [LiSa24] have proved that $A(N)=(1-1/e+o(1))N$.

OPEN
What is the size of the largest $A\subseteq \{1,\ldots,N\}$ such that for any $a,b_1,\ldots,b_k\in A$ with $k\geq 2$ we have $\frac{1}{a}\neq \frac{1}{b_1}+\cdots+\frac{1}{b_k}?$ Is there a constant $c>1/2$ such that $\lvert A\rvert >cN$ is possible for all large $N$?
The example $A=(N/2,N]\cap \mathbb{N}$ shows that $\lvert A\rvert\geq N/2$.
OPEN
Is it true that, for any $\delta>1/2$, if $N$ is sufficiently large and $A\subseteq \{1,\ldots,N\}$ has $\lvert A\rvert \geq \delta N$ then there exist $a,b,c\in A$ such that $\frac{1}{a}=\frac{1}{b}+\frac{1}{c}.$
The colouring version of this is [303], which was solved by Brown and Rödl [BrRo91].

The possible alternative question, that if $A\subseteq \mathbb{N}$ is a set of positive lower density then must there exist $a,b,c\in A$ such that $\frac{1}{a}=\frac{1}{b}+\frac{1}{c},$ has a negative answer, taking for example $A$ to be the union of $[5^k,(1+\epsilon)5^k]$ for large $k$ and sufficiently small $\epsilon>0$. This was observed by Hunter and Sawhney.

Additional thanks to: Zachary Hunter and Mehtaab Sawhney
SOLVED
Is it true that in any finite colouring of the integers there exists a monochromatic solution to $\frac{1}{a}=\frac{1}{b}+\frac{1}{c}$ with distinct $a,b,c$?
The density version of this is [302]. This colouring version is true, as proved by Brown and Rödl [BrRo91].
OPEN
For integers $1\leq a<b$ let $N(a,b)$ denote the minimal $k$ such that there exist integers $1<n_1<\cdots<n_k$ with $\frac{a}{b}=\frac{1}{n_1}+\cdots+\frac{1}{n_k}.$ Estimate $N(b)=\max_{1\leq a<b}N(a,b)$. Is it true that $N(b) \ll \log\log b$?
Erdős [Er50c] proved that $\log\log b \ll N(b) \ll \frac{\log b}{\log\log b}.$ The upper bound was improved by Vose [Vo85] to $N(b) \ll \sqrt{\log b}.$ One can also investigate the average of $N(a,b)$ for fixed $b$, and it is known that $\frac{1}{b}\sum_{1\leq a<b}N(a,b) \gg \log\log b.$

Related to [18].

SOLVED
For integers $1\leq a<b$ let $D(a,b)$ be the minimal value of $n_k$ such that there exist integers $1\leq n_1<\cdots <n_k$ with $\frac{a}{b}=\frac{1}{n_1}+\cdots+\frac{1}{n_k}.$ Estimate $D(b)=\max_{1\leq a<b}D(a,b)$. Is it true that $D(b) \ll b(\log b)^{1+o(1)}?$
Bleicher and Erdős [BlEr76] have shown that $D(b)\ll b(\log b)^2.$ If $b=p$ is a prime then $D(p) \gg p\log p.$

This was solved by Yokota [Yo88], who proved that $D(b)\ll b(\log b)(\log\log b)^4(\log\log\log b)^2.$ This was improved by Liu and Sawhney [LiSa24] to $D(b)\ll b(\log b)(\log\log b)^3(\log\log\log b)^{O(1)}.$

OPEN
Let $a/b\in \mathbb{Q}_{>0}$ with $b$ squarefree. Are there integers $1<n_1<\cdots<n_k$, each the product of two distinct primes, such that $\frac{a}{b}=\frac{1}{n_1}+\cdots+\frac{1}{n_k}?$
For $n_i$ the product of three distinct primes, this is true when $b=1$, as proved by Butler, Erdős and Graham [BEG15] (this paper is perhaps Erdős' last paper, appearing 19 years after his death).
OPEN
Are there two finite sets of primes $P,Q$ such that $1=\left(\sum_{p\in P}\frac{1}{p}\right)\left(\sum_{q\in Q}\frac{1}{q}\right)?$
Asked by Barbeau [Ba76]. Can this be done if we drop the requirement that all $p\in P$ are prime and just ask for them to be relatively coprime, and similarly for $Q$?
OPEN
Let $N\geq 1$. What is the smallest integer not representable as the sum of distinct unit fractions with denominators from $\{1,\ldots,N\}$? Is it true that the set of integers not representable as such has the shape $[m,\infty)$ for some $m$?
SOLVED
Let $N\geq 1$. How many integers can be written as the sum of distinct unit fractions with denominators from $\{1,\ldots,N\}$? Are there $o(\log N)$ such integers?
The answer to the second question is no: there are at least $(1-o(1))\log N$ many such integers, which follows from a more precise result of Croot [Cr99], who showed that every integer at most $\leq \sum_{n\leq N}\frac{1}{n}-(\tfrac{9}{2}+o(1))\frac{(\log\log N)^2}{\log N}$ can be so represented.
Additional thanks to: Zachary Hunter and Mehtaab Sawhney
SOLVED
Let $\alpha >0$ and $N\geq 1$. Is it true that for any $A\subseteq \{1,\ldots,N\}$ with $\lvert A\rvert \geq \alpha N$ there exists some $S\subseteq A$ such that $\frac{a}{b}=\sum_{n\in S}\frac{1}{n}$ with $a\leq b =O_\alpha(1)$?
Liu and Sawhney [LiSa24] observed that the main result of Bloom [Bl21] implies a positive solution to this conjecture. They prove a more precise version, that if $(\log N)^{-1/7+o(1)}\leq \alpha \leq 1/2$ then there is some $S\subseteq A$ such that $\frac{a}{b}=\sum_{n\in S}\frac{1}{n}$ with $a\leq b \leq \exp(O(1/\alpha))$. They also observe that the dependence $b\leq \exp(O(1/\alpha))$ is sharp.
OPEN
What is the minimal value of $\lvert 1-\sum_{n\in A}\frac{1}{n}\rvert$ as $A$ ranges over all subsets of $\{1,\ldots,N\}$ which contain no $S$ such that $\sum_{n\in S}\frac{1}{n}=1$? Is it $e^{-(c+o(1))N}$ for some constant $c\in (0,1)$?
It is trivially at least $1/[1,\ldots,N]$.
OPEN
Does there exist some $c>0$ such that, for any $K>1$, whenever $A$ is a sufficiently large finite multiset of integers with $\sum_{n\in A}\frac{1}{n}>K$ there exists some $S\subseteq A$ such that $1-e^{-cK} < \sum_{n\in S}\frac{1}{n}\leq 1?$
Erdős and Graham knew this with $e^{-cK}$ replaced by $c/K^2$.
OPEN
Are there infinitely many solutions to $\frac{1}{p_1}+\cdots+\frac{1}{p_k}=1-\frac{1}{m},$ where $m\geq 2$ is an integer and $p_1<\cdots<p_k$ are distinct primes?
For example, $\frac{1}{2}+\frac{1}{3}=1-\frac{1}{6}.$
SOLVED
Let $n\geq 1$ and let $m$ be minimal such that $\sum_{n\leq k\leq m}\frac{1}{k}\geq 1$. We define $\epsilon(n) = \sum_{n\leq k\leq m}\frac{1}{k}-1.$ How small can $\epsilon(n)$ be? Is it true that $\liminf n^2\epsilon(n)=0?$
This is true, and shown by Lim and Steinerberger [LiSt24] who proved that there exist infinitely many $n$ such that $\epsilon(n)n^2\ll \left(\frac{\log\log n}{\log n}\right)^{1/2}.$ Erdős and Graham (and also Lim and Steinerberger) believe that the exponent of $2$ is best possible here, in that $\liminf \epsilon(n) n^{2+\delta}=\infty$ for all $\delta>0$.
OPEN
Let $u_1=2$ and $u_{n+1}=u_n^2-u_n+1$. Let $a_1<a_2<\cdots$ be any other sequence with $\sum \frac{1}{a_k}=1$. Is it true that $\liminf a_n^{1/2^n}<\lim u_n^{1/2^n}=c_0=1.264085\cdots?$
$c_0$ is called the Vardi constant and the sequence $u_n$ is Sylvester's sequence.

In [ErGr80] this problem is stated with the sequence $u_1=1$ and $u_{n+1}=u_n(u_n+1)$, but Quanyu Tang has pointed out this is probably an error (since with that choice we do not have $\sum \frac{1}{u_n}=1$). This question with Sylvester's sequence is the most natural interpretation of what they meant to ask.

SOLVED
Is it true that if $A\subset \mathbb{N}\backslash\{1\}$ is a finite set with $\sum_{n\in A}\frac{1}{n}<2$ then there is a partition $A=A_1\sqcup A_2$ such that $\sum_{n\in A_i}\frac{1}{n}<1$ for $i=1,2$?
This is not true if $A$ is a multiset, for example $2,3,3,5,5,5,5$.

This is not true in general, as shown by Sándor [Sa97], who observed that the proper divisors of $120$ form a counterexample. More generally, Sándor shows that for any $n\geq 2$ there exists a finite set $A\subseteq \mathbb{N}\backslash\{1\}$ with $\sum_{k\in A}\frac{1}{k}<n$ and no partition into $n$ parts each of which has $\sum_{k\in A_i}\frac{1}{k}<1$.

The minimal counterexample is $\{2,3,4,5,6,7,10,11,13,14,15\}$, found by Tom Stobart.

OPEN
Is there some constant $c>0$ such that for every $n\geq 1$ there exists some $\delta_k\in \{-1,0,1\}$ for $1\leq k\leq n$ with $0< \left\lvert \sum_{1\leq k\leq n}\frac{\delta_k}{k}\right\rvert < \frac{c}{2^n}?$ Is it true that for sufficiently large $n$, for any $\delta_k\in \{-1,0,1\}$, $\left\lvert \sum_{1\leq k\leq n}\frac{\delta_k}{k}\right\rvert > \frac{1}{[1,\ldots,n]}$ whenever the left-hand side is not zero?
Inequality is obvious for the second claim, the problem is strict inequality. This fails for small $n$, for example $\frac{1}{2}-\frac{1}{3}-\frac{1}{4}=-\frac{1}{12}.$
OPEN
Let $A\subseteq \mathbb{N}$ be an infinite arithmetic progression and $f:A\to \{-1,1\}$ be a non-constant function. Must there exist a finite $S\subset A$ such that $\sum_{n\in S}\frac{f(n)}{n}=0?$ What about if $A$ is an arbitrary set of positive density? What if $A$ is the set of squares excluding $1$?
Erdős and Straus [ErSt75] proved this when $A=\mathbb{N}$. Sattler [Sat75] proved this when $A$ is the set of odd numbers. For the squares $1$ must be excluded or the result is trivially false, since $\sum_{k\geq 2}\frac{1}{k^2}<1.$
OPEN
What is the size of the largest $A\subseteq \{1,\ldots,N\}$ such that there is a function $\delta:A\to \{-1,1\}$ such that $\sum_{n\in A}\frac{\delta_n}{n}=0$ and $\sum_{n\in A'}\frac{\delta_n}{n}\neq 0$ for all $A'\subsetneq A$?
OPEN
Let $S(N)$ count the number of distinct sums of the form $\sum_{n\in A}\frac{1}{n}$ for $A\subseteq \{1,\ldots,N\}$. Estimate $S(N)$.
Bleicher and Erdős [BlEr75] proved the lower bound $\frac{N}{\log N}\prod_{i=3}^k\log_iN\leq \frac{\log S(N)}{\log 2},$ valid for $k\geq 4$ and $\log_kN\geq k$, and also [BlEr76b] proved the upper bound $\log S(N)\leq \log_r N\left(\frac{N}{\log N} \prod_{i=3}^r \log_iN\right),$ valid for $r\geq 1$ and $\log_{2r}N\geq 1$. (In these bounds $\log_in$ denotes the $i$-fold iterated logarithm.)

Additional thanks to: Boris Alexeev and Dustin Mixon
OPEN
What is the size of the largest $A\subseteq \{1,\ldots,N\}$ such that all sums $\sum_{n\in S}\frac{1}{n}$ are distinct for $S\subseteq A$?
Let $R(N)$ be the maximal such size. Results of Bleicher and Erdős from [BlEr75] and [BlEr76b] imply that $\frac{N}{\log N}\prod_{i=3}^k\log_iN\leq R(N)\leq \frac{1}{\log 2}\log_r N\left(\frac{N}{\log N} \prod_{i=3}^r \log_iN\right),$ valid for any $k\geq 4$ with $\log_kN\geq k$ and any $r\geq 1$ with $\log_{2r}N\geq 1$. (In these bounds $\log_in$ denotes the $i$-fold iterated logarithm.)

Additional thanks to: Boris Alexeev, Zachary Hunter, and Dustin Mixon
OPEN
Let $k\geq 3$ and $A\subset \mathbb{N}$ be the set of $k$th powers. What is the order of growth of $1_A^{(k)}(n)$, i.e. the number of representations of $n$ as the sum of $k$ many $k$th powers? Does there exist some $c>0$ and infinitely many $n$ such that $1_A^{(k)}(n) >n^c?$
Connected to Waring's problem. The famous Hypothesis $K$ of Hardy and Littlewood was that $1_A^{(k)}(n)\leq n^{o(1)}$, but this was disproved by Mahler [Ma36] for $k=3$, who constructed infinitely many $n$ such that $1_A^{(3)}(n)\gg n^{1/12}$ (where $A$ is the set of cubes). Erdős believed Hypothesis $K$ fails for all $k\geq 4$, but this is unknown. Hardy and Littlewood made the weaker Hypothesis $K^*$ that for all $N$ and $\epsilon>0$ $\sum_{n\leq N}1_A^{(k)}(n)^2 \ll_\epsilon N^{1+\epsilon}.$ Erdős and Graham remark: 'This is probably true but no doubt very deep. However, it would suffice for most applications.'

Independently Erdős [Er36] and Chowla proved that for all $k\geq 3$ and infinitely many $n$ $1_A^{(k)}(n) \gg n^{c/\log\log n}$ for some constant $c>0$ (depending on $k$).

OPEN
Let $1\leq m\leq k$ and $f_{k,m}(x)$ denote the number of integers $\leq x$ which are the sum of $m$ many nonnegative $k$th powers. Is it true that $f_{k,k}(x) \gg_\epsilon x^{1-\epsilon}$ for all $\epsilon>0$? Is it true that if $m<k$ then $f_{k,m}(x) \gg x^{m/k}$ for sufficiently large $x$?
This would have significant applications to Waring's problem. Erdős and Graham describe this as 'unattackable by the methods at our disposal'. The case $k=2$ was resolved by Landau, who showed $f_{2,2}(x) \sim \frac{cx}{\sqrt{\log x}}$ for some constant $c>0$.

For $k>2$ it is not known if $f_{k,k}(x)=o(x)$.

OPEN
Does there exist a polynomial $f(x)\in\mathbb{Z}[x]$ such that all the sums $f(a)+f(b)$ with $a<b$ nonnegative integers are distinct?
Erdős and Graham describe this problem as 'very annoying'. Probably $f(x)=x^5$ should work.
OPEN
Let $k\geq 3$ and $f_{k,3}(x)$ denote the number of integers $\leq x$ which are the sum of three nonnegative $k$th powers. Is it true that $f_{k,3}(x) \gg x^{3/k}$ or even $\gg_\epsilon x^{3/k-\epsilon}$?
Mahler and Erdős [ErMa38] proved that $f_{k,2}(x) \gg x^{2/k}$. For $k=3$ the best known is due to Wooley [Wo15], $f_{3,3}(x) \gg x^{0.917\cdots}.$
OPEN
Let $A\subset \mathbb{N}$ be an additive basis of order $2$. Must there exist $B=\{b_1<b_2<\cdots\}\subseteq A$ which is also a basis such that $\lim_{k\to \infty}\frac{b_k}{k^2}$ does not exist?
Erdős originally asked whether this was true with $A=B$, but this was disproved by Cassels [Ca57].
OPEN
Suppose $A\subseteq \{1,\ldots,N\}$ is such that if $a,b\in A$ and $a\neq b$ then $a+b\nmid ab$. Can $A$ be 'substantially more' than the odd numbers?

What if $a,b\in A$ with $a\neq b$ implies $a+b\nmid 2ab$? Must $\lvert A\rvert=o(N)$?

The connection to unit fractions comes from the observation that $\frac{1}{a}+\frac{1}{b}$ is a unit fraction if and only if $a+b\mid ab$.
SOLVED
Suppose $A\subseteq\mathbb{N}$ and $C>0$ is such that $1_A\ast 1_A(n)\leq C$ for all $n\in\mathbb{N}$. Can $A$ be partitioned into $t$ many subsets $A_1,\ldots,A_t$ (where $t=t(C)$ depends only on $C$) such that $1_{A_i}\ast 1_{A_i}(n)<C$ for all $1\leq i\leq t$ and $n\in \mathbb{N}$?
Asked by Erdős and Newman. Nešetřil and Rödl have shown the answer is no for all $C$ (source is cited as 'personal communication' in [ErGr80]). Erdős had previously shown the answer is no for $C=3,4$ and infinitely many other values of $C$.
OPEN
Suppose $A\subseteq \mathbb{N}$ is a Sidon set. How large can $\limsup_{N\to \infty}\frac{\lvert A\cap \{1,\ldots,N\}\rvert}{N^{1/2}}$ be?
Erdős proved that $1/2$ is possible and Krückeberg [Kr61] proved $1/\sqrt{2}$ is possible. Erdős and Turán [ErTu41] have proved this $\limsup$ is always $\leq 1$.

The fact that $1$ is possible would follow if any finite Sidon set is a subset of a perfect difference set (see [707]).

OPEN
Suppose $A\subset\mathbb{N}$ is a minimal basis with positive density. Is it true that, for any $n\in A$, the (upper) density of integers which cannot be represented without using $n$ is positive?
SOLVED
Let $A,B\subseteq \mathbb{N}$ such that for all large $N$ $\lvert A\cap \{1,\ldots,N\}\rvert \gg N^{1/2}$ and $\lvert B\cap \{1,\ldots,N\}\rvert \gg N^{1/2}.$ Is it true that there are infinitely many solutions to $a_1-a_2=b_1-b_2\neq 0$ with $a_1,a_2\in A$ and $b_1,b_2\in B$?
Ruzsa has observed that there is a simple counterexample: take $A$ to be the set of numbers whose binary representation has only non-zero digits in even places, and $B$ similarly but with non-zero digits only in odd places. It is easy to see $A$ and $B$ both grow like $\gg N^{1/2}$ and yet for any $n\geq 1$ there is exactly one solution to $n=a+b$ with $a\in A$ and $b\in B$.

Ruzsa suggests that a non-trivial variant of this problem arises if one imposes the stronger condition that $\lvert A\cap \{1,\ldots,N\}\rvert \sim c_AN^{1/2}$ for some constant $c_A>0$ as $N\to \infty$, and similarly for $B$.

OPEN
Let $A\subseteq \mathbb{N}$ and $D(A)$ be the set of those numbers which occur infinitely often as $a_1-a_2$ with $a_1,a_2\in A$. What conditions on $A$ are sufficient to ensure $D(A)$ has bounded gaps?
Prikry, Tijdeman, Stewart, and others (see the survey articles [St78] and [Ti79]) have shown that a sufficient condition is that $A$ has positive density.

One can also ask what conditions are sufficient for $D(A)$ to have positive density, or for $\sum_{d\in D(A)}\frac{1}{d}=\infty$, or even just $D(A)\neq\emptyset$.

OPEN
Let $A\subseteq \mathbb{N}$ be a set of density zero. Does there exist a basis $B$ such that $A\subseteq B+B$ and $\lvert B\cap \{1,\ldots,N\}\rvert =o(N^{1/2})$ for all large $N$?
Erdős and Newman [ErNe77] have proved this is true when $A$ is the set of squares.

OPEN
Find the best function $f(n)$ such that every $n$ can be written as $n=a+b$ where both $a,b$ are $f(n)$-smooth (that is, are not divisible by any prime $p>f(n)$.)
Erdős originally asked if even $f(n)\leq n^{1/3}$ is true. This is known, and the best bound is due to Balog [Ba89] who proved that $f(n) \ll_\epsilon n^{\frac{4}{9\sqrt{e}}+\epsilon}$ for all $\epsilon>0$. (Note $\frac{4}{9\sqrt{e}}=0.2695\ldots$.)

It is likely that $f(n)\leq n^{o(1)}$, or even $f(n)\leq e^{O(\sqrt{\log n})}$.

OPEN
Let $d(A)$ denote the density of $A\subseteq \mathbb{N}$. Characterise those $A,B\subseteq \mathbb{N}$ with positive density such that $d(A+B)=d(A)+d(B).$
One way this can happen is if there exists $\theta>0$ such that $A=\{ n>0 : \{ n\theta\} \in X_A\}\textrm{ and }B=\{ n>0 : \{n\theta\} \in X_B\}$ where $\{x\}$ denotes the fractional part of $x$ and $X_A,X_B\subseteq \mathbb{R}/\mathbb{Z}$ are such that $\mu(X_A+X_B)=\mu(X_A)+\mu(X_B)$. Are all possible $A$ and $B$ generated in a similar way (using other groups)?
OPEN
For $r\geq 2$ let $h(r)$ be the maximal finite $k$ such that there exists a basis $A\subseteq \mathbb{N}$ of order $r$ (so every large integer is the sum of at most $r$ integers from $A$) and exact order $k$ (i.e. $k$ is minimal such that every large integer is the sum of exactly $k$ integers from $A$). Find the value of $\lim_r \frac{h(r)}{r^2}.$
Erdős and Graham [ErGr80b] have shown that a basis $A$ has an exact order if and only if $a_2-a_1,a_3-a_2,a_4-a_3,\ldots$ are coprime. They also prove that $\frac{1}{4}\leq \lim_r \frac{h(r)}{r^2}\leq \frac{5}{4}.$ It is known that $h(2)=4$, but even $h(3)$ is unknown (it is $\geq 7$).
SOLVED
Let $A\subseteq \mathbb{N}$ be an additive basis (of any finite order) such that $\lvert A\cap \{1,\ldots,N\}\rvert=o(N)$. Is it true that $\lim_{N\to \infty}\frac{\lvert (A+A)\cap \{1,\ldots,N\}\rvert}{\lvert A\cap \{1,\ldots,N\}\rvert}=\infty?$
The answer is no, and a counterexample was provided by Turjányi [Tu84]. This was generalised (to the replacement of $A+A$ by the $h$-fold sumset $hA$ for any $h\geq 2$) by Ruzsa and Turjányi [RT85]. Ruzsa and Turjányi do prove (under the same hypotheses) that $\lim_{N\to \infty}\frac{\lvert (A+A+A)\cap \{1,\ldots,3N\}\rvert}{\lvert A\cap \{1,\ldots,N\}\rvert}=\infty,$ and conjecture that the same should be true with $(A+A)\cap \{1,\ldots,2N\}$ in the numerator.
Additional thanks to: Zachary Chase, Zach Hunter
OPEN
The restricted order of a basis is the least integer $t$ (if it exists) such that every large integer is the sum of at most $t$ distinct summands from $A$. What are necessary and sufficient conditions that this exists? Can it be bounded (when it exists) in terms of the order of the basis? What are necessary and sufficient conditions that this is equal to the order of the basis?
Bateman has observed that for $h\geq 3$ there is a basis of order $h$ with no restricted order, taking $A=\{1\}\cup \{x>0 : h\mid x\}.$ Kelly [Ke57] has shown that any basis of order $2$ has restricted order at most $4$ and conjectures it always has restricted order at most $3$.

The set of squares has order $4$ and restricted order $5$ (see [Pa33]) and the set of triangular numbers has order $3$ and restricted order $3$ (see [Sc54]).

Is it true that if $A\backslash F$ is a basis for all finite sets $F$ then $A$ must have a restricted order? What if they are all bases of the same order?

OPEN
Let $A\subseteq \mathbb{N}$ be a basis of order $r$. Must the set of integers representable as the sum of exactly $r$ distinct elements from $A$ have positive density?
OPEN
Let $A=\{1,2,4,8,13,21,31,45,66,81,97,\ldots\}$ be the greedy Sidon sequence: we begin with $1$ and iteratively include the next smallest integer that preserves the Sidon property. What is the order of growth of $A$? Is it true that $\lvert A\cap \{1,\ldots,N\}\rvert \gg N^{1/2-\epsilon}$ for all $\epsilon>0$ and large $N$?
Erdős and Graham [ErGr80] also ask about the difference set $A-A$, whether this has positive density, and whether this contains $22$.

This sequence is at OEIS A005282.

OPEN
Let $A=\{a_1<\cdots<a_k\}$ be a finite set of integers and extend it to an infinite sequence $\overline{A}=\{a_1<a_2<\cdots \}$ by defining $a_{n+1}$ for $n\geq k$ to be the least integer exceeding $a_n$ which is not of the form $a_i+a_j$ with $i,j\leq n$. Is it true that the sequence of differences $a_{m+1}-a_m$ is eventually periodic?
An old problem of Dickson. Even a starting set as small as $\{1,4,9,16,25\}$ requires thousands of terms before periodicity occurs.
OPEN
With $a_1=1$ and $a_2=2$ let $a_{n+1}$ for $n\geq 2$ be the least integer $>a_n$ which can be expressed uniquely as $a_i+a_j$ for $i<j\leq n$.

What can be said about this sequence? Do infinitely many pairs $a,a+2$ occur? Does this sequence eventually have periodic differences? Is the density $0$?

A problem of Ulam. The sequence is $1,2,3,4,6,8,11,13,16,18,26,28,\ldots$ at OEIS A002858.
SOLVED
If $A\subseteq \mathbb{N}$ is a multiset of integers such that $\lvert A\cap \{1,\ldots,N\}\rvert\gg N$ for all $N$ then must $A$ be subcomplete? That is, must $P(A) = \left\{\sum_{n\in B}n : B\subseteq A\textrm{ finite }\right\}$ contain an infinite arithmetic progression?
A problem of Folkman. Folkman [Fo66] showed that this is true if $\lvert A\cap \{1,\ldots,N\}\rvert\gg N^{1+\epsilon}$ for some $\epsilon>0$ and all $N$.

The original question was answered by Szemerédi and Vu [SzVu06] (who proved that the answer is yes).

This is best possible, since Folkman [Fo66] showed that for all $\epsilon>0$ there exists a multiset $A$ with $\lvert A\cap \{1,\ldots,N\}\rvert\ll N^{1+\epsilon}$ for all $N$, such that $A$ is not subcomplete.

SOLVED
If $A\subseteq \mathbb{N}$ is a set of integers such that $\lvert A\cap \{1,\ldots,N\}\rvert\gg N^{1/2}$ for all $N$ then must $A$ be subcomplete? That is, must $P(A) = \left\{\sum_{n\in B}n : B\subseteq A\textrm{ finite }\right\}$ contain an infinite arithmetic progression?
Folkman proved this under the stronger assumption that $\lvert A\cap \{1,\ldots,N\}\rvert\gg N^{1/2+\epsilon}$ for some $\epsilon>0$.

This is true, and was proved by Szemerédi and Vu [SzVu06]. The stronger conjecture that this is true under $\lvert A\cap \{1,\ldots,N\}\rvert\geq (2N)^{1/2}$ seems to be still open (this would be best possible as shown by [Er61b].

OPEN
Let $A\subseteq \mathbb{N}$ be a complete sequence, and define the threshold of completeness $T(A)$ to be the least integer $m$ such that all $n\geq m$ are in $P(A) = \left\{\sum_{n\in B}n : B\subseteq A\textrm{ finite }\right\}$ (the existence of $T(A)$ is guaranteed by completeness).

Is it true that there are infinitely many $k$ such that $T(n^k)>T(n^{k+1})$?

Erdős and Graham [ErGr80] remark that very little is known about $T(A)$ in general. It is known that $T(n)=1, T(n^2)=128, T(n^3)=12758,$ $T(n^4)=5134240,\textrm{ and }T(n^5)=67898771.$ Erdős and Graham remark that a good candidate for the $n$ in the question are $k=2^t$ for large $t$, perhaps even $t=3$, because of the highly restricted values of $n^{2^t}$ modulo $2^{t+1}$.
OPEN
Let $A=\{a_1< a_2<\cdots\}$ be a set of integers such that
• $A\backslash B$ is complete for any finite subset $B$ and
• $A\backslash B$ is not complete for any infinite subset $B$.
Is it true that if $a_{n+1}/a_n \geq 1+\epsilon$ for some $\epsilon>0$ and all $n$ then $\lim_n \frac{a_{n+1}}{a_n}=\frac{1+\sqrt{5}}{2}?$
Graham [Gr64d] has shown that the sequence $a_n=F_n-(-1)^{n}$, where $F_n$ is the $n$th Fibonacci number, has these properties. Erdős and Graham [ErGr80] remark that it is easy to see that if $a_{n+1}/a_n>\frac{1+\sqrt{5}}{2}$ then the second property is automatically satisfied, and that it is not hard to construct very irregular sequences satisfying both properties.
OPEN
Is there a sequence $A=\{a_1\leq a_2\leq \cdots\}$ of integers with $\lim \frac{a_{n+1}}{a_n}=2$ such that $P(A')= \left\{\sum_{n\in B}n : B\subseteq A'\textrm{ finite }\right\}$ has density $1$ for every cofinite subsequence $A'$ of $A$?
OPEN
For what values of $0\leq m<n$ is there a complete sequence $A=\{a_1\leq a_2\leq \cdots\}$ of integers such that
• $A$ remains complete after removing any $m$ elements, but
• $A$ is not complete after removing any $n$ elements?
The Fibonacci sequence $1,1,2,3,5,\ldots$ shows that $m=1$ and $n=2$ is possible. The sequence of powers of $2$ shows that $m=0$ and $n=1$ is possible. The case $m=2$ and $n=3$ is not known.
OPEN
For what values of $t,\alpha \in (0,\infty)$ is the sequence $\lfloor t\alpha^n\rfloor$ complete?
Even in the range $t\in (0,1)$ and $\alpha\in (1,2)$ the behaviour is surprisingly complex. For example, Graham [Gr64e] has shown that for any $k$ there exists some $t_k\in (0,1)$ such that the set of $\alpha$ such that the sequence is complete consists of at least $k$ disjoint line segments. It seems likely that the sequence is complete for all $t>0$ and all $1<\alpha < \frac{1+\sqrt{5}}{2}$. Proving this seems very difficult, since we do not even known whether $\lfloor (3/2)^n\rfloor$ is odd or even infinitely often.
SOLVED
If $A\subset\mathbb{N}$ is a finite set of integers all of whose subset sums are distinct then $\sum_{n\in A}\frac{1}{n}<2.$
This was proved by Ryavec. The stronger statement that, for all $s\geq 0$, $\sum_{n\in A}\frac{1}{n^s} <\frac{1}{1-2^{-s}},$ was proved by Hanson, Steele, and Stenger [HSS77].

OPEN
Let $p(x)\in \mathbb{Q}[x]$. Is it true that $A=\{ p(n)+1/n : n\in \mathbb{N}\}$ is strongly complete, in the sense that, for any finite set $B$, $\left\{\sum_{n\in X}n : X\subseteq A\backslash B\textrm{ finite }\right\}$ contains all sufficiently large rational numbers?
Graham [Gr64f] proved this is true when $p(n)=n$. Erdős and Graham also ask which rational functions $r(x)\in\mathbb{Z}[x]$ force $\{ r(n) : n\in\mathbb{N}\}$ to be complete?
OPEN
Let $\alpha,\beta\in \mathbb{R}_{>0}$ such that $\alpha/\beta$ is irrational. Is $\{ \lfloor \alpha\rfloor,\lfloor 2\alpha\rfloor,\lfloor 4\alpha\rfloor,\ldots\}\cup \{ \lfloor \beta\rfloor,\lfloor 2\beta\rfloor,\lfloor 4\beta\rfloor,\ldots\}$ complete? What if $2$ is replaced by some $\gamma\in(1,2)$?
SOLVED
Let $A\subseteq \mathbb{N}$ be a lacunary sequence (so that $A=\{a_1<a_2<\cdots\}$ and there exists some $\lambda>1$ such that $a_{n+1}/a_n\geq \lambda$ for all $n\geq 1$). Must $\left\{ \sum_{a\in A'}\frac{1}{a} : A'\subseteq A\textrm{ finite}\right\}$ contain all rationals in some open interval?
Bleicher and Erdős conjecture the answer is no.

Steinerberger has pointed out that as written this problem is trivial: simply take some lacunary $A$ whose prime factors are restricted (e.g. $A=\{1,2,4,8,\ldots,\}$) - clearly any finite sum of the shape $\sum_{a\in A'}\frac{1}{a}$ can only form a rational with denominator divisible by one of these restricted set of primes.

This is puzzling, since Erdős and Graham were very aware of this kind of obstruction, so it's a strange thing to miss. I assume that there was some unwritten extra assumption intended (e.g. $A$ contains a multiple of every integer).

SOLVED
Is there some $c>0$ such that, for all sufficiently large $n$, there exist integers $a_1<\cdots<a_k\leq n$ such that there are at least $cn^2$ distinct integers of the form $\sum_{u\leq i\leq v}a_i$?
This fails for $a_i=i$ for example. Erdős and Graham also ask what happens if we drop the monotonicity restriction and just ask that the $a_i$ are distinct. Perhaps some permutation of $\{1,\ldots,n\}$ has at least $cn^2$ such distinct sums (this was solved by Konieczny [Ko15], see [34]).

The original problem was solved (in the affirmative) by Beker [Be23b].

They also ask how many consecutive integers $>n$ can be represented as such a sum? Is it true that, for any $c>0$ at least $cn$ such integers are possible (for sufficiently large $n$)?

OPEN
Let $1\leq a_1<\cdots <a_k\leq n$ be integers such that all sums of the shape $\sum_{u\leq i\leq v}a_i$ are distinct. How large can $k$ be? Must $k=o(n)$?
Asked by Erdős and Harzheim. What if we remove the monotonicity and/or the distinctness constraint? Also what is the least $m$ which is not a sum of the given form? Can it be much larger than $n$? Erdős and Harzheim can show that $\sum_{x<a_i<x^2}\frac{1}{a_i}\ll 1$. Is it true that $\sum_i \frac{1}{a_i}\ll 1$?

OPEN
Is there a sequence $A=\{1\leq a_1<a_2<\cdots\}$ such that every integer is the sum of some finite number of consecutive elements of $A$? Can the number of representations of $n$ in this form tend to infinity with $n$?
Erdős and Moser [Mo63] considered the case when $A$ is the set of primes, and conjectured that the $\limsup$ of the number of such representations in this case is infinite. They could not even prove that the upper density of the set of integers representable in this form is positive.
OPEN
Let $a_1<a_2<\cdots$ be an infinite sequence of integers such that $a_1=n$ and $a_{i+1}$ is the least integer which is not a sum of consecutive earlier $a_j$s. What can be said about the density of this sequence?

In particular, in the case $n=1$, can one prove that $a_k/k\to \infty$ and $a_k/k^{1+c}\to 0$ for any $c>0$?

A problem of MacMahon, studied by Andrews [An75]. When $n=1$ this sequence begins $1,2,4,5,8,10,15,\ldots.$ Andrews conjectures $a_k\sim \frac{k\log k}{\log\log k}.$

SOLVED
Let $f(n)$ be minimal such that $\{1,\ldots,n\}$ can be partitioned into $f(n)$ classes so that $n$ cannot be expressed as a sum of distinct elements from the same class. How fast does $f(n)$ grow?
Alon and Erdős [AlEr96] proved that $f(n) = n^{1/3+o(1)}$, and more precisely $\frac{n^{1/3}}{(\log n)^{4/3}}\ll f(n) \ll \frac{n^{1/3}}{(\log n)^{1/3}}(\log\log n)^{1/3}.$ Vu [Vu07] improved the lower bound to $f(n) \gg \frac{n^{1/3}}{\log n}.$ Conlon, Fox, and Pham [CFP21] determined the order of growth of $f(n)$ up to a multiplicative constant, proving $f(n) \asymp \frac{n^{1/3}(n/\phi(n))}{(\log n)^{1/3}(\log\log n)^{2/3}}.$
OPEN
Let $c>0$ and $n$ be some large integer. What is the size of the largest $A\subseteq \{1,\ldots,\lfloor cn\rfloor\}$ such that $n$ is not a sum of a subset of $A$? Does this depend on $n$ in an irregular way?
OPEN
Let $A\subseteq \mathbb{N}$ be a finite set of size $N$. Is it true that, for any fixed $t$, there are $\ll \frac{2^N}{N^{3/2}}$ many $S\subseteq A$ such that $\sum_{n\in S}n=t$?

If we further ask that $\lvert S\rvert=l$ (for any fixed $l$) then is the number of solutions $\ll \frac{2^N}{N^2},$ with the implied constant independent of $l$ and $t$?

Erdős and Moser proved the first bound with an additional factor of $(\log n)^{3/2}$. This was removed by Sárközy and Szemerédi [SaSz65], thereby answering the first question in the affirmative. Stanley [St80] has shown that this quantity is maximised when $A$ is an arithmetic progression and $t=\tfrac{1}{2}\sum_{n\in A}n$.
SOLVED
When is the product of two or more disjoint blocks of consecutive integers a power? Is it true that there are only finitely many collections of disjoint intervals $I_1,\ldots,I_n$ of size $\lvert I_i\rvert \geq 4$ for $1\leq i\leq n$ such that $\prod_{1\leq i\leq n}\prod_{m\in I_i}m$ is a square?
Erdős and Selfridge have proved that the product of consecutive integers is never a power. The condition $\lvert I_i\rvert \geq 4$ is necessary here, since Pomerance has observed that the product of $(2^{n-1}-1)2^{n-1}(2^{n-1}+1),$ $(2^n-1)2^n(2^n+1),$ $(2^{2n-1}-2)(2^{2n-1}-1)2^{2n-1},$ and $(2^{2n-2}-2)(2^{2n}-1)2^{2n}$ is always a square.

This is false: Ulas [Ul05] has proved there are infinitely many solutions when $n=4$ or $n\geq 6$ and $\lvert I_i\rvert=4$ for $1\leq i\leq n$. Bauer and Bennett [BaBe07] proved there are infinitely many solutions when $n=3$ or $n=5$ and $\lvert I_i\rvert=4$ for $1\leq i\leq n$. Furthermore, Bennett and Van Luijk [BeVL12] have found infinitely many solutions when $n\geq 5$ and $\lvert I_i\rvert=5$ for $1\leq i\leq n$.

In general, Ulas conjectures there are infinitely many solutions for any fixed size of $\lvert I_i\rvert$, provided $n$ is sufficiently large.

Additional thanks to: Euro Vidal Sampaio and Julia Schmerling
OPEN
Are there any triples of consecutive positive integers all of which are powerful (i.e. if $p\mid n$ then $p^2\mid n$)?
Erdős originally asked Mahler whether there are infinitely many pairs of consecutive powerful numbers, but Mahler immediately observed that the answer is yes from the infinitely many solutions to the Pell equation $x^2=8y^2+1$.

OPEN
Do all pairs of consecutive powerful numbers $n$ and $n+1$ come from solutions to Pell equations? In other words, must either $n$ or $n+1$ be a square?

Is the number of such $n\leq x$ bounded by $(\log x)^{O(1)}$?

Erdős originally asked Mahler whether there are infinitely many pairs of consecutive powerful numbers, but Mahler immediately observed that the answer is yes from the infinitely many solutions to the Pell equation $x^2=2^3y^2+1$.

The list of $n$ such that $n$ and $n+1$ are both powerful is A060355 in the OEIS.

The answer to the first question is no: Golomb [Go70] observed that both $12167=23^3$ and $12168=2^33^213^2$ are powerful. Walker [Wa76] proved that the equation $7^3x^2=3^3y^2+1$ has infinitely many solutions, giving infinitely many counterexamples.

OPEN
Are there any 2-full $n$ such that $n+1$ is 3-full? That is, if $p\mid n$ then $p^2\mid n$ and if $p\mid n+1$ then $p^3\mid n+1$.
Erdős originally asked Mahler whether there are infinitely many pairs of consecutive powerful numbers, but Mahler immediately observed that the answer is yes from the infinitely many solutions to the Pell equation $x^2=8y^2+1$.

Stephan has found no such $n$ below $10^8$.

Note that $8$ is 3-full and $9$ is 2-full. Erdős and Graham asked if this is the only pair of such consecutive integers. Stephan has observed that $12167=23^3$ and $12168=2^33^213^2$ (a pair already known to Golomb [Go70]) is another example, but there are no other examples below $10^8$.

OPEN
Let $B_2(n)$ be the 2-full part of $n$ (that is, $B_2(n)=n/n'$ where $n'$ is the product of all primes that divide $n$ exactly once). Is it true that, for every fixed $k\geq 1$, $\prod_{n\leq m<n+k}B_2(m) \ll n^{2+o(1)}?$ Or perhaps even $\ll_k n^2$?
It would also be interesting to find upper and lower bounds for the analogous product with $B_r$ for $r\geq 3$, where $B_r(n)$ is the $r$-full part of $n$ (that is, the product of prime powers $p^a \mid n$ such that $p^{a+1}\nmid n$ and $a\geq r$). Is it true that, for every fixed $r,k\geq 2$ and $\epsilon>0$, $\limsup \frac{\prod_{n\leq m<n+k}B_r(m) }{n^{1+\epsilon}}\to\infty?$
OPEN
How large is the largest prime factor of $n(n+1)$?
Mahler [Ma35] showed that this is $\gg \log\log n$. Schinzel [Sc67b] observed that for infinitely many $n$ it is $\leq n^{O(1/\log\log\log n)}$. The truth is probably $\gg (\log n)^2$ for all $n$.

The largest prime factors of $n(n+1)$ are listed as A074399 in the OEIS.

OPEN
Let $\epsilon>0$ and $k\geq 2$. Is it true that, for all sufficiently large $n$, there is a sequence of $k$ consecutive integers in $\{1,\ldots,n\}$ all of which are $n^\epsilon$-smooth?
Erdős and Graham state that this is open even for $k=2$ and 'the answer should be affirmative but the problem seems very hard'.

Unfortunately the problem is trivially true as written (simply taking $\{1,\ldots,k\}$ and $n>k^{1/\epsilon}$). There are (at least) two possible variants which are non-trivial, and it is not clear which Erdős and Graham meant. Let $P$ be the sequence of $k$ consecutive integers sought for. The potential strengthenings which make this non-trivial are:

• Each $m\in P$ must be $m^\epsilon$-smooth. If this is the problem then the answer is yes, which follows from a result of Balog and Wooley [BaWo98]: for any $\epsilon>0$ and $k\geq 2$ there exist infinitely many $m$ such that $m+1,\ldots,m+k$ are all $m^\epsilon$-smooth.
• Each $m\in P$ must be in $[n/2,n]$ (say). In this case a positive answer also follows from the result of Balog and Wooley [BaWo98] for infinitely many $n$, but the case of all sufficiently large $n$ is open.

SOLVED
Are there infinitely many $n$ such that the largest prime factor of $n$ is $<n^{1/2}$ and the largest prime factor of $n+1$ is $<(n+1)^{1/2}$?
Pomerance has observed that if we replace $1/2$ in the exponent by $1/\sqrt{e}-\epsilon$ for any $\epsilon>0$ then this is true for density reasons (since the density of integers $n$ whose greatest prime factor is $\leq n^{1/\sqrt{e}}$ is $1/2$).

Steinerberger has pointed out this problem has a trivial solution: take $n=m^2-1$, and then it is obvious that the largest prime factor of $n$ is $\leq m+1\ll n^{1/2}$ and the largest prime factor of $n+1$ is $\leq m\ll (n+1)^{1/2}$ (these $\ll$ can be replaced by $<$ if we choose $m$ such that $m,m+1$ are both composite).

Given that Erdős and Graham describe the above observation of Pomerance and explicitly say about this problem that 'we know very little about this', it is strange that such a trivial obstruction was overlooked. Perhaps the problem they intended was subtly different, and the problem in this form was the result of a typographical error, but I have no good guess what was intended here.

OPEN
Let $P(n)$ denote the largest prime factor of $n$. Show that the set of $n$ with $P(n+1)>P(n)$ has density $1/2$.
Conjectured by Erdős and Pomerance [ErPo78], who proved that this set and its complement both have positive upper density.

In [Er79e] Erdős also asks whether, for every $\alpha>0$, the density of the set of $n$ where $P(n+1)>P(n)n^\alpha$ exists.

The sequence of such $n$ is A070089 in the OEIS.

SOLVED
Let $P(n)$ denote the largest prime factor of $n$. There are infinitely many $n$ such that $P(n)>P(n+1)>P(n+2)$.
Conjectured by Erdős and Pomerance [ErPo78], who proved the analogous result for $P(n)<P(n+1)<P(n+2)$. Solved by Balog [Ba01], who proved that this is true for $\gg \sqrt{x}$ many $n\leq x$ (for all large $x$).
OPEN
Show that the equation $n! = a_1!a_2!\cdots a_k!,$ with $n-1>a_1\geq a_2\geq \cdots \geq a_k$, has only finitely many solutions.
This would follow if $P(n(n+1))/\log n\to \infty$, where $P(m)$ denotes the largest prime factor of $m$ (see Problem [368]). Hickerson conjectured the largest solution is $16! = 14! 5!2!.$ The condition $a_1<n-1$ is necessary to rule out the trivial solutions when $n=a_2!\cdots a_k!$.

Surányi was the first to conjecture that the only non-trivial solution to $a!b!=n!$ is $6!7!=10!$.

OPEN
For any $m\in \mathbb{N}$, let $F(m)$ be the minimal $k\geq 2$ (if it exists) such that there are $a_1<\cdots <a_k=m$ with $a_1!\cdots a_k!$ a square. Let $D_k=\{ m : F(m)=k\}$. What is the order of growth of $\lvert D_k\cap\{1,\ldots,n\}\rvert$ for $3\leq k\leq 6$? For example, is it true that $\lvert D_6\cap \{1,\ldots,n\}\rvert \gg n$?
Studied by Erdős and Graham [ErGr76] (see also [LSS14]). It is known, for example, that:
• no $D_k$ contains a prime,
• $D_2=\{ n^2 : n>1\}$,
• $\lvert D_3\cap \{1,\ldots,n\}\rvert = o(\lvert D_4\cap \{1,\ldots,n\}\rvert)$,
• the least element of $D_6$ is $527$, and
• $D_k=\emptyset$ for $k>6$.
OPEN
Is it true that for any $n,k\geq 1$, if $n+1,\ldots,n+k$ are all composite then there are distinct primes $p_1,\ldots,p_k$ such that $p_i\mid n+i$ for $1\leq i\leq k$?
Note this is trivial when $k\leq 2$. Originally conjectured by Grimm. This is a very difficult problem, since it in particular implies $p_{n+1}-p_n <p_n^{1/2-c}$ for some constant $c>0$, in particular resolving Legendre's conjecture.

Grimm proved that this is true if $k\ll \log n/\log\log n$. Erdős and Selfridge improved this to $k\leq (1+o(1))\log n$. Ramachandra, Shorey, and Tijdeman [RST75] have improved this to $k\ll\left(\frac{\log n}{\log\log n}\right)^3.$

OPEN
Are there infinitely many $n$ such that $\binom{2n}{n}$ is coprime to $105$?
Erdős, Graham, Ruzsa, and Straus [EGRS75] have shown that, for any two primes $p$ and $q$, there are infinitely many $n$ such that $\binom{2n}{n}$ is coprime to $pq$.

The sequence of such $n$ is A030979 in the OEIS.

OPEN
Is there some absolute constant $C>0$ such that $\sum_{p\leq n}1_{p\nmid \binom{2n}{n}}\frac{1}{p}\leq C$ for all $n$?
A question of Erdős, Graham, Ruzsa, and Straus [EGRS75], who proved that if $f(n)$ is the sum in question then $\lim_{x\to \infty}\frac{1}{x}\sum_{n\leq x}f(n) = \sum_{k=2}^\infty \frac{\log k}{2^k}=\gamma_0$ and $\lim_{x\to \infty}\frac{1}{x}\sum_{n\leq x}f(n)^2 = \gamma_0^2,$ so that for almost all integers $f(m)=\gamma_0+o(1)$. They also prove that, for all large $n$, $f(n) \leq c\log\log n$ for some constant $c<1$. (It is trivial from Mertens estimates that $f(n)\leq (1+o(1))\log\log n$.)

A positive answer would imply that $\sum_{p\leq n}1_{p\mid \binom{2n}{n}}\frac{1}{p}=(1-o(1))\log\log n,$ and Erdős, Graham, Ruzsa, and Straus say there is 'no doubt' this latter claim is true.

OPEN
Let $r\geq 0$. Does the density of integers $n$ for which $\binom{n}{k}$ is squarefree for at least $r$ values of $1\leq k<n$ exist? Is this density $>0$?
Erdős and Graham state they can prove that, for $k$ fixed and large, the density of $n$ such that $\binom{n}{k}$ is squarefree is $o_k(1)$. They can also prove that there are infinitely many $n$ such that $\binom{n}{k}$ is not squarefree for $1\leq k<n$, and expect that the density of such $n$ is positive.
OPEN
Let $S(n)$ denote the largest integer such that, for all $1\leq k<n$, the binomial coefficient $\binom{n}{k}$ is divisible by $p^{S(n)}$ for some prime $p$ (depending on $k$). Is it true that $\limsup S(n)=\infty?$
If $s(n)$ denotes the largest integer such that $\binom{n}{k}$ is divisible by $p^{s(n)}$ for some prime $p$ for at least one $1\leq k<n$ then it is easy to see that $s(n)\to \infty$ as $n\to \infty$ (and in fact that $s(n) \asymp \log n$).
OPEN
We call an interval $[u,v]$ 'bad' if the greatest prime factor of $\prod_{u\leq m\leq v}m$ occurs with an exponent greater than $1$. Let $B(x)$ count the number of $n\leq x$ which are contained in at least one bad interval. Is it true that $B(x)\sim \#\{ n\leq x: p\mid n\rightarrow p\leq n^{1/2}\}?$
Erdős and Graham only knew that $B(x) > x^{1-o(1)}$. Similarly, we call an interval $[u,v]$ 'very bad' if $\prod_{u\leq m\leq v}m$ is powerful. The number of integers $n\leq x$ contained in at least one very bad interval should be $\ll x^{1/2}$. In fact, it should be asymptotic to the number of powerful numbers $\leq x$.

SOLVED
A number $n$ is highly composite if $\tau(m)<\tau(n)$ for all $m<n$, where $\tau(m)$ counts the number of divisors of $m$. Let $Q(x)$ count the number of highly composite numbers in $[1,x]$.

Is it true that $Q(x)\gg_k (\log x)^k$ for every $k\geq 1$?

Erdős [Er44] proved $Q(x)\gg (\log x)^{1+c}$ for some constant $c>0$.

The answer to this problem is no: Nicolas [Ni71] proved that $Q(x) \ll (\log x)^{O(1)}.$

OPEN
Let $u\leq v$ be such that the largest prime dividing $\prod_{u\leq m\leq v}m$ appears with exponent at least $2$. Is it true that $v-u=v^{o(1)}$? Can $v-u$ be arbitrarily large?
Erdős and Graham report it follows from results of Ramachandra that $v-u\leq v^{1/2+o(1)}$.

OPEN
Is it true that for every $k$ there are infinitely many primes $p$ such that the largest prime divisor of $\prod_{0\leq i\leq k}(p^2+i)$ is $p$?
SOLVED
If $1<k<n-1$ then $\binom{n}{k}$ is divisible by a prime $p<n/2$ (except $\binom{7}{3}=5\cdot 7$).
A conjecture of Erdős and Selfridge. Proved by Ecklund [Ec69], who made the stronger conjecture that whenever $n>k^2$ the binomial coefficient $\binom{n}{k}$ is divisible by a prime $p<n/k$. They have proved the weaker inequality $p\ll n/k^c$ for some constant $c>0$.
OPEN
Let $F(n) = \max_{\substack{m<n\\ m\textrm{ composite}}} m+p(m),$ where $p(m)$ is the least prime divisor of $m$. Is it true that $F(n)>n$ for all sufficiently large $n$? Does $F(n)-n\to \infty$ as $n\to\infty$?
A question of Erdős, Eggleton, and Selfridge, who write that 'plausible conjectures on primes' imply that $F(n)\leq n$ for only finitely many $n$, and in fact it is possible that this quantity is always at least $n+(1-o(1))\sqrt{n}$ (note that it is trivially $\leq n+\sqrt{n}$).

Tao has discussed this problem in a blog post.

OPEN
Can $\binom{n}{k}$ be the product of consecutive primes infinitely often? For example $\binom{21}{2}=2\cdot 3\cdot 5\cdot 7.$
Erdős and Graham write that 'a proof that this cannot happen infinitely often for $\binom{n}{2}$ seems hopeless; probably this can never happen for $\binom{n}{k}$ if $3\leq k\leq n-3$.'
OPEN
Is there an absolute constant $c>0$ such that, for all $1\leq k< n$, the binomial coefficient $\binom{n}{k}$ has a divisor in $(cn,n]$?
Erdős once conjectured that $\binom{n}{k}$ must always have a divisor in $(n-k,n]$, but this was disproved by Schinzel and Erdős [Sc58].
OPEN
Can one classify all solutions of $\prod_{1\leq i\leq k_1}(m_1+i)=\prod_{1\leq j\leq k_2}(m_2+j)$ where $1<k_1<k_2$ and $m_1+k_1\leq m_2$? Are there only finitely many solutions? More generally, if $k_1>2$ then for fixed $a$ and $b$ $a\prod_{1\leq i\leq k_1}(m_1+i)=b\prod_{1\leq j\leq k_2}(m_2+j)$ should have only a finite number of solutions. What if one just requires that the products have the same prime factors, say when $k_1=k_2$?
OPEN
Is it true that for every $n\geq 1$ there is a $k$ such that $n(n+1)\cdots(n+k-1)\mid (n+k)\cdots (n+2k-1)?$
Asked by Erdős and Straus. For example when $n=2$ we have $k=5$ $2\times 3 \times 4 \times 5\times 6 \mid 7 \times 8 \times 9\times 10\times 11.$ and when $n=3$ we have $k=4$: $3\times 4\times 5\times 6 \mid 7\times 8\times 9\times 10.$ Bhavik Mehta has computed the minimal such $k$ for $1\leq n\leq 18$ (now available as A375071 on the OEIS).
OPEN
Let $f(n)$ be the minimal $m$ such that $n! = a_1\cdots a_k$ with $n< a_1<\cdots <a_k=m$. Is there (and what is it) a constant $c$ such that $f(n)-2n \sim c\frac{n}{\log n}?$
Erdős, Guy, and Selfridge [EGS82] have shown that $f(n)-2n \asymp \frac{n}{\log n}.$
OPEN
Let $t(n)$ be maximal such that there is a representation $n!=a_1\cdots a_n$ with $t(n)=a_1\leq \cdots \leq a_n$. Obtain good bounds for $t(n)/n$. In particular, is it true that $\lim \frac{t(n)}{n}=\frac{1}{e}?$ Furthermore, does there exist some constant $c>0$ such that $\frac{t(n)}{n} \leq \frac{1}{e}-\frac{c}{\log n}$ for infinitely many $n$?
It is easy to see that $\lim \frac{t(n)}{n}\leq \frac{1}{e}.$ Erdős [Er96b] wrote he, Selfridge, and Straus had proved a corresponding lower bound, so that $\lim \frac{t(n)}{n}=\frac{1}{e}$, and 'believed that Straus had written up our proof. Unfortunately Straus suddenly died and no trace was ever found of his notes. Furthermore, we never could reconstruct our proof, so our assertion now can be called only a conjecture.'

Alladi and Grinstead [AlGr77] have obtained similar results when the $a_i$ are restricted to prime powers.

OPEN
Let $A(n)$ denote the least value of $t$ such that $n!=a_1\cdots a_t$ with $a_1\leq \cdots \leq a_t\leq n^2$. Is it true that $A(n)=\frac{n}{2}-\frac{n}{2\log n}+o\left(\frac{n}{\log n}\right)?$
If we change the condition to $a_t\leq n$ it can be shown that $A(n)=n-\frac{n}{\log n}+o\left(\frac{n}{\log n}\right)$ via a greedy decomposition (use $n$ as often as possible, then $n-1$, and so on). Other questions can be asked for other restrictions on the sizes of the $a_t$.
OPEN
Let $f(n)$ denote the minimal $m$ such that $n! = a_1\cdots a_t$ with $a_1<\cdots <a_t=a_1+m$. What is the behaviour of $f(n)$?
Erdős and Graham write that they do not even know whether $f(n)=1$ infinitely often (i.e. whether a factorial is the product of two consecutive integers infinitely often).
SOLVED
Let $t_k(n)$ denote the least $m$ such that $n\mid (m+1)(m+2)\cdots (m+k).$ Is it true that $\sum_{1\leq n\leq N}t_2(n)=o(N)?$
The answer is yes, proved by Hall. It is probably true that the sum is $o(N/(\log N)^c)$ for some constant $c>0$. Similar questions can be asked for other $k\geq 3$.
OPEN
Is it true that for every $k$ there exists $n$ such that $\prod_{0\leq i\leq k}(n-i) \mid \binom{2n}{n}?$
Erdős and Graham write that $n+1$ always divides $\binom{2n}{n}$ (indeed $\frac{1}{n+1}\binom{2n}{n}$ is the $n$th Catalan number), but it is quite rare that $n$ divides $\binom{2n}{n}$.

Pomerance [Po14] has shown that for any $k\geq 0$ there are infinitely many $n$ such that $n-k\mid\binom{2n}{n}$, although the set of such $n$ has upper density $<1/3$. Pomerance also shows that the set of $n$ such that $\prod_{1\leq i\leq k}(n+i)\mid \binom{2n}{n}$ has density $1$.

The smallest $n$ for each $k$ are listed as A375077 on the OEIS.

OPEN
Are there only finitely many solutions to $\prod_i \binom{2m_i}{m_i}=\prod_j \binom{2n_j}{n_j}$ with the $m_i,n_j$ distinct?
OPEN
Are the only solutions to $n!=x^2-1$ when $n=4,5,7$?
The Brocard-Ramanujan conjecture. Erdős and Graham describe this as an old conjecture, and write it 'is almost certainly true but it is intractable at present'.

Overholt [Ov93] has shown that this has only finitely many solutions assuming a weak form of the abc conjecture.

There are no other solutions below $10^9$ (see the OEIS page).

OPEN
Is it true that there are no solutions to $n! = x^k\pm y^k$ with $x,y,n\in \mathbb{N}$, with $xy>1$ and $k>2$?
Erdős and Obláth [ErOb37] proved this is true when $(x,y)=1$ and $k\neq 4$. Pollack and Shapiro [PoSh73] proved this is true when $(x,y)=1$ and $k=4$. The known methods break down without the condition $(x,y)=1$.
OPEN
For any $k\geq 2$ let $g_k(n)$ denote the maximal value of $n-(a_1+\cdots+a_k)$ where $a_1,\ldots,a_k$ are integers such that $a_1!\cdots a_k! \mid n!$. Can one show that $\sum_{n\leq x}g_k(n) \sim c_k x\log x$ for some constant $c_k$? Is it true that there is a constant $c_k$ such that for almost all $n<x$ we have $g_k(n)=c_k\log x+o(\log x)?$
Erdős and Graham write that it is easy to show that $g_k(n) \ll_k \log n$ always, but the best possible constant is unknown.

OPEN
Is there some function $\omega(r)$ such that $\omega(r)\to \infty$ as $r\to\infty$, such that for all large $n$ there exist $a_1,a_2$ with $a_1+a_2> n+\omega(r)\log n$ such that $a_1!a_2! \mid n!2^n3^n\cdots p_r^n$?
SOLVED
Prove that, for any finite set $A\subset\mathbb{N}$, there exist $a,b\in A$ such that $\mathrm{gcd}(a,b)\leq a/\lvert A\rvert.$
A conjecture of Graham [Gr70], who also conjectured that (assuming $A$ itself has no common divisor) the only cases where equality is achieved are when $A=\{1,\ldots,n\}$ or $\{L/1,\ldots,L/n\}$ (where $L=\mathrm{lcm}(1,\ldots,n)$) or $\{2,3,4,6\}$.

Proved for all sufficiently large sets (including the sharper version which characterises the case of equality) independently by Szegedy [Sz86] and Zaharescu [Za87].

Proved for all sets by Balasubramanian and Soundararajan [BaSo96].

SOLVED
Does the equation $2^m=a_1!+\cdots+a_k!$ with $a_1<a_2<\cdots <a_k$ have only finitely many solutions?
Asked by Burr and Erdős. Frankl and Lin [Li76] independently showed that the answer is yes, and the largest solution is $2^7=2!+3!+5!.$ In fact Lin showed that the largest power of $2$ which can divide a sum of distinct factorials containing $2$ is $2^{254}$, and that there are only 5 solutions to $3^m=a_1!+\cdots+a_k!$ (when $m=0,1,2,3,6$).

OPEN
Let $f(a,p)$ be the largest $k$ such that there are $a=a_1<\cdots<a_k$ such that $p^k \mid (a_1!+\cdots+a_k!).$ Is $f(a,p)$ bounded by some absolute constant? What if this constant is allowed to depend on $a$ and $p$?

Is there a prime $p$ and an infinite sequence $a_1<a_2<\cdots$ such that if $p^{m_k}$ is the highest power of $p$ dividing $\sum_{i\leq k}a_i!$ then $m_k\to \infty$?

See also [403]. Lin [Li76] has shown that $f(2,2) \leq 254$.
OPEN
Let $p$ be an odd prime. Is it true that the equation $(p-1)!+a^{p-1}=p^k$ has only finitely many solutions?
Erdős and Graham remark that it is probably true that in general $(p-1)!+a^{p-1}$ is rarely a power at all (although this can happen, for example $6!+2^6=28^2$).

Erdős and Graham ask this allowing the case $p=2$, but this is presumably an oversight, since clearly there are infinitely many solutions to this equation when $p=2$.

OPEN
Is it true that there are only finitely many powers of $2$ which have only the digits $0$ and $1$ when written in base $3$?
The only examples seem to be $4=1+3$ and $256=1+3+3^2+3^5$. If we only allow the digits $1$ and $2$ then $2^{15}$ seems to be the largest such power of $2$.

This would imply via Kummer's theorem that $3\mid \binom{2^{k+1}}{2^k}$ for all large $k$.

SOLVED
Let $w(n)$ count the number of solutions to $n=2^a+3^b+2^c3^d$ with $a,b,c,d\geq 0$ integers. Is it true that $w(n)$ is bounded by some absolute constant?
A conjecture originally due to Newman.

This is true, and was proved by Evertse, Györy, Stewart, and Tijdeman [EGST88].

OPEN
Let $\phi(n)$ be the Euler totient function and $\phi_k(n)$ be the iterated $\phi$ function, so that $\phi_1(n)=\phi(n)$ and $\phi_k(n)=\phi(\phi_{k-1}(n))$. Let $f(n) = \min \{ k : \phi_k(n)=1\}.$ Does $f(n)/\log n$ have a distribution function? Is $f(n)/\log n$ almost always constant? What can be said about the largest prime factor of $\phi_k(n)$ when, say, $k=\log\log n$?
Pillai [Pi29] was the first to investigate this function, and proved $\log_3 n < f(n) < \log_2 n$ for all large $n$. Shapiro [Sh50] proved that $f(n)$ is essentially multiplicative.

Erdős, Granville, Pomerance, and Spiro [EGPS90] have proved that the answer to the first two questions is yes, conditional on a form of the Elliott-Halberstam conjecture.

It is likely true that, if $k\to \infty$ however slowly with $n$, then for almost $n$ the largest prime factor of $\phi_k(n)$ is $\leq n^{o(1)}$.

OPEN
How many iterations of $n\mapsto \phi(n)+1$ are needed before a prime is reached? Can infinitely many $n$ reach the same prime? What is the density of $n$ which reach any fixed prime?
A problem of Finucane. One can also ask about $n\mapsto \sigma(n)-1$.

The number of iterations required is A039651 in the OEIS.

OPEN
Let $\sigma_1(n)=\sigma(n)$, the sum of divisors function, and $\sigma_k(n)=\sigma(\sigma_{k-1}(n))$. Is it true that $\lim_{k\to \infty} \sigma_k(n)^{1/k}=\infty?$
OPEN
Let $g_1=g(n)=n+\phi(n)$ and $g_k(n)=g(g_{k-1}(n))$. For which $n$ and $r$ is it true that $g_{k+r}(n)=2g_k(n)$ for all large $k$?
The known solutions to $g_{k+2}(n)=2g_k(n)$ are $n=10$ and $n=94$. Selfridge and Weintraub found solutions to $g_{k+9}(n)=9g_k(n)$ and Weintraub found $g_{k+25}(3114)=729g_k(3114)$ for all $k\geq 6$.
OPEN
Let $\sigma_1(n)=\sigma(n)$, the sum of divisors function, and $\sigma_k(n)=\sigma(\sigma_{k-1}(n))$. Is it true that, for every $m,n$, there exist some $i,j$ such that $\sigma_i(m)=\sigma_j(n)$?
In [Er79d] Erdős attributes this conjecture to van Wijngaarden, who told it to Erdős in the 1950s.

That is, there is (eventually) only one possible sequence that the iterated sum of divisors function can settle on. Selfridge reports numerical evidence which suggests the answer is no, but Erdős and Graham write 'it seems unlikely that anything can be proved about this in the near future'.

OPEN
Let $\omega(n)$ count the number of distinct primes dividing $n$. Are there infinitely many $n$ such that, for all $m<n$, we have $m+\omega(m) \leq n$?

Can one show that there exists an $\epsilon>0$ such that there are infinitely many $n$ where $m+\epsilon \omega(m)\leq n$ for all $m<n$?

In [Er79] Erdős calls such an $n$ a 'barrier' for $\omega$. Some other natural number theoretic functions (such as $\phi$ and $\sigma$) have no barriers because they increase too rapidly. Erdős believed that $\omega$ should have infinitely many barriers. In [Er79d] he proves that $F(n)=\prod k_i$, where $n=\prod p_i^{k_i}$, has infinitely many barriers (in fact the set of barriers has positive density in the integers).

Erdős also believed that $\Omega$, the count of the number of prime factors with multiplicity), should have infinitely many barriers. Selfridge found the largest barrier for $\Omega$ which is $<10^5$ is $99840$.

In [ErGr80] this problem is suggested as a way of showing that the iterated behaviour of $n\mapsto n+\omega(n)$ eventually settles into a single sequence, regardless of the starting value of $n$ (see also [412] and [414]).

Erdős and Graham report it could be attacked by sieve methods, but 'at present these methods are not strong enough'.

OPEN
Let $h_1(n)=h(n)=n+\tau(n)$ (where $\tau(n)$ counts the number of divisors of $n$) and $h_k(n)=h(h_{k-1}(n))$. Is it true, for any $m,n$, there exist $i$ and $j$ such that $h_i(m)=h_j(n)$?
Asked by Spiro. That is, there is (eventually) only one possible sequence that the iterations of $n\mapsto h(n)$ can settle on. Erdős and Graham believed the answer is yes. Similar questions can be asked by the iterates of many other functions. See also [412] and [413].
OPEN
For any $n$ let $F(n)$ be the largest $k$ such that any of the $k!$ possible ordering patterns appears in some sequence of $\phi(m+1),\ldots,\phi(m+k)$ with $m+k\leq n$. Is it true that $F(n)=(c+o(1))\log\log\log n$ for some constant $c$? Is the first pattern which fails to appear always $\phi(m+1)>\phi(m+2)>\cdots \phi(m+k)?$ Is it true that 'natural' ordering which mimics what happens to $\phi(1),\ldots,\phi(k)$ is the most likely to appear?
Erdős [Er36b] proved that $F(n)\asymp \log\log\log n,$ and similarly if we replace $\phi$ with $\sigma$ or $\tau$ or $\nu$ or any 'decent' additive or multiplicative function.
OPEN
Let $V(x)$ count the number of $n\leq x$ such that $\phi(m)=n$ is solvable. Does $V(2x)/V(x)\to 2$? Is there an asymptotic formula for $V(x)$?
Pillai [Pi29] proved $V(x)=o(x)$. Erdős [Er35b] proved $V(x)=x(\log x)^{-1+o(1)}$.

The behaviour of $V(x)$ is now almost completely understood. Maier and Pomerance [MaPo88] proved $V(x)=\frac{x}{\log x}e^{(C+o(1))(\log\log\log x)^2},$ for some explicit constant $C>0$. Ford [Fo98] improved this to $V(x)\asymp\frac{x}{\log x}e^{C_1(\log\log\log x-\log\log\log\log x)^2+C_2\log\log\log x-C_3\log\log\log\log x}$ for some explicit constants $C_1,C_2,C_3>0$. Unfortunately this falls just short of an asymptotic formula for $V(x)$ and determining whether $V(2x)/V(x)\to 2$.

In [Er79e] Erdős asks further to estimate the number of $n\leq x$ such that the smallest solution to $\phi(m)=n$ satisfies $kx<m\leq (k+1)x$.

OPEN
Let $V'(x)=\#\{\phi(m) : 1\leq m\leq x\}$ and $V(x)=\#\{\phi(m) \leq x : 1\leq m\}.$ Does $\lim V(x)/V'(x)$ exist? Is it $>1$?
It is trivial that $V'(x) \leq V(x)$. See also [416].
SOLVED
Are there infinitely many integers not of the form $n-\phi(n)$?
Asked by Erdős and Sierpiński. It follows from the Goldbach conjecture that every odd number can be written as $n-\phi(n)$. What happens for even numbers?

Erdős [Er73b] has shown that a positive density set of integers cannot be written as $\sigma(n)-n$.

This is true, as shown by Browkin and Schinzel [BrSc95], who show that any integer of the shape $2^{k}\cdot 509203$ is not of this form. It seems to be open whether there is a positive density set of integers not of this form.

SOLVED
If $\tau(n)$ counts the number of divisors of $n$, then what is the set of limit points of $\frac{\tau((n+1)!)}{\tau(n!)}?$
Erdős and Graham noted that any number of the shape $1+1/k$ for $k\geq 1$ is a limit point (and thus so too is $1$), but knew of no others.

Mehtaab Sawhney has shared the following simple argument that proves that the above limit points are in fact the only ones.

If $v_p(m)$ is the largest $k$ such that $p^k\mid m$ then $\tau(m)=\prod_p (v_p(m)+1)$ and so $\frac{\tau((n+1)!)}{\tau(n!)} = \prod_{p|n+1}\left(1+\frac{v_p(n+1)}{v_p(n!)+1}\right).$ Note that $v_p(n!)\geq n/p$, and furthermore $n+1$ has $<\log n$ prime divisors, each of which satisfy $v_p(n+1)<\log n$. It follows that the contribution from $p\leq n^{2/3}$ is at most $\left(1+\frac{\log n}{n^{1/3}}\right)^{\log n}\leq 1+o(1).$

There is at most one $p\mid n+1$ with $p\geq n^{2/3}$ which (if present) contributes exactly $\left(1+\frac{1}{\frac{n+1}{p}}\right).$ We have proved the claim, since these two facts combined show that the ratio in question is either $1+o(1)$ or $1+1/k+o(1)$, the latter occurring if $n+1=pk$ for some $p>n^{2/3}$.

After receiving Sawhney's argument I found that this had already been proved, with essentially the same argument, by Erdős, Graham, Ivić, and Pomerance [EGIP].

Additional thanks to: Zachary Chase, Mehtaab Sawhney
OPEN
If $\tau(n)$ counts the number of divisors of $n$ then let $F(f,n)=\frac{\tau((n+\lfloor f(n)\rfloor)!)}{\tau(n!)}.$ Is it true that $\lim_{n\to \infty}F((\log n)^C,n)=\infty$ for large $C$? Is it true that $F(\log n,n)$ is everywhere dense in $(1,\infty)$? More generally, if $f(n)\leq \log n$ is a monotonic function then is $F(f,n)$ everywhere dense?
Erdős and Graham write that it is easy to show that $\lim F(n^{1/2},n)=\infty$, and in fact the $n^{1/2}$ can be replaced by $n^{1/2-c}$ for some small constant $c>0$.
OPEN
Is there a sequence $1\leq d_1<d_2<\cdots$ with density $1$ such that all products $\prod_{u\leq i\leq v}d_i$ are distinct?
OPEN
Let $f(1)=f(2)=1$ and for $n>2$ $f(n) = f(n-f(n-1))+f(n-f(n-2)).$ Does $f(n)$ miss infinitely many integers? What is its behaviour?
Asked by Hofstadter. The sequence begins $1,1,2,3,3,4,\ldots$ and is A005185 in the OEIS. It is not even known whether $f(n)$ is well-defined for all $n$.
OPEN
Let $a_1=1$ and $a_2=2$ and for $k\geq 3$ we choose $a_k$ to be the least integer $>a_{k-1}$ which is the sum of at least two consecutive terms of the sequence. What is the asymptotic behaviour of this sequence?
Asked by Hofstadter. The sequence begins $1,2,3,5,6,8,10,11,\ldots$ and is A005243 in the OEIS.
OPEN
Let $a_1=2$ and $a_2=3$ and continue the sequence by appending to $a_1,\ldots,a_n$ all possible values of $a_ia_j-1$ with $i\neq j$. Is it true that the set of integers which eventually appear has positive density?
Asked by Hofstadter. The sequence begins $2,3,5,9,14,17,26,\ldots$ and is A005244 in the OEIS. This problem is also discussed in section E31 of Guy's book Unsolved Problems in Number Theory.

In [ErGr80] (and in Guy's book) this problem as written is asking for whether almost all integers appear in this sequence, but the answer to this is trivially no (as pointed out to me by Steinerberger): no integer $\equiv 1\pmod{3}$ is ever in the sequence, so the set of integers which appear has density at most $2/3$. This is easily seen by induction, and the fact that if $a,b\in \{0,2\}\pmod{3}$ then $ab-1\in \{0,2\}\pmod{3}$.

Presumably it is the weaker question of whether a positive density of integers appear (as correctly asked in [Er77c]) that was also intended in [ErGr80].

Additional thanks to: Mehtaab Sawhney, Stefan Steinerberger
OPEN
Let $F(n)$ be the maximum possible size of a subset $A\subseteq\{1,\ldots,N\}$ such that the products $ab$ are distinct for all $a<b$. Is there a constant $c$ such that $F(n)=\pi(n)+(c+o(1))n^{3/4}(\log n)^{-3/2}?$

If $A\subseteq \{1,\ldots,n\}$ is such that all products $a_1\cdots a_r$ are distinct for $a_1<\cdots <a_r$ then is it true that $\lvert A\rvert \leq \pi(n)+O(n^{\frac{r+1}{2r}})?$

Erdős [Er68] proved that there exist some constants $0<c_1\leq c_2$ such that $\pi(n)+c_1 n^{3/4}(\log n)^{-3/2}\leq F(n)\leq \pi(n)+c_2 n^{3/4}(\log n)^{-3/2}.$ Surprisingly, if we consider the corresponding problem in the reals (so consider the largest $A\subset [1,x]$ such that for any distinct $a,b,c,d\in A$ we have $\lvert ab-cd\rvert \geq 1$) then Alexander proved that $\lvert A\rvert> x/8e$ is possible (disproving an earlier conjecture of Erdős [Er73] that $m=o(x)$). Alexander's construction seems to be unpublished, and I have no idea what it is.

SOLVED
Is it true that, for every $n$ and $d$, there exists $k$ such that $d \mid p_{n+1}+\cdots+p_{n+k},$ where $p_r$ denotes the $r$th prime?
Cedric Pilatte has observed that a positive solution to this follows from a result of Shiu [Sh00]: for any $k\geq 1$ and $(a,q)=1$ there exist infinitely many $k$-tuples of consecutive primes $p_m,\ldots,p_{m+{k-1}}$ all of which are congruent to $a$ modulo $q$.

Indeed, we apply this with $k=q=d$ and $a=1$ and let $p_m,\ldots,p_{m+{d-1}}$ be consecutive primes all congruent to $1$ modulo $d$, with $m>n+1$. If $p_{n+1}+\cdots+p_{m-1}\equiv r\pmod{d}$ with $1\leq r\leq d$ then $d \mid p_{n+1}+\cdots +p_m+\cdots+p_{m+r-1}.$

OPEN
Is there a set $A\subseteq \mathbb{N}$ such that, for infinitely many $n$, all of $n-a$ are prime for all $a\in A$ with $0<a<n$ and $\liminf\frac{\lvert A\cap [1,x]\rvert}{\pi(x)}>0?$
Erdős and Graham could show this is true (assuming the prime $k$-tuple conjecture) if we replace $\liminf$ by $\limsup$.
SOLVED
Is it true that, if $A\subseteq \mathbb{N}$ is sparse enough and does not cover all residue classes modulo $p$ for any prime $p$, then there exists some $n$ such that $n+a$ is prime for all $a\in A$?
Weisenberg [We24] has shown the answer is no: $A$ can be arbitrarily sparse and missing at least one residue class modulo every prime $p$, and yet $A+n$ is not contained in the primes for any $n\in \mathbb{Z}$.
OPEN
Fix some integer $n$ and define a decreasing sequence in $[1,n)$ by $a_1=n-1$ and, for $k\geq 2$, letting $a_k$ be the greatest integer in $[1,a_{k-1})$ such that all of the prime factors of $a_k$ are $>n-a_k$. Is it true that, for sufficiently large $n$, not all of this sequence can be prime?
Erdős and Graham write 'preliminary calculations made by Selfridge indicate that this is the case but no proof is in sight'. For example if $n=8$ we have $a_1=7$ and $a_2=5$ and then must stop.
OPEN
Are there two infinite sets $A$ and $B$ such that $A+B$ agrees with the set of prime numbers up to finitely many exceptions?
A problem of Ostmann, sometimes known as the 'inverse Goldbach problem'. The answer is surely no. The best result in this direction is due to Elsholtz and Harper [ElHa15], who showed that if $A,B$ are such sets then for all large $x$ we must have $\frac{x^{1/2}}{\log x\log\log x} \ll \lvert A \cap [1,x]\rvert \ll x^{1/2}\log\log x$ and similarly for $B$.

Elsholtz [El01] has proved there are no infinite sets $A,B,C$ such that $A+B+C$ agrees with the set of prime numbers up to finitely many exceptions.

OPEN
Let $A,B\subseteq \mathbb{N}$ be two infinite sets. How dense can $A+B$ be if all elements of $A+B$ are pairwise relatively prime?
Asked by Straus, inspired by a problem of Ostmann (see [431]).
OPEN
If $A\subset \mathbb{N}$ is a finite set then let $G(A)$ denote the greatest integer which is not expressible as a finite sum of elements from $A$ (with repetitions allowed). Let $g(n,t)=\max G(A)$ where the maximum is taken over all $A\subseteq \{1,\ldots,t\}$ of size $\lvert A\rvert=n$ which has no common divisor. Is it true that $g(n,t)\sim \frac{t^2}{n-1}?$
This type of problem is associated with Frobenius. Erdős and Graham [ErGr72] proved $g(n,t)<2t^2/n$, and there are examples which show that $g(n,t) \geq \frac{t^2}{n-1}-5t$ for $n\geq 2$.

The problem is written as Erdős and Graham describe it, but presumably they had in mind the regime where $n$ is fixed and $t\to \infty$.

OPEN
Let $k\leq n$. What choice of $A\subseteq \{1,\ldots,n\}$ of size $\lvert A\rvert=k$ maximises the number of integers not representable as the sum of finitely many elements from $A$ (with repetitions allowed)? Is it $\{n,n-1,\ldots,n-k+1\}$?
Associated with problems of Frobenius.
OPEN
Let $n\in\mathbb{N}$ with $n\neq p^k$ for any prime $p$ and $k\geq 0$. What is the largest integer not of the form $\sum_{1\leq i<n}c_i\binom{n}{i}$ where the $c_i\geq 0$ are integers?
OPEN
If $p$ is a prime and $k,m\geq 2$ then let $r(k,m,p)$ be the minimal $r$ such that $r,r+1,\ldots,r+m-1$ are all $k$th power residues modulo $p$. Let $\Lambda(k,m)=\limsup_{p\to \infty} r(k,m,p).$ Is it true that $\Lambda(k,2)$ is finite for all $k$? Is $\Lambda(k,3)$ finite for all odd $k$? How large are they?
Asked by Lehmer and Lehmer [LeLe62]. For example $\Lambda(2,2)=9$ and $\Lambda(3,2)=77$. It is known that $\Lambda(k,3)=\infty$ for all even $k$ and $\Lambda(k,4)=\infty$ for all $k$.

This has been partially resolved: Hildebrand [Hi91] has shown that $\Lambda(k,2)$ is finite for all $k$.

SOLVED
Let $1\leq a_1<\cdots<a_k\leq x$. How many of the partial products $a_1,a_1a_2,\ldots,a_1\cdots a_k$ can be squares? Is it true that, for any $\epsilon>0$, there can be more than $x^{1-\epsilon}$ squares?
Erdős and Graham write it is 'trivial' that there are $o(x)$ many such squares, although this is not quite trivial, using Siegel's theorem.

A positive answer follows from work of Bui, Pratt, and Zaharescu [BPZ24], as noted by Tao in this blog post. In particular Tao shows that, if $L(x)$ is the maximal number of such squares possible, and $u(x)=(\log x\log\log x)^{1/2}$, then $x\exp(-(2^{1/2}+o(1))u(x)) \leq L(x) \leq x\exp(-(2^{-1/2}+o(1))u(x)).$

SOLVED
How large can $A\subseteq \{1,\ldots,N\}$ be if $A+A$ contains no square numbers?
Taking all integers $\equiv 1\pmod{3}$ shows that $\lvert A\rvert\geq N/3$ is possible. This can be improved to $\tfrac{11}{32}N$ by taking all integers $\equiv 1,5,9,13,14,17,21,25,26,29,30\pmod{32}$, as observed by Massias.

Lagarias, Odlyzko, and Shearer [LOS83] proved this is sharp for the modular version of the problem; that is, if $A\subseteq \mathbb{Z}/N\mathbb{Z}$ is such that $A+A$ contains no squares then $\lvert A\rvert\leq \tfrac{11}{32}N$. They also prove the general upper bound of $\lvert A\rvert\leq 0.475N$ for the integer problem.

In fact $\frac{11}{32}$ is sharp in general, as shown by Khalfalah, Lodha, and Szemerédi [KLS02], who proved that the maximal such $A$ satisfies $\lvert A\rvert\leq (\tfrac{11}{32}+o(1))N$.

SOLVED
Is it true that, in any finite colouring of the integers, there must be two integers $x\neq y$ of the same colour such that $x+y$ is a square? What about a $k$th power?
A question of Roth, Erdős, Sárközy, and Sós [ESS89] (according to some reports, although in [Er80c] Erdős claims this arose in a conversation with Silverman in 1977). Erdős, Sárközy, and Sós [ESS89] proved this for $2$ or $3$ colours.

In other words, if $G$ is the infinite graph on $\mathbb{N}$ where we connect $m,n$ by an edge if and only if $n+m$ is a square, then is the chromatic number of $G$ equal to $\aleph_0$?

This is true, as proved by Khalfalah and Szemerédi [KhSz06], who in fact prove the general result with $x+y=z^2$ replaced by $x+y=f(z)$ for any non-constant $f(z)\in \mathbb{Z}[z]$ such that $2\mid f(z)$ for some $z\in \mathbb{Z}$.

OPEN
Let $A=\{a_1<a_2<\cdots\}\subseteq \mathbb{N}$ be infinite and let $A(x)$ count the number of indices for which $\mathrm{lcm}(a_i,a_{i+1})\leq x$. Is it true that $A(x) \ll x^{1/2}$? How large can $\liminf \frac{A(x)}{x^{1/2}}$ be?
It is easy to give a sequence with $\limsup\frac{A(x)}{x^{1/2}}=c>0.$ There are related results (particularly for the more general case of $\mathrm{lcm}(a_i,a_{i+1},\ldots,a_{i+k})$) in a paper of Erdős and Szemerédi [ErSz80].
SOLVED
Let $N\geq 1$. What is the size of the largest $A\subset \{1,\ldots,N\}$ such that $\mathrm{lcm}(a,b)\leq N$ for all $a,b\in A$?

Is it attained by choosing all integers in $[1,(N/2)^{1/2}]$ together with all even integers in $[(N/2)^{1/2},(2N)^{1/2}]$?

Let $g(N)$ denote the size of the largest such $A$. The construction mentioned proves that $g(N) \geq \left(\tfrac{9}{8}n\right)^{1/2}+O(1).$ Erdős [Er51b] proved $g(N) \leq (4n)^{1/2}+O(1)$. Chen [Ch98] established the asymptotic $g(N) \sim \left(\tfrac{9}{8}n\right)^{1/2}.$ Chen and Dai [DaCh06] proved that $g(N)\leq \left(\tfrac{9}{8}n\right)^{1/2}+O\left(\left(\frac{N}{\log N}\right)^{1/2}\log\log N\right).$ In [ChDa07] the same authors prove that, infinitely often, Erdős' construction is not optimal: if $B$ is that construction and $A$ is such that $\lvert A\rvert=g(N)$ then, for infinitely many $N$, $\lvert A\rvert\geq \lvert B\rvert+t,$ where $t\geq 0$ is defined such that the $t$-fold iterated logarithm of $N$ is in $[0,1)$.
SOLVED
Is it true that if $A\subseteq\mathbb{N}$ is such that $\frac{1}{\log\log x}\sum_{n\in A\cap [1,x)}\frac{1}{n}\to \infty$ then $\left(\sum_{n\in A\cap [1,x)}\frac{1}{n}\right)^{-2} \sum_{\substack{a,b\in A\cap (1,x]\\ a<b}}\frac{1}{\mathrm{lcm}(a,b)}\to \infty?$
Tao [Ta24b] has shown this is false: there exists $A\subset\mathbb{N}$ such that $\sum_{n\in A\cap [1,x)}\frac{1}{n}\gg \exp((\tfrac{1}{2}+o(1))\sqrt{\log\log x}\log\log\log x)$ and $\left(\sum_{n\in A\cap [1,x)}\frac{1}{n}\right)^{-2} \sum_{\substack{a,b\in A\cap (1,x]\\ a<b}}\frac{1}{\mathrm{lcm}(a,b)}\ll 1.$ Moreover, Tao shows this is the best possible result, in that if $\sum_{n\in A\cap [1,x)}\frac{1}{n}$ grows faster than $\exp(O(\sqrt{\log\log x}\log\log\log x))$ then $\left(\sum_{n\in A\cap [1,x)}\frac{1}{n}\right)^{-2} \sum_{\substack{a,b\in A\cap (1,x]\\ a<b}}\frac{1}{\mathrm{lcm}(a,b)}\to \infty.$
OPEN
Let $m,n\geq 1$. What is $\# \{ k(m-k) : 1\leq k\leq m/2\} \cap \{ l(n-l) : 1\leq l\leq n/2\}?$ Can it be arbitrarily large? Is it $\leq (mn)^{o(1)}$ for all suffiicently large $m,n$?
SOLVED
Let $A\subseteq\mathbb{N}$ be infinite and $d_A(n)$ count the number of $a\in A$ which divide $n$. Is it true that, for every $k$, $\limsup_{x\to \infty} \frac{\max_{n<x}d_A(n)}{\left(\sum_{n\in A\cap[1,x)}\frac{1}{n}\right)^k}=\infty?$
The answer is yes, proved by Erdős and Sárkőzy [ErSa80].
OPEN
Is it true that, for any $c>1/2$, if $p$ is a large prime and $n$ is sufficiently large (both depending on $c$) then there exist $a,b\in(n,n+p^c)$ such that $ab\equiv 1\pmod{p}$?
An unpublished result of Heilbronn guarantees this for $c$ sufficiently close to $1$.
SOLVED
Let $\delta(n)$ denote the density of integers which are divisible by some integer in $(n,2n)$. What is the growth rate of $\delta(n)$?

If $\delta'(n)$ is the density of integers which have exactly one divisor in $(n,2n)$ then is it true that $\delta'(n)=o(\delta(n))$?

Besicovitch [Be34] proved that $\liminf \delta(n)=0$. Erdős [Er35] proved that $\delta(n)=o(1)$. Erdős [Er60] proved that $\delta(n)=(\log n)^{-\alpha+o(1)}$ where $\alpha=1-\frac{1+\log\log 2}{\log 2}=0.08607\cdots.$ This estimate was refined by Tenenbaum [Te84], and the true growth rate of $\delta(n)$ was determined by Ford [Fo08] who proved $\delta(n)\asymp \frac{1}{(\log n)^\alpha(\log\log n)^{3/2}}.$

Among many other results in [Fo08], Ford also proves that the second conjecture is false, and more generally that if $\delta_r(n)$ is the density of integers with exactly $r$ divisors in $(n,2n)$ then $\delta_r(n)\gg_r\delta(n)$.

Additional thanks to: Zachary Chase and Kevin Ford
SOLVED
Let $\tau(n)$ count the divisors of $n$ and $\tau^+(n)$ count the number of $k$ such that $n$ has a divisor in $[2^k,2^{k+1})$. Is it true that, for all $\epsilon>0$, $\tau^+(n) < \epsilon \tau(n)$ for almost all $n$?
This is false, and was disproved by Erdős and Tenenbaum [ErTe81], who showed that in fact the upper density of the set of such $n$ is $\asymp \epsilon^{1-o(1)}$ (where the $o(1)$ in the exponent $\to 0$ as $\epsilon \to 0$).

A more precise result was proved by Hall and Tenenbaum [HaTe88] (see Section 4.6), who showed that the upper density is $\ll\epsilon \log(2/\epsilon)$. Hall and Tenenbaum further prove that $\tau^+(n)/\tau(n)$ has a distribution function.

Erdős and Graham also asked whether there is a good inequality known for $\sum_{n\leq x}\tau^+(n)$. This was provided by Ford [Fo08] who proved $\sum_{n\leq x}\tau^+(n)\asymp x\frac{(\log x)^{1-\alpha}}{(\log\log x)^{3/2}}$ where $\alpha=1-\frac{1+\log\log 2}{\log 2}=0.08607\cdots.$

SOLVED
Let $r(n)$ count the number of $d_1,d_2$ such that $d_1\mid n$ and $d_2\mid n$ and $d_1<d_2<2d_1$. Is it true that, for every $\epsilon>0$, $r(n) < \epsilon \tau(n)$ for almost all $n$, where $\tau(n)$ is the number of divisors of $n$?
This is false - indeed, for any constant $K>0$ we have $r(n)>K\tau(n)$ for a positive density set of $n$. Kevin Ford has observed this follows from the negative solution to [448]: the Cauchy-Schwarz inequality implies $r(n)+\tau(n)\geq \tau(n)^2/\tau^+(n)$ where $\tau^+(n)$ is as defined in [448], and the negative solution to [448] implies the right-hand side is at least $(K+1)\tau(n)$ for a positive density set of $n$. (This argument is given for an essentially identical problem by Hall and Tenenbaum [HaTe88], Section 4.6.)

OPEN
How large must $y=y(\epsilon,n)$ be such that the number of integers in $(x,x+y)$ with a divisor in $(n,2n)$ is at most $\epsilon y$?
OPEN
Estimate $n_k$, the smallest integer such that $\prod_{1\leq i\leq k}(n_k-i)$ has no prime factor in $(k,2k)$.
Erdős and Graham write 'we can prove $n_k>k^{1+c}$ but no doubt much more is true'.

In [Er79d] Erdős writes that probably $n_k<e^{o(k)}$ but $n_k>k^d$ for all constant $d$.

OPEN
Let $\omega(n)$ count the number of distinct prime factors of $n$. What is the size of the largest interval $I\subseteq [x,2x]$ such that $\omega(n)>\log\log n$ for all $n\in I$?
Erdős [Er37] proved that the density of integers $n$ with $\omega(n)>\log\log n$ is $1/2$. The Chinese remainder theorem implies that there is such an interval with $\lvert I\rvert \geq (1+o(1))\frac{\log x}{(\log\log x)^2}.$ It could be true that there is such an interval of length $(\log x)^{k}$ for arbitrarily large $k$.
SOLVED
Is it true that, for all sufficiently large $n$, $p_n^2 \leq p_{n+i}p_{n-i}$ for all $i<n$, where $p_k$ is the $k$th prime?
Asked by Erdős and Straus. The answer is no, as shown by Pomerance [Po79].
OPEN
Let $f(n) = \min_{i<n} (p_{n+i}+p_{n-i}),$ where $p_k$ is the $k$th prime. Is it true that $\limsup_n (f(n)-2p_n)=\infty?$
Pomerance [Po79] has proved the $\limsup$ is at least $2$.
OPEN
Let $q_1<q_2<\cdots$ be a sequence of primes such that $q_{n+1}-q_n\geq q_n-q_{n-1}.$ Must $\lim_n \frac{q_n}{n^2}=\infty?$
Richter [Ri76] proved that $\liminf_n \frac{q_n}{n^2}>0.352\cdots.$
OPEN
Let $p_n$ be the smallest prime $\equiv 1\pmod{n}$ and let $m_n$ be the smallest integer such that $n\mid \phi(m_n)$. Is it true that $p_n>m_n$ for almost all $n$? Does $p_n/m_n\to \infty$ for almost all $n$? Are there infinitely many primes $p$ such that $p-1$ is the only $n$ for which $m_n=p$?
Linnik's theorem implies that $p_n\leq n^{O(1)}$. Erdős [Er79e] writes it is 'easy to show' that for infinitely many $n$ we have $p_n <m_n$.
OPEN
Is there some $\epsilon>0$ such that there are infinitely many $n$ where all primes $p\leq (2+\epsilon)\log n$ divide $\prod_{1\leq i\leq \log n}(n+i)?$
A problem of Erdős and Pomerance.

More generally, let $q(n,k)$ denote the least prime which does not divide $\prod_{1\leq i\leq k}(n+i)$. This problem asks whether $q(n,\log n)\geq (2+\epsilon)\log n$ infinitely often. Taking $n$ to be the product of primes between $\log n$ and $(2+o(1))\log n$ gives an example where $q(n,\log n)\geq (2+o(1))\log n.$

Can one prove that $q(n,\log n)<(1-\epsilon)(\log n)^2$ for all large $n$ and some $\epsilon>0$?

OPEN
Let $[1,\ldots,n]$ denote the least common multiple of $\{1,\ldots,n\}$. Is it true that, for all $k\geq 1$, $[1,\ldots,p_{k+1}-1]< p_k[1,\ldots,p_k]?$
Erdős and Graham write this is 'almost certainly' true, but the proof is beyond our ability, for two reasons (at least):
• Firstly, one has to rule out the possibility of many primes $q$ such that $p_k<q^2<p_{k+1}$. There should be at most one such $q$, which would follow from $p_{k+1}-p_k<p_k^{1/2}$, which is essentially the notorious Legendre's conjecture.
• The small primes also cause trouble.
OPEN
Let $f(u)$ be the largest $v$ such that no $m\in (u,v)$ is composed entirely of primes dividing $uv$. Estimate $f(u)$.
OPEN
Let $a_0=n$ and $a_1=1$, and in general $a_k$ is the least integer $>a_{k-1}$ for which $(n-a_k,n-a_i)=1$ for all $1\leq i<k$. Does $\sum_{i}\frac{1}{a_i}\to \infty$ as $n\to \infty$? What about if we restrict the sum to those $i$ such that $n-a_j$ is divisible by some prime $\leq a_j$, or the complement of such $i$?
This question arose in work of Eggleton, Erdős, and Selfridge.
OPEN
Let $s_t(n)$ be the $t$-smooth component of $n$ - that is, the product of all primes $p$ (with multiplicity) dividing $n$ such that $p<t$. Let $f(n,t)$ count the number of distinct possible values for $s_t(m)$ for $m\in [n+1,n+t]$. Is there an $\epsilon>0$ such that $f(n,t)>\epsilon t$ for all $t$ and $n$?
Erdős and Graham report they can show $f(n,t) \gg \frac{t}{\log t}.$
OPEN
Let $p(n)$ denote the least prime factor of $n$. There is a constant $c>0$ such that $\sum_{\substack{n<x\\ n\textrm{ not prime}}}\frac{p(n)}{n}\sim c\frac{x^{1/2}}{(\log x)^2}.$ Is it true that there exists a constant $C>0$ such that $\sum_{x\leq n\leq x+Cx^{1/2}(\log x)^2}\frac{p(n)}{n} \gg 1$ for all large $x$?
OPEN
Is there a function $f$ with $f(n)\to \infty$ as $n\to \infty$ such that, for all large $n$, there is a composite number $m$ such that $n+f(n)<m<n+p(m)?$ (Here $p(m)$ is the least prime factor of $m$.)
SOLVED
Let $A=\{a_1<a_2<\cdots\}\subset \mathbb{N}$ be a lacunary sequence (so there exists some $\lambda>1$ with $a_{k+1}\geq \lambda a_k$ for all $k$). Is there an irrational $\alpha$ such that $\{ \{\alpha a_k\} : k\geq 1\}$ is not everywhere dense in $[0,1]$ (where $\{x\}=x-\lfloor x\rfloor$ is the fractional part).
Erdős and Graham write the existence of such an $\alpha$ has 'very recently been shown', but frustratingly give neither a name nor a reference. I will try to track down this solution soon.
SOLVED
Let $N(X,\delta)$ denote the maximum number of points $P_1,\ldots,P_n$ which can be chosen in a circle of radius $X$ such that $\| \lvert P_i-P_j\rvert \| \geq \delta$ for all $1\leq i<j\leq n$. (Here $\|x\|$ is the distance from $x$ to the nearest integer.)

Is it true that, for any $0<\delta<1/2$, we have $N(X,\delta)=o(X)?$ In fact, is it true that (for any fixed $\delta>0$) $N(X,\delta)<X^{1/2+o(1)}?$

The first conjecture was proved by Sárközy [Sa76], who in fact proved $N(X,\delta) \ll \delta^{-3}\frac{X}{\log\log X}.$ See also [466].

Even a stronger statement is true, as shown by Konyagin [Ko01], who proved that $N(X,\delta) \ll_\delta N^{1/2}.$

SOLVED
Let $N(X,\delta)$ denote the maximum number of points $P_1,\ldots,P_n$ which can be chosen in a circle of radius $X$ such that $\| \lvert P_i-P_j\rvert \| \geq \delta$ for all $1\leq i<j\leq n$. (Here $\|x\|$ is the distance from $x$ to the nearest integer.)

Is there some $\delta>0$ such that $\lim_{x\to \infty}N(X,\delta)=\infty?$

Graham proved this is true, and in fact $N(X,1/10)> \frac{\log X}{10}.$ This was substantially improved by Sárközy [Sa76], who proved that for, all sufficiently small $\delta>0$, $N(X,\delta)>X^{1/2-\delta^{1/7}}.$ See also [465].
OPEN
Prove the following for all large $x$: there is a choice of congruence classes $a_p$ for all primes $p\leq x$ and a decomposition $\{p\leq x\}=A\sqcup B$ into two non-empty sets such that, for all $n<x$, there exist some $p\in A$ and $q\in B$ such that $n\equiv a_p\pmod{p}$ and $n\equiv a_q\pmod{q}$.
This is what I assume the intended problem is, although the presentation in [ErGr80] is missing some crucial quantifiers, so I may have misinterpreted it.
OPEN
For any $n$ let $D_n$ be the set of sums of the shape $d_1,d_1+d_2,d_1+d_2+d_3,\ldots$ where $1<d_1<d_2<\cdots$ are the divisors of $n$.

What is the size of $D_n\backslash \cup_{m<n}D_m$?

If $f(N)$ is the minimal $n$ such that $N\in D_n$ then is it true that $f(N)=o(N)$? Perhaps just for almost all $N$?

OPEN
Let $A$ be the set of all $n$ such that $n=d_1+\cdots+d_k$ with $d_i$ distinct proper divisors of $n$, but this is not true for any $m\mid n$ with $m<n$. Does $\sum_{n\in A}\frac{1}{n}$ converge?
The same question can be asked for those $n$ which do not have distinct sums of sets of divisors, but any proper divisor of $n$ does.
OPEN
Call $n$ weird if $\sigma(n)\geq 2n$ and $n\neq d_1+\cdots+d_k$, where the $d_i$ are distinct proper divisors of $n$. Are there any odd weird numbers? Are there infinitely many primitive weird numbers, i.e. those such that no proper divisor of $n$ is weird?
Weird numbers were investigated by Benkoski and Erdős [BeEr74], who proved that the set of weird numbers has positive density.

Melfi [Me15] has proved that there are infinitely many primitive weird numbers, conditional on various well-known conjectures on the distribution of prime gaps. For example, it would suffice to show that $p_{n+1}-p_n <\frac{1}{10}p_n^{1/2}$ for sufficiently large $n$.

The sequence of weird numbers is A006037 in the OEIS. There are no odd weird numbers below $10^{17}$.

OPEN
Given a finite set of primes $Q=Q_0$, define a sequence of sets $Q_i$ by letting $Q_{i+1}$ be $Q_i$ together with all primes formed by adding three distinct elements of $Q_i$. Is there some initial choice of $Q$ such that the $Q_i$ become arbitrarily large?
A problem of Ulam. In particular, what about $Q=\{3,5,7,11\}$?
OPEN
Given some initial finite sequence of primes $q_1<\cdots<q_m$ extend it so that $q_{n+1}$ is the smallest prime of the form $q_n+q_i-1$ for $n\geq m$. Is there an initial starting sequence so that the resulting sequence is infinite?
A problem due to Ulam. For example if we begin with $3,5$ then the sequence continues $3,5,7,11,13,17,\ldots$. It is possible that this sequence is infinite.
SOLVED
Is there a permutation $a_1,a_2,\ldots$ of the positive integers such that $a_k+a_{k+1}$ is always prime?

Watts has suggested that perhaps the obvious greedy algorithm defines such a permutation - that is, let $a_1=1$ and let $a_{n+1}=\min \{ x : a_n+x\textrm{ is prime and }x\neq a_i\textrm{ for }i\leq n\}.$ In other words, do all positive integers occur as some such $a_n$? Do all primes occur as a sum?

OPEN
Let $p$ be a prime. Given any finite set $A\subseteq \mathbb{F}_p\backslash \{0\}$, is there always a rearrangement $A=\{a_1,\ldots,a_t\}$ such that all partial sums $\sum_{1\leq k\leq m}a_{k}$ are distinct, for all $1\leq m\leq t$?
A problem of Graham, who proved it when $t=p-1$. A similar conjecture was made for arbitrary abelian groups by Alspach. Such an ordering is often called a valid ordering.

This has been proved for $t\leq 12$ (see Costa and Pellegrini [CoPe20] and the references therein) and for $p-3\leq t\leq p-1$ (see Hicks, Ollis, and Schmitt [HOS19] and the references therein). Kravitz [Kr24] has proved this for $t \leq \frac{\log p}{\log\log p}.$

SOLVED
Let $A\subseteq \mathbb{F}_p$. Let $A\hat{+}A = \{ a+b : a\neq b \in A\}.$ Is it true that $\lvert A\hat{+}A\rvert \geq \min(2\lvert A\rvert-3,p)?$
A question of Erdős and Heilbronn. Solved in the affirmative by da Silva and Hamidoune [dSHa94].
OPEN
Let $f:\mathbb{Z}\to \mathbb{Z}$ be a polynomial of degree at least $2$. Is there a set $A$ such that every $z\in \mathbb{Z}$ has exactly one representation as $z=a+f(n)$ for some $a\in A$ and $n\in \mathbb{Z}$?
Probably there is no such $A$ for any polynomial $f$.
OPEN
Let $p$ be a prime and $A_p = \{ k! \pmod{p} : 1\leq k<p\}.$ Is it true that $\lvert A_p\rvert \sim (1-\tfrac{1}{e})p?$
OPEN
Is it true that, for all $k\neq 1$, there are infinitely many $n$ such that $2^n\equiv k\pmod{n}$?
A conjecture of Graham. It is easy to see that $2^n\not\equiv 1\mod{n}$ for all $n>1$, so the restriction $k\neq 1$ is necessary. Erdős and Graham report that Graham, Lehmer, and Lehmer have proved this for $k=2^i$ for $i\geq 1$, or if $k=-1$, but I cannot find such a paper.

As an indication of the difficulty, when $k=3$ the smallest $n$ such that $2^n\equiv 3\pmod{n}$ is $n=4700063497$.

The minimal such $n$ for each $k$ is A036236 in the OEIS.

SOLVED
Let $x_1,x_2,\ldots\in [0,1]$ be an infinite sequence. Is it true that there are infinitely many $m,n$ such that $\lvert x_{m+n}-x_n\rvert \leq \frac{1}{\sqrt{5}n}?$
A conjecture of Newman. This was proved Chung and Graham, who in fact show that for any $\epsilon>0$ there must exist some $n$ such that there are infinitely many $m$ for which $\lvert x_{m+n}-x_m\rvert < \frac{1}{(c-\epsilon)n}$ where $c=1+\sum_{k\geq 1}\frac{1}{F_{2k}}=2.535\cdots$ and $F_m$ is the $m$th Fibonacci number. This constant is best possible.
OPEN
Let $a_1,\ldots,a_r,b_1,\ldots,b_r\in \mathbb{N}$ such that $\sum_{i}\frac{1}{a_i}>1$. For any finite sequence of $n$ (not necessarily distinct) integers $A=(x_1,\ldots,x_n)$ let $T(A)$ denote the sequence of length $rn$ given by $(a_ix_j+b_i)_{1\leq j\leq n, 1\leq i\leq r}.$ Prove that, if $A_1=(1)$ and $A_{i+1}=T(A_i)$, then there must be some $A_k$ with repeated elements.
Erdős and Graham write that 'it is surprising that [this problem] offers difficulty'.

The original formulation of this problem had an extra condition on the minimal element of the sequence $A_k$ being large, but Ryan Alweiss has pointed out that is trivially always satisfied since the minimal element of the sequence must grow by at least $1$ at each stage.

Additional thanks to: Ryan Alweiss, Zachary Chase
SOLVED
Define a sequence by $a_1=1$ and $a_{n+1}=\lfloor\sqrt{2}(a_n+1/2)\rfloor$ for $n\geq 1$. The difference $a_{2n+1}-2a_{2n-1}$ is the $n$th digit in the binary expansion of $\sqrt{2}$.

Find similar results for $\theta=\sqrt{m}$, and other algebraic numbers.

The result for $\sqrt{2}$ was obtained by Graham and Pollak [GrPo70]. The problem statement is open-ended, but presumably Erdős and Graham would have been satisfied with the wide-ranging generalisations of Stoll ([St05] and [St06]).
OPEN
Let $f(k)$ be the minimal $N$ such that if $\{1,\ldots,N\}$ is $k$-coloured then there is a monochromatic solution to $a+b=c$. Estimate $f(k)$. In particular, is it true that $f(k) < c^k$ for some constant $c>0$?
Schur proved that $f(k)<ek!$. See also [183].
SOLVED
Prove that there exists an absolute constant $c>0$ such that, whenever $\{1,\ldots,N\}$ is $k$-coloured (and $N$ is large enough depending on $k$) then there are at least $cN$ many integers in $\{1,\ldots,N\}$ which are representable as a monochromatic sum (that is, $a+b$ where $a,b\in \{1,\ldots,N\}$ are in the same colour class and $a\neq b$).
A conjecture of Roth.

Solved by Erdős, Sárközy, and Sós [ESS89], who in fact prove that there are at least $\frac{N}{2}-O(N^{1-1/2^{k+1}})$ many even numbers which are of this form. They also prove that if $k=2$ then there are at least $\frac{N}{2}-O(\log N)$ many even numbers which are of this form, and that $O(\log N)$ is best possible, since there is a $2$-colouring such that no power of $2$ is representable as a monochromatic sum.

A refinement of this problem appears as Problem 25 on the open problems list of Ben Green.

OPEN
Let $A\subseteq \mathbb{N}$, and for each $n\in A$ choose some $X_n\subseteq \mathbb{Z}/n\mathbb{Z}$. Let $B = \{ m\in \mathbb{N} : m\not\in X_n\pmod{n}\textrm{ for all }n\in A\}.$ Must $B$ have a logarithmic density, i.e. is it true that $\lim_{x\to \infty} \frac{1}{\log x}\sum_{\substack{m\in B\\ m<x}}\frac{1}{m}$ exists?
Davenport and Erdős [DaEr37] proved that the answer is yes when $X_n=\{0\}$ for all $n\in A$. The problem considers logarithmic density since Besicovitch [Be34] showed examples exist without a natural density, even when $X_n=\{0\}$ for all $n\in A$.
SOLVED
Let $A\subseteq \mathbb{N}$ have positive density. Must there exist distinct $a,b,c\in A$ such that $[a,b]=c$ (where $[a,b]$ is the lowest common multiple of $a$ and $b$)?
Davenport and Erdős [DaEr37] showed that there must exist an infinite sequence $a_1<a_2\cdots$ in $A$ such that $a_i\mid a_j$ for all $i\leq j$.

This is true, a consequence of the positive solution to [447] by Kleitman [Kl71].

OPEN
Let $A$ be a finite set and $B=\{ n \geq 1 : a\nmid n\textrm{ for all }a\in A\}.$ Is it true that, for every $m>n$, $\frac{\lvert B\cap [1,m]\rvert }{m}< 2\frac{\lvert B\cap [1,n]\rvert}{n}?$
The example $A=\{a\}$ and $n=2a-1$ and $m=2a$ shows that $2$ would be best possible.
OPEN
Let $A\subseteq \mathbb{N}$ be a set such that $\lvert A\cap [1,x]\rvert=o(x^{1/2})$. Let $B=\{ n\geq 1 : a\nmid n\textrm{ for all }a\in A\}.$ If $B=\{b_1<b_2<\cdots\}$ then is it true that $\lim \frac{1}{x}\sum_{b_i<x}(b_{i+1}-b_i)^2$ exists (and is finite)?
For example, when $A=\{p^2: p\textrm{ prime}\}$ then $B$ is the set of squarefree numbers, and the existence of this limit was proved by Erdős.

SOLVED
Let $A,B\subseteq \{1,\ldots,N\}$ be such that all the products $ab$ with $a\in A$ and $b\in B$ are distinct. Is it true that $\lvert A\rvert \lvert B\rvert \ll \frac{N^2}{\log N}?$
This would be best possible, for example letting $A=[1,N/2]\cap \mathbb{N}$ and $B=\{ N/2<p\leq N: p\textrm{ prime}\}$.

This is true, and was proved by Szemerédi [Sz76].

SOLVED
Let $f:\mathbb{N}\to \mathbb{R}$ be an additive function (i.e. $f(ab)=f(a)+f(b)$ whenever $(a,b)=1$). If there is a constant $c$ such that $\lvert f(n+1)-f(n)\rvert <c$ for all $n$ then must there exist some $c'$ such that $f(n)=c'\log n+O(1)?$
Erdős [Er46] proved that if $f(n+1)-f(n)=o(1)$ or $f(n+1)\geq f(n)$ then $f(n)=c\log n$ for some constant $c$.

This is true, and was proved by Wirsing [Wi70].

SOLVED
Let $A=\{a_1<a_2<\cdots\}\subseteq \mathbb{N}$ be infinite such that $a_{i+1}/a_i\to 1$. For any $x\geq a_1$ let $f(x) = \frac{x-a_i}{a_{i+1}-a_i}\in [0,1),$ where $x\in [a_i,a_{i+1})$. Is it true that, for almost all $\alpha$, the sequence $f(\alpha n)$ is uniformly distributed in $[0,1)$?
For example if $A=\mathbb{N}$ then $f(x)=\{x\}$ is the usual fractional part operator.

A problem due to Le Veque [LV53], who proved it in some special cases.

This is false is general, as shown by Schmidt [Sc69].

SOLVED
Does there exist a $k$ such that every sufficiently large integer can be written in the form $\prod_{i=1}^k a_i - \sum_{i=1}^k a_i$ for some integers $a_i\geq 2$?
Erdős attributes this question to Schinzel. Eli Seamans has observed that the answer is yes (with $k=2$) for a very simple reason: $n = 2(n+2)-(2+(n+2)).$ There may well have been some additional constraint in the problem as Schinzel posed it, but [Er61] does not record what this is.
OPEN
Let $\alpha,\beta \in \mathbb{R}$. Is it true that $\liminf_{n\to \infty} n \| n\alpha \| \| n\beta\| =0$ where $\|x\|$ is the distance from $x$ to the nearest integer?
The infamous Littlewood conjecture.
SOLVED
Let $\alpha \in \mathbb{R}$ be irrational and $\epsilon>0$. Are there positive integers $x,y,z$ such that $\lvert x^2+y^2-z^2\alpha\rvert <\epsilon?$
Originally a conjecture due to Oppenheim. Davenport and Heilbronn [DaHe46] solve the analogous problem for quadratic forms in 5 variables.

This is true, and was proved by Margulis [Ma89].

SOLVED
Let $f$ be a Rademacher multiplicative function: a random $\{-1,0,1\}$-valued multiplicative function, where for each prime $p$ we independently choose $f(p)\in \{-1,1\}$ uniformly at random, and for square-free integers $n$ we extend $f(p_1\cdots p_r)=f(p_1)\cdots f(p_r)$ (and $f(n)=0$ if $n$ is not squarefree). Does there exist some constant $c>0$ such that, almost surely, $\limsup_{N\to \infty}\frac{\sum_{m\leq N}f(m)}{\sqrt{N\log\log N}}=c?$
Note that if we drop the multiplicative assumption, and simply assign $f(m)=\pm 1$ at random, then this statement is true (with $c=\sqrt{2}$), the law of the iterated logarithm.

Wintner [Wi44] proved that, almost surely, $\sum_{m\leq N}f(m)\ll N^{1/2+o(1)},$ and Erdős improved the right-hand side to $N^{1/2}(\log N)^{O(1)}$. Lau, Tenenbaum, and Wu [LTW13] have shown that, almost surely, $\sum_{m\leq N}f(m)\ll N^{1/2}(\log\log N)^{2+o(1)}.$ Harper [Ha13] has shown that the sum is almost surely not $O(N^{1/2}/(\log\log N)^{5/2+o(1)})$, and conjectured that in fact Erdős' conjecture is false, and almost surely $\sum_{m\leq N}f(m) \ll N^{1/2}(\log\log N)^{1/4+o(1)}.$ This was proved by Caich [Ca23].

OPEN
Let $\ell(N)$ be maximal such that in any finite set $A\subset \mathbb{R}$ of size $N$ there exists a Sidon subset $S$ of size $\ell(N)$ (i.e. the only solutions to $a+b=c+d$ in $S$ are the trivial ones). Determine the order of $\ell(N)$.

In particular, is it true that $\ell(N)\sim N^{1/2}$?

Originally asked by Riddell [Ri69]. Erdős noted the bounds $N^{1/3} \ll \ell(N) \leq (1+o(1))N^{1/2}$ (the upper bound following from the case $A=\{1,\ldots,N\}$). The lower bound was improved to $N^{1/2}\ll \ell(N)$ by Komlós, Sulyok, and Szemerédi [KSS75]. The correct constant is unknown, but it is likely that the upper bound is true, so that $\ell(N)\sim N^{1/2}$.

In [AlEr85] Alon and Erdős make the stronger conjecture that perhaps $A$ can always be written as the union of at most $(1+o(1))N^{1/2}$ many Sidon sets. (This is easily verified for $A=\{1,\ldots,N\}$ using standard constructions of Sidon sets.)

OPEN
Let $F(k)$ be the minimal $N$ such that if we two-colour $\{1,\ldots,N\}$ there is a set $A$ of size $k$ such that all subset sums $\sum_{a\in S}a$ (for $\emptyset\neq S\subseteq A$) are monochromatic. Estimate $F(k)$.
The existence of $F(k)$ was established by Sanders and Folkman, and it also follows from Rado's theorem. It is commonly known as Folkman's theorem.

Erdős and Spencer [ErSp89] proved that $F(k) \geq 2^{ck^2/\log k}$ for some constant $c>0$. Balogh, Eberhrad, Narayanan, Treglown, and Wagner [BENTW17] have improved this to $F(k) \geq 2^{2^{k-1}/k}.$

SOLVED
If $\mathbb{N}$ is 2-coloured then is there some infinite set $A\subseteq \mathbb{N}$ such that all finite subset sums $\sum_{n\in S}n$ (as $S$ ranges over all non-empty finite subsets of $A$) are monochromatic?
In other words, must some colour class be an IP set. Asked by Graham and Rothschild. See also [531].

Proved by Hindman [Hi74] (for any number of colours).

OPEN
What is the largest possible subset $A\subseteq\{1,\ldots,N\}$ which contains $N$ such that $(a,b)>1$ for all $a\neq b\in A$?
A problem of Erdős and Graham (in [Er73] it was stated with $(a,b)=1$ instead but this is clearly a typo). They conjecture that this maximum is either $N/p$ (where $p$ is the smallest prime factor of $N$) or it is the number of integers $\{2t: t\leq N/2\textrm{ and }(2t,N)> 1\}$.

OPEN
Let $r\geq 3$, and let $f_r(N)$ denote the size of the largest subset of $\{1,\ldots,N\}$ such that no subset of size $r$ has the same pairwise greatest common divisor between all elements. Estimate $f_r(N)$.
Erdős [Er64] proved that $f_r(N) \leq N^{\frac{3}{4}+o(1)},$ and Abbott and Hanson [AbHa70] improved this exponent to $1/2$. Erdős [Er64] proved the lower bound $f_3(N) > N^{\frac{c}{\log\log N}}$ for some constant $c>0$, and conjectured this should also be an upper bound.

Erdős writes this is 'intimately connected' with the sunflower problem [20]. Indeed, the conjectured upper bound would follow from the following stronger version of the sunflower problem: estimate the size of the largest set of integers $A$ such that $\omega(n)=k$ for all $n\in A$ and there does not exist $a_1,\ldots,a_r\in A$ and an integer $d$ such that $(a_i,a_j)=d$ for all $i\neq j$ and $(a_i/d,d)=1$ for all $i$. The conjectured upper bound for $f_r(N)$ would follow if the size of such an $A$ must be at most $c_r^k$. The original sunflower proof of Erdős and Rado gives the upper bound $c_r^kk!$.

OPEN
Let $\epsilon>0$ and $N$ be sufficiently large. Is it true that if $A\subseteq \{1,\ldots,N\}$ has size at least $\epsilon N$ then there must be $a,b,c\in A$ such that $[a,b]=[b,c]=[a,c],$ where $[a,b]$ denotes the least common multiple?
This is false if we ask for four elements with the same pairwise least common multiple, as shown by Erdős [Er62] (with a proof given in [Er70]).

SOLVED
Let $\epsilon>0$ and $N$ be sufficiently large. If $A\subseteq \{1,\ldots,N\}$ has $\lvert A\rvert \geq \epsilon N$ then must there exist $a_1,a_2,a_3\in A$ and distinct primes $p_1,p_2,p_3$ such that $a_1p_1=a_2p_2=a_3p_3?$
A positive answer would imply [536].

Erdős describes a construction of Ruzsa which disproves this: consider the set of all squarefree numbers of the shape $p_1\cdots p_r$ where $p_{i+1}>2p_i$ for $1\leq i<r$. This set has positive density, and hence if $A$ is its intersection with $(N/2,N)$ then $\lvert A\rvert \gg N$ for all large $N$. Suppose now that $p_1a_1=p_2a_2=p_3a_3$ where $a_i\in A$ and $p_1,p_2,p_3$ are distinct primes. Without loss of generality we may assume that $a_2>a_3$ and hence $p_2<p_3$, and so since $p_2p_3\mid a_1\in A$ we must have $2<p_3/p_2$. On the other hand $p_3/p_2=a_2/a_3\in (1,2)$, a contradiction.

OPEN
Let $r\geq 2$ and suppose that $A\subseteq\{1,\ldots,N\}$ is such that, for any $m$, there are at most $r$ solutions to $m=pa$ where $p$ is prime and $a\in A$. Give the best possible upper bound for $\sum_{n\in A}\frac{1}{n}.$
Erdős observed that $\sum_{n\in A}\frac{1}{n}\sum_{p\leq N}\frac{1}{p}\leq r\sum_{m\leq N^2}\frac{1}{m}\ll r\log N,$ and hence $\sum_{n\in A}\frac{1}{n} \ll r\frac{\log N}{\log\log N}.$ See also [536] and [537].
OPEN
Let $h(n)$ be such that, for any set $A\subseteq \mathbb{N}$ of size $n$, the set $\left\{ \frac{a}{(a,b)}: a,b\in A\right\}$ has size at least $h(n)$. Estimate $h(n)$.
Erdős and Szemerédi proved that $n^{1/2} \ll h(n) \ll n^{1-c}$ for some constant $c>0$.
SOLVED
Is it true that if $A\subseteq \mathbb{Z}/N\mathbb{Z}$ has size $\gg N^{1/2}$ then there exists some non-empty $S\subseteq A$ such that $\sum_{n\in S}n\equiv 0\pmod{N}$?
A conjecture of Erdős and Heilbronn. The answer is yes, proved by Szemerédi [Sz70] (in fact for arbitrary finite abelian groups).

Erdős speculated that perhaps the correct threshold is $(2N)^{1/2}$; this is also a conjecture of Selfridge, and has been proved when $N$ is prime by Balandraud [Ba12].

OPEN
Let $a_1,\ldots,a_p$ be (not necessarily distinct) residues modulo $p$, such that there exists some $r$ so that if $S\subseteq [p]$ is non-empty and $\sum_{i\in S}a_i\equiv 0\pmod{p}$ then $\lvert S\rvert=r$. Must there be at most two distinct residues amongst the $a_i$?
A question of Graham.
SOLVED
Is it true that if $A\subseteq\{1,\ldots,n\}$ is a set such that $[a,b]>n$ for all $a\neq b$, where $[a,b]$ is the least common multiple, then $\sum_{a\in A}\frac{1}{a}\leq \frac{31}{30}.$ Is it true that there must be $\gg n$ many $m\leq n$ which do not divide any $a\in A$?
The first bound is best possible as $A=\{2,3,5\}$ demonstrates.

Resolved by Schinzel and Szekeres [ScSz59] who proved the answer to the first question is yes and the answer to the second is no, and in fact there are examples with at most $n/(\log n)^c$ many such $m$, for some constant $c>0$.

In [Er73] Erdős further speculates that in fact $\sum_{a\in A}\frac{1}{a}\leq 1+o(1),$ where the $o(1)$ term $\to 0$ as $n\to \infty$.

OPEN
Define $f(N)$ be the minimal $k$ such that the following holds: if $G$ is an abelian group of size $N$ and $A\subseteq G$ is a random set of size $k$ then, with probability $\geq 1/2$, all elements of $G$ can be written as $\sum_{x\in S}x$ for some $S\subseteq A$. Is $f(N) \leq \log_2 N+o(\log\log N)?$
Erdős and Rényi [ErRe65] proved that $f(N) \leq \log_2N+O(\log\log N).$ Erdős believed improving this to $o(\log\log N)$ is impossible.
SOLVED
Is there a covering system such that no two of the moduli divide each other?
Asked by Schinzel, motivated by a question of Erdős and Selfridge (see [7]). The answer is no, as proved by Balister, Bollobás, Morris, Sahasrabudhe, and Tiba [BBMST22].
SOLVED
What is the size of the largest $A\subseteq \{1,\ldots,N\}$ such that, for all $\emptyset\neq S\subseteq A$, $\sum_{n\in S}n$ is not a square?
Erdős observed that $\lvert A\rvert \gg N^{1/3}$ is possible, taking the first $\approx N^{1/3}$ multiples of some prime $p\approx N^{2/3}$.

Essentially solved by Nguyen and Vu [NgVu10], who proved that $\lvert A\rvert\ll N^{1/3}(\log N)^{O(1)}$.

This question was asked by Erdős to a young Terence Tao in 1985. We thank Tao for sharing this memory and a letter of Erdős describing the problem.

OPEN
Let $t\geq 1$ and $A\subseteq \{1,\ldots,N\}$ be such that whenever $a,b\in A$ with $b-a\geq t$ we have $b-a\nmid b$. How large can $\lvert A\rvert$ be? Is it true that $\lvert A\rvert \leq \left(\frac{1}{2}+o_t(1)\right)N?$
Asked by Erdős in a letter to Ruzsa in around 1980. Erdős observes that when $t=1$ the maximum possible is $\lvert A\rvert=\left\lfloor\frac{N+1}{2}\right\rfloor,$ achieved by taking $A$ to be all odd numbers in $\{1,\ldots,N\}$. He also observes that when $t=2$ there exists such an $A$ with $\lvert A\rvert \geq \frac{N}{2}+c\log N$ for some constant $c>0$: take $A$ to be the union of all odd numbers together with numbers of the shape $2^k$ with $k$ odd.
SOLVED
If $\mathbb{N}$ is 2-coloured then must there exist a monochromatic three-term arithmetic progression $x,x+d,x+2d$ such that $d>x$?
Erdös writes 'perhaps this is easy or false'. It is not true for four-term arithmetic progressions: colour the integers in $[3^{2k},3^{2k+1})$ red and all others blue.

Ryan Alweiss has provided the following simple argument showing that the answer is yes: suppose we have some red/blue colouring without this property. Without loss of generality, suppose $1$ is coloured red, and then either $3$ or $5$ must be blue.

Suppose first that $3$ is blue. If $n\geq 6$ is red then (considering $1,n,2n-1$) we deduce $2n-1$ is blue, and then (considering $3,n+1,2n-1$) we deduce that $n+1$ is red. In particular the colouring must be eventually constant, and we are done.

Now suppose that $5$ is blue. Arguing similarly (considering $1,n,2n-1$ and $5,n+2,2n-1$) we deduce that if $n\geq 8$ is red then $n+2$ is also red, and we are similarly done, since the colouring must be eventually constant on some congruence class modulo $2$.

SOLVED
Let $p_1,\ldots,p_k$ be distinct primes. Are there infinitely many $n$ such that $n!$ is divisible by an even power of each of the $p_i$?
The answer is yes, proved by Berend [Be97], who further proved that the sequence of such $n$ has bounded gaps (where the bound depends on the initial set of primes).
Additional thanks to: Euro Vidal Sampaio
OPEN
Let $\tau(n)$ count the number of divisors of $n$. Is there some $n>24$ such that $\max_{m<n}(m+\tau(m))\leq n+2?$
A problem of Erdős and Selfridge. This is true for $n=24$. The $n+2$ is best possible here since $\max(\tau(n-1)+n-1,\tau(n-2)+n-2)\geq n+2.$

In [Er79] Erdős says 'it is extremely doubtful' that there are infinitely many such $n$, and in fact suggets that $\lim_{n\to \infty}\max_{m<n}(\tau(m)+m-n)=\infty.$

In [Er79d] Erdős says it 'seems certain' that for every $k$ there are infinitely many $n$ for which $\max_{n-k<m<n}(m+\tau(m))\leq n+2,$ but 'this is hopeless with our present methods', although it follows from Schinzel's Hypothesis H.

OPEN
Let $g(n)$ denote the largest $t$ such that there exist integers $2\leq a_1<a_2<\cdots <a_t <n$ such that $P(a_1)>P(a_2)>\cdots >P(a_t)$ where $P(m)$ is the greatest prime factor of $m$. Estimate $g(n)$.
OPEN
Is it true that, for any two (odd) primes $p,q$, there exists some integer $n$ such that the largest prime factor of $n$ is $p$ and the largest prime factor of $n+1$ is $q$?
Erdős writes 'it is probably hopelessly difficult to decide about the truth of this conjecture'. The number of solutions is finite for any fixed $p,q$ since the largest prime factor of $n(n+1)$ tends to $\infty$ (Mahler [Ma35] showed that this is $\gg \log\log n$, see [368]).

Sampaio has observed that the answer to this question is no if one of the primes can be $2$ - for example this is false with $p=19$ and $q=2$, since if $n+1=2^k$ and $19\mid n$ then (since $2$ is a primitive root modulo $19$) we must have $18\mid k$, and hence $73\mid 2^{18}-1\mid n$. A similar argument works with $19$ replaced by any prime $p>13$ for which $2$ is a primitive root, using a result of Rotkiewicz [Ro64b] that for every prime $p>13$ there is a prime $q>p$ which divides $2^{p-1}-1$.

It is unclear whether Erdős was aware of this obstacle; certainly in [Er95c] he asks for any two primes, but may have intended to rule out such small prime obstacles.

More generally, one can ask about whether for any primes $p_1,\ldots,p_k$ there exists some $n$ such that the largest prime factor of $n+i$ is $p_i$. Erdős writes this is 'clearly impossible' if the $p_i$ are the first $k$ primes and $k$ is sufficiently large, but does not know what happens if all of the primes are sufficiently large compared to $k$.

Additional thanks to: Euro Vidal Sampaio
OPEN
Let $f(m)$ be such that if $A\subseteq \{1,\ldots,N\}$ has $\lvert A\rvert=m$ then every interval in $[1,\infty)$ of length $2N$ contains $\geq f(m)$ many distinct integers $b_1,\ldots,b_r$ where each $b_i$ is divisible by some $a_i\in A$, where $a_1,\ldots,a_r$ are distinct.

Estimate $f(m)$. In particular is it true that $f(m)\ll m^{1/2}$?

Erdős and Sarányi [ErSa59] proved that $f(m)\gg m^{1/2}$.
OPEN
Let $q(n,k)$ denote the least prime which does not divide $\prod_{1\leq i\leq k}(n+i)$. Is it true that, if $k$ is fixed and $n$ is sufficiently large, we have $q(n,k)<(1+o(1))\log n?$
A problem of Erdős and Pomerance.

The bound $q(n,k)<(1+o(1))k\log n$ is easy. It may be true this improved bound holds even up to $k=o(\log n)$.

OPEN
Can the product of an arithmetic progression of integers of length $\geq 4$ be a perfect power?
Erdős believed not. Erdős and Selfridge [ErSe75] proved that the product of consecutive integers is never a perfect power.

The theory of Pell equations implies that there are infinitely many pairs $x,d$ with $(x,d)=1$ such that $x(x+d)(x+2d)$ is a square.

Considering the question of whether the product of an arithmetic progression of length $k$ can be equal to an $\ell$th power:

• Euler proved this is impossible when $k=4$ and $\ell=2$,
• Obláth [Ob51] proved this is impossible when $(k,l)=(5,2),(3,3),(3,4),(3,5)$.
• Marszalek [Ma85] proved that this is only possible for $k\ll_d 1$, where $d$ is the common difference of the arithmetic progression.
SOLVED
Let $1=d_1<\cdots <d_{\tau(n)}=n$ be the divisors of $n$ and $G(n) = \sum_{1\leq i<\tau(n)}\frac{d_i}{d_{i+1}}.$ Is it true that $G(n)\to \infty$ for almost all $n$? Can one prove an asymptotic formula for $\sum_{n\leq X}G(n)$?
Erdős writes it is 'easy' to prove $\frac{1}{X}\sum_{n\leq X}G(n)\to \infty$.

Terence Tao has observed that, for any divisor $m\mid n$, $\frac{\tau(n/m)}{m} \leq G(n) \leq \tau(n),$ and hence for example $\tau(n)/4\leq G(n)\leq \tau(n)$ for even $n$. It is easy to then see that $G(n)$ grows on average, and in general behaves very similarly to $\tau(n)$ (and in particular the answer to the first question is yes). Tao suggests that this was a mistaken conjecture of Erdős, which he soon corrected a year later to [448].

SOLVED
Are there any integer solutions to $x^xy^y=z^z$ with $x,y,z>1$?
Ko [Ko40] proved there are none if $(x,y)=1$, but there are in fact infinitely many solutions in general - for example, $x=2^{12}3^6, y = 2^83^8,\textrm{ and } z = 2^{11}3^7.$ More generally, writing $a=2^{n+1}$ and $b=2^n-1$, $x = 2^{a(b-n)}b^{2b}\cdot 2^{2n},$ $y = 2^{a(b-n)}b^{2b}\cdot b^2,$ and $z = 2^{a(b-n)}b^{2b}\cdot 2^{n+1}b.$ In [Er79] Erdős asks if the infinite families found by Ko are the only solutions.
OPEN
We say that $A\subset \mathbb{N}$ has the translation property if, for every $n$, there exists some integer $t_n\geq 1$ such that $A\cap [1,n]=(A-t_n)\cap [1,n].$
• Does the set of the sums of two squares have the translation property?
• If we partition all primes into $P\sqcup Q$, such that each set contains $\gg x/\log x$ many primes $\leq x$ for all large $x$, then can the set of integers only divisible by primes from $P$ have the translation property?
• If $A$ is the set of squarefree numbers then how fast does the minimal such $t_n$ grow? Is it true that $t_n>\exp(n^c)$ for some constant $c>0$?
Elementary sieve theory implies that the set of squarefree numbers has the translation property.

More generally, Brun's sieve can be used to prove that if $B\subseteq \mathbb{N}$ is a set of pairwise coprime integers with $\sum_{b<x}\frac{1}{b}=o(\log\log x)$ then $A=\{ n: b\nmid n\textrm{ for all }b\in A\}$ has the translation property. Erdős did not know what happens if the condition on $\sum_{b<x}\frac{1}{b}$ is weakened or dropped altogether.

OPEN
Is every sufficiently large integer of the form $ap^2+b$ for some prime $p$ and integer $a\geq 1$ and $0\leq b<p$?
The sieve of Eratosthenes implies that almost all integers are of this form, and the Brun-Selberg sieve implies the number of exceptions in $[1,x]$ is $\ll x/(\log x)^c$ for some constant $c>0$. Erdős [Er79] believed it is 'rather unlikely' that all large integers are of this form.

What if the condition that $p$ is prime is omitted? Selfridge and Wagstaff made a 'preliminary computer search' and suggested that there are infinitely many $n$ not of this form even without the condition that $p$ is prime. It should be true that the number of exceptions in $[1,x]$ is $<x^c$ for some constant $c<1$.

Most generally, given some infinite set $A\subseteq \mathbb{N}$ and function $f:A\to \mathbb{N}$ one can ask for sufficient conditions on $A$ and $f$ that guarantee every large number (or almost all numbers) can be written as $am^2+b$ for some $m\in A$ and $a\geq 1$ and $0\leq b<f(m)$.

In another direction, one can ask what is the minimal $c_n$ such that $n$ can be written as $n=ap^2+b$ with $0\leq b<c_np$ for some $p\leq \sqrt{n}$. This problem asks whether $c_n\leq 1$ eventually, but in [Er79d] Erdős suggests that in fact $\limsup c_n=\infty$. Is it true that $c_n<n^{o(1)}$?

OPEN
Let $M(n,k)=[n+1,\ldots,n+k]$ be the least common multiple of $\{n+1,\ldots,n+k\}$.

Is it true that for all $m\geq n+k$ $M(n,k) \neq M(m,k)?$

The Thue-Siegel theorem implies that, for fixed $k$, there are only finitely many $m,n$ such that $m\geq n+k$ and $M(n,k)=M(m,k)$.

In general, how many solutions does $M(n,k)=M(m,l)$ have when $m\geq n+k$ and $l>1$? Erdős expects very few (and none when $l\geq k$).

The only solutions Erdős knew were $M(4,3)=M(13,2)$ and $M(3,4)=M(19,2)$.

In [Er79d] Erdős conjectures the stronger fact that (aside from a finite number of exceptions) if $k>2$ and $m\geq n+k$ then $\prod_{i\leq k}(n+i)$ and $\prod_{i\leq k}(m+i)$ cannot have the same set of prime factors.

OPEN
Let $M(n,k)=[n+1,\ldots,n+k]$ be the least common multiple of $\{n+1,\ldots,n+k\}$.

Let $k\geq 3$. Are there infinitely many $m,n$ with $m\geq n+k$ such that $M(n,k)>M(m,k+1)?$

It is easy to see that there are infinitely many solutions to $M(n,k)>M(m,k)$. The referee of [Er79] found $M(96,7)>M(104,8)$ and $M(132,7)>M(139,8)$.

If $n_k$ is the smallest $n$ with this property (for some $m$) then are there good bounds for $n_k$? Erdős writes that he could prove $n_k/k\to \infty$, but knew of no good upper bounds.

If $u_k$ is minimal such that $M(u_k,k)>M(u_k+1,k)$ and $t<\min(u_k,T)$ then is it true that $M(t,k)\leq M(T,k)$?

OPEN
Let $\epsilon>0$ and $\omega(n)$ count the number of distinct prime factors of $n$. Are there infinitely many values of $n$ such that $\omega(n-k) < (1+\epsilon)\frac{\log k}{\log\log k}$ for all $k<n$ which are sufficiently large depending on $\epsilon$ only?

Can one show the stronger version with $\omega(n-k) < \frac{\log k}{\log\log k}+O(1)$ is false?

One can ask similar questions for $\Omega$, the number of prime factors with multiplicity, where $\log k/\log\log k$ is replaced by $\log_2k$.

OPEN
Is it true that, for all sufficiently large $n$, there exists some $k$ such that $p(n+k)>k^2+1,$ where $p(m)$ denotes the least prime factor of $m$?

Can one prove this is false if we replace $k^2+1$ by $e^{(1+\epsilon)\sqrt{k}}+C_\epsilon$, for all $\epsilon>0$, where $C_\epsilon>0$ is some constant?

This follows from 'plausible assumptions on the distribution of primes' (as does the question with $k^2$ replaced by $k^d$ for any $d$); the challenge is to prove this unconditionally.

Erdős observed that Cramer's conjecture $\limsup_{k\to \infty} \frac{p_{k+1}-p_k}{(\log k)^2}=1$ implies that for all $\epsilon>0$ and all sufficiently large $n$ there exists some $k$ such that $p(n+k)>e^{(1-\epsilon)\sqrt{k}}.$ There is now evidence, however, that Cramer's conjecture is false; a more refined heuristic by Granville [Gr95] suggests this $\limsup$ is $2e^{-\gamma}\approx 1.119\cdots$, and so perhaps the $1+\epsilon$ in the second question should be replaced by $2e^{-\gamma}+\epsilon$.

OPEN
Is it true that for all large $n$ there exists $k$ such that $n+k$ is composite and $p(n+k)>k^2,$ where $p(m)$ is the least prime factor of $m$?
Related to questions of Erdős, Eggleton, and Selfridge. This may be true with $k^2$ replaced by $k^d$ for any $d$.

OPEN
Is it true that for almost all $n$ there exists some $m\in (p_n,p_{n+1})$ such that $p(m) \geq p_{n+1}-p_n,$ where $p(m)$ denotes the least prime factor of $m$?
Erdős first thought this should be true for all large $n$, but found a (conditional) counterexample: Dickson's conjecture says there are infinitely many $d$ such that $2183+30030d\textrm{ and }2201+30030d$ are both prime, and then they must necessarily be consecutive primes. These give a counterexample since $30030=2\cdot 3 \cdot 5\cdot 7\cdot 11\cdot 13$ and every integer in $[2184,2200]$ is divisible by at least one of these primes.

OPEN
Is it true that for every $0\leq k\leq n$ the largest prime divisor of $\binom{n}{k}$ is $> \min(n-k+1, k^{1+c})$ for some constant $c>0$?
A theorem of Sylvester and Schur states that this is $>k$ if $k\leq n/2$. Erdős writes it 'seems certain' that this holds for every $c>0$, with only a finite number of exceptions (depending on $c$). Standard heuristics on prime gaps suggest that the largest prime divisor of $\binom{n}{k}$ is in fact $> \min(n-k+1, e^{c\sqrt{k}})$ for some constant $c>0$.
OPEN
For $0\leq k\leq n$ write $\binom{n}{k} = uv$ where the only primes dividing $u$ are in $[2,k]$ and the only primes dividing $v$ are in $(k,n]$.

Let $f(n)$ be the smallest $k$ such that $u>n^2$. Give bounds for $f(n)$.

A classical theorem of Mahler states that for any $\epsilon>0$ and integers $k$ and $l$ then, writing $(n+1)\cdots (n+k) = ab$ where the only primes dividing $a$ are $\leq l$ and the only primes dividing $b$ are $>l$, we have $a < n^{1+\epsilon}$ for all sufficiently large (depending on $\epsilon,k,l$) $n$.

Mahler's theorem implies $f(n)\to \infty$ as $n\to \infty$, but is ineffective, and so gives no bounds on the growth of $f(n)$.

One can similarly ask for estimates on the smallest integer $f(n,k)$ such that if $m$ is the factor of $\binom{n}{k}$ containing all primes $\leq f(n,k)$ then $m > n^2$.

OPEN
Let $\epsilon>0$ and $n$ be large depending on $\epsilon$. Is it true that for all $n^\epsilon<k\leq n^{1-\epsilon}$ the number of distinct prime divisors of $\binom{n}{k}$ is $(1+o(1))k\sum_{k<p<n}\frac{1}{p}?$ Or perhaps even when $k \geq (\log n)^c$?
It is trivial that the number of prime factors is $>\frac{\log \binom{n}{k}}{\log n},$ and this inequality becomes (asymptotic) equality if $k>n^{1-o(1)}$.
OPEN
Can every integer $N\geq 2$ be written as $N=\frac{\prod_{1\leq i\leq k}(m+i)}{\prod_{1\leq i\leq k}(n+i)}$ for some $k\geq 2$ and $m\geq n+k$?
If $n$ and $k$ are fixed then can one say anything about the set of integers so represented?

OPEN
Let $Y(x)$ be the minimal $y$ such that there exists a choice of congruence classes $a_p$ for all primes $p\leq x$ such that every integer in $[1,y]$ is congruent to at least one of the $a_p\pmod{p}$.

Give good estimates for $Y(x)$. In particular, can one prove that $Y(x)=o(x^2)$ or even $Y(x)\ll x^{1+o(1)}$?

This function is closely related to the problem of gaps between primes (see [4]). The best known upper bound is due to Iwaniec [Iw78], $Y(x) \ll x^2.$ The best lower bound is due to Ford, Green, Konyagin, Maynard, and Tao [FGKMT18], $Y(x) \gg x\frac{\log x\log\log\log x}{\log\log x},$ improving on a previous bound of Rankin [Ra38].

Maier and Pomerance have conjectured that $Y(x)\ll x(\log x)^{2+o(1)}$.

OPEN
Define $\epsilon_n$ to be maximal such that there exists some choice of congruence class $a_p$ for all primes $n^{\epsilon_n}<p\leq n$ such that every integer in $[1,n]$ satisfies at least one of the congruences $\equiv a_p\pmod{p}$.

Estimate $\epsilon_n$ - in particular is it true that $\epsilon_n=o(1)$?

Erdős could prove $\epsilon_n \gg \frac{\log\log\log n}{\log\log n}.$

OPEN
Is there some choice of congruence class $a_p$ for all primes $2\leq p\leq n$ such that every integer in $[1,n]$ satisfies at least two of the congruences $\equiv a_p\pmod{p}$?
OPEN
Let $d_k(p)$ be the density of those integers whose $k$th smallest prime factor is $p$ (i.e. if $p_1<p_2<\cdots$ are the primes dividing $n$ then $p_k=p$).

For fixed $k\geq 1$ is $d_k(p)$ unimodular in $p$? That is, it first increases in $p$ until its maximum then decreases.

Erdős believes that this is not possible, but could not disprove it. He could show that $p_k$ is about $e^{e^k}$ for almost all $n$, but the maximal value of $d_k(p)$ is assumed for much smaller values of $p$, at $p=e^{(1+o(1))k}.$

A similar question can be asked if we consider the density of integers whose $k$th smallest divisor is $d$. Erdős could show that this function is not unimodular.

OPEN
Given $A\subseteq \mathbb{N}$ let $M_A=\{ n \geq 1 : a\mid n\textrm{ for some }a\in A\}$ be the set of multiples of $A$. Find a necessary and sufficient condition on $A$ for $M_A$ to have density $1$.
If $A$ is a set of prime numbers then a necessary and sufficient condition is that $\sum_{p\in A}\frac{1}{p}=\infty$.

The general situation is more complicated. For example suppose $A$ is the union of $(n_k,(1+\eta_k)n_k)\cap \mathbb{Z}$ where $1\leq n_1<n_2<\cdots$ is a lacunary sequence. If $\sum \eta_k<\infty$ then the density of $M_A$ exists and is $<1$. If $\eta_k=1/k$, so $\sum \eta_k=\infty$, then the density exists and is $<1$.

Erdős writes it 'seems certain' that there is some threshold $\alpha\in (0,1)$ such that, if $\eta_k=k^{-\beta}$, then the density of $M_A$ is $1$ if $\beta <\alpha$ and the density is $<1$ if $\beta >\alpha$.

OPEN
Let $\delta_1(n,m)$ be the density of the set of integers which exactly one divisor in $(n,m)$. Is $\delta_1(n,m)$ unimodular for $m>n+1$ (i.e. increases until some $m$ then decreases thereafter)? For fixed $n$, where does $\delta_1(n,m)$ achieve its maximum?
Erdős proved that $\delta_1(n,m) \ll \frac{1}{(\log n)^c}$ for all $m$, for some constant $c>0$. Sharper bounds (for various ranges of $n$ and $m$) were given by Ford [Fo08].

OPEN
Let $k\geq 2$ and $n$ be sufficiently large depending on $k$. Let $A=\{a_1<a_2<\cdots \}$ be the set of those integers in $[n,n^k]$ which have a divisor in $(n,2n)$. Estimate $\max_{i} a_{i+1}-a_i.$ Is this $\leq (\log n)^{O(1)}$?
OPEN
Let $f_{\max}(n)$ be the largest $m$ such that $\phi(m)=n$, and $f_{\min}(n)$ be the smallest such $m$, where $\phi$ is Euler's totient function. Investigate $\max_{n\leq x}\frac{f_{\max}(n)}{f_{\min}(n)}.$
Carmichael has asked whether there is an integer $n$ for which $\phi(m)=n$ has exactly one solution, that is, $\frac{f_{\max}(n)}{f_{\min}(n)}=1$. Erdős has proved that if such an $n$ exists then there must be infinitely many such $n$.

OPEN
Let $q_1<q_2<\cdots$ be a sequence of primes such that $q_{i+1}\equiv 1\pmod{q_i}$. Is it true that $\lim_k q_k^{1/k}=\infty?$ Does there exist such a sequence with $q_k \leq \exp(k(\log k)^{1+o(1)})?$
Linnik's theorem implies that there exists such a sequence of primes with $q_k \leq e^{e^{O(k)}}.$

OPEN
Let $h(n)$ be the largest $\ell$ such that there is a sequence of primes $p_1<\cdots p_\ell$ all dividing $n$ with $p_{i+1}\equiv 1\pmod{p_i}$. Let $H(n)$ be the largest $u$ such that there is a sequence of integers $d_1<\cdots d_u$ all dividing $n$ with $d_{i+1}\equiv 1\pmod{d_i}$.

Estimate $h(n)$ and $H(n)$. Is it true that $H(n)/h(n)\to \infty$ for almost all $n$?

Erdős writes it is 'easy to see' that $h(n)\to \infty$ for almost all $n$, and believed he could show that the normal order of $h(n)$ is $\log_*(n)$ (the iterated logarithm).

OPEN
Let $\delta(m,\alpha)$ denote the density of the set of integers which are divisible by some $d\equiv 1\pmod{m}$ with $1<d<\exp(m^\alpha)$. Does there exist some $\beta\in (1,\infty)$ such that $\lim_{m\to \infty}\delta(m,\alpha)$ is $0$ if $\alpha<\beta$ and $1$ if $\alpha>\beta$?
It is trivial that $\delta(m,\alpha)\to 0$ if $\alpha <1$, and Erdős could prove that the same is true for $\alpha=1$.

OPEN
Is there some $h(n)\to \infty$ such that for all $2\leq i<j\leq n/2$ $\textrm{gcd}\left( \binom{n}{i},\binom{n}{j}\right) \geq h(n)?$
A problem of Erdős and Szekeres, who observed that $\textrm{gcd}\left( \binom{n}{i},\binom{n}{j}\right) \geq \frac{\binom{n}{i}}{\binom{j}{i}}\geq 2^i$ (in particular the greatest common divisor is always $>1$). This inequality is sharp for $i=1$, $j=p$, and $n=2p$.
OPEN
Is it true that for every $1\leq i<j\leq n/2$ there exists some prime $p\geq i$ such that $p\mid \textrm{gcd}\left(\binom{n}{i}, \binom{n}{j}\right)?$
A problem of Erdős and Szekeres. A theorem of Sylvester and Schur says that for any $1\leq i\leq n/2$ there exists some prime $p>i$ which divides $\binom{n}{i}$.

Erdős and Szekeres further conjectured that $p\geq i$ can be improved to $p>i$ except in a few special cases. In particular this fails when $i=2$ and $n$ being some particular powers of $2$. They also found some counterexamples when $i=3$, but only one counterexample when $i\geq 4$: $\textrm{gcd}\left(\binom{28}{5},\binom{28}{14}\right)=2^3\cdot 3^3\cdot 5.$

OPEN
Let $f(n)=\min_{1<j\leq n/2}\textrm{gcd}\left(n,\binom{n}{k}\right).$
• Characterise those composite $n$ such that $f(n)=n/P(n)$, where $P(n)$ is the largest prime dividing $n$.
• Are there infinitely many composite $n$ such that $f(n)>n^{1/2}$?
• Is it true that, for every composite $n$, $f(n) \ll_A \frac{n}{(\log n)^A}$ for every $A>0$?
A problem of Erdős and Szekeres. It is easy to see that $f(n)\leq n/P(n)$ for composite $n$, since if $j=p^k$ where $p^k\mid n$ and $p^{k+1}\nmid n$ then $\textrm{gcd}\left(n,\binom{n}{k}\right)=n/p^k$. This implies $f(n) \leq (1+o(1))\frac{n}{\log n}.$

It is known that $f(n)=n/P(n)$ when $n$ is the product of two primes. Another example is $n=30$.

For the second problem, it is easy to see that for any $n$ we have $f(n)\geq p(n)$, where $p(n)$ is the smallest prime dividing $n$, and hence there are infinitely many $n$ (those $=p^2)$ such that $f(n)\geq n^{1/2}$.

OPEN - $100 Let$g(n)$be minimal such that for any$A\subseteq [2,\infty)\cap \mathbb{N}$with$\lvert A\rvert =n$then in any set$I$of$\max(A)$consecutive integers there exists some$B\subseteq I$with$\lvert B\rvert=g(n)$such that $\prod_{a\in A} a \mid \prod_{b\in B}b.$ Is it true that $g(n) \leq (2+o(1))n?$ Or perhaps even$g(n)\leq 2n$? A problem of Erdős and Surányi [ErSu59], who proved that$g(n) \geq (2-o(1))n$, and that$g(3)=4$. Their upper bound construction took$A$as the set of$p_ip_j$for$i\neq j$, where$p_1<\cdots <p_\ell$is some set of primes such that$2p_1^2>p_\ell^2$. Gallai was the first to consider problems of this type, and observed that$g(2)=2$and$g(3)\geq 4$. In [Er92c] Erdős offers '100 dollars or 1000 rupees', whichever is more, for a proof or disproof. (In 1992 1000 rupees was worth approximately \$38.60.)

Erdős and Surányi similarly asked what is the smallest $c_n\geq 1$ such that in any interval $I\subset [0,\infty)$ of length $c_n\max(A)$ there exists some $B\subseteq I\cap \mathbb{N}$ with $\lvert B\rvert=n$ such that $\prod_{a\in A} a \mid \prod_{b\in B}b.$ They prove $c_2=1$ and $c_3=\sqrt{2}$, but have no good upper or lower bounds in general.

OPEN
Let $f(n)$ be minimal such that, for any $A=\{a_1,\ldots,a_n\}\subseteq [2,\infty)\cap\mathbb{N}$ of size $n$, in any interval $I$ of $f(n)\max(A)$ consecutive integers there exist distinct $x_1,\ldots,x_n\in I$ such that $a_i\mid x_i$.

Obtain good bounds for $f(n)$, or even an asymptotic formula.

A problem of Erdős and Surányi [ErSu59], who proved $(\log n)^c \ll f(n) \ll n^{1/2}$ for some constant $c>0$.

OPEN - $78 Let$f(n)$be minimal such that in$(n,n+f(n))$there exist distinct integers$a_1,\ldots,a_n$such that$k\mid a_k$for all$1\leq k\leq n$. Obtain an asymptotic formula for$f(n)$. A problem of Erdős and Pomerance [ErPo80], who proved $n\left(\frac{\log n}{\log\log n}\right)^{1/2}\ll f(n)\leq (2+o(1))n(\log n)^{1/2}.$ In [Er92c] Erdős offered 2000 rupees for an asymptotic formula; for uniform comparison across prizes I have converted this using the 1992 exchange rates. See also [711]. OPEN -$39
Let $f(n,m)$ be minimal such that in $(m,m+f(n,m))$ there exist distinct integers $a_1,\ldots,a_n$ such that $k\mid a_k$ for all $1\leq k\leq n$. Prove that $\max_m f(n,m) \leq n^{1+o(1)}$ and that $\max_m (f(n,m)-f(n,n))\to \infty.$
A problem of Erdős and Pomerance [ErPo80], who proved that $\max_m f(n,m) \ll n^{3/2}.$

In [Er92c] Erdős offered 1000 rupees for a proof of either; for uniform comparison across prizes I have converted this using the 1992 exchange rates.

SOLVED
Let $W(3,k)$ be the van der Waerden number defined as the minimum $n$ such that in any red/blue colouring of $\{1,\ldots,n\}$ there exists either a red $3$-term arithmetic progression or a blue $k$-term arithmetic progression.

Give reasonable bounds for $W(3,k)$. In particular, give any non-trivial lower bounds for $W(3,k)$ and prove that $W(3,k) < \exp(k^c)$ for some constant $c<1$.

While we do not have a full understanding of the growth of $W(3,k)$, both of the specific challenges of Erdős have been met.

Green [Gr22] established the superpolynomial lower bound $W(3,k) \geq \exp\left( c\frac{(\log k)^{4/3}}{(\log\log k) ^{1/3}}\right)$ for some constant $c>0$ (in particular disproving a conjecture of Graham that $W(3,k)\ll k^2$). Hunter [Hu22] improved this to $W(3,k) \geq \exp\left( c\frac{(\log k)^{2}}{\log\log k}\right).$ The first to show that $W(3,k) < \exp(k^c)$ for some $c<1$ was Schoen [Sc21]. The best upper bound currently known is $W(3,k) \ll \exp\left( O((\log k)^9)\right),$ which follows from the best bounds known for sets without three-term arithmetic progressions (see [BlSi23] which improves slightly on the bounds due to Kelley and Meka [KeMe23]).

OPEN
As $n\to \infty$ ranges over integers $\sum_{p\leq n}1_{n\in (p/2,p)\pmod{p}}\frac{1}{p}\sim \frac{\log\log n}{2}.$
A conjecture of Erdős, Graham, Ruzsa, and Straus [EGRS75]. For comparison the classical estimate of Mertens states that $\sum_{p\leq n}\frac{1}{p}\sim \log\log n.$ By $n\in (p/2,p)\pmod{p}$ we mean $n\equiv r\pmod{p}$ for some integer $r$ with $p/2<r<p$.
OPEN
Let $k\geq 2$. Does $(n+k)!^2 \mid (2n)!$ for infinitely many $n$?
A conjecture of Erdős, Graham, Ruzsa, and Straus [EGRS75]. It is open even for $k=2$.

Balakran [Ba29] proved this holds for $k=1$ - that is, $(n+1)^2\mid \binom{2n}{n}$ for infinitely many $n$. It is a classical fact that $(n+1)\mid \binom{2n}{n}$ for all $n$ (see Catalan numbers).

Erdős, Graham, Ruzsa, and Straus observe that the method of Balakran can be further used to prove that there are infinitely many $n$ such that $(n+k)!(n+1)! \mid (2n)!$ (in fact this holds whenever $k<c \log n$ for some small constant $c>0$).

Erdős [Er68c] proved that if $a!b!\mid n!$ then $a+b\leq n+O(\log n)$.

OPEN
Let $\epsilon,C>0$. Are there integers $a,b,n$ such that $a>\epsilon n$, $b>\epsilon n$, $a! b! \mid n!(a+b-n)!$ and $a+b>n+C\log n$?
A question of Erdős, Graham, Ruzsa, and Straus [EGRS75]. Erdős [Er68c] proved that if $a!b!\mid n!$ then $a+b\leq n+O(\log n)$.

By Legendre's formula $a! b! \mid n!(a+b-n)!$ is true if and only if for all primes $p$ $s_p(n)+s_p(a+b-n) \leq s_p(a)+s_p(b),$ where $s_p(n)$ is the sum of the base $p$ digits of $n$.

OPEN
Let $C>0$ be a constant. Are there infinitely many integers $a,b,n$ with $a+b> n+C\log n$ such that the denominator of $\frac{n!}{a!b!}$ contains only primes $\ll_C 1$?
Erdős [Er68c] proved that if $a!b!\mid n!$ then $a+b\leq n+O(\log n)$. The proof is easy, and can be done with powers of $2$ alone: Legendre's formula implies that if $2^k$ is the highest power of $2$ dividing $n!$ then $k=n+O(\log n)$, and hence if $a!b!\mid n!$ then $a+b\leq n+O(\log n)$.

This problem is asking if $a!b!\mid n!$ 'ignoring what happens on small primes' still implies $a+b+\leq n+O(\log n)$.

OPEN
Are there infinitely many pairs of integers $n\neq m$ such that $\binom{2n}{n}$ and $\binom{2m}{m}$ have the same set of prime divisors?
A problem of Erdős, Graham, Ruzsa, and Straus [EGRS75], who believed there is 'no doubt' that the answer is yes.

For example $(87,88)$ and $(607,608)$.

Kummer's theorem implies that, for all odd primes $p$, $p\mid \binom{2n}{n}$ if and only some base $p$ digit of $n$ is $>p/2$, and hence $(n,n+1)$ has the required property if for all primes $p\leq n$ we have $n\not\in \{\frac{p-1}{2},p-1\}\pmod{p}$. Standard heuristics then predict there should be $\gg \frac{x}{(\log x)^2}$ many such $n\leq x$.

OPEN
Find some reasonable function $f(n)$ such that, for almost all integers $n$, the least integer $m$ such that $m\nmid \binom{2n}{n}$ satisfies $m\sim f(n).$
A problem of Erdős, Graham, Ruzsa, and Straus [EGRS75], who say it is 'not hard to show that', for almost all $n$, the minimal such $m$ satisfies $m=\exp((\log n)^{1/2+o(1)}).$
SOLVED
Let $f(n)$ count the number of sum-free $A\subseteq \{1,\ldots,n\}$, i.e. $A$ contains no solutions to $a=b+c$ with $a,b,c\in A$. Is it true that $f(n)=2^{(1+o(1))\frac{n}{2}}?$
The Cameron-Erdős conjecture. It is trivial to see that $f(n) \geq 2^{\frac{n}{2}}$, considering all subsets of $[n/2,n]$.

This is true, and in fact $f(n) \ll 2^{n/2}$, which was proved independently by Green [Gr04] and Sapozhenko [Sa03]. In fact, both papers prove the stronger asymptotic $f(n) \sim c_n 2^{n/2}$, where $c_n$ takes on one of two values depending on the parity of $n$.

SOLVED
Let $A\subseteq \mathbb{N}$. Can there exist some constant $c>0$ such that $\sum_{n\leq N} 1_A\ast 1_A(n) = cN+O(1)?$
A conjecture of Erdős and Turán. Erdős and Fuchs [ErFu56] proved that the answer is no in a strong form: in fact even $\sum_{n\leq N} 1_A\ast 1_A(n) = cN+o\left(\frac{N^{1/4}}{(\log N)^{1/2}}\right)$ is impossible. The error term here was improved to $N^{1/4}$ by Jurkat (unpublished) and Montgomery and Vaughan [MoVa90].
SOLVED
Let $A\subseteq \mathbb{N}$. Can there exist some constant $c>0$ such that $\sum_{n\leq N} 1_A\ast 1_A\ast 1_A(n) = cN+O(1)?$
The case of $1_A\ast 1_A(n)$ is the subject of [763].

The answer is no, proved in a strong form by Vaughan [Va72], who showed that in fact $\sum_{n\leq N} 1_A\ast 1_A\ast 1_A(n) = cN+o\left(\frac{N^{1/4}}{(\log N)^{1/2}}\right)$ is impossible. Vaughan proves a more general result that applies to any $h$-fold convolution, with different main terms permitted.

OPEN
Let $A\subset\mathbb{N}$ be the set of $n$ such that for every prime $p\mid n$ there exists some $d\mid n$ such that $d\equiv 1\pmod{p}$. Is it true that there exists some constant $c>0$ such that for all large $N$ $\frac{\lvert A\cap [1,N]\rvert}{N}=\exp(-(c+o(1))\sqrt{\log N}\log\log N).$
Erdős could prove that there exists some constant $c>0$ such that for all large $N$ $\exp(-c\sqrt{\log N}\log\log N)\leq \frac{\lvert A\cap [1,N]\rvert}{N}$ and $\frac{\lvert A\cap [1,N]\rvert}{N}\leq \exp(-(1+o(1))\sqrt{\log N\log\log N}).$ Erdős asked about this because $\lvert A\cap [1,N]\rvert$ provides an upper bound for the number of integers $n\leq N$ for which there is a non-cyclic simple group of order $n$.
OPEN
Let $c(n)$ be minimal such that if $k\geq c(n)$ then the $n$-dimensional unit cube can be decomposed into $k$ homothetic $n$-dimensional cubes. Give good bounds for $c(n)$ - in particular, is it true that $c(n) \gg n^n$?
A problem first investigated by Hadwiger, who proved the lower bound $c(n) \geq 2^n+2^{n-1}.$ It is easy to see that $c(2)=6$. Meier conjectured $c(3)=48$. Burgess and Erdős [Er74b] proved $c(n) \ll n^{n+1}.$ Erdős wrote 'I am certain that if $n+1$ is a prime then $c(n)>n^n$.'.
OPEN
Let $h(n)$ be minimal such that $2^n-1,3^n-1,\ldots,h(n)^n-1$ are mutually coprime.

Does, for every prime $p$, the density $\delta_p$ of integers with $h(n)=p$ exist? Does $\liminf h(n)=\infty$? Is it true that if $p$ is the greatest prime such that $p-1\mid n$ and $p>n^\epsilon$ then $h(n)=p$?

It is easy to see that $h(n)=n+1$ if and only if $n+1$ is prime, and that $h(n)$ is unbounded for odd $n$.

It is probably true that $h(n)=3$ for infinitely many $n$.

SOLVED
Let $f(n)$ be maximal such that, for every $m\geq 1$, there exists some $S\subseteq \{1,\ldots,n\}$ with $\lvert S\rvert=f(n)$ such that $m\neq \sum_{a\in A}a$ for all $A\subseteq S$.

Is it true that $f(n) = \left(\frac{1}{2}+o(1)\right)\frac{n}{\log n}?$

A conjecture of Erdős and Graham, who proved the lower bound $f(n)\geq \left(\frac{1}{2}+o(1)\right)\frac{n}{\log n}.$ Their proof is to note that we can assume that $m< \binom{n+1}{2}$ and then, for any $m$, take $S=\{ kp : 1\leq k<\frac{n}{p}\}$ where $p$ is the least prime that does not divide $m$ (so $p<(2+o(1))\log n$).

The complementary bound $f(n) \leq \left(\frac{1}{2}+o(1)\right)\frac{n}{\log n}$ was proved by Alon and Freiman [AlFr88], who chose $m$ as the least common multiple of $\{1,\ldots,s\}$ where $s$ is maximal such that $m\leq \frac{n^2}{20(\log n)^2}$.

SOLVED
Let $k\geq 1$ and $H_k(n)$ be the maximal $r$ such that if $A\subset\mathbb{N}$ has $\lvert A\rvert=n$ and $\| 1_A\ast 1_A\|_\infty \leq k$ then $A$ contains a Sidon set of size at least $r$.

Is it true that $H_k(n)/n^{1/2}\to \infty$? Or even $H_k(n) > n^{1/2+c}$ for some constant $c>0$?

Erdős [Er84d] proved that $H_k(n) \ll n^{2/3}$ (where the implied constant is absolute). The lower bound $H_k(n)\gg n^{1/2}$ follows from the fact that any set of size $n$ contains a Sidon set of size $\gg n^{1/2}$ (see [530]).

The answer is yes, and in fact $H_k(n) \gg_k n^{2/3},$ proved by Alon and Erdős [AlEr85]. We sketch their proof as follows: take a random subset $A'\subset A$, including each $n\in A'$ with probability $\asymp n^{-1/3}$. The number of non-trivial additive quadruples in $A$ is $\ll n^2$ and hence only $\ll n^{2/3}$ non-trivial additive quadruples remain in $A'$. Since the size of the random subset is $\gg n^{2/3}$, all of the remaining non-trivial additive quadruples can be removed by removing at most $\lvert A'\rvert/2$ (choosing the constants suitably).

OPEN
What is the size of the largest Sidon subset $A\subseteq\{1,2^2,\ldots,N^2\}$? Is it $N^{1-o(1)}$?
A question of Alon and Erdős [AlEr85], who proved $\lvert A\rvert \geq N^{2/3-o(1)}$ is possible (via a random subset), and observed that $\lvert A\rvert \ll \frac{N}{(\log N)^{1/4}},$ since (as shown by Landau) the density of the sums of two squares decays like $(\log N)^{-1/2}$.
OPEN
We call $A\subset \mathbb{N}$ dissociated if $\sum_{n\in X}n\neq \sum_{m\in Y}m$ for all finite $X,Y\subset A$ with $X\neq Y$.

Let $A\subset \mathbb{N}$ be an infinite set. We call $A$ proportionately dissocaited if every finite $B\subset A$ contains a dissociated set of size $\gg \lvert B\rvert$.

Is every proportionately dissociated set the union of a finite number of dissociated sets?

This question appears in a paper of Alon and Erdős [AlEr85], although the general topic was first considered by Pisier [Pi83], who observed that the converse holds, and proved that being proportionately dissociated is equivalent to being a 'Sidon set' in the harmonic analysis sense; that is, whenever $f:A\to \mathbb{C}$ there exists some $\theta\in [0,1]$ such that $\| f\|_1 \ll \left\lvert\sum_{n\in A} f(n)e(n\theta)\right\rvert,$ where $e(x)=e^{2\pi ix}$.

Alon and Erdős write that it 'seems unlikely that [this] is also sufficient'. They also point out the same question can be asked replacing dissociated with Sidon (in the additive combinatorial sense).

OPEN
Let $n\geq 1$ and $p_1<\cdots<p_n$ denote the first $n$ primes. Let $P=\prod_{1\leq i\leq n}p_i$. Does there always exist some prime $p$ with $p_n<p<P$ such that $P+p$ is prime?
A problem of Deaconescu. Erdős expects that the least such prime is much smaller than $P$, and in fact satisfies $p\leq n^{O(1)}$. Deaconescu has verified this conjecture for $n\leq 1000$.
OPEN
Do the squares contain arbitrarily long quasi-progressions? That is, does there exist some constant $C>0$ such that, for any $k$, the squares contain a sequence $x_1,\ldots,x_k$ where, for some $d$ and all $1\leq i<k$, $x_i+d\leq x_{i+1}\leq x_i+d+C.$ Do the squares contain arbitrarily large cubes $a+\left\{ \sum_i \epsilon_ib_i : \epsilon_i\in \{0,1\}\right\}?$
A question of Brown, Erdős, and Freedman [BEF90]. It is a classical fact that the squares do not contain arithmetic progressions of length $4$.

An affirmative answer to the first question implies an affirmative answer to the second.

Solymosi [So07] conjectured the answer to the second question is no. Cilleruelo and Granville [CiGr07] have observed that the answer to the second question is no conditional on the Bombieri-Lang conjecture.

OPEN
Fix some constant $C>0$ and let $n$ be large. Let $A\subseteq \{2,\ldots,n\}$ be such that $(a,b)=1$ for all $a\neq b\in A$ and $\sum_{n\in A}\frac{1}{n}\leq C$.

What choice of such an $A$ minimises the number of integers $m\leq n$ not divisible by any $a\in A$? Is this minimised by letting $n\geq q_1>q_2>\cdots$ be the consecutive primes in decreasing order and choosing $A=\{q_1,\ldots,q_k\}$ where $k$ is maximal such that $\sum_{i=1}^k\frac{1}{q_i}\leq C?$

OPEN
Let $C>0$ be some constant and $n$ be large. If $A\subseteq\{1,\ldots,n\}$ has $\sum_{n\in A}\frac{1}{n}\leq C$ then is there some $c$ (which may depend on $C$) such that $\{ m\leq n : a\nmid m\textrm{ for all }a\in A\}$ has size $\geq n/(\log n)^{c}$?
An example of Schinzel and Szekeres [ScSz59] shows that this would be best possible (up to the value of $c$).

OPEN
Let $\epsilon>0$. Is there some set $A\subset \mathbb{N}$ of density $>1-\epsilon$ such that $a_1\cdots a_r=b_1\cdots b_s$ with $a_i,b_j\in A$ can only hold when $r=s$?

Similarly, can one always find a set $A\subset\{1,\ldots,N\}$ with this property of size $\geq (1-o(1))N$?

An example of such a set with density $1/4$ is given by the integers $\equiv 2\pmod{4}$.

Selfridge constructed such a set with density $1/e-\epsilon$ for any $\epsilon>0$: let $p_1<\cdots<p_k$ be a sequence of large consecutive primes such that $\sum_{i=1}^k\frac{1}{p_i}<1<\sum_{i=1}^{k+1}\frac{1}{p_i},$ and let $A$ be those integers divisible by exactly one of $p_1,\ldots,p_k$.

For the second question the set of integers with a prime factor $>N^{1/2}$ give an example of a set with size $\geq (\log 2)N$. Erdős could improve this constant slightly.

OPEN
Let $F(n)$ be the maximum possible size of a subset $A\subseteq\{1,\ldots,N\}$ such that $a\nmid bc$ whenever $a,b,c\in A$ with $a\neq b$ and $a\neq c$. Is there a constant $c$ such that $F(n)=\pi(n)+(c+o(1))n^{2/3}(\log n)^{-2}?$

Erdős [Er38] proved there exist constants $0<c_1\leq c_2$ such that $\pi(n)+c_1n^{2/3}(\log n)^{-2}\leq F(n) \leq \pi(n)+c_2n^{2/3}(\log n)^{-2}.$

Erdős [Er69] gave a simple proof that $F(n) \leq \pi(n)+n^{2/3}$: we define a graph with vertex set the union of those integers in $[1,n^{2/3}]$ with all primes $p\in (n^{2/3},n]$. We have an edge $u\sim v$ if and only if $uv\in A$. It is easy to see that every $m\leq n$ can be written as $uv$ where $u\leq n^{2/3}$ and $v$ is either prime or $\leq n^{2/3}$, and hence there are $\geq \lvert A\rvert$ many edges. This graph contains no path of length $3$ and hence must be a tree and have fewer edges than vertices, and we are done. This can be improved to give the upper bound mentioned by using a subset of integers in $[1,n^{2/3}]$.

More generally, one can ask for such an asymptotic for the size of sets such that no $a\in A$ divides the product of $r$ distinct other elements of $A$, with the exponent $2/3$ replaced by $\frac{2}{r+1}$.

OPEN
Let $g(n)$ be the maximal size of $A\subseteq \{1,\ldots,n\}$ such that $\prod_{n\in S}n$ are distinct for all $S\subseteq A$. Is it true that $g(n) \leq \pi(n)+\pi(n^{1/2})+o\left(\frac{x^{1/2}}{\log n}\right)?$
Erdős proved [Er66] $g(n) \leq \pi(n)+O\left(\frac{x^{1/2}}{\log n}\right).$ This upper bound would be essentially best possible, since one could take $A$ to be all primes and squares of primes.
OPEN
Let $k\geq 2$ and let $g_k(n)$ be the largest possible size of $A\subseteq \{1,\ldots,n\}$ such that every $m$ has $<k$ solutions to $m=a_1a_2$ with $a_1<a_2\in A$.

Estimate $g_k(n)$. In particular, is it true that $g_k(n)=\frac{\log\log n}{\log n}n+(c+o(1))\frac{n}{(\log n)^2}$ for some constant $c$?

Erdős [Er64d] proved that if $2^{r-1}<k\leq 2^r$ then $g_k(n) \sim \frac{(\log\log n)^{r-1}}{(r-1)!\log n}n$ (which is the asymptotic count of those integers $\leq n$ with $r$ distinct prime factors).

In particular the asymptotics of $g_k(n)$ are known; in this problem Erdős was asking about the second order terms. For $k=3$ he could prove the existence of some $0<c_1\leq c_2$ such that $\frac{\log\log n}{\log n}n+c_1\frac{n}{(\log n)^2}\leq g_k(n)\leq \frac{\log\log n}{\log n}n+c_2\frac{n}{(\log n)^2}.$

The special case $k=2$ is the subject of [425].

OPEN
Let $H(n)$ be the smallest integer $l$ such that there exist $k<l$ with $(k^n-1,l^n-1)=1$.

Is it true that $H(n)=3$ infinitely often? (That is, $(2^n-1,3^n-1)=1$ infinitely often?)

Estimate $H(n)$. Is it true that there exists some constant $c>0$ such that, for all $\epsilon>0$, $H(n) > \exp(n^{(c-\epsilon)/\log\log n})$ for infinitely many $n$ and $H(n) < \exp(n^{(c+\epsilon)/\log\log n})$ for all large enough $n$?

Does a similar upper bound hold for the smallest $k$ such that $(k^n-1,2^n-1)=1$?

Erdős [Er74b] proved that there exists a constant $c>0$ such that $H(n) > \exp(n^{c/(\log\log n)^2})$ for infinitely many $n$.

The sequence $H(n)$ for $1\leq n\leq 10$ is $3,3,3,6,3,18,3,6,3,12.$ The sequence of $n$ for which $(2^n-1,3^n-1)=1$ is A263647 in the OEIS.

OPEN
Let $g(n)$ count the number of $m$ such that $\phi(m)=n$. Is it true that, for every $\epsilon>0$, there exist infinitely many $n$ such that $g(n) > n^{1-\epsilon}?$
Pillai proved that $\limsup g(n)=\infty$ and Erdős [Er35b] proved that there exists some constant $c>0$ such that $g(n) >n^c$ for infinitely many $n$.

This conjecture would follow if we knew that, for every $\epsilon>0$, there are $\gg_\epsilon \frac{x}{\log x}$ many primes $p<x$ such that all prime factors of $p-1$ are $<p^\epsilon$.

OPEN
Does the set of integers of the form $n+\phi(n)$ have positive density?
A similar question can be asked for $n+\sigma(n)$, where $\sigma$ is the sum of divisors function.
SOLVED
Let $\alpha\geq 1$. Is there a sequence of integers $n_k,m_k$ such that $n_k/m_k\to \alpha$ and $\sigma(n_k)=\sigma(m_k)$ for all $k\geq 1$, where $\sigma$ is the sum of divisors function?
Erdős [Er74b] writes it is 'easy to prove the analogous result for $\phi(n)$'.

The answer is yes, proved by Pollack [Po15b].

OPEN
Let $h(x)$ count the number of integers $1\leq a<b<x$ such that $(a,b)=1$ and $\sigma(a)=\sigma(b)$, where $\sigma$ is the sum of divisors function.

Is it true that $h(x)>x^{2-o(1)}$?

Erdős [Er74b] proved that $\limsup h(x)/x= \infty$, and claimed a similar proof for this problem. A complete proof that $h(x)/x\to \infty$ was provided by Pollack and Pomerance [PoPo16].

A similar question can be asked if we replace the condition $(a,b)=1$ with the condition that $a$ and $b$ are squarefree.

OPEN
Is there an absolute constant $C>0$ such that every integer $n$ with $\sigma(n)>Cn$ is the distinct sum of proper divisors of $n$?
A problem of Benkoski and Erdős. This could be true with $C=3$. We must have $C>2$ since $\sigma(70)=144$ but $70$ is not the distinct sum of integers from $\{1,2,5,7,10,14,35\}$.

Erdős suggested that as $C\to \infty$ only divisors at most $\epsilon n$ need to be used, where $\epsilon \to 0$.

OPEN
Are there infinitely many $n$ such that, for all $k\geq 1$, $\tau(n+k)\ll k?$
A stronger form of [248].
OPEN
Is it true that, for any $a\in\mathbb{Z}$, there are infinitely many $n$ such that $\phi(n) \mid n+a?$
A conjecture of Graham. Lehmer has conjectured that $\phi(n)\mid n-1$ if and only if $n$ is prime. It is an easy exercise to show that $\phi(n) \mid n$ if and only if $n=2^a3^b$.
OPEN
Let $A\subset\mathbb{N}$ be the set of cubes. Is it true that $1_A\ast 1_A(n) \ll (\log n)^{O(1)}?$
Mordell proved that $\limsup_{n\to \infty} 1_A\ast 1_A(n)=\infty$ and Mahler [Ma35b] proved $1_A\ast 1_A(n) \gg (\log n)^{1/4}$ for infinitely many $n$.
OPEN
We say that $a,b\in \mathbb{N}$ are an amicable pair if $\sigma(a)=\sigma(b)=a+b$. Are there infinitely many amicable pairs? If $A(x)$ counts the number of amicable $1\leq a\leq b\leq x$ then is it true that $A(x)>x^{1-o(1)}?$
For example $220$ and $284$. Erdős [Er55b] proved that $A(x)=o(x)$, and Pomerance [Po81] improved this to $A(x) \leq x \exp(-(\log x)^{1/3})$ and later [Po15] to $A(x) \leq x \exp(-(\tfrac{1}{2}+o(1))(\log x\log\log x)^{1/2}).$
OPEN
Let $1\leq a_1<a_2<\cdots$ be a sequence of integers such that no $a_i$ is the sum of consecutive $a_j$ for $j<i$. Is it true that $\limsup \frac{a_n}{n}=\infty?$ Or even $\lim \frac{1}{\log x}\sum_{a_n<x}\frac{1}{a_n}=0?$
It is easy to see that $\liminf a_n/n<\infty$ is possible.

SOLVED
Let $t_n$ be minimal such that $\{n+1,\ldots,n+t_n\}$ contains a subset whose product is a square number (and let $t_n=0$ if $n$ is itself square). Estimate $t_n$.
A problem of Erdős, Graham, and Selfridge. For example, $t_n=6$ since $6\cdot 8\cdot 12=24^2$. Erdős originally asked whether the set with $t_n\geq n^{1-o(1)}$ has density zero. Selfridge then proved that $t_n=P(n)$, where $P(n)$ is the largest prime divisor of $n$, if $P(n)>\sqrt{2n}+1$, and $t_n \ll n^{1/2}$ otherwise.

Bui, Pratt, and Zaharescu [BPZ24] proved that the distribution of $t_n$ continues to follow $P(n)$, in that for any fixed $c\in (0,1]$ $\lim_{x\to \infty}\frac{\lvert \{ n\leq x : t_n\leq n^c\}\rvert}{x} = \lim_{x\to \infty}\frac{\lvert \{ n\leq x : P(n)\leq n^c\}\rvert}{x}.$ They also prove that for at least $x^{1-o(1)}$ many $n\leq x$ we have $t_n \leq \exp(O(\sqrt{\log n\log\log n}))$ and for all non-square $n$ $t_n \gg (\log\log n)^{6/5}(\log\log\log n)^{-1/5}.$

OPEN
Are the squares Ramsey $2$-complete?

That is, is it true that, in any 2-colouring of the square numbers, every sufficiently large $n\in \mathbb{N}$ can be written as a monochromatic sum of distinct squares?

A problem of Burr and Erdős. A similar question can be asked for the set of $k$th powers for any $k\geq 3$.

OPEN
Let $A\subseteq \{1,\ldots,N\}$ be such that, for all $a,b\in A$, the product $ab$ is not squarefree.

Is the maximum size of such an $A$ achieved by taking $A$ to be the set of even numbers and odd non-squarefree numbers?

A problem of Erdős and Sárközy.

OPEN
Let $C>0$. Is it true that the set of integers of the form $n=b_1+\cdots+b_t\textrm{ with }b_1<\cdots<b_t$ where $b_i=2^{k_i}3^{l_i}$ for $1\leq i\leq t$ and $b_t\leq Cb_1$ has density $0$?
In [Er92b] Erdős wrote 'last year I made the following silly conjecture': every integer $n$ can be written as the sum of distinct integers of the form $2^k3^l$, none of which divide any other. 'I mistakenly thought that this was a nice and difficult conjecture but Jansen and several others found a simple proof by induction.'

Indeed, one proves (by induction) the stronger fact that such a representation always exists, and moreover if $n$ is even then all the summands can be taken to be even: if $n=2m$ we are done applying the inductive hypothesis to $m$. Otherwise if $n$ is odd then let $3^k$ be the largest power of $3$ which is $\leq n$ and apply the inductive hypothesis to $n-3^k$ (which is even).

OPEN
Let $N$ be a large integer. Is the maximum size of a set $A\subseteq \{1,\ldots,N\}$ such that $ab+1$ is never squarefree (for all $a,b\in A$) achieved by taking those $n\equiv 7\pmod{25}$?
A problem of Erdős and Sárközy.

OPEN
Is it true that, for every integer $t\geq 1$, there is some integer $a$ such that $\binom{n}{k}=a$ (with $1\leq k\leq n/2$) has exactly $t$ solutions?
Erdős [Er96b] credits this to himself and Gordon 'many years ago', but it is more commonly known as Singmaster's conjecture. For $t=3$ one could take $a=120$, and for $t=4$ one could take $a=3003$. There are no known examples for $t\geq 5$.

Both Erdős and Singmaster believed the answer to this question is no, and in fact that there exists an absolute upper bound on the number of solutions.

Matomäki, Radziwill, Shao, Tao, and Teräväinen [MRSTT22] have proved that there are always at most two solutions if we restrict $k$ to $k\geq \exp((\log n)^{2/3+\epsilon}),$ assuming $a$ is sufficiently large depending on $\epsilon>0$.

OPEN
Can there exist two distinct integers $x$ and $y$ such that $x,y$ have the same prime factors, $x+1,y+1$ have the same prime factors, and $x+2,y+2$ also have the same prime factors?
For just $x,y$ and $x+1,y+1$ one can take $x=2(2^r-1)$ and $y = x(x+2).$ Erdős also asked whether there are any other examples. Matthew Bolan has observed that $x=75$ and $y=1215$ is another example, since $75 = 3\cdot 5^2 \textrm{ and }1215 = 3^5\cdot 5$ while $76 = 2^2\cdot 19\textrm{ and }1216 = 2^6\cdot 19.$ No other exampes are known. This sequence is listed as A343101 at the OEIS.

OPEN
Let $\epsilon>0$. Is there some $r\ll_\epsilon 1$ such that the density of integers of the form $2^k+n$, where $k\geq 0$ and $n$ has at most $r$ prime divisors, is at least $1-\epsilon$?
Romanoff [Ro34] proved that the set of integers of the form $2^k+p$ (where $p$ is prime) has positive lower density.

OPEN
Let $d_n=p_{n+1}-p_n$, where $p_n$ is the $n$th prime. Let $h(x)$ be maximal such that for some $n<x$ the numbers $d_n,d_{n+1},\ldots,d_{n+h(x)-1}$ are all distinct. Estimate $h(x)$. In particular, is it true that $h(x) >(\log x)^c$ for some constant $c>0$, and $h(x)=o(\log x)?$
Brun's sieve implies $h(x) \to \infty$ as $x\to \infty$.
OPEN
Let $d_n=p_{n+1}-p_n$, where $p_n$ is the $n$th prime. Let $r(x)$ be the smallest even integer $t$ such that $d_n=t$ has no solutions for $n\leq x$.

Is it true that $r(x)\to \infty$? Or even $r(x)/\log x \to \infty$?

In [Er85c] Erdős omits the condition that $t$ be even, but this is clearly necessary.
OPEN
Let $n_k$ denote the $k$th primorial, i.e. the product of the first $k$ primes.

If $1=a_1<a_2<\cdots a_{\phi(n_k)}=n_k-1$ is the sequence of integers coprime to $n_k$, then estimate the smallest integer not of the form $a_{i+1}-a_i$.

OPEN
If $\pi(x)$ counts the number of primes in $[1,x]$ then is it true that $\pi(x+y) \leq \pi(x)+\pi(y)?$
Commonly known as the second Hardy-Littlewood conjecture. In [Er85c] Erdős describes it as 'an old conjecture of mine which was probably already stated by Hardy and Littlewood'.

This is probably false, since Hensley and Richards [HeRi73] have shown that this is false assuming the Hardy-Littlewood prime tuples conjecture.

Erdős [Er85c] reports Straus as remarking that the 'correct way' of stating this conjecture would have been $\pi(x+y) \leq \pi(x)+2\pi(y/2).$ Clark and Jarvis [ClJa01] have shown this is also incompatible with the prime tuples conjecture.

In [Er85c] Erdős conjectures the weaker result (which in particular follows from the conjecture of Straus) that $\pi(x+y) \leq \pi(x)+\pi(y)+O\left(\frac{y}{(\log y)^2}\right),$ which the Hensley and Richards result shows (conditionally) would be best possible. Richards conjectured that this is false.

Erdős and Richards further conjectured that the original inequality is true almost always - that is, the set of $x$ such that $\pi(x+y)\leq \pi(x)+\pi(y)$ for all $y<x$ has density $1$. They could only prove that this set has positive lower density.

They also conjectured that for every $x$ the inequality $\pi(x+y)\leq \pi(x)+\pi(y)$ is true provided $y \gg (\log x)^C$ for some large constant $C>0$.

Hardy and Littlewood proved $\pi(x+y) \leq \pi(x)+O(\pi(y)).$ The best known in this direction is a result of Montgomery and Vaughan [MoVa73], which shows $\pi(x+y) \leq \pi(x)+2\frac{y}{\log y}.$

OPEN
Let $k\geq 3$ and $f_k(N)$ be the maximum value of $\sum_{n\in A}\frac{1}{n}$, where $A$ ranges over all subsets of $\{1,\ldots,N\}$ which contain no subset of size $k$ with the same pairwise least common multiple.

Estimate $f_k(N)$.

Erdős [Er70] notes that $f_k(N) \ll \frac{\log N}{\log\log N}.$ Indeed, let $A$ be such a set. This in particular implies that, for every $t$, there are $<k$ solutions to $t=ap$ with $a\in A$ and $p$ prime, whence $\sum_{n\in A}\frac{1}{n}\sum_{p<N}\frac{1}{p}< k \sum_{t<N^2}\frac{1}{t} \ll \log N,$ and the bound follows since $\sum_{p<N}\frac{1}{p}\gg \log\log N$.

The analogous question with natural density in place of logarithmic density (that is, we measure $\lvert A\rvert$ in place of $\sum_{n\in A}\frac{1}{n}$) is the subject of [536]. In particular Erdős [Er70] has constructed $A\subseteq \{1,\ldots,N\}$ with $\lvert A\rvert \gg N$ where no four have the same pairwise least common multiple, and hence the interest of the natural density problem is the $k=3$ case.

A related combinatorial problem is asked at [857].

OPEN
Let $A\subseteq \{1,\ldots,N\}$ be such that there is no solution to $at=b$ with $a,b\in A$ and the smallest prime factor of $t$ is $>a$. Estimate the maximum of $\frac{1}{\log N}\sum_{n\in A}\frac{1}{n}.$
Alexander [Al66] and Erdős, Sárközi, and Szemerédi [ESS68] proved that this maximum is $o(1)$ (as $N\to \infty$). This condition on $A$ is a stronger form of the usual primitive condition. If $A$ is merely primitive then Behrend [Be35] proved $\frac{1}{\log N}\sum_{n\in A}\frac{1}{n}\ll \frac{1}{\sqrt{\log\log N}}.$

An example of such a set $A$ is the set of all integers in $[N^{1/2},N]$ divisible by some prime $>N^{1/2}$.

Let $t\geq 1$ and let $d_t$ be the density of the set of integers $n\in\mathbb{N}$ for which $t$ can be represented as the sum of distinct divisors of $n$.
Do there exist constants $c_1,c_2>0$ such that $d_t \sim \frac{c_1}{(\log t)^{c_2}}$ as $t\to \infty$?
Erdős [Er70] proved that $d_t$ always exists, and that there exist some constants $c_3,c_4>0$ such that $\frac{1}{(\log t)^{c_3}} < d_t < \frac{1}{(\log t)^{c_4}}.$