Institution: | 1. Mathematics Institute, ?itná 25, 11567 Prague 1, Czech Republic;2. Mathematics Institute, Silesian University at Opava, Na Rybní?ku 1, 74601 Opava, Czech Republic;3. Fachrichtung Mathematik, Universität des Saarlandes, Postfach 15 11 50, D-66041 Saarbrücken, Germany |