1. Department of Mathematics and Statistics, Troy University, Troy, AL, USA
2. Department of Mathematics and Statistics, Quy Nhon University, Quy Nhon, Vietnam
3. Department of Mathematics, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA
4. University of Science, Vietnam National University, Hanoi, Vietnam
5. Faculty of Economics and Law, University of Finance and Marketing, Ho Chi Minh City, Vietnam