1. Institute of Analysis and Number Theory, Graz University of Technology, Kopernikusgasse 24/II, A-8010 Graz, Austria
2. Institute of Mathematics, University of Debrecen, H-4010 Debrecen, P.O. Box 12, Hungary
3. University of Salzburg, Hellbrunnerstrasse 34/I, A-5020 Salzburg, Austria