1. Division of Mathematics, Thang Long University, Nghiem Xuan Yem Street, Hoang Mai, Hanoi, Vietnam
2. Faculty of Education, An Giang University, Vietnam National University Ho Chi Minh City, Vietnam
3. 18 Ung Van Khiem, Dong Xuyen, Long Xuyen, An Giang, Vietnam
4. Department of Mathematics, Hanoi University of Civil Engineering, 55 Giai Phong Street, Hai Ba Trung, Hanoi, Vietnam