1. Laboratory of Biophysics, Department of Physics, Faculty of Science; University of Yaoundé I; P.O. Box 812 Yaoundé Cameroon
2. Theoretical Physics Group, Department of Physics, Faculty of Science; University of Stellenbosch, Private Bag X1 Matieland; Stellenbosch 7602 South Africa
3. The African Institute for Mathematical Sciences; 6-8 Melrose Rd Muizenberg 7945 South Africa
4. Laboratory of Mechanics, Department of Physics, Faculty of Science; University of Yaoundé I; P.O. Box 812 Yaoundé Cameroon