1. IRMAR, CNRS UMR 6625, Université de Rennes 1; Campus de Beaulieu; 263 avenue du Général Leclerc, CS 74205 35042 Rennes cedex France
2. Department of Mathematics; University of Turin; Via Carlo Alberto 10 10123 Torino (TO) Italy and RUDN University, 6 Miklukho-Maklay St., Moscow, 117198, Russia
3. Department of Mathematics; Linnaeus University; SE-35195 Växjö Sweden