# Library Algebra.Sgroup_cat

Set Implicit Arguments.
Unset Strict Implicit.
Require Export Subcat.
Require Export Set_cat.
Title "The category of semi-groups."

Definition law_of_composition (E : SET) := Hom (cart E E:SET) E.

Definition associative (E : SET) (f : law_of_composition E) :=
forall x y z : E,
Equal (f (couple (f (couple x y)) z)) (f (couple x (f (couple y z)))).

Record sgroup_on (E : SET) : Type :=
{sgroup_law_map : law_of_composition E;
sgroup_assoc_prf : associative sgroup_law_map}.

Record sgroup : Type :=
{sgroup_set :> Setoid; sgroup_on_def :> sgroup_on sgroup_set}.
Coercion Build_sgroup : sgroup_on >-> sgroup.
Set Strict Implicit.
Unset Implicit Arguments.

Definition sgroup_law (E : sgroup) : E -> E -> E :=
fun x y : E:Setoid => sgroup_law_map E (couple x y).
Set Implicit Arguments.
Unset Strict Implicit.
Section Hom.
Variable E F : sgroup.

Definition sgroup_hom_prop (f : Hom (E:SET) F) :=
forall x y : E, Equal (f (sgroup_law _ x y)) (sgroup_law _ (f x) (f y)).

Record sgroup_hom : Type :=
{sgroup_map :> Map E F; sgroup_hom_prf : sgroup_hom_prop sgroup_map}.
End Hom.

Definition sgroup_hom_comp :
forall E F G : sgroup, sgroup_hom F G -> sgroup_hom E F -> sgroup_hom E G.
intros E F G g f; try assumption.
apply (Build_sgroup_hom (sgroup_map:=comp_map_map g f)).
unfold sgroup_hom_prop in |- *; auto with algebra.
simpl in |- *.
unfold comp_map_fun in |- *.
intros x y; try assumption.
apply
Trans
with
(Ap (sgroup_map g)
(sgroup_law _ (Ap (sgroup_map f) x) (Ap (sgroup_map f) y)));
auto with algebra.
cut
(Equal (Ap (sgroup_map f) (sgroup_law _ x y))
(sgroup_law _ (Ap (sgroup_map f) x) (Ap (sgroup_map f) y))).
auto with algebra.
apply (sgroup_hom_prf f).
apply (sgroup_hom_prf g).
Defined.

Definition sgroup_id : forall E : sgroup, sgroup_hom E E.
intros E; try assumption.
apply (Build_sgroup_hom (sgroup_map:=Id E)).
red in |- *.
simpl in |- *; auto with algebra.
Defined.

Definition SGROUP : category.
apply
(subcat (C:=SET) (C':=sgroup) (i:=sgroup_set)
(homC':=fun E F : sgroup =>
Build_subtype_image (E:=MAP E F)
(subtype_image_carrier:=sgroup_hom E F)
(sgroup_map (E:=E) (F:=F))) (CompC':=sgroup_hom_comp)
(idC':=sgroup_id)).
simpl in |- *.
intros a; try assumption.
red in |- *.
auto with algebra.
simpl in |- *.
intros a b c g f; try assumption.
red in |- *.
auto with algebra.
Defined.