1. Laboratory of Mathematics, Department of Mathematics and Computer Science Faculty of Sciences, University of Douala, P. O. Box 24157, Douala, Cameroon
2. Laboratoire de Mathématiques Informatique et Applications (LAMIA), Université des Antilles (UA), Guadeloupe, France
3. Institute of Research for Development, Sorbonne University, UMMISCO, F-93143, Bondy, France
4. AIMS Cameroon South West Region, Crystal Garden, P. O. Box 608, Limbe, Cameroon
5. Faculty of Economics Sciences and Applied Management, University of Douala, Douala, Cameroon