1. Department of Mathematics, GMR Institute of Technology, Rajam 532 127, Andhra Pradesh, India
2. Institute for Groundwater Studies, University of the Free State, Bloemfontein 9300, South Africa
3. Department of Mathematics and Sciences, Prince Sultan University, P.O. Box 66833, Riyadh 11586, Saudi Arabia
4. Department of Medical Research, China Medical University, Taichung, Taiwan