1. School of Mathematical Sciences , Chongqing Normal University , Chongqing 401331; and School of Mathematics and Statistics, Tianshui Normal University, Tianshui 741001 , P. R. China
2. College of Mathematics and Information , 56714 China West Normal University , Nanchong 637009 ; and Key Laboratory of Optimization Theory and Applications at China West Normal University of Sichuan Province , P. R. China
3. School of Mathematics and Information , 56714 China West Normal University , Nanchong 637009 , P. R. China
4. School of Mathematics and Statistics , [26463]Southwest University , Chongqing 400715; and School of Mathematics and Information Science, North Minzu University, Yinchuan 750021 , P. R. China
5. School of Mathematics and Statistics , [26463]Southwest University , Chongqing 400715 , P. R. China
6. School of Mathematics and Statistics , 118420 Tianshui Normal University , Tianshui 741001 , P. R. China