A graph $G$ is Ramsey for a $H$ if every colouring of the edges in two colours contains monochromatic copy $H$. Two graphs $H_1$ and $H_2$ are equivalent any only it $H_2$. parameter $s$ distinguishing $s(H_1)\neq s(H_2)$ implies that not equivalent. In this paper we show chromatic number parameter. We also extend to multi-colour case use similar idea find another which distinguishing.

We study the hat chromatic number of a graph defined in following way: there is one player at each vertex G , an adversary places K colors on head player, two players can see other's hats if and only they are adjacent vertices. All simultaneously try to guess color their hat. The cannot communicate but collectively determine strategy before placed. number, denoted by HG ( ) largest such that ab...

A pseudo-disk is a subset of the plane that is homeomorphic to a closed disk. Consider a family F of pseudo-disks whose interiors are pairwise disjoint, and such that any two pseudo-disks intersect in at most one point. If any point of the plane is contained in at most k elements of F (with k sufficiently large), then we show that the elements of F can be colored with at most k + 1 colors so th...

A graph G is (a, b)-choosable if for any assignment of a list of a colors to each of its vertices there is a subset of b colors of each list so that subsets corresponding to adjacent vertices are disjoint. It is shown that for every graph G, the minimum ratio a/b where a, b range over all pairs of integers for which G is (a, b)-choosable is equal to the fractional chromatic number of G.