Affiliation: | a Faculty of Economics and Commerce, Hoa Sen University, 08 Nguyen Van Trang Str., Dist. 1, Ho Chi Minh City, Viet Nam b Nhatrang Educational College, 01 Nguyen Chanh Str., Nhatrang City, Viet Nam c Department of Mathematics and Computer Science, University of Natural Science, Vietnam National University, 227 Nguyen Van Cu Str., Dist. 5, Ho Chi Minh City, Viet Nam |