1. College of Mathematics and Statistics, Yangtze Normal University , Chongqing, P.R. China.
2. Mianyang Dongchen International School , Mianyang, P.R. China.
3. Department of Applied Mathematics, Chongqing University of Posts and Telecommunications , Chongqing, P.R. China.
4. College of Mathematics and Statistics, Chongqing University , Chongqing, P.R. China.