Merging the A- and Q-spectral theories for digraphs

Abstract: Let $G$ be a digraph and $A(G)$ be the adjacency matrix of $G$. Let $D(G)$ be the diagonal matrix with outdegrees of vertices of $G$. For any real $\alpha\in[0,1]$, Liu et al. \cite{LWCL} defined the matrix $A_\alpha(G)$ as $$A_\alpha(G)=\alpha D(G)+(1-\alpha)A(G).$$ The largest modulus of the eigenvalues of $A_\alpha(G)$ is called the $A_\alpha$ spectral radius of $G$. In this paper, we determine the digraphs which attain the maximum (or minimum) $A_\alpha$ spectral radius among all strongly connected digraphs with given parameters such as girth, clique number, vertex connectivity or arc connectivity. We also discuss a number of open problems.