1. Institut für Mathematik; Universität Zürich; Winterthurerstrasse 190 CH-8057 Zürich Switzerland
2. INRIA Bordeaux-Sud-Ouest; 200 Avenue de la Vieille Tour 33405 Talence Cedex France
3. Flow Physics and Computational Engineering Group, Mechanical Engineering Department; Stanford University; 488 Escondido Mall CA 94305-3035 Stanford USA