1. Institute of Applied Mathematics, Friedrich Schiller Unversity Jena, Ernst-Abbe-Platz 2, 07743 Jena, Germany
2. Institute of Psychology, Friedrich Schiller Unversity Jena, Am Steiger 3, 07743 Jena, Germany
3. Department of Mathematics, Brandenburg Techn. University Cottbus, PO box 10 13 44, 03013 Cottbus, Germany
4. Department of Information Science, Tokyo University of Science, Noda City, Chiba 278-8510, Japan