1. Institute of Mathematics, University of Kassel, Kassel, Germany.
2. African Institute for Mathematical Sciences, AIMS-Cameroon, Limbé, Cameroon.
3. Department of Mathematics, Higher Teachers’ Training College, University of Yaounde I, Yaoundé, Cameroon.
4. Faculty of Science, Department of Mathematics, University of Yaounde I, Yaoundé, Cameroon.
5. Departamento de Matemática Aplicada II, E.E. Aeronáutica e do Espazo, Universidade de Vigo, Ourense, Spain.