On the signless Laplacian spectral radius of $K_{s,t}$-minor free graphs

Abstract

In this paper, we prove that if $G$ is a $K_{2,t}$-minor free graph of order $n\geq t^2+4t+1$ with $t\geq 3$, the signless Laplacian spectral radius $q(G)\leq \frac{1}{2}(n+2t-2+\sqrt{(n-2t+2)^2+8t-8}\ )$ with equality if and only if $n\equiv 1~(\mathrm{mod}~t)$ and $G=F_{2,t}(n)$, where $F_{s,t}(n):=K_{s-1}\vee (p\cdot K_t\cup K_r)$ for $n-s+1=pt+r$ and $0\leq r<t$. In particular, if $t=3$ and $n\geq 22$, then $F_{2,3}(n)$ is the unique $K_{2,3}$-minor free graph of order $n$ with the maximum signless Laplacian spectral radius. In addition, $F_{3,3}(n)$ is the unique extremal graph with the maximum signless Laplacian spectral radius among all $K_{3,3}$-minor free graphs of order $n\ge 1186$.