Affiliation: | 1. Ninh Thuan Branch , Nong Lam University , Ninh Thuan , Vietnam;2. Training and Scientific Research Department , Ninh Thuan College of Pedagogy , Ninh Thuan , Vietnam latuan02@yahoo.com;4. Department of Mathematical Analysis , Hanoi Institute of Mathematics , Hanoi , Vietnam;5. Faculty of Economic Information Systems , Hanoi University of Commerce , Hanoi , Vietnam |