1. Institute of Mathematics and Physics, College of Science, Central South University of Forestry and Technology, Changsha, Hunan, People's Republic of China
2. Key Laboratory of Computing and Stochastic Mathematics (Ministry of Education), School of Mathematics and Statistics, Hunan Normal University, Changsha, Hunan, People's Republic of China
3. School of Mathematics and Statistics, Changsha University of Science and Technology, Changsha, Hunan, People's Republic of China