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Mathlib.CategoryTheory.Square

The category of commutative squares #

In this file, we define a bundled version of CommSq which allows to consider commutative squares as objects in a category Square C.

The four objects in a commutative square are numbered as follows:

X₁ --> X₂
|      |
v      v
X₃ --> X₄

We define the flip functor, and two equivalences with the category Arrow (Arrow C), depending on whether we consider a commutative square as a horizontal morphism between two vertical maps (arrowArrowEquivalence) or a vertical morphism between two horizontal maps (arrowArrowEquivalence').

The category of commutative squares in a category.

  • X₁ : C

    the top-left object

  • X₂ : C

    the top-right object

  • X₃ : C

    the bottom-left object

  • X₄ : C

    the bottom-right object

  • f₁₂ : self.X₁ self.X₂

    the top morphism

  • f₁₃ : self.X₁ self.X₃

    the left morphism

  • f₂₄ : self.X₂ self.X₄

    the right morphism

  • f₃₄ : self.X₃ self.X₄

    the bottom morphism

  • fac : CategoryTheory.CategoryStruct.comp self.f₁₂ self.f₂₄ = CategoryTheory.CategoryStruct.comp self.f₁₃ self.f₃₄
Instances For
    theorem CategoryTheory.Square.commSq {C : Type u} [CategoryTheory.Category.{v, u} C] (sq : CategoryTheory.Square C) :
    CategoryTheory.CommSq sq.f₁₂ sq.f₁₃ sq.f₂₄ sq.f₃₄

    A morphism between two commutative squares consists of 4 morphisms which extend these two squares into a commuting cube.

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      theorem CategoryTheory.Square.Hom.ext {C : Type u} {inst✝ : CategoryTheory.Category.{v, u} C} {sq₁ sq₂ : CategoryTheory.Square C} {x y : sq₁.Hom sq₂} (τ₁ : x.τ₁ = y.τ₁) (τ₂ : x.τ₂ = y.τ₂) (τ₃ : x.τ₃ = y.τ₃) (τ₄ : x.τ₄ = y.τ₄) :
      x = y
      @[simp]
      theorem CategoryTheory.Square.Hom.comm₁₃_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] {sq₁ sq₂ : CategoryTheory.Square C} (self : sq₁.Hom sq₂) {Z : C} (h : sq₂.X₃ Z) :
      @[simp]
      theorem CategoryTheory.Square.Hom.comm₁₂_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] {sq₁ sq₂ : CategoryTheory.Square C} (self : sq₁.Hom sq₂) {Z : C} (h : sq₂.X₂ Z) :
      @[simp]
      theorem CategoryTheory.Square.Hom.comm₃₄_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] {sq₁ sq₂ : CategoryTheory.Square C} (self : sq₁.Hom sq₂) {Z : C} (h : sq₂.X₄ Z) :
      @[simp]
      theorem CategoryTheory.Square.Hom.comm₂₄_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] {sq₁ sq₂ : CategoryTheory.Square C} (self : sq₁.Hom sq₂) {Z : C} (h : sq₂.X₄ Z) :

      The identity of a commutative square.

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        def CategoryTheory.Square.Hom.comp {C : Type u} [CategoryTheory.Category.{v, u} C] {sq₁ sq₂ sq₃ : CategoryTheory.Square C} (f : sq₁.Hom sq₂) (g : sq₂.Hom sq₃) :
        sq₁.Hom sq₃

        The composition of morphisms of squares.

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          @[simp]
          theorem CategoryTheory.Square.Hom.comp_τ₂ {C : Type u} [CategoryTheory.Category.{v, u} C] {sq₁ sq₂ sq₃ : CategoryTheory.Square C} (f : sq₁.Hom sq₂) (g : sq₂.Hom sq₃) :
          (f.comp g).τ₂ = CategoryTheory.CategoryStruct.comp f.τ₂ g.τ₂
          @[simp]
          theorem CategoryTheory.Square.Hom.comp_τ₁ {C : Type u} [CategoryTheory.Category.{v, u} C] {sq₁ sq₂ sq₃ : CategoryTheory.Square C} (f : sq₁.Hom sq₂) (g : sq₂.Hom sq₃) :
          (f.comp g).τ₁ = CategoryTheory.CategoryStruct.comp f.τ₁ g.τ₁
          @[simp]
          theorem CategoryTheory.Square.Hom.comp_τ₃ {C : Type u} [CategoryTheory.Category.{v, u} C] {sq₁ sq₂ sq₃ : CategoryTheory.Square C} (f : sq₁.Hom sq₂) (g : sq₂.Hom sq₃) :
          (f.comp g).τ₃ = CategoryTheory.CategoryStruct.comp f.τ₃ g.τ₃
          @[simp]
          theorem CategoryTheory.Square.Hom.comp_τ₄ {C : Type u} [CategoryTheory.Category.{v, u} C] {sq₁ sq₂ sq₃ : CategoryTheory.Square C} (f : sq₁.Hom sq₂) (g : sq₂.Hom sq₃) :
          (f.comp g).τ₄ = CategoryTheory.CategoryStruct.comp f.τ₄ g.τ₄
          @[simp]
          theorem CategoryTheory.Square.category_comp_τ₄ {C : Type u} [CategoryTheory.Category.{v, u} C] {X✝ Y✝ Z✝ : CategoryTheory.Square C} (f : X✝.Hom Y✝) (g : Y✝.Hom Z✝) :
          @[simp]
          theorem CategoryTheory.Square.category_comp_τ₁ {C : Type u} [CategoryTheory.Category.{v, u} C] {X✝ Y✝ Z✝ : CategoryTheory.Square C} (f : X✝.Hom Y✝) (g : Y✝.Hom Z✝) :
          @[simp]
          theorem CategoryTheory.Square.category_comp_τ₂ {C : Type u} [CategoryTheory.Category.{v, u} C] {X✝ Y✝ Z✝ : CategoryTheory.Square C} (f : X✝.Hom Y✝) (g : Y✝.Hom Z✝) :
          @[simp]
          theorem CategoryTheory.Square.category_comp_τ₃ {C : Type u} [CategoryTheory.Category.{v, u} C] {X✝ Y✝ Z✝ : CategoryTheory.Square C} (f : X✝.Hom Y✝) (g : Y✝.Hom Z✝) :
          theorem CategoryTheory.Square.hom_ext {C : Type u} [CategoryTheory.Category.{v, u} C] {sq₁ sq₂ : CategoryTheory.Square C} {f g : sq₁ sq₂} (h₁ : f.τ₁ = g.τ₁) (h₂ : f.τ₂ = g.τ₂) (h₃ : f.τ₃ = g.τ₃) (h₄ : f.τ₄ = g.τ₄) :
          f = g
          def CategoryTheory.Square.isoMk {C : Type u} [CategoryTheory.Category.{v, u} C] {sq₁ sq₂ : CategoryTheory.Square C} (e₁ : sq₁.X₁ sq₂.X₁) (e₂ : sq₁.X₂ sq₂.X₂) (e₃ : sq₁.X₃ sq₂.X₃) (e₄ : sq₁.X₄ sq₂.X₄) (comm₁₂ : CategoryTheory.CategoryStruct.comp sq₁.f₁₂ e₂.hom = CategoryTheory.CategoryStruct.comp e₁.hom sq₂.f₁₂) (comm₁₃ : CategoryTheory.CategoryStruct.comp sq₁.f₁₃ e₃.hom = CategoryTheory.CategoryStruct.comp e₁.hom sq₂.f₁₃) (comm₂₄ : CategoryTheory.CategoryStruct.comp sq₁.f₂₄ e₄.hom = CategoryTheory.CategoryStruct.comp e₂.hom sq₂.f₂₄) (comm₃₄ : CategoryTheory.CategoryStruct.comp sq₁.f₃₄ e₄.hom = CategoryTheory.CategoryStruct.comp e₃.hom sq₂.f₃₄) :
          sq₁ sq₂

          Constructor for isomorphisms in Square c

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            Flipping a square by switching the top-right and the bottom-left objects.

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              theorem CategoryTheory.Square.flip_f₁₂ {C : Type u} [CategoryTheory.Category.{v, u} C] (sq : CategoryTheory.Square C) :
              sq.flip.f₁₂ = sq.f₁₃
              @[simp]
              theorem CategoryTheory.Square.flip_f₃₄ {C : Type u} [CategoryTheory.Category.{v, u} C] (sq : CategoryTheory.Square C) :
              sq.flip.f₃₄ = sq.f₂₄
              @[simp]
              theorem CategoryTheory.Square.flip_f₂₄ {C : Type u} [CategoryTheory.Category.{v, u} C] (sq : CategoryTheory.Square C) :
              sq.flip.f₂₄ = sq.f₃₄
              @[simp]
              theorem CategoryTheory.Square.flip_f₁₃ {C : Type u} [CategoryTheory.Category.{v, u} C] (sq : CategoryTheory.Square C) :
              sq.flip.f₁₃ = sq.f₁₂
              @[simp]
              @[simp]

              The functor which flips commutative squares.

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                Flipping commutative squares is an auto-equivalence.

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                  The functor Square C ⥤ Arrow (Arrow C) which sends a commutative square sq to the obvious arrow from the left morphism of sq to the right morphism of sq.

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                    The functor Arrow (Arrow C) ⥤ Square C which sends a morphism Arrow.mk f ⟶ Arrow.mk g to the commutative square with f on the left side and g on the right side.

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                      The equivalence Square C ≌ Arrow (Arrow C) which sends a commutative square sq to the obvious arrow from the left morphism of sq to the right morphism of sq.

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                        The functor Square C ⥤ Arrow (Arrow C) which sends a commutative square sq to the obvious arrow from the top morphism of sq to the bottom morphism of sq.

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                          The functor Arrow (Arrow C) ⥤ Square C which sends a morphism Arrow.mk f ⟶ Arrow.mk g to the commutative square with f on the top side and g on the bottom side.

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                            The equivalence Square C ≌ Arrow (Arrow C) which sends a commutative square sq to the obvious arrow from the top morphism of sq to the bottom morphism of sq.

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                              The top-left evaluation Square C ⥤ C.

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                                The top-right evaluation Square C ⥤ C.

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                                  The bottom-left evaluation Square C ⥤ C.

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                                    The bottom-right evaluation Square C ⥤ C.

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                                      The map Square C → Square Cᵒᵖ which switches X₁ and X₃, but does not move X₂ and X₃.

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                                        @[simp]
                                        theorem CategoryTheory.Square.op_f₃₄ {C : Type u} [CategoryTheory.Category.{v, u} C] (sq : CategoryTheory.Square C) :
                                        sq.op.f₃₄ = sq.f₁₃.op
                                        @[simp]
                                        theorem CategoryTheory.Square.op_f₂₄ {C : Type u} [CategoryTheory.Category.{v, u} C] (sq : CategoryTheory.Square C) :
                                        sq.op.f₂₄ = sq.f₁₂.op
                                        @[simp]
                                        theorem CategoryTheory.Square.op_f₁₃ {C : Type u} [CategoryTheory.Category.{v, u} C] (sq : CategoryTheory.Square C) :
                                        sq.op.f₁₃ = sq.f₃₄.op
                                        @[simp]
                                        theorem CategoryTheory.Square.op_f₁₂ {C : Type u} [CategoryTheory.Category.{v, u} C] (sq : CategoryTheory.Square C) :
                                        sq.op.f₁₂ = sq.f₂₄.op

                                        The map Square Cᵒᵖ → Square C which switches X₁ and X₃, but does not move X₂ and X₃.

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                                          theorem CategoryTheory.Square.unop_f₁₂ {C : Type u} [CategoryTheory.Category.{v, u} C] (sq : CategoryTheory.Square Cᵒᵖ) :
                                          sq.unop.f₁₂ = sq.f₂₄.unop
                                          @[simp]
                                          theorem CategoryTheory.Square.unop_f₂₄ {C : Type u} [CategoryTheory.Category.{v, u} C] (sq : CategoryTheory.Square Cᵒᵖ) :
                                          sq.unop.f₂₄ = sq.f₁₂.unop
                                          @[simp]
                                          theorem CategoryTheory.Square.unop_f₃₄ {C : Type u} [CategoryTheory.Category.{v, u} C] (sq : CategoryTheory.Square Cᵒᵖ) :
                                          sq.unop.f₃₄ = sq.f₁₃.unop
                                          @[simp]
                                          theorem CategoryTheory.Square.unop_f₁₃ {C : Type u} [CategoryTheory.Category.{v, u} C] (sq : CategoryTheory.Square Cᵒᵖ) :
                                          sq.unop.f₁₃ = sq.f₃₄.unop

                                          The functor (Square C)ᵒᵖ ⥤ Square Cᵒᵖ.

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                                            @[simp]
                                            theorem CategoryTheory.Square.opFunctor_map_τ₂ {C : Type u} [CategoryTheory.Category.{v, u} C] {X✝ Y✝ : (CategoryTheory.Square C)ᵒᵖ} (φ : X✝ Y✝) :
                                            (CategoryTheory.Square.opFunctor.map φ).τ₂ = φ.unop.τ₂.op
                                            @[simp]
                                            theorem CategoryTheory.Square.opFunctor_map_τ₄ {C : Type u} [CategoryTheory.Category.{v, u} C] {X✝ Y✝ : (CategoryTheory.Square C)ᵒᵖ} (φ : X✝ Y✝) :
                                            (CategoryTheory.Square.opFunctor.map φ).τ₄ = φ.unop.τ₁.op
                                            @[simp]
                                            theorem CategoryTheory.Square.opFunctor_map_τ₃ {C : Type u} [CategoryTheory.Category.{v, u} C] {X✝ Y✝ : (CategoryTheory.Square C)ᵒᵖ} (φ : X✝ Y✝) :
                                            (CategoryTheory.Square.opFunctor.map φ).τ₃ = φ.unop.τ₃.op
                                            @[simp]
                                            theorem CategoryTheory.Square.opFunctor_map_τ₁ {C : Type u} [CategoryTheory.Category.{v, u} C] {X✝ Y✝ : (CategoryTheory.Square C)ᵒᵖ} (φ : X✝ Y✝) :
                                            (CategoryTheory.Square.opFunctor.map φ).τ₁ = φ.unop.τ₄.op

                                            The functor (Square Cᵒᵖ)ᵒᵖ ⥤ Square Cᵒᵖ.

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                                              The equivalence (Square C)ᵒᵖ ≌ Square Cᵒᵖ.

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                                                The image of a commutative square by a functor.

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                                                  The functor Square C ⥤ Square D induced by a functor C ⥤ D.

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                                                    @[simp]
                                                    theorem CategoryTheory.Functor.mapSquare_map_τ₁ {C : Type u} [CategoryTheory.Category.{v, u} C] {D : Type u'} [CategoryTheory.Category.{v', u'} D] (F : CategoryTheory.Functor C D) {X✝ Y✝ : CategoryTheory.Square C} (φ : X✝ Y✝) :
                                                    (F.mapSquare.map φ).τ₁ = F.map φ.τ₁
                                                    @[simp]
                                                    theorem CategoryTheory.Functor.mapSquare_map_τ₃ {C : Type u} [CategoryTheory.Category.{v, u} C] {D : Type u'} [CategoryTheory.Category.{v', u'} D] (F : CategoryTheory.Functor C D) {X✝ Y✝ : CategoryTheory.Square C} (φ : X✝ Y✝) :
                                                    (F.mapSquare.map φ).τ₃ = F.map φ.τ₃
                                                    @[simp]
                                                    theorem CategoryTheory.Functor.mapSquare_map_τ₂ {C : Type u} [CategoryTheory.Category.{v, u} C] {D : Type u'} [CategoryTheory.Category.{v', u'} D] (F : CategoryTheory.Functor C D) {X✝ Y✝ : CategoryTheory.Square C} (φ : X✝ Y✝) :
                                                    (F.mapSquare.map φ).τ₂ = F.map φ.τ₂
                                                    @[simp]
                                                    theorem CategoryTheory.Functor.mapSquare_map_τ₄ {C : Type u} [CategoryTheory.Category.{v, u} C] {D : Type u'} [CategoryTheory.Category.{v', u'} D] (F : CategoryTheory.Functor C D) {X✝ Y✝ : CategoryTheory.Square C} (φ : X✝ Y✝) :
                                                    (F.mapSquare.map φ).τ₄ = F.map φ.τ₄

                                                    The natural transformation F.mapSquare ⟶ G.mapSquare induces by a natural transformation F ⟶ G.

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                                                      The functor (C ⥤ D) ⥤ Square C ⥤ Square D.

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