Department of Mathematics and Statistics, University of Tromsø, 9037 Tromsø, Norway. steinar.thorvaldsen@uit.no|Department of Mathematics and Statistics, University of Tromsø, 9037 Tromsø, Norway. steinar.thorvaldsen@uit.no|Department of Mathematics and Statistics, University of Tromsø, 9037 Tromsø, Norway. steinar.thorvaldsen@uit.no