1. Simula Research Laboratory, 0164 Oslo, Norway.
2. Department of Mathematics, Tufts University, Medford, MA 02155 USA.
3. Simula Research Laboratory, 0164 Oslo, Norway, and Department of Mathematics, University of Oslo, Blindern, 0316 Oslo, Norway.
4. Department of Mathematics, Pennsylvania State University, University Park, PA 16802 USA.