1. School of Mathematics and Computational Science, Sun Yat-sen University, Guangzhou 510275, P. R. China
2. Department of Mathematics, Huizhou University, Guangdong, 516007, P. R. China
3. Sun Yat-sen Business School, Sun Yat-sen University, Guangzhou 510275, P. R. China
4. Lingnan College, Sun Yat-sen University, Guangzhou 510275, P. R. China