Affiliation: | a Johann Bernoulli Institute for Mathematics and Computer Science, University of Groningen, P.O. Box 407, 9700 AK Groningen, The Netherlands b Institut für Mathematik, Technische Universität Ilmenau, Curiebau, Weimarer Str. 25, 98693 Ilmenau, Germany c Institute of Mathematics, Jagiellonian University, ?ojasiewicza 6, 30-348 Kraków, Poland |