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Mathlib.CategoryTheory.Comma.StructuredArrow

The category of "structured arrows" #

For T : C ⥤ D, a T-structured arrow with source S : D is just a morphism S ⟶ T.obj Y, for some Y : C.

These form a category with morphisms g : Y ⟶ Y' making the obvious diagram commute.

We prove that 𝟙 (T.obj Y) is the initial object in T-structured objects with source T.obj Y.

The category of T-structured arrows with domain S : D (here T : C ⥤ D), has as its objects D-morphisms of the form S ⟶ T Y, for some Y : C, and morphisms C-morphisms Y ⟶ Y' making the obvious triangle commute.

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    Construct a structured arrow from a morphism.

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      To construct a morphism of structured arrows, we need a morphism of the objects underlying the target, and to check that the triangle commutes.

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        @[simp]
        theorem CategoryTheory.StructuredArrow.homMk'_right {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {S : D} {Y' : C} {T : CategoryTheory.Functor C D} (f : CategoryTheory.StructuredArrow S T) (g : f.right Y') :
        (f.homMk' g).right = g

        Given a structured arrow X ⟶ T(Y), and an arrow Y ⟶ Y', we can construct a morphism of structured arrows given by (X ⟶ T(Y)) ⟶ (X ⟶ T(Y) ⟶ T(Y')).

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          To construct an isomorphism of structured arrows, we need an isomorphism of the objects underlying the target, and to check that the triangle commutes.

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            The converse of this is true with additional assumptions, see mono_iff_mono_right.

            Eta rule for structured arrows. Prefer StructuredArrow.eta for rewriting, since equality of objects tends to cause problems.

            A morphism between source objects S ⟶ S' contravariantly induces a functor between structured arrows, StructuredArrow S' T ⥤ StructuredArrow S T.

            Ideally this would be described as a 2-functor from D (promoted to a 2-category with equations as 2-morphisms) to Cat.

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              The identity structured arrow is initial.

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                If F is an equivalence, then so is the functor (S, F ⋙ G) ⥤ (S, G).

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                The functor (S, F) ⥤ (G(S), F ⋙ G).

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                  If G is fully faithful, then post S F G : (S, F) ⥤ (G(S), F ⋙ G) is an equivalence.

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                  The functor StructuredArrow L R ⥤ StructuredArrow L' R' that is deduced from a natural transformation R ⋙ G ⟶ F ⋙ R' and a morphism L' ⟶ G.obj L.

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                    noncomputable instance CategoryTheory.StructuredArrow.isEquivalenceMap₂ {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {A : Type u₃} [CategoryTheory.Category.{v₃, u₃} A] {B : Type u₄} [CategoryTheory.Category.{v₄, u₄} B] {L : D} {R : CategoryTheory.Functor C D} {L' : B} {R' : CategoryTheory.Functor A B} {F : CategoryTheory.Functor C A} {G : CategoryTheory.Functor D B} (α : L' G.obj L) (β : R.comp G F.comp R') [F.IsEquivalence] [G.Faithful] [G.Full] [CategoryTheory.IsIso α] [CategoryTheory.IsIso β] :
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                    @[reducible, inline]

                    A structured arrow is called universal if it is initial.

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                      The family of morphisms out of a universal arrow.

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                        Any structured arrow factors through a universal arrow.

                        theorem CategoryTheory.StructuredArrow.IsUniversal.hom_ext {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {S : D} {T : CategoryTheory.Functor C D} {f : CategoryTheory.StructuredArrow S T} (h : f.IsUniversal) {c : C} {η : f.right c} {η' : f.right c} (w : CategoryTheory.CategoryStruct.comp f.hom (T.map η) = CategoryTheory.CategoryStruct.comp f.hom (T.map η')) :
                        η = η'

                        Two morphisms out of a universal T-structured arrow are equal if their image under T are equal after precomposing the universal arrow.

                        The category of S-costructured arrows with target T : D (here S : C ⥤ D), has as its objects D-morphisms of the form S Y ⟶ T, for some Y : C, and morphisms C-morphisms Y ⟶ Y' making the obvious triangle commute.

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                          Construct a costructured arrow from a morphism.

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                            To construct a morphism of costructured arrows, we need a morphism of the objects underlying the source, and to check that the triangle commutes.

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                              Given a costructured arrow S(Y) ⟶ X, and an arrow Y' ⟶ Y', we can construct a morphism of costructured arrows given by (S(Y) ⟶ X) ⟶ (S(Y') ⟶ S(Y) ⟶ X).

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                                Variant of homMk' where both objects are applications of mk.

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                                  To construct an isomorphism of costructured arrows, we need an isomorphism of the objects underlying the source, and to check that the triangle commutes.

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                                    The converse of this is true with additional assumptions, see epi_iff_epi_left.

                                    Eta rule for costructured arrows. Prefer CostructuredArrow.eta for rewriting, as equality of objects tends to cause problems.

                                    A morphism between target objects T ⟶ T' covariantly induces a functor between costructured arrows, CostructuredArrow S T ⥤ CostructuredArrow S T'.

                                    Ideally this would be described as a 2-functor from D (promoted to a 2-category with equations as 2-morphisms) to Cat.

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                                      The identity costructured arrow is terminal.

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                                        If F is an equivalence, then so is the functor (F ⋙ G, S) ⥤ (G, S).

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                                        The functor (F, S) ⥤ (F ⋙ G, G(S)).

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                                          If G is fully faithful, then post F G S : (F, S) ⥤ (F ⋙ G, G(S)) is an equivalence.

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                                          The functor CostructuredArrow S T ⥤ CostructuredArrow U V that is deduced from a natural transformation F ⋙ U ⟶ S ⋙ G and a morphism G.obj T ⟶ V

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                                            noncomputable instance CategoryTheory.CostructuredArrow.isEquivalenceMap₂ {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {T : D} {S : CategoryTheory.Functor C D} {A : Type u₃} [CategoryTheory.Category.{v₃, u₃} A] {B : Type u₄} [CategoryTheory.Category.{v₄, u₄} B] {U : CategoryTheory.Functor A B} {V : B} {F : CategoryTheory.Functor C A} {G : CategoryTheory.Functor D B} (α : F.comp U S.comp G) (β : G.obj T V) [F.IsEquivalence] [G.Faithful] [G.Full] [CategoryTheory.IsIso α] [CategoryTheory.IsIso β] :
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                                            @[reducible, inline]

                                            A costructured arrow is called universal if it is terminal.

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                                              The family of morphisms into a universal arrow.

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                                                Any costructured arrow factors through a universal arrow.

                                                theorem CategoryTheory.CostructuredArrow.IsUniversal.hom_ext {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {T : D} {S : CategoryTheory.Functor C D} {f : CategoryTheory.CostructuredArrow S T} (h : f.IsUniversal) {c : C} {η : c f.left} {η' : c f.left} (w : CategoryTheory.CategoryStruct.comp (S.map η) f.hom = CategoryTheory.CategoryStruct.comp (S.map η') f.hom) :
                                                η = η'

                                                Two morphisms into a universal S-costructured arrow are equal if their image under S are equal after postcomposing the universal arrow.

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                                                theorem CategoryTheory.Functor.toStructuredArrow_obj {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {E : Type u₃} [CategoryTheory.Category.{v₃, u₃} E] (G : CategoryTheory.Functor E C) (X : D) (F : CategoryTheory.Functor C D) (f : (Y : E) → X F.obj (G.obj Y)) (h : ∀ {Y Z : E} (g : Y Z), CategoryTheory.CategoryStruct.comp (f Y) (F.map (G.map g)) = f Z) (Y : E) :
                                                (G.toStructuredArrow X F f h).obj Y = CategoryTheory.StructuredArrow.mk (f Y)
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                                                theorem CategoryTheory.Functor.toStructuredArrow_map {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {E : Type u₃} [CategoryTheory.Category.{v₃, u₃} E] (G : CategoryTheory.Functor E C) (X : D) (F : CategoryTheory.Functor C D) (f : (Y : E) → X F.obj (G.obj Y)) (h : ∀ {Y Z : E} (g : Y Z), CategoryTheory.CategoryStruct.comp (f Y) (F.map (G.map g)) = f Z) :
                                                ∀ {X_1 Y : E} (g : X_1 Y), (G.toStructuredArrow X F f h).map g = CategoryTheory.StructuredArrow.homMk (G.map g)

                                                Given X : D and F : C ⥤ D, to upgrade a functor G : E ⥤ C to a functor E ⥤ StructuredArrow X F, it suffices to provide maps X ⟶ F.obj (G.obj Y) for all Y making the obvious triangles involving all F.map (G.map g) commute.

                                                This is of course the same as providing a cone over F ⋙ G with cone point X, see Functor.toStructuredArrowIsoToStructuredArrow.

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                                                  def CategoryTheory.Functor.toStructuredArrowCompProj {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {E : Type u₃} [CategoryTheory.Category.{v₃, u₃} E] (G : CategoryTheory.Functor E C) (X : D) (F : CategoryTheory.Functor C D) (f : (Y : E) → X F.obj (G.obj Y)) (h : ∀ {Y Z : E} (g : Y Z), CategoryTheory.CategoryStruct.comp (f Y) (F.map (G.map g)) = f Z) :
                                                  (G.toStructuredArrow X F f ).comp (CategoryTheory.StructuredArrow.proj X F) G

                                                  Upgrading a functor E ⥤ C to a functor E ⥤ StructuredArrow X F and composing with the forgetful functor StructuredArrow X F ⥤ C recovers the original functor.

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                                                    theorem CategoryTheory.Functor.toStructuredArrow_comp_proj {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {E : Type u₃} [CategoryTheory.Category.{v₃, u₃} E] (G : CategoryTheory.Functor E C) (X : D) (F : CategoryTheory.Functor C D) (f : (Y : E) → X F.obj (G.obj Y)) (h : ∀ {Y Z : E} (g : Y Z), CategoryTheory.CategoryStruct.comp (f Y) (F.map (G.map g)) = f Z) :
                                                    (G.toStructuredArrow X F f ).comp (CategoryTheory.StructuredArrow.proj X F) = G
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                                                    theorem CategoryTheory.Functor.toCostructuredArrow_map {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {E : Type u₃} [CategoryTheory.Category.{v₃, u₃} E] (G : CategoryTheory.Functor E C) (F : CategoryTheory.Functor C D) (X : D) (f : (Y : E) → F.obj (G.obj Y) X) (h : ∀ {Y Z : E} (g : Y Z), CategoryTheory.CategoryStruct.comp (F.map (G.map g)) (f Z) = f Y) :
                                                    ∀ {X_1 Y : E} (g : X_1 Y), (G.toCostructuredArrow F X f h).map g = CategoryTheory.CostructuredArrow.homMk (G.map g)
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                                                    theorem CategoryTheory.Functor.toCostructuredArrow_obj {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {E : Type u₃} [CategoryTheory.Category.{v₃, u₃} E] (G : CategoryTheory.Functor E C) (F : CategoryTheory.Functor C D) (X : D) (f : (Y : E) → F.obj (G.obj Y) X) (h : ∀ {Y Z : E} (g : Y Z), CategoryTheory.CategoryStruct.comp (F.map (G.map g)) (f Z) = f Y) (Y : E) :
                                                    (G.toCostructuredArrow F X f h).obj Y = CategoryTheory.CostructuredArrow.mk (f Y)

                                                    Given F : C ⥤ D and X : D, to upgrade a functor G : E ⥤ C to a functor E ⥤ CostructuredArrow F X, it suffices to provide maps F.obj (G.obj Y) ⟶ X for all Y making the obvious triangles involving all F.map (G.map g) commute.

                                                    This is of course the same as providing a cocone over F ⋙ G with cocone point X, see Functor.toCostructuredArrowIsoToCostructuredArrow.

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                                                      def CategoryTheory.Functor.toCostructuredArrowCompProj {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {E : Type u₃} [CategoryTheory.Category.{v₃, u₃} E] (G : CategoryTheory.Functor E C) (F : CategoryTheory.Functor C D) (X : D) (f : (Y : E) → F.obj (G.obj Y) X) (h : ∀ {Y Z : E} (g : Y Z), CategoryTheory.CategoryStruct.comp (F.map (G.map g)) (f Z) = f Y) :
                                                      (G.toCostructuredArrow F X f ).comp (CategoryTheory.CostructuredArrow.proj F X) G

                                                      Upgrading a functor E ⥤ C to a functor E ⥤ CostructuredArrow F X and composing with the forgetful functor CostructuredArrow F X ⥤ C recovers the original functor.

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                                                        theorem CategoryTheory.Functor.toCostructuredArrow_comp_proj {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {E : Type u₃} [CategoryTheory.Category.{v₃, u₃} E] (G : CategoryTheory.Functor E C) (F : CategoryTheory.Functor C D) (X : D) (f : (Y : E) → F.obj (G.obj Y) X) (h : ∀ {Y Z : E} (g : Y Z), CategoryTheory.CategoryStruct.comp (F.map (G.map g)) (f Z) = f Y) :
                                                        (G.toCostructuredArrow F X f ).comp (CategoryTheory.CostructuredArrow.proj F X) = G

                                                        For a functor F : C ⥤ D and an object d : D, we obtain a contravariant functor from the category of structured arrows d ⟶ F.obj c to the category of costructured arrows F.op.obj c ⟶ (op d).

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                                                          For a functor F : C ⥤ D and an object d : D, we obtain a contravariant functor from the category of structured arrows op d ⟶ F.op.obj c to the category of costructured arrows F.obj c ⟶ d.

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                                                            For a functor F : C ⥤ D and an object d : D, we obtain a contravariant functor from the category of costructured arrows F.obj c ⟶ d to the category of structured arrows op d ⟶ F.op.obj c.

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                                                              For a functor F : C ⥤ D and an object d : D, we obtain a contravariant functor from the category of costructured arrows F.op.obj c ⟶ op d to the category of structured arrows d ⟶ F.obj c.

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                                                                For a functor F : C ⥤ D and an object d : D, the category of structured arrows d ⟶ F.obj c is contravariantly equivalent to the category of costructured arrows F.op.obj c ⟶ op d.

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                                                                  For a functor F : C ⥤ D and an object d : D, the category of costructured arrows F.obj c ⟶ d is contravariantly equivalent to the category of structured arrows op d ⟶ F.op.obj c.

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