1. Key Laboratory of Computing and Stochastic Mathematics (Ministry of Education of China), School of Mathematics and Statistics, Hunan Normal University, Changsha, Hunan 410081, P. R. China
2. Key Laboratory of Computing and Stochastic Mathematics (Ministry of Education of China), School of Mathematics and Statistics, Hunan Normal University, Changsha, Hunan 410081, P. R. Chin
3. School of Economy and Trade, Hunan University, Changsha, Hunan 410082, China
4. Beijing Research Center for Science of Science, Beijing 100054, China