1. Department of Mathematics, Beijing Jiaotong University, Beijing 100044, P. R. China
2. Department of MEEM, City University of Hong Kong, Kowloon, Hong Kong SAR, China
3. School of Mechanical and Electrical Engineering, Central South University, 410083, P. R. China
4. Lab of CompSys, Institute of Automation, CAS, Beijing 100190, P. R. China