English

Suns in triangle-free graphs of large chromatic number

Combinatorics 2026-05-11 v2

Abstract

For an integer t4t\geq 4, a tt-sun is a graph obtained from a tt-vertex cycle CC by adding a degree-one neighbor for each vertex of CC. Trotignon asked whether every triangle-free graph of sufficiently large chromatic number has an induced subgraph that is a tt-sun for some t4t\geq 4. This remains open, but we show that every triangle-free graph of chromatic number at least 4848 has an induced subgraph that is either a tt-sun for some t5t\geq 5, or a 44-sun with a single degree-one vertex deleted. In fact, we prove that for all 5\ell\geq 5, there exists c=c()Nc=c(\ell)\in \mathbb{N} such that every triangle-free graph of chromatic number at least cc has an induced subgraph that is either a tt-sun for some tt\geq \ell, or a 44-sun with a single degree-one vertex deleted.

Keywords

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}
}