A note on Ramsey numbers for Berge-G hyper graphs

Abstract

For a graph G=(V,E), a hypergraph H is called Berge-G if there is a bijection f from E(G) to E(H) such that for each e in E(G), e is a subset of f(e). The set of all Berge-G hypergraphs is denoted B(G). For integers k>1, r>1, and a graph G, let the Ramsey number R_r(B(G), k) be the smallest integer n such that no matter how the edges of a complete r-uniform n-vertex hypergraph are colored with k colors, there is a copy of a monochromatic Berge-G subhypergraph. Furthermore, let R(B(G),k) be the smallest integer n such that no matter how all subsets an n-element set are colored with k colors, there is a monochromatic copy of a Berge-G hypergraph. We give an upper bound for R_r(B(G),k) in terms of graph Ramsey numbers. In particular, we prove that when G becomes acyclic after removing some vertex, R_r(B(G),k)\le 4k|V(G)|+r-2, in contrast with classical multicolor Ramsey numbers. When G is a triangle or a K_4, we find sharper bounds and some exact results and determine some `small' Ramsey numbers: k/2 - o(k) < R_3(B(K_3)), k) < 3k/4+ o(k), For any odd integer t\neq 3, R(B(K_3),2^t-1)=t+2, 2^{ck} < R_3(B(K_4),k) < e(1+o(1))(k-1)k!, R_3(B(K_3),2)=R_3(B(K_3),3)=5, R_3(B(K_3),4)=6, R_3(B(K_3),5)=7, R_3(B(K_3),6)=8, R_3(B(K_3,8)=9, R_3(B(K_4),2)=6.