1. Department of Mechanics; Nanjing University of Aeronautics and Astronautics; Nanjing 210016 PR China
2. School of Mathematics and Physics; Anhui University of Technology; Ma'anshan 243002 PR China
3. Department of Mathematics; Nanjing University of Aeronautics and Astronautics; Nanjing 210016 PR China
4. College of Mathematics and Systems Science; Shandong University of Science and Technology; Qingdao 266590 PR China