1. Department of Mathematics, Pan African University Institute for Basic Sciences, Technology and Innovation, P.O. Box 62000, Nairobi 00200, Kenya
2. Department of Mathematics, Machakos University, P.O. Box 136, Machakos 90100, Kenya
3. Department of Statistics and Actuarial Sciences, Kenyatta University, P.O. Box 43844, Nairobi 00100, Kenya