Affiliation: | 1 Department of Algebra and Geometry, Faculty of Science, Palacký University, Tomkova 40, 779 00 Olomouc, Czech Republic 2 Department of Mathematics, La Trobe University, Bundoora, Victoria 3086, Australia; Institute of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlá?ská 2, 611 37 Brno, Czech Republic |