1. School of Applied Mathematics, Chengdu University of Information Technology, Chengdu, Sichuan 610225, P. R. China
2. Department of Electrical Engineering, City University of Hong Kong, Kowloon, Hong Kong SAR, P. R. China
3. School of Mathematical Science, Huaqiao University, Quanzhou, Fujian 362021, P. R. China
4. Department of Mathematics, Zhejiang Normal University, Jinhua, Zhejing 321004, P. R. China