[1] Univ Buenos Aires, Dept Matemat, C1428EGA, Buenos Aires, DF, Argentina
[2] Univ Buenos Aires, IMAS, C1428EGA, Buenos Aires, DF, Argentina
[3] Consejo Nacl Invest Cient & Tecn, C1428EGA, Buenos Aires, DF, Argentina
[4] Univ Nacl Cuyo, CNEA, Inst Balseiro, R8402AGP, San Carlos De Bariloche, Rio Negro, Argentina
[5] Consejo Nacl Invest Cient & Tecn, R8402AGP, San Carlos De Bariloche, Rio Negro, Argentina
[6] Univ Politecn Valencia, Inst Univ Matemat Pura & Aplicada, E-46022 Valencia, Spain