1. Committed Mathematics Team, Research Unit in Mathematics and Applications ; and Departement of Mathematics and Computer Science , Faculty of Science , 362702 University of Dschang , P.O. Box 67 , Dschang , Cameroon ; and Institute of Mathematics University of Mainz, Staudingersweg 9, 55128 Mainz, Germany
2. Committed Mathematics Team, Research Unit in Mathematics and Applications ; and Departement of Mathematics and Computer Science , Faculty of Science , 362702 University of Dschang , P.O. Box 67 , Dschang , Cameroon