1. MTA-SZTE Analysis and Stochastics Research Group, Bolyai Institute, University of Szeged, Aradi vértanúk tere 1, Szeged, H-6720, Hungary
2. Department of Mathematics, University of Klagenfurt, Universitätsstraße 65-67, Klagenfurt am Wörthersee, 9020, Austria
3. Bolyai Institute, University of Szeged, Aradi vértanúk tere 1, Szeged, H-6720, Hungary