1. INRIA Saclay - Ile de France; Laboratoire de Recherche en Informatique Université Paris-Sud 11; France
2. INRIA Rocquencourt; Alpines, B.P. 105 F-78153 Le Chesnay Cedex France
3. UPMC Univ Paris 06, CNRS UMR 7598; Laboratoire Jacques-Louis Lions; F-75005 Paris France
4. Department of Mathematics, College of Sciences; Shanghai University; Shanghai 200444 China