1. Laboratory of Electronics, Department of Physics, Faculty of Science, University of Dschang, P.O. Box 67, Dschang, Cameroon
2. Laboratory of Applied Mathematics, Department of Mathematics and Computer Science, Faculty of Science, University of Douala, P.O. Box 24157, Douala, Cameroon
3. Instituto de Física Teórica-UNESP, Universidade Estadual Paulista, Rua Dr. Bento Teobaldo Ferraz 271, Bloco II, Barra Funda, 01140-070 São Paulo, SP, Brazil