1. Department of Mathematics, Hangzhou Normal University, Hangzhou 311121, P. R. China
2. Department of Mathematics, Zhejiang Normal University, Jinhua, Zhejing 321004, P. R. China
3. School of Mathematical Science, Huaqiao University, Quanzhou, Fujian 362021, P. R. China
4. Department of Electrical Engineering, City University of Hong Kong, Hong Kong SAR, P. R. China