1. School of Mathematics and Statistics, Southwest University, Chongqing 400715, P. R. China
2. College of Mathematics and Statistics, Chongqing University, Chongqing 401331, P. R. China
3. College of Science, Chongqing University of Posts and Telecommunications, Chongqing 400065, P. R. China
4. College of Mathematics and Statistics, Yunnan University, Kunming 650091, P. R. China