End Stabiliser.
(* Centralizer subgroup and central homomorphisms. *)
Section CentHom.
Variable f : 'M[R]_n.
Definition rcent := [set x in G | f *m rG x == rG x *m f].

(* Specifications: *)
Lemma rcent_sub : rcent \subset G.
Canonical rcent_group := Group rcent_group_set.