1. Division of Mathematical Sciences, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore
2. Department of Information Engineering, Computer Science and Mathematics, University of L'Aquila, via Vetoio snc, Coppito (L'Aquila), 67010, Italy
3. Department of Economic Studies, University of Chieti-Pescara, Viale Pindaro 42, Pescara, 65127, Italy