1. Department of Applied Mathematics and Statistics, Midlands State University, Gweru 9055, Zimbabwe
2. School of Mathematics, Statistics and Computer Science, University of KwaZulu-Natal, Howard College Campus, Durban 4041, South Africa
3. School of Mathematics, Statistics and Computer Science, University of KwaZulu-Natal, Pietermaritzburg Campus, Pietermaritzburg 3209, South Africa