1. Department of Computer Systems and Mathematics , Ardhi Universty , Dar es Salaam ; and Department of Mathematics, College of Natural and Applied Sciences, University of Dar es Salaam, Dar es Salaam, Tanzania
2. Department of Mathematics , College of Natural and Applied Sciences , University of Dar es Salaam , Dar es Salaam , Tanzania ; and Department of Mathematics, North Eastern Hill University, Shillong, Meghalaya, India
3. Department of Medical Research , China Medical University Hospital , China Medical University (Taiwan) , Taichung , Taiwan ; and Department of Mathematics, Aligarh Muslim University, India