1. Informetrics Research Group Ton Duc Thang University Ho Chi Minh City Vietnam
2. Faculty of Mathematics and Statistics Ton Duc Thang University Ho Chi Minh City Vietnam
3. Department of Mathematics, Faculty of Science University of Tabuk P.O. Box 741 Tabuk 71491 Saudi Arabia
4. School of Finance and Economics Jiangsu University Zhenjiang 212013 Jiangsu People's Republic of China