1. Associate Professor Department of Mathematics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China e-mail:
2. Professor Department of Mathematics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China e-mail:
3. Department of Mechanics, Tianjin University, Tianjin 300072, China e-mail: