1. Institute of Fundamentals and Theory in Electrical Engineering, Graz University of Technology, Inffeldgasse 18, 8010 Graz, Austria
2. Department of Mathematics, AAU Klagenfurt, Universitätsstrasse 65-67, 9020 Klagenfurt, Austria
3. Department of Mathematics, TU Darmstadt, Dolivostraße 15, 64293 Darmstadt, Germany