1. University of Michigan, Department of Mathematics, Ann Arbor, Michigan, USA.
2. Institut Polytechnique de Paris, Centre de Mathématiques Appliquées, Ecole Polytechnique, Palaiseau, France.
3. University of Houston, Department of Mathematics, Houston, Texas, USA.
4. University of Michigan, Department of Mathematics, Ann Arbor, Michigan, USA and Uppsala Universitet, Department of Information Technology, Division of Scientific Computing, Uppsala, Sweden. (corresponding author)