On December 30, 2020, Paul Seymour from Princeton University was the speaker of the Virtual Discrete Math Colloquium. He presented his recent breakthrough on the Erdős-Hajnal conjecture with Maria Chudnovsky, Alex Scott, and Sophie Spirkl, which in particular proves that the Erdős-Hajnal conjecture holds for the cycle of length 5. The title of his talk was “The Erdős-Hajnal conjecture is true for excluding a five-cycle“.

## Paul Seymour, The Erdős-Hajnal conjecture is true for excluding a five-cycle

In an n-vertex graph, there must be a clique or stable set of size at least $C\log n$, and there are graphs where this bound is attained. But if we look at graphs not containing a fixed graph H as an induced subgraph, the largest clique or stable set is bigger.

Erdős and Hajnal conjectured in 1977 that for every graph H, there exists c>0 such that every H-free graph has a clique or stable set of size at least $|G|^c$ (“H-free” means not containing H as an induced subgraph, and |G| means the number of vertices of G). This is still open, even for some five-vertex graphs H; and the case that has attracted most attention is when H is a cycle of length five.

It is true in that case. We will give a sketch of the proof, which is via applying a lemma about bipartite graphs, a variant of a theorem of I. Tomon.

This lemma has several other applications to the Erdős-Hajnal conjecture. For instance, it implies that for every cycle C and forest T, there exists c>0 such that every graph that is both C-free and T’-free (where T’ is the complement of T) has a clique or stable set of size $|G|^c$. (Until now this was open when C has length five and T is a 5-vertex path.)

Joint work with Maria Chudnovsky, Alex Scott and Sophie Spirkl.