Spectral Tur\'{a}n problem of non-bipartite graphs: Forbidden books

TL;DR

This paper investigates the maximum spectral radius of non-bipartite graphs that do not contain a book graph $B_{r+1}$, identifying extremal graphs for different values of $r$ using novel spectral techniques.

Contribution

It introduces a new spectral extremal approach and characterizes the extremal non-bipartite $B_{r+1}$-free graphs, revealing different extremal structures for $r=0$ and $r eq0$.

Findings

Spectral radius of non-bipartite $B_{r+1}$-free graphs is maximized by a specific constructed graph.

Extremal graphs differ significantly between the cases $r=0$ and $r eq0$.

The paper establishes a spectral Turán-type theorem for these graphs.

Abstract

A book graph $B_{r+1}$ is a set of $r+1$ triangles with a common edge, where $r\geq0$ is an integer. Zhai and Lin [J. Graph Theory 102 (2023) 502-520] proved that for $n\geq\frac{13}{2}r$, if $G$ is a $B_{r+1}$-free graph of order $n$, then $\rho(G)\leq\rho(T_{n,2})$, with equality if and only if $G\cong T_{n,2}$. Note that the extremal graph $T_{n,2}$ is bipartite. Motivated by the above elegant result, we investigate the spectral Tur\'{a}n problem of non-bipartite $B_{r+1}$-free graphs of order $n$. For general $r\geq1$, let $K_{\lfloor\frac{n-1}{2}\rfloor,\lceil\frac{n-1}{2}\rceil}^{r, r}$ be the graph obtained from $K_{\lceil\frac{n-1}{2}\rceil,\lfloor\frac{n-1}{2}\rfloor}$ by adding a new vertex $v_{0}$ such that $v_{0}$ has exactly $r$ neighbours in each part of $K_{\lceil\frac{n-1}{2}\rceil,\lfloor\frac{n-1}{2}\rfloor}$. By adopting a different technique named the residual index, Chv\'{a}tal-Hanson theorem and typical spectral extremal methods, we in this paper prove that: If $G$ is a non-bipartite $B_{r+1}$-free graph of order $n$, then $\rho(G)\leq\rho\Big(K_{\lfloor\frac{n-1}{2}\rfloor,\lceil\frac{n-1}{2}\rceil}^{r, r}\Big)$ , with equality if and only if $G\cong K_{\lfloor\frac{n-1}{2}\rfloor,\lceil\frac{n-1}{2}\rceil}^{r, r}$. An interesting phenomenon is that the spectral extremal graphs are completely different for $r=0$ and general $r\geq1$.