Section Def.
Variable G G' : GROUP.
Variable f : Hom G G'.
Definition kernel_part : part_set G.
apply
(Build_Predicate (E:=G)
(Pred_fun:=fun x : G => Equal (f x) (monoid_unit G'))).
red in |- *.
intros x y H' H'0; try assumption.
apply Trans with (Ap (sgroup_map (monoid_sgroup_hom f)) x); auto with algebra.
Defined.
Definition Ker : subgroup G.
apply (BUILD_SUB_GROUP (G:=G) (H:=kernel_part)).
simpl in |- *.
intros x y H' H'0; try assumption.
apply
Trans
with
(sgroup_law _ (Ap (sgroup_map (monoid_sgroup_hom f)) x)
(Ap (sgroup_map (monoid_sgroup_hom f)) y));
auto with algebra.
apply Trans with (sgroup_law G' (monoid_unit G') (monoid_unit G'));
auto with algebra.
simpl in |- *.
auto with algebra.
simpl in |- *.
intros x H'; try assumption.
apply Trans with (group_inverse _ (Ap (sgroup_map (monoid_sgroup_hom f)) x));
auto with algebra.
apply Trans with (group_inverse _ (monoid_unit G')); auto with algebra.
Defined.
Definition coKer : subgroup G'.
apply (BUILD_SUB_GROUP (G:=G') (H:=image f (full G))).
intros x y H' H'0; try assumption.
elim H'0; intros x0 E; elim E; intros H'1 H'2; try exact H'2; clear E H'0.
elim H'; intros x1 E; elim E; intros H'0 H'3; try exact H'3; clear E H'.
exists (sgroup_law _ x1 x0); split; [ try assumption | idtac ].
apply
Trans
with
(sgroup_law G' (Ap (sgroup_map (monoid_sgroup_hom f)) x1)
(Ap (sgroup_map (monoid_sgroup_hom f)) x0));
auto with algebra.
simpl in |- *.
exists (monoid_unit G); auto with algebra.
simpl in |- *.
intros x H'; try assumption.
elim H'; intros x0 E; elim E; intros H'0 H'1; try exact H'1; clear E H'.
exists (group_inverse _ x0); split; [ try assumption | idtac ].
apply
Trans with (group_inverse G' (Ap (sgroup_map (monoid_sgroup_hom f)) x0));
auto with algebra.
Defined.

Lemma kernel_normal : normal Ker.