following PðkÞkg for large k, the degree exponent g is scattered between 2.1 and 3. These results first offered the evidence that some large networks can self-organize into a stationary scale-free state. This is a significant discovery in complex networks. The power-law degree distribution is a heavy-tailed distribution. Consequently, scale-free networks are inhomogeneous, leading over time to a few nodes that have large connectivity, becoming hubs in the networks, but
most others have much less connections. Then, scale-free networks can exhibit robustness and fragility [9].