More on the normalized Laplacian Estrada index

Abstract

Let $G$ be a simple graph of order $N$. The normalized Laplacian Estrada index of $G$ is defined as $NEE(G)=\sum_{i=1}^Ne^{\lambda_i}$, where $\lambda_1,\lambda_2,\cdots,\lambda_N$ are the normalized Laplacian eigenvalues of $G$. In this paper, we give a tight lower bound for $NEE$ of general graphs. We also calculate $NEE$ for a class of treelike fractals, which contain some classical chemical trees as special cases. It is shown that $NEE$ scales linearly with the order of the fractal, in line with a best possible lower bound for connected bipartite graphs.