Ordering digraphs with maximum outdegrees by their $A_{\alpha}$ spectral radius

Abstract

Let $G$ be a strongly connected digraph with $n$ vertices and $m$ arcs. For any real $\alpha\in[0,1]$, the $A_\alpha$ matrix of a digraph $G$ is defined as $$A_\alpha(G)=\alpha D(G)+(1-\alpha)A(G),$$ where $A(G)$ is the adjacency matrix of $G$ and $D(G)$ is the outdegrees diagonal matrix of $G$. The eigenvalue of $A_\alpha(G)$ with the largest modulus is called the $A_\alpha$ spectral radius of $G$, denoted by $\lambda_{\alpha}(G)$. In this paper, we first obtain an upper bound on $\lambda_{\alpha}(G)$ for $\alpha\in[\frac{1}{2},1)$. Employing this upper bound, we prove that for two strongly connected digraphs $G_1$ and $G_2$ with $n\ge4$ vertices and $m$ arcs, and $\alpha\in [\frac{1}{\sqrt{2}},1)$, if the maximum outdegree $\Delta^+(G_1)\ge 2\alpha(1-\alpha)(m-n+1)+2\alpha$ and $\Delta^+(G_1)>\Delta^+(G_2)$, then $\lambda_\alpha(G_1)>\lambda_\alpha(G_2)$. Moreover, We also give another upper bound on $\lambda_{\alpha}(G)$ for $\alpha\in[\frac{1}{2},1)$. Employing this upper bound, we prove that for two strongly connected digraphs with $m$ arcs, and $\alpha\in[\frac{1}{2},1)$, if the maximum outdegree $\Delta^+(G_1)>\frac{2m}{3}+1$ and $\Delta^+(G_1)>\Delta^+(G_2)$, then $\lambda_\alpha(G_1)+\frac{1}{4}>\lambda_\alpha(G_2)$.