1. Department of Mathematics, Faculty of Sciences, King Abdulaziz University, Jeddah, Saudi Arabia
2. Politecnico di Torino, 10129 Torino, Italy
3. Istituto di Matematica Applicata e Tecnologie Informatiche del CNR, Pavia, Italy
4. Department of Mathematics, University of Roma, “La Sapienza”, Italy
5. M&MOCS, International Research Center on Mathematics and Mechanics of Complex Systems, University of l’Aquila, Palazzo Caetani, Cisterna di Latina, (LT) 04012, Italy