1. University of Turin, Department of Mathematics “G. Peano”, via Carlo Alberto 10, I-10123 Torino, Italy
2. Department of Mathematics, Faculty of Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium
3. University of Ferrara, Department of Mathematics and Computer Science, Via Machiavelli 30, Ferrara, I-44121, Italy