Department of Mathematical Sciences, University of South Africa, UNISA 0003, Republic of South Africa; Corresponding author.
T.S. Moretlo
Department of Mathematical Sciences, Sol Plaatje University, Private Bag X5008, Kimberley 8300, Republic of South Africa; Department of Mathematical Sciences, North-West University, Private Bag X 2046, Mmabatho 2735, Republic of South Africa
B. Muatjetjeja
Department of Mathematical Sciences, North-West University, Private Bag X 2046, Mmabatho 2735, Republic of South Africa; Department of Mathematics, Faculty of Science, University of Botswana, Private Bag 22, Gaborone, Botswana