Logo
FAQ Tags Prizes Definitions Links
All Problems Random Solved Random Open
3 solved out of 11 shown
$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 $n$.
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.
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
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).
$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 [ErHaSz82]. 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 $\aleph_1$.
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 [ErHaSz82].
For every $k\geq 3$ and $n\geq 2$ is there some finite $f(n,k)$ such that every graph of chromatic number $\geq f(n,k)$ contains a subgraph of girth at least $k$ and chromatic number at least $n$?
Conjectured by Erdős and Hajnal. Rödl [Ro77] has proved the $k=3$ 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.
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 [ErHaSz82]. This fails if the graph has chromatic number $\aleph_0$.
Let $c>0$ and $G$ be a graph of chromatic number $\aleph_1$. Are there infinitely many $n$ such that $G$ contains a subgraph on $n$ vertices which cannot be made bipartite by deleting at most $cn$ edges?
Conjectured by Erdős, Hajnal, and Szemerédi [ErHaSz82].
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?
Conjectured by Andrásfai and Erdős. It is possible that such a graph could contain an infinite complete graph.
Let $G$ be the infinite graph on $\mathbb{N}$ where we connect $m,n$ by an edge if and only if $n+m$ is a square. Is the chromatic number of $G$ equal to $\aleph_0$? What if $n+m$ is required to be a $k$th power?