1. Department of Mechanics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China
2. Faculty of Mathematics and Physics, Huaiyin Institute of Technology, Huaian 223003, P. R. China
3. Department of Mathematics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China
4. College of Mathematics and Systems Science, Shandong University of Science and Technology, Qingdao 266590, P. R. China