A signless Laplacian spectral Erdös-Stone-Simonovits theorem

Abstract

The celebrated Erdős--Stone--Simonovits theorem states that $\mathrm{ex}(n,F)=(1-\frac{1}{\chi (F)-1}+o(1))\frac{n^2}{2}$, where $\chi (F)$ is the chromatic number of $F$. In 2009, Nikiforov proved a spectral extension of the Erdős--Stone--Simonovits theorem in terms of the adjacency spectral radius. In this paper, we shall establish a unified extension in terms of the signless Laplacian spectral radius. Let $q(G)$ be the signless Laplacian spectral radius of $G$ and we denote $\mathrm{ex}_{q}(n,F) =\max \{q(G):|G|=n ~\mbox{and}~F\nsubseteq G\}$. It is known that the Erdős--Stone--Simonovits type result for the signless Laplacian spectral radius does not hold for even cycles. We prove that if $F$ is a graph with $\chi (F)\ge 3$, then ${\mathrm{ex}}_q(n,F)=(1-\frac{1}{\chi (F)-1}+o(1))2n$. This solves a problem proposed by Li, Liu and Feng (2022), which gives an entirely satisfactory answer to the problem of estimating ${\mathrm{ex}}_q(n,F)$. Furthermore, it extends the aforementioned result of Erdős, Stone and Simonovits as well as the spectral result of Nikiforov. Our result indicates that the Erdős--Stone--Simonovits type result regarding the signless Laplacian spectral radius is valid in general.