1. Department of Engineering and Computer Science and Mathematics, University of L’Aquila, Via Vetoio 1, 67010 Coppito (AQ), Italy
2. Department of Mathematics, Computer Science and Physics, University of Udine, Via delle Scienze 206, 33100 Udine, Italy
3. Department of Mathematics and Computer Science, University of Ferrara, Via Machiavelli 30, 44121 Ferrara, Italy