1. African Institute for Mathematical Sciences (AIMS), Limbe, Cameroon
2. Laboratoire LAMIA, Université des Antilles, Pointe-à-Pitre, Guadeloupe (FWI)
3. Laboratoire MAINEGE, Université Ouaga 3S, Ouagadougou, Burkina Faso
4. Department Mathematik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany
5. Department of Mathematics, University of Buea, Buea, Cameroon