1. Department of Mathematics, University of Turku, FI-20014 Turku, Finland, michelangelo.bucci@utu.fi
2. Dipartimento di Scienze Fisiche, Università degli Studi di Napoli Federico II, via Cintia, Monte S. Angelo, I-80126 Napoli, Italy
3. Department of Mathematics, University of Turku, FI-20014 Turku, Finland, alessandro.deluca@utu.fi
4. Université de Lyon, Université Lyon 1, CNRS UMR 5208 Institut Camille Jordan, Bâtiment du Doyen Jean Braconnier, 43, blvd du 11 novembre 1918, F-69622 Villeurbanne Cedex, France
5. Department of Mathematics, University of Turku, FI-20014 Turku, Finland, luca.zamboni@utu.fi