1. Department of Mathematics, Hanoi National University of Education, No. 136 Xuan Thuy, Cau Giay, Hanoi, Vietnam
2. Institute of Mathematics, Vietnam Academy of Science and Technology, No. 18 Hoang Quoc Viet, Hanoi, Vietnam
3. Department of Mathematics, Hoa Lu University, Ninh Nhat, Ninh Binh, Vietnam