1. Department of Mathematics, Southeast University, Nanjing 210096, P. R. China
2. College of Information and Electrical Engineering, Shandong University of Science and Technology, Qingdao 266510, P. R. China
3. Department of Mechanical Engineering, University of Hong Kong, Hong Kong
4. Department of Automation, Nanjing University of Science and Technology, Nanjing 210094, P. R. China