1. School of Data and Computer Science, Sun Yat-sen University, Guangzhou 510006, P. R. China
2. Department of Mathematics, The Hong Kong University of Science and Technology, Kowloon, Hong Kong, P. R. China
3. School of Mathematics (Zhuhai), Sun Yat-sen University, Zhuhai, P. R. China
4. Guangdong Province Key Laboratory of Computational Science, Sun Yat-sen University, Zhuhai, P. R. China