Affiliation: | (1) JATE Bolyai Institute, Szeged, Aradi Vértanúk tere 1, H-6720, Hungary, e-mail: czedli@math.u-szeged.hu, a.szendrei@math.u-szeged.hu, HU;(2) Department of Algebra and Geometry, Palacky University Olomouc, Tomkova 40, 779 00 Olomouc, Czech Republic, e-mail: halas@risc.upol.cz, CZ;(3) Department of Mathematics, University of Louisville, Louisville, KY 40292, USA, e-mail: kearnes@louisville.edu, US;(4) Mathematical Institute of the Hungarian Academy of Sciences, Budapest, P.O.Box 127, H-1364, Hungary, e-mail: ppp@math-inst.hu, HU |