25 solved out of 72 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 $N\geq (\tfrac{1}{4}-o(1))\frac{2^n}{\sqrt{n}}.$ 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of the constant have been given (see [St23] for a history), with the current record$\sqrt{2/\pi}$first proved in unpublished work of Elkies and Gleason.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". See also [350]. Additional thanks to: Zachary Hunter 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 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
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]).

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 $\{1,\ldots,N\}$ (any number of the form $2^{2^m}-1$ with $m\geq 3$ suffices). Pan [Pa11] improved this to $\gg_\epsilon N^{1-\epsilon}$ for any $\epsilon>0$. Erdős believes this cannot be proved by covering systems, i.e. integers of the form $p+2^k+2^l$ exist in every infinite arithmetic progression.

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 $2$).

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 $97$.
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 $h(n!)<n^{o(1)}$?

It is easy to see that almost all numbers are not practical. This may be true with $m=n!$. Erdős originally showed that $h(n!) <n$. Vose [Vo85] has shown that $h(n!)\ll n^{1/2}$.

Related to [304].

SOLVED - $500 If$G$is an edge-disjoint union of$n$copies of$K_n$then is$\chi(G)=n$? Conjectured by Faber, Lovász, and Erdős. Kahn [Ka92] proved that$\chi(G)\leq (1+o(1))n$. Hindman has proved the conjecture for$n<10$. Kang, Kelly, Kühn, Methuku, and Osthus [KKKMO21] have proved the answer is yes for all sufficiently large$n$. In [Er97d] Erdős asks how large$\chi(G)$can be if instead of asking for the copies of$K_n$to be edge disjoint we only ask for their intersections to be triangle free, or to contain at most one edge. OPEN -$1000
Let $f(n,k)$ be minimal such that every $\mathcal{F}$ family of $n$-uniform sets with $\lvert F\rvert \geq f(n,k)$ contains a $k$-sunflower. Is it true that $f(n,k) < c_k^n$ for some constant $c_k>0$?
Erdős and Rado [ErRa60] originally proved $f(n,k)\leq (k-1)^nn!$. Kostochka [Ko97] improved this slightly (in particular establishing an upper bound of $o(n!)$, for which Erdős awarded him the consolation prize of \$100), but the bound stood at$n^{(1+o(1))n}$for a long time until Alweiss, Lovett, Wu, and Zhang [ALWZ20] proved $f(n,k) < (Ck\log n\log\log n)^n$ for some constant$C>1$. This was refined slightly, independently by Rao [Ra20], Frankston, Kahn, Narayanan, and Park [FKNP19], and Bell, Chueluecha, and Warnke [BCW21], leading to the current record of $f(n,k) < (Ck\log n)^n$ for some constant$C>1$. The usual focus is on the regime where$k=O(1)$is fixed (say$k=3$) and$n$is large, although for the opposite regime Kostochka, Rödl, and Talysheva [KoRoTa99] have shown $f(n,k)=(1+O_n(k^{-1/2^n}))k^n.$ Additional thanks to: Zachary Chase 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 has found a counterexample (according to Erdős in [Er95], although I cannot find a paper in which this appears). 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).

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$. See also [40]. 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$, $h(N) = N^{1/2}+O_\epsilon(N^\epsilon)?$ 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$N$. Additional thanks to: Zachary Hunter OPEN -$50
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$1_A\ast 1_A(n)=O(1)$. 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 $\liminf \frac{\lvert A\cap \{1,\ldots,N\}\rvert}{N^{1/2}}=0.$ Additional thanks to: Zachary Chase 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 $(1-\epsilon)M^{1/2}$?
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 $c>0$.

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].
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 $t$.
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$o(\lvert A\rvert^2)$. 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, it is reasonable to conjecture (although I cannot find a record of Erdős himself actually doing so) that for any$m\geq 2$and finite set of integers (or reals, etc)$A$that $\max( \lvert mA\rvert,\lvert A^m\rvert)\gg \lvert A\rvert^{m-o(1)}.$ See [53] for more on this generalisation. 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 $N$.

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. See also [54]. Additional thanks to: Mehtaab Sawhney 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 [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$k$). Additional thanks to: Zachary Chase SOLVED If$G$is a graph with infinite chromatic number and$a_1<a_2<\cdots $are lengths of the odd cycles of$G$then$\sum \frac{1}{a_i}=\infty$. Conjectured by Hajnal and Erdős and solved by Liu and Montgomery [LiMo20]. The lower density of the set can be$0$since there are graphs of arbitrarily large chromatic number and girth. OPEN Does every infinite graph with infinite chromatic number contain a cycle of length$2^n$for infinitely many$n$? Conjectured by Mihók and Erdős. It is likely that$2^n$can be replaced by any sufficiently quickly growing sequence (e.g. the squares). OPEN -$1000
Does every graph with minimum degree at least 3 contain a cycle of length $2^k$ for some $k\geq 2$?
Conjectured by Erdős and Gyárfás, who believed the answer must be negative, and in fact for every $r$ there must be a graph of minimum degree at least $r$ without a cycle of length $2^k$ for any $k\geq 2$.

This was solved in the affirmative if the minimum degree is larger than some absolute constant by Liu and Montgomery [LiMo20] (therefore disproving the above stronger conjecture of Erdős and Gyárfás). Liu and Montgomery prove a much stronger result: if the average degree of $G$ is sufficiently large then there is some large integer $\ell$ such that for every even integer $m\in [(\log \ell)^8,\ell]$, $G$ contains a cycle of length $m$.

OPEN
Let $G$ be a graph with $n$ vertices and $kn$ edges, and $a_1<a_2<\cdots$ be the lengths of cycles in $G$. Is the sum $\sum\frac{1}{a_i}$ minimised when $G$ is a complete bipartite graph?
A problem of Erdős and Hajnal. Gyárfás, Komlós, and Szemerédi [GyKoSz84] have proved that this sum is $\gg \log k$. Liu and Montgomery [LiMo20] have proved the asymptotically sharp lower bound of $\geq (\tfrac{1}{2}-o(1))\log k$.
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. SOLVED Is it true that for every pair$a,b\geq 1$such that either$a$is even or both$a$and$b$are odd then there is$c=c(a,b)$such that every graph with average degree at least$c$contains a cycle whose length is$\equiv a\pmod{b}$? This has been proved by Bollobás [Bo77]. The best dependence of the constant$c(a,b)$is unknown. SOLVED -$100
Is there a set $A\subset \mathbb{N}$ of density $0$ and a constant $c>0$ such that every graph on sufficiently many vertices with average degree $\geq c$ contains a cycle whose length is in $A$?
Bollobás proved that such a $c$ does exist if $A$ is an infinite arithmetic progression containing even numbers. Erdős was 'almost certain' that if $A$ is the set of powers of $2$ then no such $c$ exists (although conjectures that $n$ vertices and average degree $\gg (\log n)^{C}$ suffices for some $C=O(1)$). If $A$ is the set of squares (or the set of $p\pm 1$ for $p$ prime) then he had no guess.

Solved by Verstraëte [Ve05], who gave a non-constructive proof that such a set $A$ exists.

Liu and Montgomery [LiMo20] proved that in fact this is true when $A$ is the set of powers of $2$ (more generally any set of even numbers which doesn't grow too quickly) - in particular this contradicts the previous belief of Erdős.

SOLVED
Let $k\geq 0$. Let $G$ be a graph on $n$ vertices such that every subgraph $H\subseteq G$ contains an independent set of size $\geq \frac{1}{2}\lvert H\rvert-k$. Must $G$ be the union of a bipartite graph and $O_k(1)$ many vertices?
Proved by Reed [Re99]. (Thanks also to Reed for pointing out that the case $k=0$ is trivial, since if $G$ is not bipartite then $G$ contains an odd cycle.)
OPEN - $500 Let$f(n)\to \infty$(possibly very slowly). Is there a graph of infinite chromatic number such that every finite subgraph on$n$vertices can be made bipartite by deleting at most$f(n)$edges? Conjectured by Erdős, Hajnal, and Szemerédi [EHS82]. Rödl [Ro82] has proved this for hypergraphs. It is open even for$f(n)=\sqrt{n}$. Erdős offered \$500 for a proof but only \$250 for a counterexample. This fails (even with$f(n)\gg n$) if the graph has chromatic number$\aleph_1$(see [111]). OPEN Is there a graph of chromatic number$\aleph_1$such that for all$\epsilon>0$if$n$is sufficiently large and$H$is a subgraph on$n$vertices then$H$contains an independent set of size$>n^{1-\epsilon}$? Conjectured by Erdős, Hajnal, and Szemerédi [EHS82]. SOLVED Is it true that in any$2$-colouring of the edges of$K_n$there must exist at least $(1+o(1))\frac{n^2}{12}$ many edge-disjoint monochromatic triangles? Conjectured by Erdős, Faudreee, and Ordman. This would be best possible, as witnessed by dividing the vertices of$K_n$into two equal parts and colouring all edges between the parts red and all edges inside the parts blue. The answer is yes, proved by Gruslys and Letzter [GrLe20]. In [Er97d] Erdős also asks for a lower bound for the count of edge-disjoint monochromatic triangles in single colour (the colour chosen to maximise this quantity), and speculates that the answer is$\geq cn^2$for some constant$c?1/24$. Additional thanks to: Tuan Tran OPEN -$250
Find the value of $\lim_{k\to \infty}R(k)^{1/k}$.
Erdős offered \$100 for just a proof of the existence of this constant, without determining its value. He also offered \$1000 for a proof that the limit does not exist, but says 'this is really a joke as [it] certainly exists'. Erdős proved $\sqrt{2}\leq \liminf_{k\to \infty}R(k)^{1/k}\leq \limsup_{k\to \infty}R(k)^{1/k}\leq 4.$ The upper bound has been improved to $4-\tfrac{1}{128}$ by Campos, Griffiths, Morris, and Sahasrabudhe [CGMS23].
OPEN - $100 Give a constructive proof that$R(k)>C^k$for some constant$C>1$. Erdős gave a simple probabilistic proof that$R(k) \gg k2^{k/2}$. Equivalently, this question asks for an explicit construction of a graph on$n$vertices which does not contain any clique or independent set of size$\geq c\log n$for some constant$c>0$. Cohen [Co15] (see the introduction for further history) constructed a graph on$n$vertices which does not contain any clique or independent set of size $\geq 2^{(\log\log n)^{C}}$ for some constant$C>0$. Li [Li23b] has recently improved this to $\geq (\log n)^{C}$ for some constant$C>0$. Additional thanks to: Jesse Goodman, Mehtaab Sawhney OPEN We say$G$is Ramsey size linear if$R(G,H)\ll m$for all graphs$H$with$m$edges and no isolated vertices. Are there infinitely many graphs$G$which are not Ramsey size linear but such that all of its subgraphs are? Asked by Erdős, Faudree, Rousseau, and Schelp [EFRS93].$K_4$is the only known example of such a graph. OPEN Let$G$be a chordal graph on$n$vertices - that is,$G$has no induced cycles of length greater than$3$. Can the edges of$G$be partitioned into$n^2/6+O(n)$many cliques? Asked by Erdős, Ordman, and Zalcstein [EOZ93], who proved an upper bound of$(1/4-\epsilon)n^2$many cliques (for some very small$\epsilon>0$). The example of all edges between a complete graph on$n/3$vertices and an empty graph on$2n/3$vertices show that$n^2/6+O(n)$is sometimes necessary. A split graph is one where the vertices can be split into a clique and an independent set. Every split graph is chordal. Chen, Erdős, and Ordman [CEO94] have shown that any split graph can be partitioned into$\frac{3}{16}n^2+O(n)$many cliques. OPEN Let$F(n)$be maximal such that every graph on$n$vertices contains a regular induced subgraph on at least$F(n)$vertices. Prove that$F(n)/\log n\to \infty$. Conjectured by Erdős, Fajtlowicz, and Stanton. It is known that$F(5)=3$and$F(7)=4$. Ramsey's theorem implies that$F(n)\gg \log n$. Bollobás observed that$F(n)\ll n^{1/2+o(1)}$. Alon, Krivelevich, and Sudakov [AKS07] have improved this to$n^{1/2}(\log n)^{O(1)}$. Additional thanks to: Zachary Hunter SOLVED -$500
Suppose that we have a family $\mathcal{F}$ of subsets of $[4n]$ such that $\lvert A\rvert=2n$ for all $A\in\mathcal{F}$ and for every $A,B\in \mathcal{F}$ we have $\lvert A\cap B\rvert \geq 2$. Then $\lvert \mathcal{F}\rvert \leq \frac{1}{2}\left(\binom{4n}{2n}-\binom{2n}{n}^2\right).$
Conjectured by Erdős, Ko, and Rado [ErKoRa61]. This inequality would be best possible, as shown by taking $\mathcal{F}$ to be the collection of all subsets of $[4n]$ of size $2n$ containing at least $n+1$ elements from $[2n]$.

Proved by Ahlswede and Khachatrian [AhKh97], who more generally showed the following. Let $2\leq t\leq k\leq m$ and let $r\geq 0$ be such that $\frac{1}{r+1}\leq \frac{m-2k+2t-2}{(t-1)(k-t+1)}< \frac{1}{r}.$ The largest possible family of subsets of $[m]$ of size $k$, such that the pairwise intersections have size at least $t$, is the family of all subsets of $[m]$ of size $k$ which contain at least $t+r$ elements from $\{1,\ldots,t+2r\}$.

SOLVED
The cycle set of a graph $G$ on $n$ vertices is a set $A\subseteq \{3,\ldots,n\}$ such that there is a cycle in $G$ of length $\ell$ if and only if $\ell \in A$. Let $f(n)$ count the number of possible such $A$. Prove that $f(n)=o(2^n)$.
Conjectured by Erdős and Faudree, who showed that $f(n) > 2^{n/2}$, and further speculate that $f(n)/2^{n/2}\to \infty$. This conjecture was proved by Verstraëte [Ve04], who proved the number of such sets is $\ll 2^{n-n^c}$ for some constant $c>0$.

There are interesting further questions (including $f(n)2^{-n/2}\to \infty$ and the existence of $\lim f(n)^{1/n}$) which remain open.

OPEN
Let $f(n)$ be such that every graph on $n$ vertices with minimal degree $\geq f(n)$ contains a $C_4$. Is it true that $f(n+1)\geq f(n)$?
A weaker version of the conjecture asks for some constant $c$ such that $f(m)>f(n)-c$ for all $m>n$. This question can be asked for other graphs than $C_4$.
OPEN - $100 Let$Q_n$be the$n$-dimensional hypercube graph (so that$Q_n$has$2^n$vertices and$n2^{n-1}$edges). Is it true that every subgraph of$Q_n$with $\geq \left(\frac{1}{2}+o(1)\right)n2^{n-1}$ many edges contains a$C_4$? The best known result is due to Balogh, Hu, Lidicky, and Liu [BHLL14], who proved that$0.6068 n2^{n-1}$edges suffice. A similar question can be asked for other even cycles. Additional thanks to: Casey Tompkins OPEN Let$\epsilon >0$. Is it true that, if$k$is sufficiently large, then $R(G)>(1-\epsilon)^kR(k)$ for every graph$G$with chromatic number$\chi(G)=k$? Even stronger, is there some$c>0$such that, for all large$k$,$R(G)>cR(k)$for every graph$G$with chromatic number$\chi(G)=k$? Erdős originally conjectured that$R(G)\geq R(k)$, which is trivial for$k=3$, but fails already for$k=4$, as Faudree and McKay [FaMc93] showed that$R(W)=17$for the pentagonal wheel$W$. Since$R(k)\leq 4^k$this is trivial for$\epsilon\geq 3/4$. Yuval Wigderson points out that$R(G)\gg 2^{k/2}$for any$G$with chromatic number$k$(via a random colouring), which asymptotically matches the best-known lower bounds for$R(k)$. Additional thanks to: Yuval Wigderson SOLVED -$100
For any $\epsilon>0$ there exists $\delta=\delta(\epsilon)>0$ such that if $G$ is a graph on $n$ vertices with no independent set or clique of size $\geq \epsilon\log n$ then $G$ contains an induced subgraph with $m$ edges for all $m\leq \delta n^2$.
Conjectured by Erdős and McKay, who proved it with $\delta n^2$ replaced by $\delta (\log n)^2$. Solved by Kwan, Sah, Sauermann, and Sawhney [KSSS22]. Erdős' original formulation also had the condition that $G$ has $\gg n^2$ edges, but an old result of Erdős and Szemerédi says that this follows from the other condition anyway.
Additional thanks to: Zachary Hunter and Mehtaab Sawhney
OPEN - $500 Does every set of$n$distinct points in$\mathbb{R}^2$determine$\gg n/\sqrt{\log n}$many distinct distances? A$\sqrt{n}\times\sqrt{n}$integer grid shows that this would be the best possible. Nearly solved by Guth and Katz [GuKa15] who proved that there are always$\gg n/\log n$many distinct distances. A stronger form (see [604]) may be true: is there a single point which determines$\gg n/\sqrt{\log n}$distinct distances, or even$\gg n$many such points, or even that this is true averaged over all points. See also [661]. OPEN -$500
Does every set of $n$ distinct points in $\mathbb{R}^2$ contain at most $n^{1+O(1/\log\log n)}$ many pairs which are distance 1 apart?
The unit distance problem. This would be the best possible, as is shown by a set of lattice points. It is easy to show that there are $O(n^{3/2})$ many such pairs. The best known upper bound is $O(n^{4/3})$, due to Spencer, Szemerédi, and Trotter [SST84]. In [Er83c] and [Er85] Erdős offers \$250 for an upper bound of the form$n^{1+o(1)}$. Part of the difficulty of this problem is explained by a result of Valtr (see [Sz16]), who constructed a metric on$\mathbb{R}^2$and a set of$n$points with$\gg n^{4/3}$unit distance pairs (with respect to this metric). The methods of the upper bound proof of Spencer, Szemerédi, and Trotter [SST84] generalise to include this metric. Therefore to prove an upper bound better than$n^{4/3}$some special feature of the Euclidean metric must be exploited. See a survey by Szemerédi [Sz16] for further background and related results. See also [96] and [605]. OPEN -$500
Let $\epsilon>0$. Given any set of $n$ distinct points in $\mathbb{R}^2$ prove that each point is equidistant from $O_\epsilon(n^\epsilon)$ many other points.
Erdős only offers \$100 for a counterexample. SOLVED If$n$distinct points in$\mathbb{R}^2$form a convex polygon then they determine at least$\lfloor \frac{n+1}{2}\rfloor$distinct distances. Solved by Altman [Al63]. The stronger variant that says there is one point which determines at least$\lfloor \frac{n+1}{2}\rfloor$distinct distances is still open. Fishburn in fact conjectures that if$R(x)$counts the number of distinct distances from$x$then $\sum_{x\in A}R(x) \geq \binom{n}{2}.$ Szemerédi conjectured (see [Er97e]) that this stronger variant remains true if we only assume that no three points are on a line, and proved this with the weaker bound of$n/3$. See also [660]. SOLVED Suppose$n$points in$\mathbb{R}^2$determine a convex polygon and the set of distances between them is$\{u_1,\ldots,u_t\}$. Suppose$u_i$appears as the distance between$f(u_i)$many pairs of points. Then $\sum_i f(u_i)^2 \ll n^3.$ Solved by Fishburn [Al63]. Note it is trivial that$\sum f(u_i)=\binom{n}{2}$. The stronger conjecture that$\sum f(u_i)^2$is maximal for the regular$n$-gon (for large enough$n$) is still open. SOLVED -$500
Let $x_1,\ldots,x_n\in\mathbb{R}^2$ determine the set of distances $\{u_1,\ldots,u_t\}$. Suppose $u_i$ appears as the distance between $f(u_i)$ many pairs of points. Then for all $\epsilon>0$ $\sum_i f(u_i)^2 \ll_\epsilon n^{3+\epsilon}.$
The case when the points determine a convex polygon was been solved by Fishburn [Al63]. Note it is trivial that $\sum f(u_i)=\binom{n}{2}$. Solved by Guth and Katz [GuKa15] who proved the upper bound $\sum_i f(u_i)^2 \ll n^3\log n.$
OPEN - $100 Does every convex polygon have a vertex with no other$4$vertices equidistant from it? Erdős originally conjectured this with no$3$vertices equidistant, but Danzer found a convex polygon on 9 points such that every vertex has three vertices equidistant from it (but this distance depends on the vertex), and Fishburn and Reeds [FiRe92] have found a convex polygon on 20 points such that every vertex has three vertices equidistant from it (and this distance is the same for all vertices). If this fails for$4$, perhaps there is some constant for which it holds? Erdős suggested this as an approach to solve [96]. Indeed, if this problem holds for$k+1$vertices then, by induction, this implies an upper bound of$kn$for [96]. The answer is no if we omit the requirement that the polygon is convex (I thank Boris Alexeev and Dustin Mixon for pointing this out), since for any$d$there are graphs with minimum degree$d$which can be embedded in the plane such that each edge has length one (for example one can take the$d$-dimensional hypercube graph on$2^d$vertices). One can then connect the vertices in a cyclic order so that there are no self-intersections and no three consecutive vertices on a line, thus forming a (non-convex) polygon. Additional thanks to: Boris Alexeev and Dustin Mixon OPEN -$100
Let $A\subseteq\mathbb{R}^2$ be a set of $n$ points with minimum distance equal to 1, chosen to minimise the diameter of $A$. If $n$ is sufficiently large then must there be three points in $A$ which form an equilateral triangle of size 1?
The minimal such diameter is achieved (asymptotically) by the points in a triangular lattice intersected with a circle. In general Erdős believed such a set must have very large intersection with the triangular lattice (perhaps as many as $(1-o(1))n$).

The stated problem is false for $n=4$, for example taking the points to be vertices of a square. The behaviour of such sets for small $n$ is explored by Bezdek and Fodor [BeFo99].

Additional thanks to: Boris Alexeev and Dustin Mixon
OPEN
Let $A\subseteq\mathbb{R}^2$ be a set of $n$ points with distance set $D=\{\lvert x-y\rvert : x\neq y \in A\}$. Suppose that if $d_1\neq d_2\in D$ then $\lvert d_1-d_2\rvert \geq 1$. Is there some constant $c>0$ such that the diameter of $A$ must be at least $cn$?
Perhaps the diameter is $\geq n-1$ for all large $n$ (Piepmeyer has an example of $9$ such points with diameter $<5$).
OPEN - $100 Given$n$points in$\mathbb{R}^2$, no five of which are on a line, the number of lines containing four points is$o(n^2)$. There are examples of sets of$n$points with$\sim n^2/6$many collinear triples and no four points on a line. Such constructions are given by Burr, Grünbaum, and Sloane [BGS74] and Füredi and Palásti [FuPa84]. Grünbaum [Gr76] constructed an example with$\gg n^{3/2}$such lines. Erdős speculated this may be the correct order of magnitude. This is false: Solymosi and Stojaković [SoSt13] have constructed a set with no five on a line and at least $n^{2-O(1/\sqrt{\log n})}$ many lines containing exactly four points. See also [102]. A generalisation of this problem is asked in [588]. Additional thanks to: Zach Hunter OPEN Let$c>0$and$h_c(n)$be such that for any$n$points in$\mathbb{R}^2$such that there are$\geq cn^2$lines each containing more than three points, there must be some line containing$h_c(n)$many points. Estimate$h_c(n)$. Is it true that, for fixed$c>0$, we have$h_c(n)\to \infty$? A problem of Erdős and Purdy. It is not even known if$h_c(n)\geq 5$(see [101]). It is easy to see that$h_c(n) \ll_c n^{1/2}$, and Erdős originally suggested that perhaps a similar lower bound$h_c(n)\gg_c n^{1/2}$holds. Zach Hunter has pointed out that this is false, even replacing$>3$points on each line with$>k$points: consider the set of points in$\{1,\ldots,m\}^d$where$n\approx m^d$. These intersect any line in$\ll_d n^{1/d}$points, and have$\gg_d n^2$many pairs of points each of which determine a line with at least$k$points. This is a construction in$\mathbb{R}^d$, but a random projection into$\mathbb{R}^2$preserves the relevant properties. This construction shows that$h_c(n) \ll n^{1/\log(1/c)}$. Additional thanks to: Zach Hunter OPEN Given$n$points in$\mathbb{R}^2$the number of distinct unit circles containing at least three points is$o(n^2)$. In [Er81d] Erdős proved that$\gg n$many circles is possible, and that there cannot be more than$n(n-1)$many circles. Elekes [El84] has a simple construction of a set with$\gg n^{3/2}$such circles. This may be the correct order of magnitude. OPEN -$50
Let $A,B\subset \mathbb{R}^2$ be disjoint sets of size $n$ and $n-3$ respectively, with not all of $A$ contained on a single line. Is there a line which contains at least two points from $A$ and no points from $B$?
Conjectured by Erdős and Purdy [ErPu95] (the prize is for a proof or disproof). A construction of Hickerson shows that this fails with $n-2$. A result independently proved by Beck [Be83] and Szemerédi and Trotter [SzTr83] (see [211]) implies it is true with $n-3$ replaced by $cn$ for some constant $c>0$.
Draw $n$ squares inside the unit square with no common interior point. Let $f(n)$ be the maximum possible total perimeter of the squares. Is $f(k^2+1)=4k$?
It is trivial from the Cauchy-Schwarz inequality that $f(k^2)=4k$. Erdős also asks for which $n$ is it true that $f(n+1)=f(n)$.
OPEN - $500 Let$f(n)$be minimal such that any$f(n)$points in$\mathbb{R}^2$, no three on a line, contain$n$points which form the vertices of a convex$n$-gon. Prove that$f(n)=2^{n-2}+1$. The Erdős-Klein-Szekeres 'Happy Ending' problem. The problem originated in 1931 when Klein observed that$f(4)=5$. Turán and Makai showed$f(5)=9$. Erdős and Szekeres proved the bounds $2^{n-2}+1\leq f(n)\leq \binom{2n-4}{n-2}+1.$ ([ErSz60] and [ErSz35] respectively). There were several improvements of the upper bound, but all of the form$4^{(1+o(1))n}$, until Suk [Su17] proved $f(n) \leq 2^{(1+o(1))n}.$ The current best bound is due to Holmsen, Mojarrad, Pach, and Tardos [HMPT20], who prove $f(n) \leq 2^{n+O(\sqrt{n\log n})}.$ In [Er97e] Erdős clarifies that the \$500 is for a proof, and only offers \$100 for a disproof. See also the entry in the graphs problem collection and [216] and [651]. Additional thanks to: Casey Tompkins OPEN Let$G$be either$Q_3$or$K_{3,3}$or$H_5$(the last formed by adding two vertex-disjoint chords to$C_5$). Is it true that, if$H$has$m$edges and no isolated vertices, then $R(G,H)\ll m?$ In other words, is$G$Ramsey size linear? A special case of [566]. In [Er95] Erdős specifically asks about the case$G=K_{3,3}$. The graph$H_5$can also be described as$K_4^*$, obtained from$K_4$by subdividing one edge. ($K_4$itself is not Ramsey size linear, since$R(4,n)\gg n^{3-o(1)}$, see [166].) Bradać, Gishboliner, and Sudakov [BGS23] have shown that every subdivision of$K_4$on at least$6$vertices is Ramsey size linear, and also that$R(H_5,H) \ll m$whenever$H$is a bipartite graph with$m$edges and no isolated vertices. OPEN -$500
Given $n$ distinct points $A\subset\mathbb{R}^2$ must there be a point $x\in A$ such that $\#\{ d(x,y) : y \in A\} \gg n^{1-o(1)}?$ Or even $\gg n/\sqrt{\log n}$?
The pinned distance problem, a stronger form of [89]. The example of an integer grid show that $n/\sqrt{\log n}$ would be best possible.
It may be true that there are $\gg n$ many such points, or that this is true on average. In [Er97e] Erdős offers \$500 for a solution to this problem, but it is unclear whether he intended this for proving the eixstence of a single such point or for$\gg n$many such points. In [Er97e] Erdős wrote that he initially 'overconjectured' and thought that the answer to this problem is the same as for the number of distinct distances between all pairs (see [89]), but this was disproved by Harborth. It could be true that the answers are the same up to an additive factor of$n^{o(1)}\$.
The best known bound is $\gg n^{c-o(1)},$ due to Katz and Tardos [KaTa04], where $c=\frac{48-14e}{55-16e}=0.864137\cdots.$