English

Perfect divisions in ($P_2 \cup P_4$, bull)-free graphs

Combinatorics 2025-08-12 v2

Abstract

A graph GG has a perfect division if its vertex set can be partitioned into two sets AA, BB such that G[A]G[A] is perfect and ω(G[B])<ω(G)\omega(G[B]) < \omega(G). We call GG perfectly divisible if every induced subgraph of GG admits a perfect division. We prove that every (P2P4P_2 \cup P_4, bull)-free graph GG with ω(G)3\omega(G) \geq 3 has a perfect division if GG contains no homogeneous set. The clique-number condition is tight: a counterexample exists for ω(G)=2\omega(G) = 2. Additionally, we present a short proof of the perfect divisibility of (P5P_5, bull)-free graphs, originally established by Chudnovsky and Sivaraman [J. Graph Theory 90 (2019), 54-60.].

Keywords

Cite

@article{arxiv.2507.18506,
  title  = {Perfect divisions in ($P_2 \cup P_4$, bull)-free graphs},
  author = {Lizhong Chen and Hongyang Wang},
  journal= {arXiv preprint arXiv:2507.18506},
  year   = {2025}
}