1. Department of Applied Mathematics, University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands.
2. Department of Mathematics, Utrecht University, Budapestlaan 6, 3584 CD Utrecht, The Netherlands, and Department of Applied Mathematics, University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands.
3. Bolyai Institute, University of Szeged, Aradi vértanúk tere 1, Szeged, H-6720, Hungary, and HUN-REN-ELTE Numerical Analysis and Large Networks Research Group, Budapest, Hungary.