1. Department of Physics, Faculty of Science, University of Yaounde I, P.O. Box 812, Cameroon
2. Ecole Nationale Supérieure Polytechnique, University of Yaounde I, P.O. Box 8390, Cameroon
3. The Abdus Salam International Centre for Theoretical Physics (ICTP), P.O. Box 586, Strada Costiera, II-34014, Trieste, Italy