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• Alejandro Alvarado-Garc´ıa, C´esar Cejudo-Castilla, Hugo Alberto Rinc´on-Mej´ıa, Ivan Fernando Vilchis-Montalvo Departmento de Matem´aticas 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 G.) cesarcc@ciencias.unam.mx (C. Cejudo C.) hurincon@gmail.com (H. A. Rinc´on M.) vilchis.f@gmail.com (I.F. Vilchis M.)