1. School of Mathematics, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. This research was done when the author was affiliated with the Institut für Mathematik, Freie Universität Berlin, 14195 Berlin, Germany.
2. Hamburg University of Technology, Institute of Mathematics, Hamburg, 21073, Germany.
3. Faculty of Computer Science and Mathematics, University of Passau, Passau, 94032, Germany. This research was done when the author was affiliated with the Hamburg University of Technology, Institute of Mathematics, Hamburg, 21073, Germany.
4. FernUniversität in Hagen, Fakultät für Mathematik und Informatik, Hagen, 58084, Germany.