Institution: | 1. Fachbereich Mathematik, Universit?t Dortmund, 44221 Dortmund, GermanyPhone: +49 231 755 3099, Fax: +49 231 755 3064;2. Faculty of Informatics, University of Debrecen, Pf. 12, H–4010 Debrecen, Hungary;3. Phone: +36 52 316666 22826, Fax: +36 52 416857 |