Affiliation: | 1.Institut für Angewandte Mathematik, Universität Bonn, Wegelerstr. 6, D-53115, Bonn, (Germany) ;2.SFB 256, Bonn; BiBoS, Bielefeld-Bonn; CERFIM, Locarno and USI, (Switzerland) ;3.Institute of Mathematics of the National Academy of Sciences of Ukraine, Tereshchenkovskaya 3, 01601, Kiev (Ukraine) ; |