English

Chromatic number is not tournament-local

Combinatorics 2023-12-05 v2

Abstract

Scott and Seymour conjectured the existence of a function f ⁣:NNf \colon \mathbb{N} \to \mathbb{N} such that, for every graph GG and tournament TT on the same vertex set, χ(G)f(k)\chi(G) \geqslant f(k) implies that χ(G[NT+(v)])k\chi(G[N_T^+(v)]) \geqslant k for some vertex vv. In this note we disprove this conjecture even if vv is replaced by a vertex set of size O(logV(G))\mathcal{O}(\log{\lvert V(G)\rvert}). As a consequence, we answer in the negative a question of Harutyunyan, Le, Thomass\'{e}, and Wu concerning the corresponding statement where the graph GG is replaced by another tournament, and disprove a related conjecture of Nguyen, Scott, and Seymour. We also show that the setting where chromatic number is replaced by degeneracy exhibits a quite different behaviour.

Keywords

Cite

@article{arxiv.2305.15585,
  title  = {Chromatic number is not tournament-local},
  author = {António Girão and Kevin Hendrey and Freddie Illingworth and Florian Lehner and Lukas Michel and Michael Savery and Raphael Steiner},
  journal= {arXiv preprint arXiv:2305.15585},
  year   = {2023}
}

Comments

7 pages; funding information added

R2 v1 2026-06-28T10:45:18.617Z