1. 1Institute for Groundwater Studies, Faculty of Natural and Agricultural Sciences, University of the Free State, 9300, Bloemfontein, Africa
2. 2Institute of Space Sciences, Magurele, Bucharest, Romania
3. 3Department of Mathematics and Computer Sciences, Faculty of Arts and Sciences, Cankara University, 06530 Ankara, Turkey
4. 4Department of Mathematics, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Arabia