The signless Laplacian spectral radius of subgraphs of regular graphs

Abstract

Let $q(H)$ be the signless Laplacian spectral radius of a graph $H$. In this paper, we prove that \\1. Let $H$ be a proper subgraph of a $\Delta$-regular graph $G$ with $n$ vertices and diameter $D$. Then $$2\Delta - q(H)>\frac{1}{n(D-\frac{1}{4})}.$$ \\2. Let $H$ be a proper subgraph of a $k$-connected $\Delta$-regular graph $G$ with $n$ vertices, where $k\geq 2$. Then $$2\Delta-q(H)>\frac{2(k-1)^{2}}{2(n-\Delta)(n-\Delta+2k-4)+(n+1)(k-1)^{2}}.$$ Finally, we compare the two bounds. We obtain that when $k>2\sqrt{\frac{(n-\Delta)(n+\Delta-4)}{n(4D-3)-2}}+1$, the second bound is always better than the first. On the other hand, when $k<\frac{2(n-\Delta)}{\sqrt{n(4D-3)-2}}+1$, the first bound is always better than the second.