1. Department of Mathematics, Sun Yat-sen University, Guangzhou 510275, P. R. China
2. Department of Mathematics and Statistics, University of Swat, Swat, KP, Pakistan
3. School of Statistics and Mathematics, Guangdong University of Finance and Economics, Big data and Educational Statistics Application Laboratory, Guangzhou 510320, P. R. China
4. School of Statistics and Mathematics, Guangdong University of Finance and Economics, Guangzhou 510320, P. R. China