1. School of Mathematics and Actuarial Science, University of Leicester, Leicester LE1 7RH, United Kingdom and Department of Mathematics and Statistics, Taif University, P.O. Box 888, Taif, Saudi Arabia
2. School of Engineering, University of Leicester, Leicester LE1 7RH, United Kingdom
3. Dipartimento di Ingegneria Aerospaziale, Universitá di Pisa, 56122 Pisa, Italy