### Rutgers Discrete Mathematics Seminar

Title: Gyarfas-Sumner meets Erdos-HajnalGyarfas-Sumner meets Erdos-Hajnal

Speaker: **Paul Seymour**, Princeton

Date: Monday, February 19, 2018 2:00 pm

Location: Hill Center, Room 705, Rutgers University, Busch Campus, Piscataway, NJ

Abstract:
The Gyarfas-Sumner conjecture says that every graph with huge (enough) chromatic number and bounded clique number contains any given forest as an induced subgraph. The Erdos-Hajnal conjecture says that for every graph H, all graphs not containing H as an induced subgraph have a clique or stable set of polynomial size. This talk is about a third problem related to both of these, the following. Say an n-vertex graph is "c-coherent'' if every vertex has degree < cn, and every two disjoint vertex subsets of size at least cn have an edge between them. To prove a given graph H satisfies the Erdos-Hajnal conjecture, it is enough to prove H satisfies the conjecture in all c-coherent graphs and their complements, where c>0 is fixed and as small as we like. But for some graphs H, all c-coherent graphs contain H if c is small enough, so half of the task is done for free. Which graphs H have this property? Paths do (a theorem of Bousquet, Lagoutte, and Thomasse), and non-forests don't. Maybe all forests do? In other words, do all c-coherent graphs with c small enough contain any given forest as an induced subgraph? That question is the topic of the talk. It looks much like the Gyarfas-Sumner conjecture, but it seems easier, and there are already several pretty results. For instance the conjecture is true for all subdivided caterpillars (which is more than we know for Gyarfas-Sumner), and all trees of radius two.

Joint work with Maria Chudnovsky, Anita Liebenau, Marcin Pilipczuk, Alex Scott and Sophie Spirkl.

See: http://sites.math.rutgers.edu/~ajr224/DM