Affiliation: | 1. School of Applied Mathematics and Informatics, Hanoi University of Science and Technology, Hanoi, Vietnamthuy.lequang@hust.edu.vn;3. Department of Mathematics, Vietnam National University, Hanoi, Vietnam;4. TIMAS, Thang Long University, Hanoi, Vietnam;5. School of Applied Mathematics and Informatics, Hanoi University of Science and Technology, Hanoi, Vietnam |