1. a Institut für Angewandte Mathematik , Universität Bonn , Bonn, Germany
2. b SFB 611, Bonn, BiBoS, Bielefeld-Bonn , Germany
3. c CERFIM , Locarno, Switzerland
4. d USI , Mendrisio, Switzerland
5. e Institute of Mathematics , Ukrainian National Academy of Sciences , Kyiv, Ukraine