1. Faculty of Mathematics and Computer Science, University of Science, 227 Nguyen Van Cu, D. 5, Ho Chi Minh City, Vietnam
2. Vietnam National University, Ho Chi Minh City, Vietnam. E-mail: bltthanh@hcmus.edu.vn
3. ShanghaiTech University, 393 Middle Huaxia Road, Pudong, Shanghai, 201210, China. E-mail: qhnguyen@shanghaitech.edu.cn