First Author:   Hugues Tchantcho Faculty of Sciences, UY I, Yaounde, Cameroon thuguesfr@yahoo.fr   BP 47 Yaounde, Cameroon       Second Author: Issofa Moyouwou ENS, UY I, Yaounde, Cameroon imoyouwou2@yahoo.fr   BP 47 Yaounde, Cameroon       Third Author: Nicolas G. Andjiga ENS, UY I, Yaounde andjiga2002@yahoo.fr   ENS, Yaounde, BP 47 Yaounde, Cameroon