Affiliation: | 1. Department of Engineering Mathematics , University of Bristol , Bristol BS8 1TR , UK;2. Bolyai Institute, University of Szeged , Aradi vértanúk tere 1, H-6720 Szeged , Hungary;3. Bolyai Institute, University of Szeged , Aradi vértanúk tere 1, H-6720 Szeged , Hungary |