Affiliation: | 1 Institut für Angewandte Mathematik, Universität Bonn, Wegelerstr. 6, D-53115 Bonn, Germany SFB 256, Bonn, BiBoS, Bielefeld-Bonn 2 CERFIM, Locarno and Acc. Arch. (USI) (Switzerland) 3 Kyiv Taras Shevchenko University, Cybernetics Department, Volodymyrska 64, Kyiv 01033, Ukraine 4 Chalmers Tekniska Högskola, Matematiska Vetenskaper, SE 412 96 Göteborg, Sweden |