Energy, Laplacian energy of double graphs and new families of equienergetic graphs

Abstract: For a graph $G$ with vertex set $V(G)={v_1, v_2, \cdots, v_n}$, the extended double cover $G^*$ is a bipartite graph with bipartition (X, Y), $X={x_1, x_2, \cdots, x_n}$ and $Y={y_1, y_2, \cdots, y_n}$, where two vertices $x_i$ and $y_j$ are adjacent if and only if $i=j$ or $v_i$ adjacent to $v_j$ in $G$. The double graph $D[G]$ of $G$ is a graph obtained by taking two copies of $G$ and joining each vertex in one copy with the neighbours of corresponding vertex in another copy. In this paper we study energy and Laplacian energy of the graphs $G^*$ and $D[G]$, $L$-spectra of $G^{k*}$ the $k$-th iterated extended double cover of $G$. We obtain a formula for the number of spanning trees of $G^*$. We also obtain some new families of equienergetic and $L$-equienergetic graphs.