1. Institute of Optoelectronic Technology, and Department of Photoelectric Engineering, Lishui University, Lishui 323000, P. R. China
2. Institute of Nonlinear Analysis and Department of Mathematics, Lishui University, Lishui 323000, P. R. China
3. Department of Mathematics, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China