1. Institut für Angewandte Mathematik; Universität Bonn, Endenicher Allee 60, 53115 Bonn, Germany; HCM, Bonn, Germany; BiBoS, Bielefeld-Bonn, Germany; CERFIM; Locarno, Switzerland; IZKS, Bonn Germany
2. Fakultät für Mathematik; Universität Bielefeld, Postfach 10 01 31; 33501 Bielefeld Germany; BiBoS, Bielefeld-Bonn Germany
3. National Dragomanov University; , Pyrogova 9, 01601 Kyiv, Ukraine; Institute of Mathematics of the Polish Academy of Science, Śniadeckich 8; 00-656 Warsaw Poland
4. National Dragomanov University; , Pyrogova 9, 01601 Kyiv, Ukraine; Institute for Mathematics of National Academy of Sciences of Ukraine, Tereshchenkivs'ka 3; 01601 Kyiv Ukraine