1. Laboratoire PMMH, Ecole Supérieure de Physique et Chimie Industrielle, 10 rue Vauquelin, F-75231 Paris, France
2. Institute for Theoretical Physics, Cologne University, D-50923 Köln, Euroland, Germany
3. School of Engineering and Science, International University Bremen, P. O. Box 750561, D-28725 Bremen, Germany