1. Center for Mathematical Sciences & School of Mathematics and Statistics, Huazhong University of Science and Technology, Wuhan 430074, China
2. College of Mathematics and Statistics, Chongqing University, Chongqing 401331, China
3. Wuhan National Laboratory for Optoelectronics, Wuhan 430074, China
4. Department of Applied Mathematics, Illinois Institute of Technology, Chicago, Illinois 60616, USA