1. MIVEGEC, CNRS, IRD University of Montpellier Montpellier France
2. Department of Mathematics, Faculty of Sciences University of Yaoundé I Yaoundé Cameroon
3. Département Tronc Commun École Polytechnique de Thiès Sénégal
4. National Advanced School of Engineering of Yaoundé University of Yaoundé I Cameroon