English

Induced subgraphs of induced subgraphs of large chromatic number

Combinatorics 2023-09-19 v4

Abstract

We prove that, for every graph FF with at least one edge, there is a constant cFc_F such that there are graphs of arbitrarily large chromatic number and the same clique number as FF in which every FF-free induced subgraph has chromatic number at most cFc_F. This generalises recent theorems of Bria\'{n}ski, Davies and Walczak, and Carbonero, Hompe, Moore and Spirkl. Our results imply that for every r3r\geq 3 the class of KrK_r-free graphs has a very strong vertex Ramsey-type property, giving a vast generalisation of a result of Folkman from 1970. We also prove related results for tournaments, hypergraphs and infinite families of graphs, and show an analogous statement for graphs where clique number is replaced by odd girth.

Keywords

Cite

@article{arxiv.2203.03612,
  title  = {Induced subgraphs of induced subgraphs of large chromatic number},
  author = {António Girão and Freddie Illingworth and Emil Powierski and Michael Savery and Alex Scott and Youri Tamitegama and Jane Tan},
  journal= {arXiv preprint arXiv:2203.03612},
  year   = {2023}
}

Comments

26 pages; v4: final version incorporating the suggestions of referees and Jarik Ne\v{s}et\v{r}il, including simplified constructions and a new open question