1. Department of Physics, East China Normal University, Shanghai 200062, P. R. China
2. Center for Network Science, Central European University, Nador u. 9, 1051 Budapest, Hungary
3. Web Science Center, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China
4. Department of Mathematics, Kyungpook National University, Daegu 702-701, South Korea