20 solved out of 43 shown (show only solved or open)
SOLVED - $500 If$G$is an edge-disjoint union of$n$copies of$K_n$then is$\chi(G)=n$? Conjectured by Faber, Lovász, and Erdős (apparently 'at a party in Boulder, Colarado in September 1972' [Er81]). Kahn [Ka92] proved that$\chi(G)\leq (1+o(1))n$(for which Erdős gave him a 'consolation prize' of \$100). 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.

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 Erdős and Hajnal, and solved by Liu and Montgomery [LiMo20]. In [Er81] Erdős asks whether the $a_i$ must in fact have positive upper density. The lower density of the set can be $0$ since there are graphs of arbitrarily large chromatic number and girth.

SOLVED
If $G$ is a graph which contains odd cycles of $\leq k$ different lengths then $\chi(G)\leq 2k+2$, with equality if and only if $G$ contains $K_{2k+2}$.
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.

Additional thanks to: David Penman
SOLVED
Does every 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).

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 $[(\log \ell)^8,\ell]$.

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)$edges? Conjectured by Erdős, Hajnal, and Szemerédi [EHS82]. Rödl [Ro82] has proved this for hypergraphs, and also proved there is such a graph (with chromatic number$\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$(see [111]). OPEN Is there a graph of chromatic number$\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}$? Conjectured by Erdős, Hajnal, and Szemerédi [EHS82]. See also [750]. OPEN For every$r\geq 4$and$k\geq 2$is there some finite$f(k,r)$such that every graph of chromatic number$\geq f(k,r)$contains a subgraph of girth$\geq r$and chromatic number$\geq k$? Conjectured by Erdős and Hajnal. Rödl [Ro77] has proved the$r=4$case. The infinite version (whether every graph of infinite chromatic number contains a subgraph of infinite chromatic number whose girth is$>k$) is also open. In [Er79b] Erdős also asks whether $\lim_{k\to \infty}\frac{f(k,r+1)}{f(k,r)}=\infty.$ OPEN Is there some$F(n)$such that every graph with chromatic number$\aleph_1$has, for all large$n$, a subgraph with chromatic number$n$on at most$F(n)$vertices? Conjectured by Erdős, Hajnal, and Szemerédi [EHS82]. This fails if the graph has chromatic number$\aleph_0$. OPEN If$G$is a graph let$h_G(n)$be defined such that any subgraph of$G$on$n$vertices can be made bipartite after deleting at most$h_G(n)$edges. What is the behaviour of$h_G(n)$? Is it true that$h_G(n)/n\to \infty$for every graph$G$with chromatic number$\aleph_1$? A problem of Erdős, Hajnal, and Szemerédi [EHS82]. Every$G$with chromatic number$\aleph_1$must have$h_G(n)\gg n$since$G$must contain, for some$r$,$\aleph_1$many vertex disjoint odd cycles of length$2r+1$. On the other hand, Erdős, Hajnal, and Szemerédi proved that there is a$G$with chromatic number$\aleph_1$such that$h_G(n)\ll n^{3/2}$. In [Er81] Erdős conjectured that this can be improved to$\ll n^{1+\epsilon}$for every$\epsilon>0$. See also [74]. OPEN Let$A\subset\mathbb{R}^2$be an infinite set which contains no three points on a line and no four points on a circle. Consider the graph with vertices the points in$A$, where two vertices are joined by an edge if and only if they are an integer distance apart. How large can the chromatic number and clique number of this graph be? In particular, can the chromatic number be infinite? Asked by Andrásfai and Erdős. Erdős [Er97b] also asked where such a graph could contain an infinite complete graph, but this is impossible by an earlier result of Anning and Erdős [AnEr45]. See also [213]. Additional thanks to: Noga Alon OPEN -$1000
The cochromatic number of $G$, denoted by $\zeta(G)$, is the minimum number of colours needed to colour the vertices of $G$ such that each colour class induces either a complete graph or empty graph. Let $\chi(G)$ denote the chromatic number.
If $G$ is a random graph with $n$ vertices and each edge included independently with probability $1/2$ then is it true that almost surely $\chi(G) - \zeta(G) \to \infty$ as $n\to \infty$?
A problem of Erdős and Gimbel (see also [Gi16]). At a conference on random graphs in Poznan, Poland (most likely in 1989) Erdős offered \$100 for a proof that this is true, and \$1000 for a proof that this is false (although later told Gimbel that \$1000 was perhaps too much). It is known that almost surely $\frac{n}{2\log_2n}\leq \zeta(G)\leq \chi(G)\leq (1+o(1))\frac{n}{2\log_2n}.$ (The final upper bound is due to Bollobás [Bo88]. The first inequality follows from the fact that almost surely$G$has clique number and independence number$< 2\log_2n$.) Heckel [He24] and, independently, Steiner [St24b] have shown that it is not the case that$\chi(G)-\zeta(G)$is bounded with high probability, and in fact if$\chi(G)-\zeta(G) \leq f(n)$with high probability then$f(n)\geq n^{1/2-o(1)}$along an infinite sequence of$n$. Heckel conjectures that, with high probability, $\chi(G)-\zeta(G) \asymp \frac{n}{(\log n)^3}.$ Additional thanks to: John Gimbel and Zach Hunter OPEN Let$k\geq 4$and$g_k(n)$denote the largest$m$such that there is a graph on$n$vertices with chromatic number$k$and girth$m$. Does $\lim_{n\to \infty}\frac{g_k(n)}{\log n}$ exist? 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]. Additional thanks to: David Penman OPEN Let$\omega(G)$denote the clique number of$G$and$\chi(G)$the chromatic number. If$f(n)$is the maximum value of$\chi(G)/\omega(G)$, as$G$ranges over all graphs on$n$vertices, then does $\lim_{n\to\infty}\frac{f(n)}{n/(\log n)^2}$ exist? Tutte and Zykov [Zy52] independently proved that for every$k$there is a graph with$\omega(G)=2$and$\chi(G)=k$. Erdős [Er61d] proved that for every$n$there is a graph on$n$vertices with$\omega(G)=2$and$\chi(G)\gg n^{1/2}/\log n$, whence$f(n) \gg n^{1/2}/\log n$. Erdős [Er67c] proved that $f(n) \asymp \frac{n}{(\log n)^2}$ and that the limit in question, if it exists, must be in $(\log 2)^2\cdot [1/4,1].$ OPEN Let$G$be a graph with chromatic number$k$containing no$K_k$. If$a,b\geq 2$and$a+b=k+1$then must there exist two disjoint subgraphs of$G$with chromatic numbers$\geq a$and$\geq b$respectively? This property is sometimes called being$(a,b)$-splittable. A question of Erdős and Lovász (often called the Erdős-Lovász Tihany conjecture). Erdős [Er68b] originally asked about$a=b=3$which was proved by Brown and Jung [BrJu69] (who in fact prove that$G$must contain two vertex disjoint odd cycles). Balogh, Kostochka, Prince, and Stiebitz [BKPS09] have proved the full conjecture for quasi-line graphs and graphs with independence number$2$. OPEN The list chromatic number$\chi_L(G)$is defined to be the minimal$k$such that for any assignment of a list of$k$colours to each vertex of$G$(perhaps different lists for different vertices) a colouring of each vertex by a colour on its list can be chosen such that adjacent vertices receive distinct colours. Determine the minimal number of vertices$n(k)$of a bipartite graph$G$such that$\chi_L(G)>k$. A problem of Erdős, Rubin, and Taylor [ERT80], who proved that $2^{k-1}<n(k) <k^22^{k+2}.$ They also prove that if$m(k)$is the size of the smallest family of$k$-sets without property B (i.e. the smallest number of$k$-sets in a graph with chromatic number$3$) then$m(k)\leq n(k)\leq m(k+1)$. Erdős, Rubin, and Taylor [ERT80] proved$n(2)=6$and Hanson, MacGillivray, and Toft [HMT96] proved$n(3)=14$and $n(k) \leq kn(k-2)+2^k.$ Additional thanks to: Zach Hunter SOLVED The list chromatic number$\chi_L(G)$is defined to be the minimal$k$such that for any assignment of a list of$k$colours to each vertex of$G$(perhaps different lists for different vertices) a colouring of each vertex by a colour on its list can be chosen such that adjacent vertices receive distinct colours. Does every planar bipartite graph$G$have$\chi_L(G)\leq 3$? A problem of Erdős, Rubin, and Taylor [ERT80]. The answer is yes, proved by Alon and Tarsi [AlTa92]. See also [631]. SOLVED The list chromatic number$\chi_L(G)$is defined to be the minimal$k$such that for any assignment of a list of$k$colours to each vertex of$G$(perhaps different lists for different vertices) a colouring of each vertex by a colour on its list can be chosen such that adjacent vertices receive distinct colours. Does every planar graph$G$have$\chi_L(G)\leq 5$? Is this best possible? A problem of Erdős, Rubin, and Taylor [ERT80]. The answer to both is yes: Thomassen [Th94] proved that$\chi_L(G)\leq 5$if$G$is planar, and Voigt [Vo93] constructed a planar graph with$\chi_L(G)=5$. A simpler construction was given by Gutner [Gu96]. See also [630]. SOLVED A graph is$(a,b)$-choosable if for any assignment of a list of$a$colours to each of its vertices there is a subset of$b$colours from each list such that the subsets of adjacent vertices are disjoint. If$G$is$(a,b)$-choosable then$G$is$(am,bm)$-choosable for every integer$m\geq 1$. A problem of Erdős, Rubin, and Taylor [ERT80]. Note that$G$is$(a,1)$-choosable corresponds to being$a$-choosable, that is, the list chromatic number satisfies$\chi_L(G)\leq a$. This is false: Dvořák, Hu, and Sereni [DHS19] construct a graph which is$(4,1)$-choosable but not$(8,2)$-choosable. Additional thanks to: David Penman and Raphael Steiner OPEN Is there some function$f$such that for all$k\geq 3$if a finite graph$G$has chromatic number$\geq f(k)$then$G$must contain some odd cycle whose vertices span a graph of chromatic number$\geq k$? A problem of Erdős and Hajnal. OPEN Let$G_n$be the unit distance graph in$\mathbb{R}^n$, with two vertices joined by an edge if and only if the distance between them is$1$. Estimate the chromatic number$\chi(G_n)$. Does it grow exponentially in$n$? Does $\lim_{n\to \infty}\chi(G_n)^{1/n}$ exist? A generalisation of the Hadwiger-Nelson problem (which addresses$n=2$). Frankl and Wilson [FrWi81] proved exponential growth: $\chi(G_n) \geq (1+o(1))1.2^n.$ The trivial colouring (by tiling with cubes) gives $\chi(G_n) \leq (2+\sqrt{n})^n.$ Larman and Rogers [LaRo72] improved this to $\chi(G_n) \leq (3+o(1))^n,$ and conjecture the truth may be$(2^{3/2}+o(1))^n$. Prosanov [Pr20] has given an alternative proof of this upper bound. See also [508], [705], and [706]. OPEN Let$G$be a finite unit distance graph in$\mathbb{R}^2$(i.e. the vertices are a finite collection of points in$\mathbb{R}^2$and there is an edge between two points f and only if the distance between them is$1$). Is there some$k$such that if$G$has girth$\geq k$(i.e.$G$contains no cycles of length$<k$) then$\chi(G)\leq 3$? The maximal value of$\chi(G)$(without a girth condition) is the Hadwiger-Nelson problem. There are unit distance graphs (e.g. the Moser spindle) with$\chi(G)=4$of girth$3$. de Grey [dG18] has constructed a unit distance graph$G$with$\chi(G)=5$. (I do not know what the largest girth achieved is by these recent constructions.) Wormald [Wo79] has constructed a unit distance graph with$\chi(G)=4$and girth$5$, with$6448$vertices. O'Donnell [OD94] has constructed a unit distance graph with$\chi(G)=4$and girth$4$, with$56$vertices. Chilakamarri [Ch95] has constructed an infinite family of unit distance graphs with$\chi(G)=4$and girth$4$, the smallest of which has$47$vertices. See also [508], [704], and [706]. OPEN Let$L(r)$be such that if$G$is a graph formed by taking a finite set of points$P$in$\mathbb{R}^2$and some set$A\subset (0,\infty)$of size$r$, where the vertex set is$P$and there is an edge between two points if and only if their distance is a member of$A$, then$\chi(G)\leq L(r)$. Estimate$L(r)$. In particular, is it true that$L(r)\leq r^{O(1)}$? The case$r=1$is the Hadwiger-Nelson problem, for which it is known that$5\leq L(1)\leq 7$. See also [508], [704], and [705]. OPEN Let$G$be a graph with chromatic number$\aleph_1$. Is there, for any integer$m\geq 1$, some graph$G_m$of chromatic number$m$such that every finite subgraph of$G_m$is a subgraph of$G$? A conjecture of Walter Taylor. More generally, Erdős asks to characterise families$\mathcal{F}_\alpha$of finite graphs such that there is a graph of chromatic number$\aleph_\alpha$all of whose finite subgraphs are in$\mathcal{F}_\alpha$. OPEN Let$G$be a graph with chromatic number$\aleph_1$. Must there exist an edge$e$such that, for all large$n$,$G$contains a cycle of length$n$containing$e$? A problem of Erdős, Hajnal, and Shelah [EHS74], who proved that$G$must contain all sufficiently large cycles (see [594]). OPEN If$G$has infinite chromatic number and is triangle-free (contains no$K_3$) then must$G$contain every tree as an induced subgraph? A conjecture of Gyárfás. OPEN Let$\mathfrak{m}$be an infinite cardinal and$G$be a graph with chromatic number$\mathfrak{m}$. Is it true that, for every infinite cardinal$\mathfrak{n}< \mathfrak{m}$, there exists a subgraph of$G$with chromatic number$\mathfrak{n}$? A question of Galvin, who proved that the answer is no if we ask for the subgraph to be induced (assuming$\aleph_1 < 2^{\aleph_0}$). OPEN Let$\mathfrak{m}$be an infinite cardinal and$G$be a graph with chromatic number$\mathfrak{m}$. Let$C\geq 1$. Must$G$contain a subgraph of chromatic number$\mathfrak{m}$which does not contain any odd cycle of length$\leq C$? A question of Erdős and Hajnal. Rödl proved this is true if$\mathfrak{m}=\aleph_0$and$C=3$. In [Er81] Erdős also asks the same question but with girth (i.e. the subgraph does not contain any cycle at all of length$\leq C$). SOLVED Let$k$be a large fixed constant. Let$f_k(n)$be the minimal$m$such that there exists a graph$G$on$n$vertices with chromatic number$k$, such that every proper subgraph has chromatic number$<k$, and$G$can be made bipartite by deleting$m$edges. Is it true that$f_k(n)\to \infty$as$n\to \infty$? In particular, is it true that$f_4(n) \gg \log n$? A problem of Erdős, Hajnal, and Szemerédi [EHS82]. Odd cycles show that$f_3(n)=1$, but they expected$f_4(n)\to \infty$. Gallai [Ga68] gave a construction which shows $f_4(n) \ll n^{1/2},$ and Lovász extended this to show $f_k(n) \ll n^{1-\frac{1}{k-2}}.$ This conjecture was disproved by Rödl and Tuza [RoTu85], who proved that in fact$f_k(n)=\binom{k-1}{2}$(for all sufficiently large$n$). Additional thanks to: Raphael Steiner OPEN Let$f(m)$be some function such that$f(m)\to \infty$as$m\to \infty$. Does there exist a graph$G$of infinite chromatic number such that every subgraph on$m$vertices contains an independent set of size at least$\frac{m}{2}-f(m)$? Erdős and Hajnal proved this is true if$f(m)=\epsilon m$for any fixed$\epsilon>0$. See also [75]. SOLVED Let$G$be a graph with chromatic number$\chi(G)=4$. If$m_1<m_2<\cdots$are the lengths of the cycles in$G$then can$\min(m_{i+1}-m_i)$be arbitrarily large? Can this happen if the girth of$G$is large? The answer is no: Bondy and Vince [BoVi98] proved that every graph with minimum degree at least$3$has two cycles whose lengths differ by at most$2$, and hence the same is true for every graph with chromatic number$4$. Additional thanks to: Raphael Steiner SOLVED The list chromatic number$\chi_L(G)$is defined to be the minimal$k$such that for any assignment of a list of$k$colours to each vertex of$G$(perhaps different lists for different vertices) a colouring of each vertex by a colour on its list can be chosen such that adjacent vertices receive distinct colours. Does there exist some constant$c>0$such that $\chi_L(G)+\chi_L(G^c)> n^{1/2+c}$ for every graph$G$on$n$vertices (where$G^c$is the complement of$G$)? A problem of Erdős, Rubin, and Taylor. The answer is no: Alon [Al92] proved that, for every$n$, there exists a graph$G$on$n$vertices such that $\chi_L(G)+\chi_L(G^c)\ll (n\log n)^{1/2},$ where the implied constant is absolute. SOLVED The cochromatic number of$G$, denoted by$\zeta(G)$, is the minimum number of colours needed to colour the vertices of$G$such that each colour class induces either a complete graph or empty graph. Let$z(n)$be the maximum value of$\zeta(G)$over all graphs$G$with$n$vertices. Determine$z(n)$for small values of$z(n)$. In particular is it true that$z(12)=4$? A question of Erdős and Gimbel, who knew that$4\leq z(12)\leq 5$and$5\leq z(15)\leq 6$. The equality$z(12)=4$would follow from proving that if$G$is a graph on$12$vertices such that both$G$and its complement are$K_4$-free then either$\chi(G)\leq 4$or$\chi(G^c)\leq 4$. In fact there do exist such graphs - Bhavik Mehta found computationally that there is exactly one (up to taking the complement) graph on$12$vertices such that both$G$and its complement are$K_4$-free with chromatic number$\geq 5$. This graph was explicitly checked to have cochromatic number$4$, and hence this proves that indeed$z(12)=4$. The values of$z(n)$are now known for$1\leq n\leq 19$: $1,1,2,2,3,3,3,3,4,4,4,4,5,5,5,6,6,6,6.$ (The only significant difficulty here is proving$z(12)=4$- the others follow from easy inductive arguments and the facts that$R(3)=6$and$R(4)=18$.) It is unknown whether$z(20)=6$or$7$. Gimbel [Gi86] has shown that$z(n) \asymp \frac{n}{\log n}$. Additional thanks to: Bhavik Mehta SOLVED The cochromatic number of$G$, denoted by$\zeta(G)$, is the minimum number of colours needed to colour the vertices of$G$such that each colour class induces either a complete graph or empty graph. Let$z(S_n)$be the maximum value of$\zeta(G)$over all graphs$G$which can be embedded on$S_n$, the orientable surface of genus$n$. Determine the growth rate of$z(S_n)$. A problem of Erdős and Gimbel. Gimbel [Gi86] proved that $\frac{\sqrt{n}}{\log n}\ll z(S_n) \ll \sqrt{n}.$ Solved by Gimbel and Thomassen [GiTh97], who proved $z(S_n) \asymp \frac{\sqrt{n}}{\log n}.$ Additional thanks to: Raphael Steiner SOLVED The cochromatic number of$G$, denoted by$\zeta(G)$, is the minimum number of colours needed to colour the vertices of$G$such that each colour class induces either a complete graph or empty graph. If$G$is a graph with chromatic number$\chi(G)=m$then must$G$contain a subgraph$H$with $\zeta(H) \gg \frac{m}{\log m}?$ A problem of Erdős and Gimbel, who proved that there must exist a subgraph$H$with $\zeta(H) \gg \left(\frac{m}{\log m}\right)^{1/2}.$ The proposed bound would be best possible, as shown by taking$G$to be a complete graph. The answer is yes, proved by Alon, Krivelevich, and Sudakov [AKS97]. OPEN The cochromatic number of$G$, denoted by$\zeta(G)$, is the minimum number of colours needed to colour the vertices of$G$such that each colour class induces either a complete graph or empty graph. The dichromatic number of$G$, denoted by$\delta(G)$, is the minimum number$k$of colours required such that, in any orientation of the edges of$G$, there is a$k$-colouring of the vertices of$G$such that there are no monochromatic oriented cycles. Must a graph with large chromatic number have large dichromatic number? Must a graph with large cochromatic number contain a graph with large dichromatic number? The first question is due to Erdős and Neumann-Lara. The second question is due to Erdős and Gimbel. A positive answer to the second question implies a positive answer to the first via the bound mentioned in [760]. SOLVED The cochromatic number of$G$, denoted by$\zeta(G)$, is the minimum number of colours needed to colour the vertices of$G$such that each colour class induces either a complete graph or empty graph. Is it true that if$G$has no$K_5$and$\zeta(G)\geq 4$then$\chi(G) \leq \zeta(G)+2$? A conjecture of Erdős, Gimbel, and Straight [EGS90], who proved that for every$n>2$there exists some$f(n)$such that if$G$contains no clique on$n$vertices then$\chi(G)\leq \zeta(G)+f(n)$. This has been disproved by Steiner [St24b], who constructed a graph$G$with$\omega(G)=4$,$\zeta(G)=4$, and$\chi(G)=7$. SOLVED Suppose$n\geq kr+(t-1)(k-1)$and the edges of the complete$r$-uniform hypergraph on$n$vertices are$t$-coloured. Prove that some colour class must contain$k$pairwise disjoint edges. In other words, this problem asks to determine the chromatic number of the Kneser graph. This would be best possible: if$n=kr-1+(t-1)(k-1)$then decomposing$[n]$as one set$X_1$of size$kr-1$and$t-1$sets$X_2,\ldots,X_{t}$of size$k-1$, a colouring without$k$pairwise disjoint edges is given colouring all subsets of$X_0$in colour$1$and assigning an edge with colour$2\leq i\leq t$if$i$is minimal such that$X_i$intersects the edge. When$k=2$this was conjectured by Kneser and proved by Lovász [Lo78]. The general case was proved by Alon, Frankl, and Lovász [AFL86]. SOLVED Let$f(d)$be the maximal acyclic chromatic number of any graph with maximum degree$d$- that is, the vertices of any graph with maximum degree$d$can be coloured with$f(d)$colours such that there is no edge between vertices of the same colour and no cycle containing only two colours. Estimate$f(d)$. In particular is it true that$f(d)=o(d^2)$? It is easy to see that$f(d)\leq d^2+1$using a greedy colouring. Erdős had shown$f(d)\geq d^{4/3-o(1)}$. Resolved by Alon, McDiarmid, and Reed [AMR91] who showed $\frac{d^{4/3}}{(\log d)^{1/3}}\ll f(d) \ll d^{4/3}.$ Additional thanks to: Noga Alon SOLVED The list chromatic number$\chi_L(G)$is defined to be the minimal$k$such that for any assignment of a list of$k$colours to each vertex of$G$(perhaps different lists for different vertices) a colouring of each vertex by a colour on its list can be chosen such that adjacent vertices receive distinct colours. Is it true that$\chi_L(G)=o(n)$for almost all graphs on$n$vertices? A problem of Erdős, Rubin and Taylor. The answer is yes: Alon [Al92] proved that in fact the random graph on$n$vertices with edge probability$1/2$has $\chi_L(G) \ll \frac{\log\log n}{\log n}n$ almost surely. Alon, Krivelevich, and Sudakov [AKS99] improved this to $\chi_L(G) \asymp \frac{n}{\log n}$ almost surely. Additional thanks to: David Penman SOLVED Let$r\geq 3$and$k$be sufficiently large in terms of$r$. Is it true that every$r$-uniform hypergraph with chromatic number$k$has at least $\binom{(r-1)(k-1)+1}{r}$ edges, with equality only for the complete graph on$(r-1)(k-1)+1$vertices? When$r=2$it is a classical fact that chromatic number$k$implies at least$\binom{k}{2}$edges. Erdős asked for$k$to be large in this conjecture since he knew it to be false for$r=k=3$, as witnessed by the Steiner triples with$7$vertices and$7$edges. This was disproved by Alon [Al85], who proved, for example, that there exists some absolute constant$C>0$such that if$r\geq C$and$k\geq Cr$then there exists an$r$-uniform hypergraph with chromatic number$\geq k$with at most $\leq (7/8)^r\binom{(r-1)(k-1)+1}{r}$ many edges. In general, Alon gave an upper bound for the minimal number of edges using Turán numbers. Using known bounds for Turán numbers then suffices to disprove this conjecture for all$r\geq 4$. The validity of this conjecture for$r=3$remains open. If$m(r,k)$denotes the minimal number of edges of any$r$-uniform hypergraph with chromatic number$>k$then Akolzin and Shabanov [AkSh16] have proved $\frac{r}{\log r}k^r \ll m(r,k) \ll (r^3\log r) k^r,$ where the implied constants are absolute. Cherkashin and Petrov [ChPe20] have proved that, for fixed$r$,$m(r,k)/k^r$converges to some limit as$k\to \infty$. Additional thanks to: Zach Hunter SOLVED Does there exist an absolute constant$c>0$such that, for all$r\geq 2$, in any$r$-uniform hypergraph with chromatic number$3$there is a vertex contained in at least$(1+c)^r$many edges? In general, determine the largest integer$f(r)$such that every$r$-uniform hypergraph with chromatic number$3$has a vertex contained in at least$f(r)$many edges. It is easy to see that$f(2)=2$and$f(3)=3$. Erdős did not know the value of$f(4)$. This was solved by Erdős and Lovász [ErLo75], who proved in particular that there is a vertex contained in at least $\frac{2^{r-1}}{4r}$ many edges. Additional thanks to: Noga Alon and Zach Hunter OPEN Let$r\geq 2$and$G$be a$r$-uniform hypergraph with chromatic number$3$, such that any two edges have non-empty intersection. Must$G$contain$O(r^2)$many vertices? Must there be two edges which meet in$\gg r$many vertices? A problem of Erdős and Shelah. The Fano geometry gives an example where there are no two edges which meet in$r-1$vertices. Are there any other examples? Erdős and Lovász [ErLo75] proved that there must be two edges which meet in$\gg \frac{r}{\log r}$many vertices. SOLVED Let$G$be a graph on$3n$vertices formed by taking$n$vertex disjoint triangles and adding a Hamiltonian cycle (with all new edges) between these vertices. Does$G$have chromatic number at most$3\$?