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Mathlib.CategoryTheory.Adjunction.Basic

Adjunctions between functors #

F ⊣ G represents the data of an adjunction between two functors F : C ⥤ D and G : D ⥤ C. F is the left adjoint and G is the right adjoint.

We provide various useful constructors:

There are also typeclasses IsLeftAdjoint / IsRightAdjoint, carrying data witnessing that a given functor is a left or right adjoint. Given [IsLeftAdjoint F], a right adjoint of F can be constructed as rightAdjoint F.

Adjunction.comp composes adjunctions.

toEquivalence upgrades an adjunction to an equivalence, given witnesses that the unit and counit are pointwise isomorphisms. Conversely Equivalence.toAdjunction recovers the underlying adjunction from an equivalence.

structure CategoryTheory.Adjunction {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] (F : CategoryTheory.Functor C D) (G : CategoryTheory.Functor D C) :
Type (max (max (max u₁ u₂) v₁) v₂)

F ⊣ G represents the data of an adjunction between two functors F : C ⥤ D and G : D ⥤ C. F is the left adjoint and G is the right adjoint.

To construct an adjunction between two functors, it's often easier to instead use the constructors mkOfHomEquiv or mkOfUnitCounit. To construct a left adjoint, there are also constructors leftAdjointOfEquiv and adjunctionOfEquivLeft (as well as their duals) which can be simpler in practice.

Uniqueness of adjoints is shown in CategoryTheory.Adjunction.Opposites.

See https://stacks.math.columbia.edu/tag/0037.

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    The notation F ⊣ G stands for Adjunction F G representing that F is left adjoint to G

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      class CategoryTheory.IsLeftAdjoint {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] (left : CategoryTheory.Functor C D) :
      Type (max (max (max u₁ u₂) v₁) v₂)

      A class giving a chosen right adjoint to the functor left.

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        class CategoryTheory.IsRightAdjoint {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] (right : CategoryTheory.Functor D C) :
        Type (max (max (max u₁ u₂) v₁) v₂)

        A class giving a chosen left adjoint to the functor right.

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          The adjunction associated to a functor known to be a left adjoint.

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            The adjunction associated to a functor known to be a right adjoint.

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              theorem CategoryTheory.Adjunction.counit_naturality {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G : CategoryTheory.Functor D C} (adj : F G) {X : D} {Y : D} (f : X Y) :
              CategoryTheory.CategoryStruct.comp (F.map (G.map f)) (adj.counit.app Y) = CategoryTheory.CategoryStruct.comp (adj.counit.app X) f
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              theorem CategoryTheory.Adjunction.unit_naturality {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G : CategoryTheory.Functor D C} (adj : F G) {X : C} {Y : C} (f : X Y) :
              CategoryTheory.CategoryStruct.comp (adj.unit.app X) (G.map (F.map f)) = CategoryTheory.CategoryStruct.comp f (adj.unit.app Y)
              theorem CategoryTheory.Adjunction.homEquiv_apply_eq {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G : CategoryTheory.Functor D C} (adj : F G) {A : C} {B : D} (f : F.obj A B) (g : A G.obj B) :
              theorem CategoryTheory.Adjunction.eq_homEquiv_apply {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G : CategoryTheory.Functor D C} (adj : F G) {A : C} {B : D} (f : F.obj A B) (g : A G.obj B) :

              This is an auxiliary data structure useful for constructing adjunctions. See Adjunction.mkOfHomEquiv. This structure won't typically be used anywhere else.

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                This is an auxiliary data structure useful for constructing adjunctions. See Adjunction.mkOfUnitCounit. This structure won't typically be used anywhere else.

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                  Construct an adjunction between F and G out of a natural bijection between each F.obj X ⟶ Y and X ⟶ G.obj Y.

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                    Construct an adjunction between functors F and G given a unit and counit for the adjunction satisfying the triangle identities.

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                      The adjunction between the identity functor on a category and itself.

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                        • CategoryTheory.Adjunction.instInhabitedAdjunctionId = { default := CategoryTheory.Adjunction.id }

                        If F and G are naturally isomorphic functors, establish an equivalence of hom-sets.

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                          If G and H are naturally isomorphic functors, establish an equivalence of hom-sets.

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                            Transport an adjunction along a natural isomorphism on the left.

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                              Transport an adjunction along a natural isomorphism on the right.

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                                Composition of adjunctions.

                                See https://stacks.math.columbia.edu/tag/0DV0.

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                                  If F and G are left adjoints then F ⋙ G is a left adjoint too.

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                                  If F and G are right adjoints then F ⋙ G is a right adjoint too.

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                                  theorem CategoryTheory.Adjunction.leftAdjointOfEquiv_obj {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {G : CategoryTheory.Functor D C} {F_obj : CD} (e : (X : C) → (Y : D) → (F_obj X Y) (X G.obj Y)) (he : ∀ (X : C) (Y Y' : D) (g : Y Y') (h : F_obj X Y), ↑(e X Y') (CategoryTheory.CategoryStruct.comp h g) = CategoryTheory.CategoryStruct.comp (↑(e X Y) h) (G.map g)) :
                                  ∀ (a : C), (CategoryTheory.Adjunction.leftAdjointOfEquiv e he).obj a = F_obj a
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                                  theorem CategoryTheory.Adjunction.leftAdjointOfEquiv_map {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {G : CategoryTheory.Functor D C} {F_obj : CD} (e : (X : C) → (Y : D) → (F_obj X Y) (X G.obj Y)) (he : ∀ (X : C) (Y Y' : D) (g : Y Y') (h : F_obj X Y), ↑(e X Y') (CategoryTheory.CategoryStruct.comp h g) = CategoryTheory.CategoryStruct.comp (↑(e X Y) h) (G.map g)) {X : C} {X' : C} (f : X X') :
                                  (CategoryTheory.Adjunction.leftAdjointOfEquiv e he).map f = (e X (F_obj X')).symm (CategoryTheory.CategoryStruct.comp f (↑(e X' (F_obj X')) (CategoryTheory.CategoryStruct.id (F_obj X'))))
                                  def CategoryTheory.Adjunction.leftAdjointOfEquiv {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {G : CategoryTheory.Functor D C} {F_obj : CD} (e : (X : C) → (Y : D) → (F_obj X Y) (X G.obj Y)) (he : ∀ (X : C) (Y Y' : D) (g : Y Y') (h : F_obj X Y), ↑(e X Y') (CategoryTheory.CategoryStruct.comp h g) = CategoryTheory.CategoryStruct.comp (↑(e X Y) h) (G.map g)) :

                                  Construct a left adjoint functor to G, given the functor's value on objects F_obj and a bijection e between F_obj X ⟶ Y and X ⟶ G.obj Y satisfying a naturality law he : ∀ X Y Y' g h, e X Y' (h ≫ g) = e X Y h ≫ G.map g. Dual to rightAdjointOfEquiv.

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                                    theorem CategoryTheory.Adjunction.adjunctionOfEquivLeft_homEquiv {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {G : CategoryTheory.Functor D C} {F_obj : CD} (e : (X : C) → (Y : D) → (F_obj X Y) (X G.obj Y)) (he : ∀ (X : C) (Y Y' : D) (g : Y Y') (h : F_obj X Y), ↑(e X Y') (CategoryTheory.CategoryStruct.comp h g) = CategoryTheory.CategoryStruct.comp (↑(e X Y) h) (G.map g)) (X : C) (Y : D) :
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                                    theorem CategoryTheory.Adjunction.adjunctionOfEquivLeft_unit_app {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {G : CategoryTheory.Functor D C} {F_obj : CD} (e : (X : C) → (Y : D) → (F_obj X Y) (X G.obj Y)) (he : ∀ (X : C) (Y Y' : D) (g : Y Y') (h : F_obj X Y), ↑(e X Y') (CategoryTheory.CategoryStruct.comp h g) = CategoryTheory.CategoryStruct.comp (↑(e X Y) h) (G.map g)) (X : C) :
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                                    theorem CategoryTheory.Adjunction.adjunctionOfEquivLeft_counit_app {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {G : CategoryTheory.Functor D C} {F_obj : CD} (e : (X : C) → (Y : D) → (F_obj X Y) (X G.obj Y)) (he : ∀ (X : C) (Y Y' : D) (g : Y Y') (h : F_obj X Y), ↑(e X Y') (CategoryTheory.CategoryStruct.comp h g) = CategoryTheory.CategoryStruct.comp (↑(e X Y) h) (G.map g)) (Y : D) :
                                    (CategoryTheory.Adjunction.adjunctionOfEquivLeft e he).counit.app Y = (e (G.obj Y) Y).symm (CategoryTheory.CategoryStruct.id (G.obj Y))
                                    def CategoryTheory.Adjunction.adjunctionOfEquivLeft {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {G : CategoryTheory.Functor D C} {F_obj : CD} (e : (X : C) → (Y : D) → (F_obj X Y) (X G.obj Y)) (he : ∀ (X : C) (Y Y' : D) (g : Y Y') (h : F_obj X Y), ↑(e X Y') (CategoryTheory.CategoryStruct.comp h g) = CategoryTheory.CategoryStruct.comp (↑(e X Y) h) (G.map g)) :

                                    Show that the functor given by leftAdjointOfEquiv is indeed left adjoint to G. Dual to adjunctionOfRightEquiv.

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                                      theorem CategoryTheory.Adjunction.rightAdjointOfEquiv_map {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G_obj : DC} (e : (X : C) → (Y : D) → (F.obj X Y) (X G_obj Y)) (he : ∀ (X' X : C) (Y : D) (f : X' X) (g : F.obj X Y), ↑(e X' Y) (CategoryTheory.CategoryStruct.comp (F.map f) g) = CategoryTheory.CategoryStruct.comp f (↑(e X Y) g)) {Y : D} {Y' : D} (g : Y Y') :
                                      (CategoryTheory.Adjunction.rightAdjointOfEquiv e he).map g = ↑(e (G_obj Y) Y') (CategoryTheory.CategoryStruct.comp ((e (G_obj Y) Y).symm (CategoryTheory.CategoryStruct.id (G_obj Y))) g)
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                                      theorem CategoryTheory.Adjunction.rightAdjointOfEquiv_obj {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G_obj : DC} (e : (X : C) → (Y : D) → (F.obj X Y) (X G_obj Y)) (he : ∀ (X' X : C) (Y : D) (f : X' X) (g : F.obj X Y), ↑(e X' Y) (CategoryTheory.CategoryStruct.comp (F.map f) g) = CategoryTheory.CategoryStruct.comp f (↑(e X Y) g)) :
                                      ∀ (a : D), (CategoryTheory.Adjunction.rightAdjointOfEquiv e he).obj a = G_obj a
                                      def CategoryTheory.Adjunction.rightAdjointOfEquiv {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G_obj : DC} (e : (X : C) → (Y : D) → (F.obj X Y) (X G_obj Y)) (he : ∀ (X' X : C) (Y : D) (f : X' X) (g : F.obj X Y), ↑(e X' Y) (CategoryTheory.CategoryStruct.comp (F.map f) g) = CategoryTheory.CategoryStruct.comp f (↑(e X Y) g)) :

                                      Construct a right adjoint functor to F, given the functor's value on objects G_obj and a bijection e between F.obj X ⟶ Y and X ⟶ G_obj Y satisfying a naturality law he : ∀ X Y Y' g h, e X' Y (F.map f ≫ g) = f ≫ e X Y g. Dual to leftAdjointOfEquiv.

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                                        theorem CategoryTheory.Adjunction.adjunctionOfEquivRight_homEquiv {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G_obj : DC} (e : (X : C) → (Y : D) → (F.obj X Y) (X G_obj Y)) (he : ∀ (X' X : C) (Y : D) (f : X' X) (g : F.obj X Y), ↑(e X' Y) (CategoryTheory.CategoryStruct.comp (F.map f) g) = CategoryTheory.CategoryStruct.comp f (↑(e X Y) g)) (X : C) (Y : D) :
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                                        theorem CategoryTheory.Adjunction.adjunctionOfEquivRight_unit_app {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G_obj : DC} (e : (X : C) → (Y : D) → (F.obj X Y) (X G_obj Y)) (he : ∀ (X' X : C) (Y : D) (f : X' X) (g : F.obj X Y), ↑(e X' Y) (CategoryTheory.CategoryStruct.comp (F.map f) g) = CategoryTheory.CategoryStruct.comp f (↑(e X Y) g)) (X : C) :
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                                        theorem CategoryTheory.Adjunction.adjunctionOfEquivRight_counit_app {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G_obj : DC} (e : (X : C) → (Y : D) → (F.obj X Y) (X G_obj Y)) (he : ∀ (X' X : C) (Y : D) (f : X' X) (g : F.obj X Y), ↑(e X' Y) (CategoryTheory.CategoryStruct.comp (F.map f) g) = CategoryTheory.CategoryStruct.comp f (↑(e X Y) g)) (Y : D) :
                                        (CategoryTheory.Adjunction.adjunctionOfEquivRight e he).counit.app Y = (e (G_obj Y) Y).symm (CategoryTheory.CategoryStruct.id (G_obj Y))
                                        def CategoryTheory.Adjunction.adjunctionOfEquivRight {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G_obj : DC} (e : (X : C) → (Y : D) → (F.obj X Y) (X G_obj Y)) (he : ∀ (X' X : C) (Y : D) (f : X' X) (g : F.obj X Y), ↑(e X' Y) (CategoryTheory.CategoryStruct.comp (F.map f) g) = CategoryTheory.CategoryStruct.comp f (↑(e X Y) g)) :

                                        Show that the functor given by rightAdjointOfEquiv is indeed right adjoint to F. Dual to adjunctionOfEquivRight.

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                                          theorem CategoryTheory.Adjunction.toEquivalence_unitIso_hom_app {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G : CategoryTheory.Functor D C} (adj : F G) [∀ (X : C), CategoryTheory.IsIso (adj.unit.app X)] [∀ (Y : D), CategoryTheory.IsIso (adj.counit.app Y)] (X : C) :
                                          (CategoryTheory.Adjunction.toEquivalence adj).unitIso.hom.app X = adj.unit.app X
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                                          theorem CategoryTheory.Adjunction.toEquivalence_unitIso_inv_app {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G : CategoryTheory.Functor D C} (adj : F G) [∀ (X : C), CategoryTheory.IsIso (adj.unit.app X)] [∀ (Y : D), CategoryTheory.IsIso (adj.counit.app Y)] (X : C) :
                                          (CategoryTheory.Adjunction.toEquivalence adj).unitIso.inv.app X = CategoryTheory.inv (adj.unit.app X)
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                                          theorem CategoryTheory.Adjunction.toEquivalence_counitIso_inv_app {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G : CategoryTheory.Functor D C} (adj : F G) [∀ (X : C), CategoryTheory.IsIso (adj.unit.app X)] [∀ (Y : D), CategoryTheory.IsIso (adj.counit.app Y)] (X : D) :
                                          (CategoryTheory.Adjunction.toEquivalence adj).counitIso.inv.app X = CategoryTheory.inv (adj.counit.app X)
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                                          theorem CategoryTheory.Adjunction.toEquivalence_counitIso_hom_app {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G : CategoryTheory.Functor D C} (adj : F G) [∀ (X : C), CategoryTheory.IsIso (adj.unit.app X)] [∀ (Y : D), CategoryTheory.IsIso (adj.counit.app Y)] (X : D) :
                                          (CategoryTheory.Adjunction.toEquivalence adj).counitIso.hom.app X = adj.counit.app X
                                          noncomputable def CategoryTheory.Adjunction.toEquivalence {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [CategoryTheory.Category.{v₂, u₂} D] {F : CategoryTheory.Functor C D} {G : CategoryTheory.Functor D C} (adj : F G) [∀ (X : C), CategoryTheory.IsIso (adj.unit.app X)] [∀ (Y : D), CategoryTheory.IsIso (adj.counit.app Y)] :
                                          C D

                                          If the unit and counit of a given adjunction are (pointwise) isomorphisms, then we can upgrade the adjunction to an equivalence.

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                                            If the unit and counit for the adjunction corresponding to a right adjoint functor are (pointwise) isomorphisms, then the functor is an equivalence of categories.

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                                              The adjunction given by an equivalence of categories. (To obtain the opposite adjunction, simply use e.symm.toAdjunction.

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                                                If F is an equivalence, it's a left adjoint.

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