1. Department of Mathematics , Faculty of Science , University of Botswana , Private Bag 22 , Gaborone , Botswana
2. Department of Mathematics , Faculty of Science , University of Botswana , Private Bag 22 , Gaborone , Botswana ; and Department of Mathematical Sciences, North-West University, Mafikeng Campus, Private Bag X2046, Mmabatho 2735, Republic of South Africa
3. Department of Mathematical Sciences , University of South Africa , UNISA 0003 , Pretoria , South Africa