On the construction of l-equienergetic graphs

Abstract

For a graph with $n$ vertices and $m$ edges, having Laplacian spectrum $\mu_1, \mu_2, \cdots,\mu_n$ and signless Laplacian spectrum $\mu^+_1,\mu^+_2, \cdots,\mu^+_n$, the Laplacian energy and signless Laplacian energy of $G$ are respectively, defined as $LE(G)=\sum_{i=1}^{n}|\mu_i-\frac{2m}{n}|$ and $LE^+(G)=\sum_{i=1}^{n}|\mu^+_i-\frac{2m}{n}|$. Two graphs $G_1$ and $G_2$ of same order are said to be $L$-equienergetic if $LE(G_1)=LE(G_2)$ and $Q$-equienergetic if $LE^{+}(G_1)=LE^{+}(G_2)$. The problem of constructing graphs having same Laplacian energy has been considered by Stevanovic for threshold graphs and by Liu and Liu for those graphs whose order is $n\equiv 0$ (mod 7). In general the problem of constructing $L$-equienergetic graphs from any pair of given graphs is still not solved, and this work is an attempt in that direction. We construct sequences of non-cospectral (Laplacian, signless Laplacian) $L$-equienergetic and $Q$-equienergetic graphs from any pair of graphs having same number of vertices and edges.