1. Eötvös Loránd University, Mathematics Institute, Department of Computer Science, Pázmány Péter sétány 1/C, H-1117 Budapest, Hungary; Alfréd Rényi Institute of Mathematics, Reáltanoda utca 13-15, 1053 Budapest, Hungary
2. Department of Stochastics, Institute of Mathematics, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary; MTA-BME Stochastics Research Group, Műegyetem rkp. 3., H-1111 Budapest, Hungary; Alfréd Rényi Institute
3. Department of Stochastics, Institute of Mathematics, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary; Alfréd Rényi Institute of Mathematics, Reáltanoda utca 13-15, 1053 Budapest, Hungary