1. School of Statistics and Mathematics, Zhongnan University of Economics and Law, Wuhan, Hubei 430073, P. R. China
2. School of Mathematics and Statistics, Wuhan University, Wuhan, Hubei 430072, P. R. China
3. School of Mathematics and Statistics, Shenzhen University, Shenzhen, Guangdong 518060, P. R. China
4. Department of Statistics and Finance, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China