On the $A_{\alpha}$-spectra of graphs

TL;DR

This paper investigates the eigenvalues of the matrix $A_{\alpha}(G)$, a convex combination of the degree matrix and adjacency matrix, providing bounds and characterizations for graphs with certain properties.

Contribution

It offers new bounds on the eigenvalues of $A_{\alpha}(G)$ for $\alpha > 1/2$, including extremal graph characterizations and eigenvalue inequalities.

Findings

Eigenvalue monotonicity when adding edges for $\alpha > 1/2$

Upper bounds on $\lambda_k(A_{\alpha}(G))$ for $2 \leq k \leq n$

Lower bound on $\lambda_n(A_{\alpha}(G))$ for graphs without isolated vertices

Abstract

Let $G$ be a graph with adjacency matrix $A(G)$ and let $D(G)$ be the diagonal matrix of the degrees of $G$. For any real $\alpha\in [0,1]$, Nikiforov \cite{VN1} defined the matrix $A_{\alpha}(G)$ as $$A_{\alpha}(G)=\alpha D(G)+(1-\alpha)A(G).$$ In this paper, we give some results on the eigenvalues of $A_{\alpha}(G)$ with $\alpha>1/2$. In particular, we show that for each $e\notin E(G)$, $\lambda_i(A_{\alpha}(G+e))\geq\lambda_i(A_{\alpha}(G))$. By utilizing the result, we prove have $\lambda_k(A_{\alpha}(G))\leq\alpha n-1$ for $2\leq k\leq n$. Moreover, we characterize the extremal graphs with equality holding. Finally, we show that $\lambda_n(A_{\alpha}({G}))\geq 2\alpha-1$ if $G$ contains no isolated vertices.