1. Institute of Mathematics, University of Debrecen, P. O. Box 12, 4010 Debrecen, Hungary. csvincze@science.unideb.hu
2. Institute of Mathematics, MTA-DE Research Group ”Equations Functions and Curves”, Hungarian Academy of Sciences and University of Debrecen, P. O. Box 12, 4010 Debrecen, Hungary. abris.nagy@science.unideb.hu