Spectral Tur\'an-type problem in non-$r$-partite graphs: Forbidden generalized book graph $B_{r,k}$

TL;DR

This paper determines the unique extremal graphs with maximum spectral radius among non-r-partite graphs avoiding a generalized book graph $B_{r,k}$, extending previous results to broader parameters using spectral stability and structural analysis.

Contribution

It generalizes the characterization of extremal graphs avoiding $B_{r,k}$ for $r eq 2$, providing a solution to an open problem in spectral graph theory.

Findings

Identifies the unique extremal graph in $ ext{SPEX}_{r + 1}(n, B_{r,k})$ for large $n$

Extends spectral Turán-type results to non-$r$-partite graphs with forbidden generalized book subgraphs

Utilizes spectral stability, local structure, and characteristic equations in the proof

Abstract

Given a graph $H$, a graph is said to be $H$-free if it does not contain $H$ as a subgraph. A graph is color-critical when it has an edge whose removal leads to a reduction in its chromatic number. For a graph $H$ with a chromatic number of \(r + 1\), we use \(\text{spex}_{r + 1}(n, H)\) to represent the maximum spectral radius among non-$r$-partite $H$-free graphs of order $n$. The set of all non-$r$-partite $H$-free graphs of order $n$ that have a spectral radius of \(\text{spex}_{r + 1}(n, H)\) is denoted as \(\text{SPEX}_{r + 1}(n, H)\). For \(r\geq2\) and \(k\geq1\), we define \(B_{r,k}\) as the graph constructed by connecting each vertex of \(K_r\) to every vertex of an independent set with size $k$. We refer to \(B_{r,k}\) as a book graph (in the case of \(r = 2\)) or a generalized book graph (when \(r\geq3\)). It should be noted that \(B_{r,k}\) is a color-critical graph with a chromatic number of \(r + 1\). Lin, Ning, and Wu (2021) identified the unique extremal graph within \(\text{SPEX}_3(n, B_{2,1})\); Li and Peng (2023) determined the unique extremal graph in \(\text{SPEX}_{r + 1}(n, B_{r,1})\) for all \(r\geq2\). Quite recently, Liu and Miao (2025) specified the unique extremal graph in \(\text{SPEX}_3(n, B_{2,k})\) for all \(k\geq2\). Inspired by these remarkable results, this paper, relying on spectral stability theory, local structure characterization, along with the theory of characteristic equations and Rayleigh quotient equations, aims to determine the unique extremal graph in \(\text{SPEX}_{r + 1}(n, B_{r,k})\) for \(r\geq3\), \(k\geq1\), and sufficiently large $n$. This work partially addresses an open problem put forward in [38].