1. College of Information Systems and Management, National University of Defense Technology, Changsha 410073, P. R. China
2. School of Economics and Business Administration, Beijing Normal University, Beijing 100875, P. R. China
3. School of Computer, National University of Defense Technology, Changsha 410073, P. R. China
4. Department of Electrical and Computer Engineering, Michigan Technological University, Houghton, Michigan 49931, USA