1. Alfréd Rényi Institute of Mathematics Reáltanoda u. 13‐15 Budapest 1053 Hungary
2. Department of Geometry Bolyai Institute University of Szeged Aradi vértanúk tere 1 Szeged 6720 Hungary
3. Karlsruhe Institute of Technology (KIT) Karlsruhe D‐76128 Germany