1. BiBoS, Universität Bielefeld, D-33615 Bielefeld, Germany
2. Institute of Mathematics, Kiev, Ukraine
3. Institute für Angewandte Mathematik, Universität Bonn, D-53115 Bonn, Germany
4. CCM, Universidade da Madeira, P-9000 Funchal, Portugal
5. Universidade da Madeira, P-9000 Funchal, Portugal
6. Department of Mechanics and Mathematics, Kiev University, 252033 Kiev, Ukraine