1. College of Teacher Education, Qujing Normal University , Qujing , Yunnan 655011 , P. R. China
2. Three Gorges Mathematical Research Center, China Three Gorges University , Yichang , Hubei 443002 , P. R. China
3. College of Science, China Three Gorges University , Yichang , Hubei 443002 , P. R. China
4. Department of Mathematics, Suzhou University of Science and Technology , Suzhou 215009 , P. R. China
5. School of Mathematics and Statistics, HNP-LAMA, Central South University , Changsha , Hunan 410083 , P. R. China