1. Institute of Systems Science, Academy of Mathematics and System Sciences, Chinese Academy of Sciences, Beijing 100080, P. R. China
2. Department of Mathematical Sciences, Tsinghua University, Beijing 100084, P. R. China
3. Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, P. R. China
4. Institute of Applied Mathematics, Academy of Mathematics and System Sciences, Chinese Academy of Sciences, Beijing 100080, P. R. China