Ecole Nationale Supérieure Polytechnique;
University of Yaounde I;
PO Box 8390;
Cameroon;
Department of Physics;
Faculty of Science;
University of Yaounde I;
PO Box 812;
Cameroon;
The Abdus Salam International Centre for Theoretical Physics;
PO Box 586;
Strada Costiera;
II-34014;
Trieste;
Italy;