1. University of Yaounde I, Faculty of Sciences, Department of Mathematics, P.O. Box 812, Yaounde, Cameroon and African institute for Mathematical Sciences Ghana, P.O. Box LGDTD 20046, Summerhill Estates, East Legon Hills, Santoe, Acrra, Ghana
2. École Polytechnique de Thiès, Département Tronc Commun, BP A10, Thiès, Sénégal
3. Institute for Financial and Actuarial Mathematics (IFAM), Department of Mathematical Sciences, University of Liverpool, Liverpool L69 7ZL, UK and African institute for Mathematical Sciences Ghana, P.O. Box LGDTD 20046, Summerhill Estates, East Legon