1. Institute of Applied Analysis and Numerical Simulation, University of Stuttgart, Pfaffenwaldring 57, D-70569 Stuttgart, Germany
2. Department of Mathematics, University of Bergen, Johs. Brunsgt. 12, N-5008 Bergen, Norway
3. Centre of Mathematics for Applications (CMA), Department of Mathematics, University of Oslo, P.O. Box 1053, Blindern, N-0316 Oslo, Norway