1. Department of Mathematics and Applied Mathematics, University of Venda, Private Bag X5050, Thohoyandou 0950, South Africa
2. Mathematics Department, University of Swaziland, Private Bag 4, M201 Kwaluseni, Swaziland
3. Department of Mathematics and Applied Mathematics, University of Natal, Private Bag X01, Scottsville , Pietermaritzburg 3209, South Africa