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Tournaments and the Strong Erdos-Hajnal Property

Eli Berger
Maria Chudnovsky
Shira Zerbib
European Journal of Combinatorics (2021)


A conjecture of Alon, Pach and Solymosi, which is equivalent to the celebrated Erdős-Hajnal Conjecture, states that for every tournament S there exists epsilon(S)>0 such that if T is an n-vertex tournament that does not contains S as a subtournament, then T contains a transitive subtournament on at least n^{epsilon(S)} vertices. Let C_5 be the unique five-vertex tournament where every vertex has two inneighbors and two outneighbors. The Alon-Pach-Solymosi conjecture is known to be true for the case when S=C_5. Here we prove a strengthening of this result, showing that in every tournament $T$ with no subtournament isomorphic to C_5 there exist disjoint vertex subsets A and B, each containing a linear proportion of the vertices of T, and such that every vertex of A is adjacent to every vertex of B.