English

Odd clique minors in graphs with independence number two

Combinatorics 2025-05-16 v2

Abstract

A KtK_t-expansion consists of tt vertex-disjoint trees, every two of which are joined by an edge. We call such an expansion odd if its vertices can be two-colored so that the edges of the trees are bichromatic but the edges between trees are monochromatic. A graph contains an odd KtK_t minor or an odd clique minor of order tt if it contains an odd KtK_t-expansion. Gerards and Seymour from 1995 conjectured that every graph GG contains an odd Kχ(G)K_{\chi(G)} minor, where χ(G)\chi(G) denotes the chromatic number of GG. This conjecture is referred to as ``Odd Hadwiger's Conjecture". Let α(G)\alpha(G) denote the independence number of a graph GG. In this paper we investigate the Odd Hadwiger's Conjecture for graphs GG with α(G)2\alpha(G)\le2. We first observe that a graph GG on nn vertices with α(G)2\alpha(G)\le2 contains an odd Kχ(G)K_{\chi(G)} minor if and only if GG contains an odd clique minor of order n/2\lceil n/2\rceil. We then prove that every graph GG on nn vertices with α(G)2\alpha(G)\le 2 contains an odd clique minor of order n/2\lceil n/2\rceil if GG contains a clique of order n/4n/4 when nn is even and (n+3)/4(n+3)/4 when nn is odd, or GG does not contain HH as an induced subgraph, where α(H)2\alpha(H)\le 2 and HH is an induced subgraph of K1+P4K_1 + P_4, K2+(K1K3)K_2+(K_1\cup K_3), K1+(K1K4)K_1+(K_1\cup K_4), K7K_7^-, K7K_7, or the kite graph.

Keywords

Cite

@article{arxiv.2505.07727,
  title  = {Odd clique minors in graphs with independence number two},
  author = {Yuqing Ji and Zi-Xia Song and Evan Weiss and Xia Zhang},
  journal= {arXiv preprint arXiv:2505.07727},
  year   = {2025}
}