1. CNRS, UMR 7598, Laboratoire Jacques-Louis Lions, France
2. UPMC Univ Paris 06, UMR 7598, Laboratoire Jacques-Louis Lions, France
3. CMAP, Ecole Polytechnique, CNRS, France
4. Dip. di Matematica, Università di Padova, via Trieste 63, 35121 Padova, Italy