1. School of Mathematics and Statistics , Guizhou University , Guiyang ; and Department of Mathematics, Zhejiang University, Hangzhou , P. R. China
2. Department of Mathematics , Zhejiang University , Hangzhou , P. R. China
3. College of Science , Nanjing University of Aeronautics and Astronautics , Nanjing , P. R. China
4. School of Mathematics and Statistics , Beijing Institute of Technology , Beijing ; and Shenzhen MSU-BIT University, Shenzhen , P. R. China