Institution: | 1 Mathematisches Institut A, Universität Stuttgart, Postfach 80 11 40, D-70511 Stuttgart, Germany 2 Department of Mathematics, Faculty of Education, Iwate University, Morioka 020-8550, Japan 3 Department of Mathematics, University of Michigan, East Hall, 525 East University Avenue, Ann Arbor, Michigan 48109-1109, USA |