1. School of Applied Mathematics and Informatics, Hanoi University of Science and Technology, Hanoi, Vietnam
2. Department of Basic Science, Tantrao University, Tuyenquang, Vietnam
3. Department of Mathematics, Hanoi Pedagogical University 2, Vinhphuc, Vietnam
4. Department of Basic Science, Electric Power University, Hanoi, Vietnam
5. HUS High School for Gifted Students, Vietnam National University, Hanoi, Vietnam