1. University of Electronic Science and Technology of China Chengdu Sichuan 611731 China
2. School of Management and Economics, Center for West African Studies University of Electronic Science and Technology of China, Sichuan 611731 China
3. East Asian Institute National University of Singapore Singapore 259770 Singapore
4. Applied Mathematics Department, Faculty of Applied Science and Technology Koforidua Technical University Koforidua Ghana
5. Faculté des sciences économiques et de gestion Université de Lomé (Togo) (FASEG)/01 BP 1515 Lomé 01 Togo