1. Section of Mathematics, Universitá Telematica Internazionale Uninettuno, 00186 Rome, Italy
2. Departamento de Ciencias Naturales y Exactas, Universidad de la Costa, Barranquilla 00928-1345, Colombia
3. Syed Babar Ali School of Science and Engineering, Lahore University of Management Sciences, Lahore 54792, Pakistan
4. Department of Mathematics and Natural Sciences, Prince Mohammad Bin Fahd University, P.O. Box 1664, Al Khobar 31952, Saudi Arabia