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arXiv · 2602.04403

The optimal chromatic bound for even-hole-free graphs without induced seven-vertex paths

Shenwei Huang·Yidong Zhou·Yeonsu Chang
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Abstract

The class of even-hole-free graphs has been extensively studied on its own and on its relation to perfect graphs. In this paper, we study the $\chi$-boundedness of even-hole-free graphs which itself is an important topic in graph theory. In particular, we prove that every even-hole-free graph $G$ without induced 7-vertex paths satisfies $\chi(G)\le \lceil\frac{5}{4}\omega(G)\rceil$, where $\chi(G)$ and $\omega(G)$ denote the chromatic number and clique number of $G$, respectively. This bound is optimal. Our result strictly extends the result of Karthick and Maffary \cite{KM19} on even-hole-free graphs without induced 6-vertex paths, and implies that even-hole-free graphs without induced 7-vertex paths satisfy Reed's Conjecture. Our proof relies on a heavy structural analysis on a maximal substructure called a nice blowup of a five-cycle and can be viewed for graphs in which all holes are of length five (graphs with all holes having the same length gain increasing interest in recent years \cite{COOK202496}). Our result gives a partial answer to a conjecture of Wang and Wu \cite{WW25} on graphs in which all holes are of length 5. One of the key technical ingredients is a technical lemma proved via clique cutset argument combined with the idea of Infinite Descent Method (often used in number theory).

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Shenwei Huang, Yidong Zhou, Yeonsu Chang. 2026-02-04. The optimal chromatic bound for even-hole-free graphs without induced seven-vertex paths. https://arxiv.org/abs/2602.04403

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