1. College of Mathematics and Statistics, Nanjing University of Information Science and Technology , Nanjing , 210044 , P.R. China
2. School of Mathematics, Southeast University , Nanjing , Jiangsu 210023 , P.R. China
3. School of Statistics and Mathematics, Zhongnan University of Economics and Law Wuhan , Hubei , 430073 , P.R. China