Affiliation: | 1. Department of Mathematical Sciences, Kent State University, 4314 Mahoning Avenue, Warren, OH 44483, USA;2. Department of Mathematics, Vinh University, Vinh city, Nghe An, Viet Nam;3. Faculty of Economics, Chiang Mai University, Chiang Mai 52000, Thailand;4. Department of Personnel and Organization, Vinh University of Technology Education, Vinh city, Nghe An, Viet Nam |