Section CentHom.
Variable f : 'M[R]_n.
Definition rcent := [set x in G | f *m rG x == rG x *m f].
Canonical rcent_group := Group rcent_group_set.
Definition centgmx := G \subset rcent.

Lemma centgmxP : reflect (forall x, x \in G -> f *m rG x = rG x *m f) centgmx.