HepLean Documentation

Mathlib.CategoryTheory.Comma.Basic

Comma categories #

A comma category is a construction in category theory, which builds a category out of two functors with a common codomain. Specifically, for functors L : A ⥤ T and R : B ⥤ T, an object in Comma L R is a morphism hom : L.obj left ⟶ R.obj right for some objects left : A and right : B, and a morphism in Comma L R between hom : L.obj left ⟶ R.obj right and hom' : L.obj left' ⟶ R.obj right' is a commutative square

L.obj left  ⟶  L.obj left'
      |               |
  hom |               | hom'
      ↓               ↓
R.obj right ⟶  R.obj right',

where the top and bottom morphism come from morphisms left ⟶ left' and right ⟶ right', respectively.

Main definitions #

References #

Tags #

comma, slice, coslice, over, under, arrow

The objects of the comma category are triples of an object left : A, an object right : B and a morphism hom : L.obj left ⟶ R.obj right.

  • left : A
  • right : B
  • hom : L.obj self.left R.obj self.right
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    A morphism between two objects in the comma category is a commutative square connecting the morphisms coming from the two objects using morphisms in the image of the functors L and R.

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      theorem CategoryTheory.CommaMorphism.ext {A : Type u₁} {inst✝ : CategoryTheory.Category.{v₁, u₁} A} {B : Type u₂} {inst✝¹ : CategoryTheory.Category.{v₂, u₂} B} {T : Type u₃} {inst✝² : CategoryTheory.Category.{v₃, u₃} T} {L : CategoryTheory.Functor A T} {R : CategoryTheory.Functor B T} {X Y : CategoryTheory.Comma L R} {x y : CategoryTheory.CommaMorphism X Y} (left : x.left = y.left) (right : x.right = y.right) :
      x = y
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      theorem CategoryTheory.Comma.hom_ext {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] {L : CategoryTheory.Functor A T} {R : CategoryTheory.Functor B T} {X Y : CategoryTheory.Comma L R} (f g : X Y) (h₁ : f.left = g.left) (h₂ : f.right = g.right) :
      f = g

      The functor sending an object X in the comma category to X.left.

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        The functor sending an object X in the comma category to X.right.

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          We can interpret the commutative square constituting a morphism in the comma category as a natural transformation between the functors fst ⋙ L and snd ⋙ R from the comma category to T, where the components are given by the morphism that constitutes an object of the comma category.

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            Extract the isomorphism between the left objects from an isomorphism in the comma category.

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              Extract the isomorphism between the right objects from an isomorphism in the comma category.

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                def CategoryTheory.Comma.isoMk {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] {L₁ : CategoryTheory.Functor A T} {R₁ : CategoryTheory.Functor B T} {X Y : CategoryTheory.Comma L₁ R₁} (l : X.left Y.left) (r : X.right Y.right) (h : CategoryTheory.CategoryStruct.comp (L₁.map l.hom) Y.hom = CategoryTheory.CategoryStruct.comp X.hom (R₁.map r.hom) := by aesop_cat) :
                X Y

                Construct an isomorphism in the comma category given isomorphisms of the objects whose forward directions give a commutative square.

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                • CategoryTheory.Comma.isoMk l r h = { hom := { left := l.hom, right := r.hom, w := h }, inv := { left := l.inv, right := r.inv, w := }, hom_inv_id := , inv_hom_id := }
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                  @[simp]
                  theorem CategoryTheory.Comma.isoMk_hom_left {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] {L₁ : CategoryTheory.Functor A T} {R₁ : CategoryTheory.Functor B T} {X Y : CategoryTheory.Comma L₁ R₁} (l : X.left Y.left) (r : X.right Y.right) (h : CategoryTheory.CategoryStruct.comp (L₁.map l.hom) Y.hom = CategoryTheory.CategoryStruct.comp X.hom (R₁.map r.hom) := by aesop_cat) :
                  (CategoryTheory.Comma.isoMk l r h).hom.left = l.hom
                  @[simp]
                  theorem CategoryTheory.Comma.isoMk_inv_left {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] {L₁ : CategoryTheory.Functor A T} {R₁ : CategoryTheory.Functor B T} {X Y : CategoryTheory.Comma L₁ R₁} (l : X.left Y.left) (r : X.right Y.right) (h : CategoryTheory.CategoryStruct.comp (L₁.map l.hom) Y.hom = CategoryTheory.CategoryStruct.comp X.hom (R₁.map r.hom) := by aesop_cat) :
                  (CategoryTheory.Comma.isoMk l r h).inv.left = l.inv
                  @[simp]
                  theorem CategoryTheory.Comma.isoMk_hom_right {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] {L₁ : CategoryTheory.Functor A T} {R₁ : CategoryTheory.Functor B T} {X Y : CategoryTheory.Comma L₁ R₁} (l : X.left Y.left) (r : X.right Y.right) (h : CategoryTheory.CategoryStruct.comp (L₁.map l.hom) Y.hom = CategoryTheory.CategoryStruct.comp X.hom (R₁.map r.hom) := by aesop_cat) :
                  (CategoryTheory.Comma.isoMk l r h).hom.right = r.hom
                  @[simp]
                  theorem CategoryTheory.Comma.isoMk_inv_right {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] {L₁ : CategoryTheory.Functor A T} {R₁ : CategoryTheory.Functor B T} {X Y : CategoryTheory.Comma L₁ R₁} (l : X.left Y.left) (r : X.right Y.right) (h : CategoryTheory.CategoryStruct.comp (L₁.map l.hom) Y.hom = CategoryTheory.CategoryStruct.comp X.hom (R₁.map r.hom) := by aesop_cat) :
                  (CategoryTheory.Comma.isoMk l r h).inv.right = r.inv

                  The functor Comma L R ⥤ Comma L' R' induced by three functors F₁, F₂, F and two natural transformations F₁ ⋙ L' ⟶ L ⋙ F and R ⋙ F ⟶ F₂ ⋙ R'.

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                    @[simp]
                    theorem CategoryTheory.Comma.map_map_right {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] {A' : Type u₄} [CategoryTheory.Category.{v₄, u₄} A'] {B' : Type u₅} [CategoryTheory.Category.{v₅, u₅} B'] {T' : Type u₆} [CategoryTheory.Category.{v₆, u₆} T'] {L : CategoryTheory.Functor A T} {R : CategoryTheory.Functor B T} {L' : CategoryTheory.Functor A' T'} {R' : CategoryTheory.Functor B' T'} {F₁ : CategoryTheory.Functor A A'} {F₂ : CategoryTheory.Functor B B'} {F : CategoryTheory.Functor T T'} (α : F₁.comp L' L.comp F) (β : R.comp F F₂.comp R') {X Y : CategoryTheory.Comma L R} (φ : X Y) :
                    ((CategoryTheory.Comma.map α β).map φ).right = F₂.map φ.right
                    @[simp]
                    theorem CategoryTheory.Comma.map_obj_left {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] {A' : Type u₄} [CategoryTheory.Category.{v₄, u₄} A'] {B' : Type u₅} [CategoryTheory.Category.{v₅, u₅} B'] {T' : Type u₆} [CategoryTheory.Category.{v₆, u₆} T'] {L : CategoryTheory.Functor A T} {R : CategoryTheory.Functor B T} {L' : CategoryTheory.Functor A' T'} {R' : CategoryTheory.Functor B' T'} {F₁ : CategoryTheory.Functor A A'} {F₂ : CategoryTheory.Functor B B'} {F : CategoryTheory.Functor T T'} (α : F₁.comp L' L.comp F) (β : R.comp F F₂.comp R') (X : CategoryTheory.Comma L R) :
                    ((CategoryTheory.Comma.map α β).obj X).left = F₁.obj X.left
                    @[simp]
                    theorem CategoryTheory.Comma.map_obj_right {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] {A' : Type u₄} [CategoryTheory.Category.{v₄, u₄} A'] {B' : Type u₅} [CategoryTheory.Category.{v₅, u₅} B'] {T' : Type u₆} [CategoryTheory.Category.{v₆, u₆} T'] {L : CategoryTheory.Functor A T} {R : CategoryTheory.Functor B T} {L' : CategoryTheory.Functor A' T'} {R' : CategoryTheory.Functor B' T'} {F₁ : CategoryTheory.Functor A A'} {F₂ : CategoryTheory.Functor B B'} {F : CategoryTheory.Functor T T'} (α : F₁.comp L' L.comp F) (β : R.comp F F₂.comp R') (X : CategoryTheory.Comma L R) :
                    ((CategoryTheory.Comma.map α β).obj X).right = F₂.obj X.right
                    @[simp]
                    theorem CategoryTheory.Comma.map_map_left {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] {A' : Type u₄} [CategoryTheory.Category.{v₄, u₄} A'] {B' : Type u₅} [CategoryTheory.Category.{v₅, u₅} B'] {T' : Type u₆} [CategoryTheory.Category.{v₆, u₆} T'] {L : CategoryTheory.Functor A T} {R : CategoryTheory.Functor B T} {L' : CategoryTheory.Functor A' T'} {R' : CategoryTheory.Functor B' T'} {F₁ : CategoryTheory.Functor A A'} {F₂ : CategoryTheory.Functor B B'} {F : CategoryTheory.Functor T T'} (α : F₁.comp L' L.comp F) (β : R.comp F F₂.comp R') {X Y : CategoryTheory.Comma L R} (φ : X Y) :
                    ((CategoryTheory.Comma.map α β).map φ).left = F₁.map φ.left
                    instance CategoryTheory.Comma.faithful_map {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] {A' : Type u₄} [CategoryTheory.Category.{v₄, u₄} A'] {B' : Type u₅} [CategoryTheory.Category.{v₅, u₅} B'] {T' : Type u₆} [CategoryTheory.Category.{v₆, u₆} T'] {L : CategoryTheory.Functor A T} {R : CategoryTheory.Functor B T} {L' : CategoryTheory.Functor A' T'} {R' : CategoryTheory.Functor B' T'} {F₁ : CategoryTheory.Functor A A'} {F₂ : CategoryTheory.Functor B B'} {F : CategoryTheory.Functor T T'} (α : F₁.comp L' L.comp F) (β : R.comp F F₂.comp R') [F₁.Faithful] [F₂.Faithful] :
                    (CategoryTheory.Comma.map α β).Faithful
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                    instance CategoryTheory.Comma.full_map {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] {A' : Type u₄} [CategoryTheory.Category.{v₄, u₄} A'] {B' : Type u₅} [CategoryTheory.Category.{v₅, u₅} B'] {T' : Type u₆} [CategoryTheory.Category.{v₆, u₆} T'] {L : CategoryTheory.Functor A T} {R : CategoryTheory.Functor B T} {L' : CategoryTheory.Functor A' T'} {R' : CategoryTheory.Functor B' T'} {F₁ : CategoryTheory.Functor A A'} {F₂ : CategoryTheory.Functor B B'} {F : CategoryTheory.Functor T T'} (α : F₁.comp L' L.comp F) (β : R.comp F F₂.comp R') [F.Faithful] [F₁.Full] [F₂.Full] [CategoryTheory.IsIso α] [CategoryTheory.IsIso β] :
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                    instance CategoryTheory.Comma.essSurj_map {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] {A' : Type u₄} [CategoryTheory.Category.{v₄, u₄} A'] {B' : Type u₅} [CategoryTheory.Category.{v₅, u₅} B'] {T' : Type u₆} [CategoryTheory.Category.{v₆, u₆} T'] {L : CategoryTheory.Functor A T} {R : CategoryTheory.Functor B T} {L' : CategoryTheory.Functor A' T'} {R' : CategoryTheory.Functor B' T'} {F₁ : CategoryTheory.Functor A A'} {F₂ : CategoryTheory.Functor B B'} {F : CategoryTheory.Functor T T'} (α : F₁.comp L' L.comp F) (β : R.comp F F₂.comp R') [F₁.EssSurj] [F₂.EssSurj] [F.Full] [CategoryTheory.IsIso α] [CategoryTheory.IsIso β] :
                    (CategoryTheory.Comma.map α β).EssSurj
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                    noncomputable instance CategoryTheory.Comma.isEquivalenceMap {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] {A' : Type u₄} [CategoryTheory.Category.{v₄, u₄} A'] {B' : Type u₅} [CategoryTheory.Category.{v₅, u₅} B'] {T' : Type u₆} [CategoryTheory.Category.{v₆, u₆} T'] {L : CategoryTheory.Functor A T} {R : CategoryTheory.Functor B T} {L' : CategoryTheory.Functor A' T'} {R' : CategoryTheory.Functor B' T'} {F₁ : CategoryTheory.Functor A A'} {F₂ : CategoryTheory.Functor B B'} {F : CategoryTheory.Functor T T'} (α : F₁.comp L' L.comp F) (β : R.comp F F₂.comp R') [F₁.IsEquivalence] [F₂.IsEquivalence] [F.Faithful] [F.Full] [CategoryTheory.IsIso α] [CategoryTheory.IsIso β] :
                    (CategoryTheory.Comma.map α β).IsEquivalence
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                    @[simp]

                    The equality between map α β ⋙ fst L' R' and fst L R ⋙ F₁, where α : F₁ ⋙ L' ⟶ L ⋙ F.

                    The isomorphism between map α β ⋙ fst L' R' and fst L R ⋙ F₁, where α : F₁ ⋙ L' ⟶ L ⋙ F.

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                      @[simp]

                      The equality between map α β ⋙ snd L' R' and snd L R ⋙ F₂, where β : R ⋙ F ⟶ F₂ ⋙ R'.

                      The isomorphism between map α β ⋙ snd L' R' and snd L R ⋙ F₂, where β : R ⋙ F ⟶ F₂ ⋙ R'.

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                        A natural transformation L₁ ⟶ L₂ induces a functor Comma L₂ R ⥤ Comma L₁ R.

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                          theorem CategoryTheory.Comma.mapLeft_map_right {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] (R : CategoryTheory.Functor B T) {L₁ L₂ : CategoryTheory.Functor A T} (l : L₁ L₂) {X✝ Y✝ : CategoryTheory.Comma L₂ R} (f : X✝ Y✝) :
                          ((CategoryTheory.Comma.mapLeft R l).map f).right = f.right
                          @[simp]
                          theorem CategoryTheory.Comma.mapLeft_map_left {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] (R : CategoryTheory.Functor B T) {L₁ L₂ : CategoryTheory.Functor A T} (l : L₁ L₂) {X✝ Y✝ : CategoryTheory.Comma L₂ R} (f : X✝ Y✝) :
                          ((CategoryTheory.Comma.mapLeft R l).map f).left = f.left

                          The functor Comma L R ⥤ Comma L R induced by the identity natural transformation on L is naturally isomorphic to the identity functor.

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                            The functor Comma L₁ R ⥤ Comma L₃ R induced by the composition of two natural transformations l : L₁ ⟶ L₂ and l' : L₂ ⟶ L₃ is naturally isomorphic to the composition of the two functors induced by these natural transformations.

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                              Two equal natural transformations L₁ ⟶ L₂ yield naturally isomorphic functors Comma L₁ R ⥤ Comma L₂ R.

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                                A natural isomorphism L₁ ≅ L₂ induces an equivalence of categories Comma L₁ R ≌ Comma L₂ R.

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                                  theorem CategoryTheory.Comma.mapLeftIso_inverse_map_right {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] (R : CategoryTheory.Functor B T) {L₁ L₂ : CategoryTheory.Functor A T} (i : L₁ L₂) {X✝ Y✝ : CategoryTheory.Comma L₂ R} (f : X✝ Y✝) :
                                  ((CategoryTheory.Comma.mapLeftIso R i).inverse.map f).right = f.right
                                  @[simp]
                                  theorem CategoryTheory.Comma.mapLeftIso_functor_map_right {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] (R : CategoryTheory.Functor B T) {L₁ L₂ : CategoryTheory.Functor A T} (i : L₁ L₂) {X✝ Y✝ : CategoryTheory.Comma L₁ R} (f : X✝ Y✝) :
                                  ((CategoryTheory.Comma.mapLeftIso R i).functor.map f).right = f.right
                                  @[simp]
                                  theorem CategoryTheory.Comma.mapLeftIso_inverse_map_left {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] (R : CategoryTheory.Functor B T) {L₁ L₂ : CategoryTheory.Functor A T} (i : L₁ L₂) {X✝ Y✝ : CategoryTheory.Comma L₂ R} (f : X✝ Y✝) :
                                  ((CategoryTheory.Comma.mapLeftIso R i).inverse.map f).left = f.left
                                  @[simp]
                                  theorem CategoryTheory.Comma.mapLeftIso_functor_map_left {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] (R : CategoryTheory.Functor B T) {L₁ L₂ : CategoryTheory.Functor A T} (i : L₁ L₂) {X✝ Y✝ : CategoryTheory.Comma L₁ R} (f : X✝ Y✝) :
                                  ((CategoryTheory.Comma.mapLeftIso R i).functor.map f).left = f.left

                                  A natural transformation R₁ ⟶ R₂ induces a functor Comma L R₁ ⥤ Comma L R₂.

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                                    theorem CategoryTheory.Comma.mapRight_map_right {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] (L : CategoryTheory.Functor A T) {R₁ R₂ : CategoryTheory.Functor B T} (r : R₁ R₂) {X✝ Y✝ : CategoryTheory.Comma L R₁} (f : X✝ Y✝) :
                                    ((CategoryTheory.Comma.mapRight L r).map f).right = f.right
                                    @[simp]
                                    theorem CategoryTheory.Comma.mapRight_map_left {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] (L : CategoryTheory.Functor A T) {R₁ R₂ : CategoryTheory.Functor B T} (r : R₁ R₂) {X✝ Y✝ : CategoryTheory.Comma L R₁} (f : X✝ Y✝) :
                                    ((CategoryTheory.Comma.mapRight L r).map f).left = f.left

                                    The functor Comma L R ⥤ Comma L R induced by the identity natural transformation on R is naturally isomorphic to the identity functor.

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                                      The functor Comma L R₁ ⥤ Comma L R₃ induced by the composition of the natural transformations r : R₁ ⟶ R₂ and r' : R₂ ⟶ R₃ is naturally isomorphic to the composition of the functors induced by these natural transformations.

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                                        Two equal natural transformations R₁ ⟶ R₂ yield naturally isomorphic functors Comma L R₁ ⥤ Comma L R₂.

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                                          A natural isomorphism R₁ ≅ R₂ induces an equivalence of categories Comma L R₁ ≌ Comma L R₂.

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                                            @[simp]
                                            theorem CategoryTheory.Comma.mapRightIso_functor_map_left {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] (L : CategoryTheory.Functor A T) {R₁ R₂ : CategoryTheory.Functor B T} (i : R₁ R₂) {X✝ Y✝ : CategoryTheory.Comma L R₁} (f : X✝ Y✝) :
                                            ((CategoryTheory.Comma.mapRightIso L i).functor.map f).left = f.left
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                                            theorem CategoryTheory.Comma.mapRightIso_inverse_map_right {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] (L : CategoryTheory.Functor A T) {R₁ R₂ : CategoryTheory.Functor B T} (i : R₁ R₂) {X✝ Y✝ : CategoryTheory.Comma L R₂} (f : X✝ Y✝) :
                                            ((CategoryTheory.Comma.mapRightIso L i).inverse.map f).right = f.right
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                                            theorem CategoryTheory.Comma.mapRightIso_inverse_map_left {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] (L : CategoryTheory.Functor A T) {R₁ R₂ : CategoryTheory.Functor B T} (i : R₁ R₂) {X✝ Y✝ : CategoryTheory.Comma L R₂} (f : X✝ Y✝) :
                                            ((CategoryTheory.Comma.mapRightIso L i).inverse.map f).left = f.left
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                                            theorem CategoryTheory.Comma.mapRightIso_functor_map_right {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] (L : CategoryTheory.Functor A T) {R₁ R₂ : CategoryTheory.Functor B T} (i : R₁ R₂) {X✝ Y✝ : CategoryTheory.Comma L R₁} (f : X✝ Y✝) :
                                            ((CategoryTheory.Comma.mapRightIso L i).functor.map f).right = f.right

                                            The functor (F ⋙ L, R) ⥤ (L, R)

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                                              Comma.preLeft is a particular case of Comma.map, but with better definitional properties.

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                                                If F is an equivalence, then so is preLeft F L R.

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                                                The functor (F ⋙ L, R) ⥤ (L, R)

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                                                  Comma.preRight is a particular case of Comma.map, but with better definitional properties.

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                                                    If F is an equivalence, then so is preRight L F R.

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                                                    The functor (L, R) ⥤ (L ⋙ F, R ⋙ F)

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                                                      Comma.post is a particular case of Comma.map, but with better definitional properties.

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                                                        If F is an equivalence, then so is post L R F.

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                                                        The canonical functor from the product of two categories to the comma category of their respective functors into Discrete PUnit.

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                                                          Taking the comma category of two functors into Discrete PUnit results in something is equivalent to their product.

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                                                            The canonical functor from Comma L R to (Comma R.op L.op)ᵒᵖ.

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                                                              The canonical functor from Comma L.op R.op to (Comma R L)ᵒᵖ.

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                                                                theorem CategoryTheory.Comma.unopFunctor_map {A : Type u₁} [CategoryTheory.Category.{v₁, u₁} A] {B : Type u₂} [CategoryTheory.Category.{v₂, u₂} B] {T : Type u₃} [CategoryTheory.Category.{v₃, u₃} T] (L : CategoryTheory.Functor A T) (R : CategoryTheory.Functor B T) {X✝ Y✝ : CategoryTheory.Comma L.op R.op} (f : X✝ Y✝) :
                                                                (CategoryTheory.Comma.unopFunctor L R).map f = Opposite.op { left := f.right.unop, right := f.left.unop, w := }

                                                                The canonical equivalence between Comma L R and (Comma R.op L.op)ᵒᵖ.

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