More on Nosal's spectral theorem: Books and $4$-cycles

TL;DR

This paper advances spectral graph theory by solving three open problems related to eigenvalues and structural properties, including counts of triangles, cliques, and 4-cycles, confirming conjectures and establishing optimal bounds.

Contribution

It proves new spectral extremal results, confirming conjectures, and provides structural insights into graphs with large spectral radii, extending previous bounds and counting results.

Findings

Graphs with spectral radius > √m contain many triangles sharing an edge.

Graphs with spectral radius > √((1-1/r)2m) contain many K_{r+1} sharing r vertices.

Graphs with spectral radius > √m contain approximately m^2/8 4-cycles, with optimal bounds.

Abstract

Spectral graph theory studies how the eigenvalues of a graph relate to the structural properties of a graph. In this paper, we solve three open problems in spectral extremal graph theory which generalize the classical Tur\'{a}n-type supersaturation results. (a) We prove that every $m$-edge graph $G$ with the spectral radius $\lambda (G) > \sqrt{m}$ contains at least $\frac{1}{144} \sqrt{m}$ triangles sharing a common edge. This result confirms a conjecture of Nikiforov, and Li and Peng. Moreover, the bound is optimal up to a constant factor. (b) Next, for $m$-edge graph $G$ with $\lambda (G) > \sqrt{(1-\frac{1}{r})2m}$, we show that it must contain $\Omega_r (\sqrt{m})$ copies of $K_{r+1}$ sharing $r$ common vertices. This confirms a conjecture of Li, Liu and Feng and unifies a series of spectral extremal results on books and cliques. Moreover, we also show that such a graph $G$ contains $\Omega_r (m^{\frac{r-1}{2}})$ copies of $K_{r+1}$. This extends a result of Ning and Zhai for counting triangles. (c) We prove that every $m$-edge graph $G$ with $\lambda (G) > \sqrt{m}$ contains at least $(\frac{1}{8}-o(1)) m^2$ copies of 4-cycles, and we provide two constructions showing that the constant $\frac{1}{8}$ is the best possible. This result settles a problem raised by Ning and Zhai, and it gives the first asymptotics for counting degenerate bipartite graphs. The key to our proof are two structural results we obtain for graphs with large spectral radii on their maximum degree and on existence of large structured subgraphs, which we believe to be of independent interest.