On the double covers of a line graph

Abstract

Let $L(X)$ be the line graph of graph $X$. Let $X^{\prime \prime }$ be the Kronecker product of $X$ by $K_2$. In this paper, we see that $L(X^{\prime \prime })$ is a double cover of $L(X)$. We define the symmetric edge graph of $X$, denoted as $\gamma (X)$ which is also a double cover of $L(X)$. We study various properties of $\gamma (X)$ in relation to $X$ and the relationship amongst the three double covers of $L(X)$ that are $L(X^{\prime \prime }),\gamma (X)$ and $L(X{)}^{\prime \prime }$. With the help of these double covers, we show that for any integer $k\ge 5$, there exist two equienergetic graphs of order $2k$ that are not cospectral.

17 pages