[1] Univ La Laguna, Dept Anal Matemat, C Astrofis Francisco Sanchez S-N, San Cristobal la Laguna 38200, Spain
[2] Univ Torcuato Tella, CONICET, Dept Matemat & Estadist, Ave Figueroa Alcorta 7350,C1428BCW, Ca De Buenos Aires, Argentina
[3] Univ La Laguna, Inst Univ Estudios Avanzados IUdEA Fis Atom Mol &, C Astrofis Francisco Sanchez S-N, San Cristobal la Laguna 38200, Spain
[4] Univ Tecn Federico Santa Maria, Dept Matemat, Casilla 5-110,Avda Espana, Valparaiso 1680, Chile