The Signless Laplacian Spectral Radius of $t{K}_3$-Free Graphs

Abstract

The signless Laplacian matrix of a graph $G$ is $Q(G)=D(G)+A(G)$, where $D(G)$ and $A(G)$ are the diagonal degree matrix and the adjacency matrix of $G$, respectively. The signless Laplacian spectral radius of $G$ is the largest eigenvalue of $Q(G)$. For a positive integer $t$, a graph is called $t{K}_3$-free if it contains no $t$ vertex-disjoint triangles. In this paper, for every fixed $t\ge 2$ and all $n\ge 28t-17$, we determine the unique graph achieving the maximum signless Laplacian spectral radius among all $t{K}_3$-free graphs of order $n$.

16 pages