Affiliation: | 1. Department of Mathematics, University of Copenhagen, Universitetsparken 5, DK-2100, Denmark;2. Institute of Mathematics, University of Wroc?aw, pl. Grunwaldzki 2/4, 50-384 Wroc?aw, Poland;3. Institute of Mathematics and Computer Science, University of Opole, ul. Oleska 48, 45-052 Opole, Poland |