Institution: | a Abteilung Reine Mathematik, Universität Ulm, Ulm 89069, Germany;b Fachbereich Mathematik, Universität Oldenburg, Oldenburg 26111, Germany;c Mathematics Department, University of California Davis, Davis, CA 95616, USA;d Information Sciences Research, AT&T Shannon Labs, Florham Park, NJ 07932-0971, USA |