1. Department of Mathematics, Shanghai Normal University, Shanghai 200234, P. R. China
2. Department of Applied Mathematics, Western University, London, Ontario, N6A 5B7, Canada
3. School of Mathematics Sciences, Qufu Normal University, Qufu 273165, P. R. China
4. School of Mathematical Sciences, Huaqiao University, Quanzhou, Fujian 362021, P. R. China