Affiliation: | 1. Department of Mathematics, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi 10000, Vietnam.anhpk@vnu.edu.vn;3. Vietnamese Academy of Science &4. Technology, Institute of Information Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Vietnam.;5. Department of Mathematics, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi 10000, Vietnam. |