1. School of Mathematics and Statistics, Chongqing Jiaotong University, Chongqing 400074, P. R. China
2. School of Mathematics and Physics, China University of Geosciences, Wuhan, Hubei 430074, P. R. China
3. College of Mathematics and Econometrics, Hunan University, Changsha, Hunan 410082, P. R. China