[1] Univ Pittsburgh, Dept Math, Pittsburgh, PA 15260 USA
[2] CNRS, Lab Math Anal Numer & EDP, UMR 8628, F-91405 Orsay, France
[3] Univ Paris 11, F-91405 Orsay, France
[4] Univ Cergy Pontoise, CNRS, UMR 8088, F-95302 Cergy Pontoise, France
[5] Univ Cergy Pontoise, Dept Math, F-95302 Cergy Pontoise, France