1. School of Applied Mathematics, Nanjing University of Finance and Economics, Nanjing 210023, P. R. China
2. Institute of Applied System Analysis, Jiangsu University, Zhenjiang 212013, P. R. China
3. School of Statistics and Mathematics, Nanjing Audit University, Nanjing 211815, P. R. China
4. Department of Mathematics, Nanjing University, Nanjing 210093, P. R. China