1. Department of Mathematics, University of Osnabrück, Osnabrück, Germany. Current address: Dipartimento di Matematica, Politecnico di Milano, Milan, Italy.
2. Department of Mathematics, University of Osnabrück, Osnabrück, Germany.
3. Department of Mathematics, KTH Royal Institute of Technology, Stockholm, Sweden, and Department of Mathematics, University of Pisa, Pisa, Italy.