1. Department of Mathematical Sciences, Florida Institute of Technology, 150 West University Boulevard, Melbourne Florida 32901, USA
2. KFUPM Chair Professor, Mathematics and Statistics Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
3. Department of Mathematics, National University of Ireland, Galway, Ireland
4. Université Cadi-Ayyad, Laboratoire de Mathématiques, et de Dynamique de Populations, Marrakech, Maroc