A signless Laplacian spectral Erdös-Stone-Simonovits theorem

Abstract: The celebrated Erd\H{o}s--Stone--Simonovits theorem states that $\mathrm{ex}(n,F)= \big(1-\frac{1}{\chi(F)-1}+o(1) \big)\frac{n^{2}}{2}$, where $\chi(F)$ is the chromatic number of $F$. In 2009, Nikiforov proved a spectral extension of the Erd\H{o}s--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 Erd\H{o}s--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 $\chi(F)\geq 3$, then $\mathrm{ex}{q}(n,F)=\big(1-\frac{1}{\chi(F)-1}+o(1) \big)2n$. 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 Erd\H{o}s, Stone and Simonovits as well as the spectral result of Nikiforov. Our result indicates that the Erd\H{o}s--Stone--Simonovits type result regarding the signless Laplacian spectral radius is valid in general.