1. School of Sciences, Henan University of Technology, Zhengzhou, Henan 450001, P.R. China
2. College of Mathematics and Informatics and Center for Applied Mathematics of Fujian Province, Fujian Normal University, Fuzhou, Fujian 350117, P.R. China
3. School of Mathematical Sciences, Xiamen University, Xiamen, Fujian 361005, P.R. China
4. School of Mathmatical Sciences, Shanxi University, Taiyuan, Shanxi 030006, P.R. China