1. Department of Mathematics, Florida State University, Tallahassee, Florida, 32306-4510, USA
2. The Institute for Scientific Computing and Applied Mathematics, Indiana University, 831 East Third Street, Bloomington, Indiana 47405, USA
3. Université de Genève, Section de Mathématiques, Genève, Switzerland
4. Laboratoire d'Analyse Numérique et EDP, Université de Paris-Sud, Orsay, France