[1] Univ Clermont Ferrand, Math Lab, UMR 6620, CNRS, F-63171 Aubiere, France
[2] Univ Valencia, Dept Matemat Aplicada, E-46100 Burjassot, Spain
[3] Univ Lorraine, Inst Elie Cartan Lorraine, UMR 7502, F-54506 Vandoeuvre Les Nancy, France
[4] CNRS, Inst Elie Cartan Lorraine, UMR 7502, F-54506 Vandoeuvre Les Nancy, France