(1) Fachbereich Mathematik, Universität Frankfurt, 60054 Frankfurt, Germany; (2) Institut für Theoretische Physik, Universität Zürich, 8057 Zürich, Switzerland; scharf(at)physik.unizh.ch
Received 25 October 2001, accepted 19 November 2001 by K. Fredenhagen
Departamento de Fisica Teorica 1, Universidad Complutense de Madrid, 28040 Madrid, Spain; Ralvarez(at)eucmax.sim.ucm.es
Received 10 July 2001, accepted 27 November 2001 by G. Röpke
Hahn-Meitner-Institut Berlin, Glienicker Str. 100, 14109 Berlin, Germany; froebrich(at)hmi.de
Received 3 January 2002, accepted 14 January 2002 by G. Röpke