On the $A_\alpha$ spectral radius of strongly connected digraphs

TL;DR

This paper investigates the extremal properties of the $A_\alpha$ spectral radius in strongly connected digraphs, generalizing previous results and characterizing digraphs with extremal spectral radii for various classes.

Contribution

It extends the study of spectral radii to the $A_\alpha$ matrix for digraphs, providing extremal characterizations for maximum and minimum spectral radii in multiple digraph classes.

Findings

Characterized extremal digraphs with maximum/minimum $A_\alpha$ spectral radius.

Determined second and third minimum spectral radii in strongly connected bicyclic digraphs.

Identified digraphs with specific minimum spectral radii among bipartite strongly connected digraphs.

Abstract

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.