1. School of Mathematical Science, Huaiyin Normal University, Huaian, Jiangsu 223300, P. R. China
2. Department of Mathematics, University of Miami, Coral Gables, FL 33124-4250, USA
3. Department of Mathematics, Harbin Institute of Technology, Harbin, Heilongjiang 150001, P. R. China
4. School of Mathematics and Information Science, Henan University, Kaifeng, Henan 475001, P. R. China