Subdivision-free graphs with the maximum spectral radius

TL;DR

This paper characterizes the structure of graphs with maximum spectral radius that avoid certain subdivisions, showing they contain a specific spanning subgraph formed by a clique joined with an independent set.

Contribution

It extends previous results by providing a structural characterization of spectral extremal graphs avoiding subdivisions of a given family of graphs.

Findings

Graphs in the extremal set contain a spanning subgraph isomorphic to a join of a clique and an independent set.

The characterization applies to graphs avoiding subdivisions of any graph in the family.

The result generalizes recent work on spectral extremal problems for minor-free graphs.

Abstract

Given a graph family $\mathbb{H}$, let ${\rm SPEX}(n,\mathbb{H}_{\rm sub})$ denote the set of $n$-vertex $\mathbb{H}$-subdivision-free graphs with the maximum spectral radius. In this paper, we investigate the problem of graph subdivision from a spectral extremal perspective, with a focus on the structural characterization of graphs in ${\rm SPEX}(n,\mathbb{H}_{\rm sub})$. For any graph $H \in \mathbb{H}$, let $\alpha(H)$ denote its independence number. Define $\gamma_\mathbb{H}:=\min_{H\in \mathbb{H}}\{|H| - \alpha(H) - 1\}$. We prove that every graph in ${\rm SPEX}(n,\mathbb{H}_{\rm sub})$ contains a spanning subgraph isomorphic to $K_{\gamma_\mathbb{H}}\vee (n-\gamma_\mathbb{H})K_1$, which is obtained by joining a $\gamma_\mathbb{H}$-clique with an independent set of $n-\gamma_\mathbb{H}$ vertices. This extends a recent result by Zhai, Fang, and Lin concerning spectral extremal problems for $\mathbb{H}$-minor-free graphs.