Let $G$ be a digraph with adjacency matrix $A(G)$. Let $D(G)$ be the diagonal matrix with outdegrees of vertices of $G$. Nikiforov \cite{ Niki} proposed to study the convex combinations of the adjacency matrix and diagonal matrix of the degrees of undirected graphs. Liu et al. \cite{ LWCL} extended the definition to digraphs. For any real $\alpha\in[0,1]$, the matrix $A_\alpha(G)$ of a digraph $G$ is defined 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 $G$, denoted by $\lambda_\alpha(G)$. This paper proves some extremal results about the spectral radius $\lambda_\alpha(G)$ that generalize previous results about $\lambda_0(G)$ and $\lambda_{ \frac{ 1}{ 2}}(G)$. In particular, we characterize the extremal digraph with the maximum (or minimum) $A_\alpha$ spectral radius among all $\widetilde{ \infty}$-digraphs and $\widetilde{ \theta}$-digraphs on $n$ vertices. Furthermore, we determine the digraphs with the second and the third minimum $A_\alpha$ spectral radius among all strongly connected bicyclic digraphs. For $0\leq\alpha\leq\frac{ 1}{ 2}$, we also determine the digraphs with the second, the third and the fourth minimum $A_\alpha$ spectral radius among all strongly connected digraphs on $n$ vertices. Finally, we characterize the digraph with the minimum $A_\alpha$ spectral radius among all strongly connected bipartite digraphs which contain a complete bipartite subdigraph.
Categories: math.CO
