1. School of Science, Hunan University of Technology, Zhuzhou, Hunan 412007, P. R. China
2. Key Laboratory of High Performance Computing and Stochastic, Information Processing (Ministry of Education of China), College of Mathematics and Statistics, Hunan Normal University, Changsha, Hunan 410081, P. R. China
3. College of Mathematics and Econometrics, Hunan University, Changsha, Hunan 410082, P. R. China