Affiliation: | 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. |