1. a Department of Mathematics and Applied Mathematics , University of KwaZulu-Natal , Westville Campus, P. Bag X54001, Durban, 4000, South Africa
2. b Department of Mathematics , University of Zimbabwe , Box MP 167, Mount Pleasant, Harare, Zimbabwe
3. c Department of Mathematics , Center of Mathematics for Applications (CMA), University of Oslo , P.O. Box 1053 Blindern, N-0316, Oslo, Norway