1. Department of Mathematics, Zhejiang Normal University, Jinhua 312004, P. R. China
2. Department of Mathematics and Statistics, University of South Florida, Tampa, FL 33620-5700, USA
3. School of Mathematical Science, Xuzhou Normal University, Xuzhou 221116, P. R. China
4. Department of Applied Mathematics, Northwestern Polytechnical University, Xi'an 710072, P. R. China