English

($P_2+P_4$, $K_4-e$)-free graphs are nearly $\omega$-colorable

Combinatorics 2025-08-08 v3 Discrete Mathematics

Abstract

For a graph GG, χ(G)\chi(G) and ω(G)\omega(G) respectively denote the chromatic number and clique number of GG. In this paper, we show the following results: (i) If GG is a (P2+P4P_2+P_4, K4eK_4-e)-free graph with ω(G)3\omega(G)\geq 3, then χ(G)max{6,ω(G)}\chi(G)\leq \max\{6, \omega(G)\}, and the bound is tight for each ω(G){4,5}\omega(G)\notin \{4,5\}. (ii) If GG is a (P2+P4P_2+P_4, K4eK_4-e)-free graph with ω(G)=4\omega(G)= 4, then χ(G)=4\chi(G)= 4. These results extend the chromatic bounds known for the class of (P2+P2P_2+P_2, K4eK_4-e)-free graphs and for the class of (P2+P3P_2+P_3, K4eK_4-e)-free graphs, improve the bound of Chen and Zhang [arXiv:2412.14524 [math.CO], 2024] given for the class of (P2+P4P_2+P_4, K4eK_4-e)-free graphs, partially answer a question of Ju and the third author [Theor. Comp. Sci. 993 (2024) Article No.: 114465] on `near optimal colorable graphs', and a question of Schiermeyer (unpublished) on the chromatic bound for (P7P_7, K4eK_4-e)-free graphs.

Keywords

Cite

@article{arxiv.2501.02543,
  title  = {($P_2+P_4$, $K_4-e$)-free graphs are nearly $\omega$-colorable},
  author = {C. U. Angeliya and T. Karthick and Shenwei Huang},
  journal= {arXiv preprint arXiv:2501.02543},
  year   = {2025}
}

Comments

Revised version includes the new result for the case Omega(G)=4