This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Jósé L. Ramírez
Instituto de Matemáticas y sus Aplicaciones Universidad Sergio Arboleda, Colombia
Gustavo N. Rubiano
Departamento de Matemáticas Universidad Nációnál de Colombia, Colombia