1. School of Mathematical Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa
2. Faculty of Military Science, Stellenbosch University, Private Bag X2, Saldanha 7395, South Africa
3. Department of Mathematics & Applied Mathematics, University of Venda, Private Bag X5050, Thohoyandou 0950, South Africa