1. Lab d’Analyze Numerique, Univ Paris VI et CNRS, 4, place Jussieu, Paris Cedex 05, 75252, France
2. Dept de Mathematique, Univ de Metz, Ile du Saulcy, Metz, Cedex 01, 57045, France
3. Inst of Fund Tech Res, Polish Acad of Sci, Swietokrzyska 21, 00-049 Warsaw, Poland