1. Department of Mathematics, University of Kaiserslautern, Erwin-Schrödinger-Straße, 67663 Kaiserslautern, Germany.
2. Fraunhofer Institute for Industrial Mathematics ITWM, Fraunhofer-Platz 1, 67663 Kaiserslautern, Germany.
3. Department IV – Mathematics, University of Trier, Universitätsring 19, 54296Trier, Germany.