Affiliation: | (1) Fakultät für Mathematik, Ruhr Universität, 44780 Bochum, Germany: CERFIM;(2) BiBoS-Research Centre, Universität Bielefeld, 33615 Bielefeld, Germany;(3) Laboratoire de Physique Mathématique et géométrie EP 0004, Université Paris VII (Denis Diderot), 2 place Jussieu, 75251 Paris Cedex 05, France;(4) Present address: Department of Pure Mathematics, The University of Hull, HU6 7RX Hull, UK |