1. Department of Mathematics, College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China
2. School of Mathematical Sciences, Anhui University, Hefei 230601, P. R. China
3. School of Mathematics and Physics, Jiangsu University of Technology, Changzhou 213001, P. R. China
4. School of Science, Jiujiang University, Jiujiang 332005, P. R. China