[1] Pontificia Univ Catolica Chile, Fac Matemat, Dept Matemat, Santiago 22, Chile
[2] Univ La Laguna, Dept Anal Matemat, San Cristobal la Laguna 38271, Spain
[3] Univ La Laguna, Fac Fis, Inst Univ Estudios Avanzados IUdEA Fis Atom Mol &, San Cristobal la Laguna 38203, Spain
[4] Univ Chile, Dept Ingn Matemat, Santiago, Chile
[5] Univ Chile, CNRS UChile, UMI 2807, Ctr Modelamiento Matemat, Santiago 3, Chile