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.