Affiliation: | 1. Palacky University Olomouc, Department of Algebra and Geometry, 17. listopadu 12, 771 46, Olomouc, Czech Republic 2. University of Szeged, Bolyai Institute, Aradi vértanúk tere 1, Szeged, 6720, Hungary 3. Department of Mathematics, Tor Vergata University of Rome, I-00133, Rome, Italy
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