  We consider the problem of characterizing, for certain natural number $m$,
the local $\mathcal{C}^m$-non-integrability near elliptic fixed points of
smooth planar measure preserving maps. Our criterion relates this
non-integrability with the existence of some Lie Symmetries associated to the
maps, together with the study of the finiteness of its periodic points. One of
the steps in the proof uses the regularity of the period function on the whole
period annulus for non-degenerate centers, question that we believe that is
interesting by itself. The obtained criterion can be applied to prove the local
non-integrability of the Cohen map and of several rational maps coming from
second order difference equations.
