Localized Turán-type inequalities for $Q$-index

Abstract

For a connected graph \(G\), let $q(G)$ denote the $Q$-index of $G$, i.e., the largest eigenvalue of its signless Laplacian matrix. Abreu and Nikiforov (2013) showed that \[ q(G) \leq 2n\left(1-\frac{1}{ω(G)}\right), \] where $ω(G)$ denotes the clique number of $G$. We first give a short algebraic proof of this result. For a vertex $v\in V(G)$, let \(c(v)\) denote the order of the largest clique of \(G\) containing \(v\). Our main result is the following vertex localized bound that refines the result of Abreu and Nikiforov: \[ q(G) \leq 2\sum_{v\in V(G)}\left(1-\frac{1}{c(v)}\right). \] Equality holds precisely for complete bipartite graphs when \(ω(G)=2\), and for regular complete \(ω(G)\)-partite graphs when \(ω(G)\geq 3\). As a consequence, we also obtain an analogous localized inequality for the $A_α$-matrix of $G$. Finally, we generalize the above localized inequality to vertex-weighted signed graphs. This contributes to the localization program for spectral Turán-type results.