Yoneda embedding of Grp C

We show that group objects are exactly those whose yoneda presheaf is a presheaf of groups, by constructing the yoneda embedding Grp C ⥤ Cᵒᵖ ⥤ GrpCat.{v} and showing that it is fully faithful and its (essential) image is the representable functors.

def Grp.homMk {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H : C} [GrpObj G] [GrpObj H] (f : G ⟶ H) [IsMonHom f] : { X := G, grp := inst✝ } ⟶ { X := H, grp := inst✝¹ }

Construct a morphism G ⟶ H of Grp C C from a map f : G ⟶ H and a IsMonHom f instance.

Equations

• Grp.homMk f = { hom := f, isMonHom_hom := inst✝ }

@[simp]

theorem Grp.homMk_hom {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H : C} [GrpObj G] [GrpObj H] (f : G ⟶ H) [IsMonHom f] : (homMk f).hom = f

@[deprecated Grp.homMk (since := "2025-10-13")]

def Grp_.homMk {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H : C} [GrpObj G] [GrpObj H] (f : G ⟶ H) [IsMonHom f] : { X := G, grp := inst✝ } ⟶ { X := H, grp := inst✝¹ }

Alias of Grp.homMk.

---

Construct a morphism G ⟶ H of Grp C C from a map f : G ⟶ H and a IsMonHom f instance.

Equations

• @Grp_.homMk = @Grp.homMk

@[simp]

theorem Grp.homMk_hom' {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H : Grp C} (f : G ⟶ H) : homMk f.hom = f

@[deprecated Grp.homMk_hom' (since := "2025-10-13")]

theorem Grp_.homMk_hom' {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H : Grp C} (f : G ⟶ H) : Grp.homMk f.hom = f

Alias of Grp.homMk_hom'.

def GrpObj.ofRepresentableBy {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] (X : C) (F : CategoryTheory.Functor Cᵒᵖ GrpCat) (α : (F.comp (CategoryTheory.forget GrpCat)).RepresentableBy X) : GrpObj X

If X represents a presheaf of monoids, then X is a monoid object.

Equations

• One or more equations did not get rendered due to their size.

@[deprecated GrpObj.ofRepresentableBy (since := "2025-09-13")]

def Grp_Class.ofRepresentableBy {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] (X : C) (F : CategoryTheory.Functor Cᵒᵖ GrpCat) (α : (F.comp (CategoryTheory.forget GrpCat)).RepresentableBy X) : GrpObj X

Alias of GrpObj.ofRepresentableBy.

---

If X represents a presheaf of monoids, then X is a monoid object.

Equations

• @Grp_Class.ofRepresentableBy = @GrpObj.ofRepresentableBy

@[reducible, inline]

abbrev Hom.group {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G X : C} [GrpObj G] : Group (X ⟶ G)

If G is a group object, then Hom(X, G) has a group structure.

Equations

• One or more equations did not get rendered due to their size.

theorem Hom.inv_def {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G X : C} [GrpObj G] (f : X ⟶ G) : f⁻¹ = CategoryTheory.CategoryStruct.comp f GrpObj.inv

def yonedaGrpObj {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] (G : C) [GrpObj G] : CategoryTheory.Functor Cᵒᵖ GrpCat

If G is a group object, then Hom(-, G) is a presheaf of groups.

Equations

• One or more equations did not get rendered due to their size.

@[simp]

theorem yonedaGrpObj_obj_coe {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] (G : C) [GrpObj G] (X : Cᵒᵖ) : ↑((yonedaGrpObj G).obj X) = (Opposite.unop X ⟶ G)

@[simp]

theorem yonedaGrpObj_map {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] (G : C) [GrpObj G] {X✝ Y✝ : Cᵒᵖ} (φ : X✝ ⟶ Y✝) : (yonedaGrpObj G).map φ = GrpCat.ofHom (MonCat.Hom.hom ((yonedaMonObj G).map φ))

def yonedaGrpObjRepresentableBy {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] (G : C) [GrpObj G] : ((yonedaGrpObj G).comp (CategoryTheory.forget GrpCat)).RepresentableBy G

If G is a monoid object, then Hom(-, G) as a presheaf of monoids is represented by G.

Equations

• yonedaGrpObjRepresentableBy G = CategoryTheory.Functor.representableByEquiv.symm (CategoryTheory.Iso.refl (CategoryTheory.yoneda.obj G))

theorem GrpObj.ofRepresentableBy_yonedaGrpObjRepresentableBy {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] (G : C) [GrpObj G] : ofRepresentableBy G (yonedaGrpObj G) (yonedaGrpObjRepresentableBy G) = inst✝

@[deprecated GrpObj.ofRepresentableBy_yonedaGrpObjRepresentableBy (since := "2025-09-13")]

theorem Grp_Class.ofRepresentableBy_yonedaGrpObjRepresentableBy {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] (G : C) [GrpObj G] : GrpObj.ofRepresentableBy G (yonedaGrpObj G) (yonedaGrpObjRepresentableBy G) = inst✝

Alias of GrpObj.ofRepresentableBy_yonedaGrpObjRepresentableBy.

def yonedaGrpObjIsoOfRepresentableBy {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] (X : C) (F : CategoryTheory.Functor Cᵒᵖ GrpCat) (α : (F.comp (CategoryTheory.forget GrpCat)).RepresentableBy X) : yonedaGrpObj X ≅ F

If X represents a presheaf of groups F, then Hom(-, X) is isomorphic to F as a presheaf of groups.

Equations

• yonedaGrpObjIsoOfRepresentableBy X F α = CategoryTheory.NatIso.ofComponents (fun (Y : Cᵒᵖ) => (let __Equiv := α.homEquiv; { toEquiv := __Equiv, map_mul' := ⋯ }).toGrpIso) ⋯

@[simp]

theorem yonedaGrpObjIsoOfRepresentableBy_inv {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] (X : C) (F : CategoryTheory.Functor Cᵒᵖ GrpCat) (α : (F.comp (CategoryTheory.forget GrpCat)).RepresentableBy X) : (yonedaGrpObjIsoOfRepresentableBy X F α).inv = { app := fun (X_1 : Cᵒᵖ) => GrpCat.ofHom ↑{ toEquiv := α.homEquiv.symm, map_mul' := ⋯ }, naturality := ⋯ }

@[simp]

theorem yonedaGrpObjIsoOfRepresentableBy_hom {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] (X : C) (F : CategoryTheory.Functor Cᵒᵖ GrpCat) (α : (F.comp (CategoryTheory.forget GrpCat)).RepresentableBy X) : (yonedaGrpObjIsoOfRepresentableBy X F α).hom = { app := fun (X_1 : Cᵒᵖ) => GrpCat.ofHom ↑{ toEquiv := α.homEquiv, map_mul' := ⋯ }, naturality := ⋯ }

def yonedaGrp {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] : CategoryTheory.Functor (Grp C) (CategoryTheory.Functor Cᵒᵖ GrpCat)

The yoneda embedding of Grp_C into presheaves of groups.

Equations

• One or more equations did not get rendered due to their size.

@[simp]

theorem yonedaGrp_obj {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] (G : Grp C) : yonedaGrp.obj G = yonedaGrpObj G.X

@[simp]

theorem yonedaGrp_map_app {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H : Grp C} (ψ : G ⟶ H) (Y : Cᵒᵖ) : (yonedaGrp.map ψ).app Y = GrpCat.ofHom (MonCat.Hom.hom ((yonedaMon.map ψ).app Y))

theorem yonedaGrp_naturality {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H X Y : C} [GrpObj G] [GrpObj H] (α : yonedaGrpObj G ⟶ yonedaGrpObj H) (f : X ⟶ Y) (g : Y ⟶ G) : (CategoryTheory.ConcreteCategory.hom (α.app (Opposite.op X))) (CategoryTheory.CategoryStruct.comp f g) = CategoryTheory.CategoryStruct.comp f ((CategoryTheory.ConcreteCategory.hom (α.app (Opposite.op Y))) g)

theorem yonedaGrp_naturality_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H X Y : C} [GrpObj G] [GrpObj H] (α : yonedaGrpObj G ⟶ yonedaGrpObj H) (f : X ⟶ Y) (g : Y ⟶ G) {Z : C} (h : H ⟶ Z) : CategoryTheory.CategoryStruct.comp ((CategoryTheory.ConcreteCategory.hom (α.app (Opposite.op X))) (CategoryTheory.CategoryStruct.comp f g)) h = CategoryTheory.CategoryStruct.comp f (CategoryTheory.CategoryStruct.comp ((CategoryTheory.ConcreteCategory.hom (α.app (Opposite.op Y))) g) h)

def yonedaGrpFullyFaithful {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] : yonedaGrp.FullyFaithful

The yoneda embedding for Grp_C is fully faithful.

Equations

• One or more equations did not get rendered due to their size.

instance instFullGrpFunctorOppositeGrpCatYonedaGrp {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] : yonedaGrp.Full

instance instFaithfulGrpFunctorOppositeGrpCatYonedaGrp {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] : yonedaGrp.Faithful

theorem essImage_yonedaGrp {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] : yonedaGrp.essImage = (fun (x : CategoryTheory.Functor Cᵒᵖ GrpCat) => x.comp (CategoryTheory.forget GrpCat)) ⁻¹' setOf CategoryTheory.Functor.IsRepresentable

theorem GrpObj.inv_comp {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H X : C} [GrpObj G] [GrpObj H] (f : X ⟶ G) (g : G ⟶ H) [IsMonHom g] : CategoryTheory.CategoryStruct.comp f⁻¹ g = (CategoryTheory.CategoryStruct.comp f g)⁻¹

theorem GrpObj.inv_comp_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H X : C} [GrpObj G] [GrpObj H] (f : X ⟶ G) (g : G ⟶ H) [IsMonHom g] {Z : C} (h : H ⟶ Z) : CategoryTheory.CategoryStruct.comp f⁻¹ (CategoryTheory.CategoryStruct.comp g h) = CategoryTheory.CategoryStruct.comp (CategoryTheory.CategoryStruct.comp f g)⁻¹ h

@[deprecated GrpObj.inv_comp (since := "2025-09-13")]

theorem Grp_Class.inv_comp {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H X : C} [GrpObj G] [GrpObj H] (f : X ⟶ G) (g : G ⟶ H) [IsMonHom g] : CategoryTheory.CategoryStruct.comp f⁻¹ g = (CategoryTheory.CategoryStruct.comp f g)⁻¹

Alias of GrpObj.inv_comp.

theorem GrpObj.div_comp {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H X : C} [GrpObj G] [GrpObj H] (f g : X ⟶ G) (h : G ⟶ H) [IsMonHom h] : CategoryTheory.CategoryStruct.comp (f / g) h = CategoryTheory.CategoryStruct.comp f h / CategoryTheory.CategoryStruct.comp g h

theorem GrpObj.div_comp_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H X : C} [GrpObj G] [GrpObj H] (f g : X ⟶ G) (h : G ⟶ H) [IsMonHom h] {Z : C} (h✝ : H ⟶ Z) : CategoryTheory.CategoryStruct.comp (f / g) (CategoryTheory.CategoryStruct.comp h h✝) = CategoryTheory.CategoryStruct.comp (CategoryTheory.CategoryStruct.comp f h / CategoryTheory.CategoryStruct.comp g h) h✝

@[deprecated GrpObj.div_comp (since := "2025-09-13")]

theorem Grp_Class.div_comp {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H X : C} [GrpObj G] [GrpObj H] (f g : X ⟶ G) (h : G ⟶ H) [IsMonHom h] : CategoryTheory.CategoryStruct.comp (f / g) h = CategoryTheory.CategoryStruct.comp f h / CategoryTheory.CategoryStruct.comp g h

Alias of GrpObj.div_comp.

theorem GrpObj.zpow_comp {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H X : C} [GrpObj G] [GrpObj H] (f : X ⟶ G) (n : ℤ) (g : G ⟶ H) [IsMonHom g] : CategoryTheory.CategoryStruct.comp (f ^ n) g = CategoryTheory.CategoryStruct.comp f g ^ n

theorem GrpObj.zpow_comp_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H X : C} [GrpObj G] [GrpObj H] (f : X ⟶ G) (n : ℤ) (g : G ⟶ H) [IsMonHom g] {Z : C} (h : H ⟶ Z) : CategoryTheory.CategoryStruct.comp (f ^ n) (CategoryTheory.CategoryStruct.comp g h) = CategoryTheory.CategoryStruct.comp (CategoryTheory.CategoryStruct.comp f g ^ n) h

@[deprecated GrpObj.zpow_comp (since := "2025-09-13")]

theorem Grp_Class.zpow_comp {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G H X : C} [GrpObj G] [GrpObj H] (f : X ⟶ G) (n : ℤ) (g : G ⟶ H) [IsMonHom g] : CategoryTheory.CategoryStruct.comp (f ^ n) g = CategoryTheory.CategoryStruct.comp f g ^ n

Alias of GrpObj.zpow_comp.

theorem GrpObj.comp_inv {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G X Y : C} [GrpObj G] (f : X ⟶ Y) (g : Y ⟶ G) : CategoryTheory.CategoryStruct.comp f g⁻¹ = (CategoryTheory.CategoryStruct.comp f g)⁻¹

theorem GrpObj.comp_inv_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G X Y : C} [GrpObj G] (f : X ⟶ Y) (g : Y ⟶ G) {Z : C} (h : G ⟶ Z) : CategoryTheory.CategoryStruct.comp f (CategoryTheory.CategoryStruct.comp g⁻¹ h) = CategoryTheory.CategoryStruct.comp (CategoryTheory.CategoryStruct.comp f g)⁻¹ h

@[deprecated GrpObj.comp_inv (since := "2025-09-13")]

theorem Grp_Class.comp_inv {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G X Y : C} [GrpObj G] (f : X ⟶ Y) (g : Y ⟶ G) : CategoryTheory.CategoryStruct.comp f g⁻¹ = (CategoryTheory.CategoryStruct.comp f g)⁻¹

Alias of GrpObj.comp_inv.

theorem GrpObj.comp_div {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G X Y : C} [GrpObj G] (f : X ⟶ Y) (g h : Y ⟶ G) : CategoryTheory.CategoryStruct.comp f (g / h) = CategoryTheory.CategoryStruct.comp f g / CategoryTheory.CategoryStruct.comp f h

theorem GrpObj.comp_div_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G X Y : C} [GrpObj G] (f : X ⟶ Y) (g h : Y ⟶ G) {Z : C} (h✝ : G ⟶ Z) : CategoryTheory.CategoryStruct.comp f (CategoryTheory.CategoryStruct.comp (g / h) h✝) = CategoryTheory.CategoryStruct.comp (CategoryTheory.CategoryStruct.comp f g / CategoryTheory.CategoryStruct.comp f h) h✝

@[deprecated GrpObj.comp_div (since := "2025-09-13")]

theorem Grp_Class.comp_div {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G X Y : C} [GrpObj G] (f : X ⟶ Y) (g h : Y ⟶ G) : CategoryTheory.CategoryStruct.comp f (g / h) = CategoryTheory.CategoryStruct.comp f g / CategoryTheory.CategoryStruct.comp f h

Alias of GrpObj.comp_div.

theorem GrpObj.comp_zpow {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G X Y : C} [GrpObj G] (f : X ⟶ Y) (g : Y ⟶ G) (n : ℤ) : CategoryTheory.CategoryStruct.comp f (g ^ n) = CategoryTheory.CategoryStruct.comp f g ^ n

theorem GrpObj.comp_zpow_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G X Y : C} [GrpObj G] (f : X ⟶ Y) (g : Y ⟶ G) (n : ℤ) {Z : C} (h : G ⟶ Z) : CategoryTheory.CategoryStruct.comp f (CategoryTheory.CategoryStruct.comp (g ^ n) h) = CategoryTheory.CategoryStruct.comp (CategoryTheory.CategoryStruct.comp f g ^ n) h

@[deprecated GrpObj.comp_zpow (since := "2025-09-13")]

theorem Grp_Class.comp_zpow {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G X Y : C} [GrpObj G] (f : X ⟶ Y) (g : Y ⟶ G) (n : ℤ) : CategoryTheory.CategoryStruct.comp f (g ^ n) = CategoryTheory.CategoryStruct.comp f g ^ n

Alias of GrpObj.comp_zpow.

theorem GrpObj.inv_eq_inv {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G : C} [GrpObj G] : inv = (CategoryTheory.CategoryStruct.id G)⁻¹

@[simp]

theorem GrpObj.one_inv {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G : C} [GrpObj G] : CategoryTheory.CategoryStruct.comp MonObj.one inv = MonObj.one

@[simp]

theorem GrpObj.one_inv_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G : C} [GrpObj G] {Z : C} (h : G ⟶ Z) : CategoryTheory.CategoryStruct.comp MonObj.one (CategoryTheory.CategoryStruct.comp inv h) = CategoryTheory.CategoryStruct.comp MonObj.one h

@[deprecated GrpObj.inv_eq_inv (since := "2025-09-13")]

theorem Grp_Class.inv_eq_inv {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G : C} [GrpObj G] : GrpObj.inv = (CategoryTheory.CategoryStruct.id G)⁻¹

Alias of GrpObj.inv_eq_inv.

instance instIsMonHomInvOfIsCommMonObj {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G : C} [GrpObj G] [CategoryTheory.BraidedCategory C] [IsCommMonObj G] : IsMonHom GrpObj.inv

instance instIsMonHomInvHomOfIsCommMonObj {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {M G : C} [MonObj M] [GrpObj G] [CategoryTheory.BraidedCategory C] [IsCommMonObj G] {f : M ⟶ G} [IsMonHom f] : IsMonHom f⁻¹

@[reducible, inline]

abbrev Hom.commGroup {C : Type u} [CategoryTheory.Category.{v, u} C] [CategoryTheory.CartesianMonoidalCategory C] {G X : C} [GrpObj G] [CategoryTheory.BraidedCategory C] [IsCommMonObj G] : CommGroup (X ⟶ G)

If G is a commutative group object, then Hom(X, G) has a commutative group structure.

Equations

• Hom.commGroup = { toGroup := Hom.group, mul_comm := ⋯ }