Conjectured by Erdős and Hajnal [ErHa66], and solved by Liu and Montgomery [LiMo20]. In [Er81] Erdős asks whether the $a_i$ must in fact have positive upper density, and in [Er95d] he speculates the upper density (or even upper logarithmic density) must be $\geq 1/2$.

The lower density of the set can be $0$ since there are graphs of arbitrarily large chromatic number and girth.

See also [65].

Conjectured by Bollobás and Erdős. Bollobás and Shelah have confirmed this for $k=1$. Proved by Gyárfás [Gy92], who proved the stronger result that, if $G$ is 2-connected, then $G$ is either $K_{2k+2}$ or contains a vertex of degree at most $2k$.

A stronger form was established by Gao, Huo, and Ma [GaHuMa21], who proved that if a graph $G$ has chromatic number $\chi(G)\geq 2k+3$ then $G$ contains cycles of $k+1$ consecutive odd lengths.

Conjectured by Erdős and Simonovits, who could not even prove that at least $2$ copies of $C_4$ are guaranteed.

He, Ma, and Yang [HeMaYa21] have proved this conjecture when $n=q^2+q+1$ for some even integer $q$.

Conjectured by Mihók and Erdős. It is likely that $2^n$ can be replaced by any sufficiently quickly growing sequence (e.g. the squares).

David Penman has observed that this is certainly true if the graph has uncountable chromatic number, since by a result of Erdős and Hajnal [ErHa66] such a graph must contain arbitrarily large finite complete bipartite graphs (see also Theorem 3.17 of Reiher [Re24]).

Zach Hunter has observed that this follows from the work of Liu and Montgomery [LiMo20]: if $G$ has infinite chromatic number then, for infinitely many $r$, it must contain some finite connected subgraph $G_r$ with chromatic number $r$ (via the de Bruijn-Erdős theorem [dBEr51]). Each $G_r$ contains some subgraph $H_r$ with minimum degree at least $r-1$, and hence via Theorem 1.1 of [LiMo20] there exists some $\ell_r\geq r^{1-o(1)}$ such that $H_r$ contains a cycle of every even length in $[(\log \ell)^8,\ell]$.

See also [64].

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 $m$.

An infinite tree with minimum degree $3$ shows that the answer is trivially false for infinite graphs.

See also the entry in the graphs problem collection.

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 $\geq (\tfrac{1}{2}-o(1))\log k$.

See also the entry in the graphs problem collection.

See also [57].

In [Er82e] Erdős credits this conjecture to himself and Burr. This has been proved by Bollobás [Bo77]. The best dependence of the constant $c(P)$ is unknown.

See also [72].

Bollobás [Bo77] proved that such a $c$ does exist if $A$ is an infinite arithmetic progression containing even numbers (see [71]).

Erdős was 'almost certain' that if $A$ is the set of powers of $2$ then no such $c$ exists (although he conjectured 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.

See also the entry in the graphs problem collection.

Conjectured by Erdős, Hajnal, and Szemerédi [EHS82].

Rödl [Ro82] has proved this for hypergraphs, and also proved there is such a graph (with chromatic number $\aleph_0$) if $f(n)=\epsilon n$ for any fixed constant $\epsilon>0$.

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]).

Conjectured by Erdős and Faudree, who showed that $2^{n/2}<f(n) \leq 2^{n-2}$. The first problem was solved by Verstraëte [Ve04], who proved \[f(n)\ll 2^{n-n^{1/10}}.\] This was improved by Nenadov [Ne25] to \[f(n) \ll 2^{n-n^{1/2-o(1)}}.\]

One can also ask about the existence and value of $\lim f(n)^{1/n}$.

Conjectured by Erdős and Hajnal. Rödl [Ro77] has proved the $r=4$ case (see [923]). The infinite version (whether every graph of infinite chromatic number contains a subgraph of infinite chromatic number whose girth is $>k$) is also open.

In [Er79b] Erdős also asks whether \[\lim_{k\to \infty}\frac{f(k,r+1)}{f(k,r)}=\infty.\]

See also the entry in the graphs problem collection and [740] for the infinitary version.

Conjectured by Erdős, Hajnal, and Szemerédi [EHS82]. This fails if the graph has chromatic number $\aleph_0$.

A theorem of de Bruijn and Erdős [dBEr51] implies that, if $G$ has infinite chromatic number, then $G$ has a finite subgraph of chromatic number $n$ for every $n\geq 1$.

In [Er95d] Erdős suggests this is true, although such an $F$ must grow faster than the $k$-fold iterated exponential function for any $k$.

Conjectured by Erdős and Gallai, who proved that $O(n\log n)$ many cycles and edges suffices.

The best bound available is due to Bucić and Montgomery [BM22], who prove that $O(n\log^*n)$ many cycles and edges suffice, where $\log^*$ is the iterated logarithm function.

Conlon, Fox, and Sudakov [CFS14] proved that $O_\epsilon(n)$ cycles and edges suffice if $G$ has minimum degree at least $\epsilon n$, for any $\epsilon>0$.

See also [583].

A problem of Erdős, Duke, and Rödl. Duke and Erdős [DuEr83], who proved the first if $n$ is sufficiently large depending on $\delta$. The real challenge is to prove this when $\delta=n^{-c}$ for some $c>0$. Duke, Erdős, and Rödl [DER84] proved the first statement with a $\delta^5$ in place of a $\delta^3$.

Fox and Sudakov [FoSu08b] have proved the second statement when $\delta >n^{-1/5}$.

See also the entry in the graphs problem collection.

Pyber, Rödl, and Szemerédi [PRS95] constructed such a graph with $\gg n\log\log n$ edges.

Chakraborti, Janzer, Methuku, and Montgomery [CJMM24] have shown that such a graph can have at most $n(\log n)^{O(1)}$ many edges. Indeed, they prove that there exists a constant $C>0$ such that for any $k\geq 2$ there is a $c_k$ such that if a graph has $n$ vertices and at least $c_kn(\log n)^{C}$ many edges then it contains $k$ pairwise edge-disjoint cycles with the same vertex set.

It is known that \[\frac{1}{4\log k}\log n\leq g_k(n) \leq \frac{2}{\log(k-2)}\log n+1,\] the lower bound due to Kostochka [Ko88] and the upper bound to Erdős [Er59b].

Erdős [Er59b] proved that \[\lim_{n\to \infty}\frac{\log h^{(m)}(n)}{\log n}\gg \frac{1}{m}\] and, for odd $m$, \[\lim_{n\to \infty}\frac{\log h^{(m)}(n)}{\log n}\leq \frac{2}{m+1},\] and conjectured this is sharp. He had no good guess for the value of the limit for even $m$, other that it should lie in $[\frac{2}{m+2},\frac{2}{m}]$, but could not prove this even for $m=4$.

See also the entry in the graphs problem collection.

A problem of Hamburger and Szegedy.

A question of Erdős, Faudree, and Schelp, who proved it when $s=2$.

The answer is yes, proved by Sudakov and Verstraëte [SuVe08], who in fact proved that under the assumption of average degree $k$ and girth $>2s$ there are at least $\gg k^s$ many consecutive even integers which are cycle lengths in $G$.

This would be a stronger form of the result of Dirac [Di60] that every such graph contains a subgraph homeomorphic to $K_4$.

The answer is yes, as proved by Thomassen [Th74].

A question of Erdős and Gallai. Gallai [Ga63] proved that \[f_4(n) \gg n^{1/2}\] and Erdős (unpublished) proved $f_4(n) \ll n^{1/2}$.

This was proved for all $k\geq 4$ by Kierstead, Szemerédi, and Trotter [KST84].