Institution: | 1. College of Mathematics and Statistics , Chongqing University , Chongqing , 401331 , PR China;2. College of Mathematics and Computer Sciences , Yangtze Normal University , Fuling , 408100 , Chongqing , PR China kaikai1981@yahoo.com.cn;4. College of Mathematics , Yibin College , Yibin , 644000 , Sichuan;5. College of Mathematics and Statistics , Chongqing University , Chongqing , 401331 , PR China |