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• Alejandro Alvarado-Garc´ıa, Hugo Alberto Rinc´on-Mej´ıa, Bertha Tom´e-Arreola Facultad de Ciencias Universidad Nacional Aut´onoma de M´exico Circuito Exterior, C.U. M´exico D.F. e-mails: alejandroalvaradogarcia@gmail.com (A. Alvarado) hurincon@gmail.com (H. Rinc´on) bta@hp.fciencias.unam.mx (B. Tom´e) Jos´e R´ıos-Montes Instituto de Matem´aticas Universidad Nacional Aut´onoma de M´exico ´ Area de la Investigaci´on Cient´ıfica Circuito Exterior, C.U. M´exico D.F. e-mail: jrios@matem.unam.mx