1. Introduction
Complex networks describe a wide range of systems in nature and society. For example, the World Wide Web (WWW), Internet, genetic networks, ecological networks, citation networks and social networks, etc. Complex networks play an important role in the understanding of many systems. Researchers are interested in unravelling the structure and dynamics of complex networks [1–3]. Erdo¨s and Re´nyi [4] studied the evolution of random networks and indicated the
theory of random graphs. Watts and Strogatz [5] proposed a small-world network, in which the average path length is surprisingly small, leading to a small-world character. A common feature of the ER and WS models is that the degree
distribution PðkÞ decreases exponentially with k. Consequently, the topological
structure of the networks is homogeneous, and all nodes have approximately the
same number of links.