Large cliques or cocliques in hypergraphs with forbidden order-size pairs

Abstract

The well-known Erdős-Hajnal conjecture states that for any graph $F$ , there exists $\epsilon \gt 0$ such that every $n$ -vertex graph $G$ that contains no induced copy of $F$ has a homogeneous set of size at least $n^{\epsilon }$ . We consider a variant of the Erdős-Hajnal problem for hypergraphs where we forbid a family of hypergraphs described by their orders and sizes. For graphs, we observe that if we forbid induced subgraphs on $m$ vertices and $f$ edges for any positive $m$ and $0\leq f \leq \binom{m}{2}$ , then we obtain large homogeneous sets. For triple systems, in the first nontrivial case $m=4$ , for every $S \subseteq \{0,1,2,3,4\}$ , we give bounds on the minimum size of a homogeneous set in a triple system where the number of edges spanned by every four vertices is not in $S$ . In most cases the bounds are essentially tight. We also determine, for all $S$ , whether the growth rate is polynomial or polylogarithmic. Some open problems remain.