Resumen de Proper periods of normal N.E.C. subgroups with even index.
By a non-Euclidean crystallographic (N.E.C.) group we shall mean a discrete subgroup G of isometries of the non-Euclidean plane including those reverse orientation, reflections and glide-reflections.
In [1] we computed the proper periods of normal N.E.C. subgroups of an N.E.C. group, when the index of the group with respect to the subgroup is odd. In this paper we shall compute the proper period of normal N.E.C. subgroups, when the index is even.
The corresponding problem for Fuchsian groups, which contain only orientable transformations, is essentially solved in the work of Maclachan [4].
