1. Faculty of Sciences, Department of Mathematics, University of Yaounde I, Yaounde, Cameroon.
2. Faculty of Sciences and Technology, Department of Mathematics, University of Coimbra, Coimbra, Portugal.
3. Department of Mathematics of the Higher Teachers’ Training College, University of Yaounde I, Yaounde, Cameroon.
4. African Institute for Mathematical Sciences, AIMS-Cameroon, Limbe, Cameroon.
5. Departamento de Matemática Aplicada II of the E.E. Aeronáutica e do Espazo, Universidade de Vigo, Ourense, Spain.