1. Department of Mathematics and Applied Mathematics, Sefako Makgatho Health Sciences University, Pretoria, South Africa
2. DST-NRF Center of Excellence in Mathematical and Statistical Sciences (CoE-MaSS), Johannesburg, South Africa
3. School of Mathematics, Statistics and Computer Science, University of KwaZulu-Natal, Durban, South Africa
4. Department of Mathematics, Usmanu Danfodiyo University Sokoto, Sokoto, Nigeria