Affiliation: | 1. Departament d?Àlgebra i Geometria, Universitat de Barcelona, Gran Via de les Corts Catalanes 585, 08007 Barcelona, Spain;2. Faculty of Mathematics and Computer Science, Jagiellonian University, ul. ?ojasiewicza 6, 30-348 Kraków, Poland;3. Faculty of Applied Mathematics, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland |