1. Mathematics Area, SISSA – International School for Advanced Studies, Via Bonomea 265, I-34136 Trieste, Italy
2. Department of Mathematical, Physical and Computer Sciences, Università di Parma, Parco Area delle Scienze 53/A, 43124 Parma, Italy
3. Department of Mathematical Sciences "G. L. Lagrange", Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy