1. Leopold-Franzens-Universität Innsbruck, Engineering Mathematics, Technikerstrasse 13, 6020 Innsbruck, Austria
2. Department of Mathematical, Physical and Computer Sciences, University of Parma, 43124 Parma, Italy
3. Institut für Mathematik, Martin-Luther-Universität Halle-Wittenberg, 06120 Halle (Saale), Germany
4. Maxwell Institute for Mathematical Sciences and Department of Mathematics, Heriot-Watt University, Edinburgh EH14 4AS, UK