An arithmetic criterion for graphs being determined by their generalized $A_α$-spectrum (2103.04010v1)

Abstract: Let $G$ be a graph on $n$ vertices, its adjacency matrix and degree diagonal matrix are denoted by $A(G)$ and $D(G)$, respectively. In 2017, Nikiforov \cite{0007} introduced the matrix $A_{\alpha}(G)=\alpha D(G)+(1-\alpha)A(G)$ for $\alpha\in [0, 1].$ The $A_\alpha$-spectrum of a graph $G$ consists of all the eigenvalues (including the multiplicities) of $A_\alpha(G).$ A graph $G$ is said to be determined by the generalized $A_{\alpha}$-spectrum (or, DGA$\alpha$S for short) if whenever $H$ is a graph such that $H$ and $G$ share the same $A{\alpha}$-spectrum and so do their complements, then $H$ is isomorphic to $G$. In this paper, when $\alpha$ is rational, we present a simple arithmetic condition for a graph being DGA$\alpha$S. More precisely, put $A{c_\alpha}:={c_\alpha}A_\alpha(G),$ here ${c_\alpha}$ is the smallest positive integer such that $A_{c_\alpha}$ is an integral matrix. Let $\tilde{W}{{\alpha}}(G)=\left[{\bf 1},\frac{A{c_\alpha}{\bf 1}}{c_\alpha},\ldots, \frac{A_{c_\alpha}^{n-1}{\bf 1}}{c_\alpha}\right]$, where ${\bf 1}$ denotes the all-ones vector. We prove that if $\frac{\det \tilde{W}{{\alpha}}(G)}{2^{\lfloor\frac{n}{2}\rfloor}}$ is an odd and square-free integer and the rank of $\tilde{W}{{\alpha}}(G)$ is full over $\mathbb{F}p$ for each odd prime divisor $p$ of $c\alpha$, then $G$ is DGA$\alpha$S except for even $n$ and odd $c\alpha\,(\geqslant 3)$. By our obtained results in this paper we may deduce the main results in \cite{0005} and \cite{0002}.