The distance spectrum of the bipartite double cover of strongly regular graphs

Abstract

A strongly regular graph with parameters $(n,d,a,c)$ is a $d$-regular graph of order $n$, in which every pair of adjacent vertices has exactly $a$ common neighbor(s) and every pair of nonadjacent vertices has exactly $c$ common neighbor(s). Let $n$ be the number of vertices of the graph $G=(V,E)$. The distance matrix $D=D(G)$ of $G$ is an $n \times n $ matrix with the rows and columns indexed by $V$ such that $D_{uv} = d_{G}(u, v)=d(u,v)$, where $d_{G}(u, v)$ is the distance between the vertices $u$ and $v$ in the graph $G$. In this paper, we are interested in determining the distance spectrum of the bipartite double cover of the family of strongly regular graphs. In other words, let $G=(V,E)$ be a strongly regular graph with parameters $(n,k,a,c)$. We show that there is a close relationship between the spectrum of $G$ and the distance spectrum of $B(G)$, where $B(G)$ is the double cover of $G$. We explicitly determine the distance spectrum of the graph $B(G)$, according to the spectrum of $G$. In fact, according to the parameters of the graph $G$.