Suns in triangle-free graphs of large chromatic number
Combinatorics
2026-05-11 v2
Abstract
For an integer , a -sun is a graph obtained from a -vertex cycle by adding a degree-one neighbor for each vertex of . Trotignon asked whether every triangle-free graph of sufficiently large chromatic number has an induced subgraph that is a -sun for some . This remains open, but we show that every triangle-free graph of chromatic number at least has an induced subgraph that is either a -sun for some , or a -sun with a single degree-one vertex deleted. In fact, we prove that for all , there exists such that every triangle-free graph of chromatic number at least has an induced subgraph that is either a -sun for some , or a -sun with a single degree-one vertex deleted.
Cite
@article{arxiv.2506.10227,
title = {Suns in triangle-free graphs of large chromatic number},
author = {Sepehr Hajebi and Sophie Spirkl},
journal= {arXiv preprint arXiv:2506.10227},
year = {2026}
}