1. School of Mathematics, University of the Witwatersrand , Private Bag 3, Johannesburg 2050 , South Africa
2. Department of Mathematics, University of Eswatini , Private Bag 4, Kwaluseni , Eswatini
3. Department of Mathematics and Applied Mathematics, Sefako Makgatho Health Sciences University , P.O. Box 94 Medunsa 0204, Pretoria , South Africa
4. Federal University of Agriculture , Abeokuta , Nigeria