1. Department of Mathematics, College of Education, Hue University, 34 Le Loi, Hue City, Vietnam
2. Hong Bang International University, 215 Dien Bien Phu Street, Binh Thanh, Ho Chi Minh City, Vietnam
3. Department of Mathematics, The University of Danang — University of Science and Education, 459 Ton Duc Thang, Danang City, Vietnam