The relationship between the negative inertia index of graph $G$ and its girth $g$ and diameter $d$

Abstract

Let $G$ be a simple connected graph. We use $n(G)$, $p(G)$, and $\eta (G)$ to denote the number of negative eigenvalues, positive eigenvalues, and zero eigenvalues of the adjacency matrix $A(G)$ of $G$, respectively. In this paper, we prove that $2n(G)\ge d(G)+1$ when $d(G)$ is odd, and $n(G)\ge \lceil \frac{g}{2}\rceil -1$ for a graph containing cycles, where $d(G)$ and $g$ are the diameter and girth of the graph $G$, respectively. Furthermore, we characterize the extremal graphs for the cases of $2n(G)=d(G)+1$, $n(G)=\lceil \frac{g}{2}\rceil$, and $n(G)=\lceil \frac{g}{2}\rceil -1$.