1. Department of Mathematics, Hanoi National University of Education, 136-Xuan Thuy, Cau Giay, Hanoi, Vietnam
2. Thang Long Institute of Mathematics and Applied Sciences, Nghiem Xuan Yem, Hoang Mai, Hanoi, Vietnam
3. Nguyen Thuong Hien High School, 544. CMT-8, F.4, Tan Binh, Ho Chi Minh City, Vietnam