1. Department of Mathematics and Applied Mathematics, University Pretoria, Lynnwood Road, Pretoria 0002, South Africa
2. Institute of Research and Development of Process IIDP, Faculty of Science and Technology, University of the Basque Country, Spain
3. Division of Applied Mathematics, School of Education, Culture & Communication, Mälardalen University, 72123 Västerås, Sweden
4. Department of Mathematics, COMSATS Institute of Information Technology, Abbottabad 22060, Pakistan