Let be the normalized Laplacian eigenvalues of a graph G with n vertices. Also, let χ and α be the chromatic number and the independence number of a graph G, respectively. In this paper, we discuss some properties of graphs with . In particular, we characterize all the graphs with when the maximum degree is n−1. Moreover, we obtain an upper bound on the multiplicity of normalized Laplacian eigenvalues in terms of n and α, and also characterize graphs for which the bound is attained.

Identifying influential spreaders in complex networks is a crucial issue that can help control the propagation process in complex networks. An aviation network is a typical complex network, and accurately identifying the key city nodes in the aviation network can help us better prevent network attacks and control the spread of diseases. In this paper, a method for identifying key nodes in undirected weighted networks, called weighted Laplacian energy centrality, was proposed and applied to an aviation network constructed from real flight data. Based on the analysis of the topological structure of the network, the paper recognized critical cities in this network, then simulation experiments were conducted on key city nodes from the perspectives of network dynamics and robustness. The results indicated that, compared with other methods, weighted Laplacian energy centrality can identify the city nodes with the most spreading influence in the network. From the perspective of network robustness, the identified key nodes also have the characteristics of accurately and quickly destroying network robustness.

Let $G$ be a graph with vertex set $V(G) = \{v_1, v_2, \ldots, v_n\}$ and edge set $E(G)$. For any vertex $v_i \in V(G)$, let $d_i$ denote the degree of $v_i$. The normalized Laplacian matrix of the graph $G$ is the matrix $\mathcal{L} = (\mathcal{L}_{ij})$ given by\[\mathcal{L}_{ij} =\begin{cases}1 &\textrm{if $i = j$ and $d_{i} \neq 0$} \\-\frac{1}{\sqrt{d_{i} \,d_{j}}} &\textrm{if $v_i v_j \in E(G)$} \\0 &\textrm{otherwise}.\end{cases}\] In this paper, we obtain some bounds on the second smallest normalized Laplacian eigenvalue of tree $T$ in terms of graph parameters and characterize the extremal trees. Utilizing these results we present some lower bounds on the normalized Laplacian energy (or Randić energy) of tree $T$ and characterize trees for which the bound is attained.