1. Department of Mathematics University College London London UK‐WC1E 6BT UK
2. Department of Mechanical Engineering Jönköping University SE‐55111 Jönköping Sweden
3. Department of Mathematics and Mathematical Statistics Umeå University SE‐901 87 Umeå Sweden
4. Center for Biomedical Computing Simula Research Laboratory PO Box 134 NO‐1325 Lysaker Norway