1. Department of Mathematics, Faculty of Sciences, University of Yaounde 1 1 , P.O. Box 812, Yaounde, Cameroon and , 072 B.P. 50, Cotonou, Republic of Benin
2. International Chair in Mathematical Physics and Applications (ICMPA-UNESCO Chair) University of Abomey-Calavi 1 , P.O. Box 812, Yaounde, Cameroon and , 072 B.P. 50, Cotonou, Republic of Benin
3. Mathematisches Institut der Westfälischen Wilhelms-Universität 2 , Einsteinstr. 62, 48149 Münster, Germany