Adjoint lifting

This file gives two constructions for building left adjoints: the adjoint triangle theorem and the adjoint lifting theorem.

The adjoint triangle theorem concerns a functor U : B ⥤ C with a left adjoint F such that ε_X : FUX ⟶ X is a regular epi. Then for any category A with coequalizers of reflexive pairs, a functor R : A ⥤ B has a left adjoint if (and only if) the composite R ⋙ U does. Note that the condition on U regarding ε_X is automatically satisfied in the case when U is a monadic functor, giving the corollary: isRightAdjoint_triangle_lift_monadic, i.e. if U is monadic, A has reflexive coequalizers then R : A ⥤ B has a left adjoint provided R ⋙ U does.

The adjoint lifting theorem says that given a commutative square of functors (up to isomorphism):

      Q
    A → B
  U ↓   ↓ V
    C → D
      R

where V is monadic, U has a left adjoint, and A has reflexive coequalizers, then if R has a left adjoint then Q has a left adjoint.

Implementation

It is more convenient to prove this theorem by assuming we are given the explicit adjunction rather than just a functor known to be a right adjoint. In docstrings, we write (η, ε) for the unit and counit of the adjunction adj₁ : F ⊣ U and (ι, δ) for the unit and counit of the adjunction adj₂ : F' ⊣ R ⋙ U.

This file has been adapted to Mathlib.CategoryTheory.Adjunction.Lifting.Right. Please try to keep them in sync.

TODO

• Dualise to lift right adjoints through monads (by reversing 2-cells).
• Investigate whether it is possible to give a more explicit description of the lifted adjoint, especially in the case when the isomorphism comm is Iso.refl _

References

• https://ncatlab.org/nlab/show/adjoint+triangle+theorem
• https://ncatlab.org/nlab/show/adjoint+lifting+theorem
• Adjoint Lifting Theorems for Categories of Algebras (PT Johnstone, 1975)
• A unified approach to the lifting of adjoints (AJ Power, 1988)

def CategoryTheory.LiftLeftAdjoint.counitCoequalises {B : Type u₂} {C : Type u₃} [CategoryTheory.Category.{v₂, u₂} B] [CategoryTheory.Category.{v₃, u₃} C] {U : CategoryTheory.Functor B C} {F : CategoryTheory.Functor C B} (adj₁ : F ⊣ U) [(X : B) → CategoryTheory.RegularEpi (adj₁.counit.app X)] (X : B) : CategoryTheory.Limits.IsColimit (CategoryTheory.Limits.Cofork.ofπ (adj₁.counit.app X) ⋯)

To show that ε_X is a coequalizer for (FUε_X, ε_FUX), it suffices to assume it's always a coequalizer of something (i.e. a regular epi).

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def CategoryTheory.LiftLeftAdjoint.otherMap {A : Type u₁} {B : Type u₂} {C : Type u₃} [CategoryTheory.Category.{v₁, u₁} A] [CategoryTheory.Category.{v₂, u₂} B] [CategoryTheory.Category.{v₃, u₃} C] {U : CategoryTheory.Functor B C} {F : CategoryTheory.Functor C B} (R : CategoryTheory.Functor A B) (F' : CategoryTheory.Functor C A) (adj₁ : F ⊣ U) (adj₂ : F' ⊣ R.comp U) (X : B) : F'.obj (U.obj (F.obj (U.obj X))) ⟶ F'.obj (U.obj X)

(Implementation) To construct the left adjoint, we use the coequalizer of F' U ε_Y with the composite

F' U F U X ⟶ F' U F U R F U' X ⟶ F' U R F' U X ⟶ F' U X

where the first morphism is F' U F ι_UX, the second is F' U ε_RF'UX, and the third is δ_F'UX. We will show that this coequalizer exists and that it forms the object map for a left adjoint to R.

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instance CategoryTheory.LiftLeftAdjoint.instIsReflexivePairMapAppCounitOtherMap {A : Type u₁} {B : Type u₂} {C : Type u₃} [CategoryTheory.Category.{v₁, u₁} A] [CategoryTheory.Category.{v₂, u₂} B] [CategoryTheory.Category.{v₃, u₃} C] {U : CategoryTheory.Functor B C} {F : CategoryTheory.Functor C B} (R : CategoryTheory.Functor A B) (F' : CategoryTheory.Functor C A) (adj₁ : F ⊣ U) (adj₂ : F' ⊣ R.comp U) (X : B) : CategoryTheory.IsReflexivePair (F'.map (U.map (adj₁.counit.app X))) (CategoryTheory.LiftLeftAdjoint.otherMap R F' adj₁ adj₂ X)

(F'Uε_X, otherMap X) is a reflexive pair: in particular if A has reflexive coequalizers then this pair has a coequalizer.

noncomputable def CategoryTheory.LiftLeftAdjoint.constructLeftAdjointObj {A : Type u₁} {B : Type u₂} {C : Type u₃} [CategoryTheory.Category.{v₁, u₁} A] [CategoryTheory.Category.{v₂, u₂} B] [CategoryTheory.Category.{v₃, u₃} C] {U : CategoryTheory.Functor B C} {F : CategoryTheory.Functor C B} (R : CategoryTheory.Functor A B) (F' : CategoryTheory.Functor C A) (adj₁ : F ⊣ U) (adj₂ : F' ⊣ R.comp U) [CategoryTheory.Limits.HasReflexiveCoequalizers A] (Y : B) : A

Construct the object part of the desired left adjoint as the coequalizer of F'Uε_Y with otherMap.

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noncomputable def CategoryTheory.LiftLeftAdjoint.constructLeftAdjointEquiv {A : Type u₁} {B : Type u₂} {C : Type u₃} [CategoryTheory.Category.{v₁, u₁} A] [CategoryTheory.Category.{v₂, u₂} B] [CategoryTheory.Category.{v₃, u₃} C] {U : CategoryTheory.Functor B C} {F : CategoryTheory.Functor C B} (R : CategoryTheory.Functor A B) (F' : CategoryTheory.Functor C A) (adj₁ : F ⊣ U) (adj₂ : F' ⊣ R.comp U) [CategoryTheory.Limits.HasReflexiveCoequalizers A] [(X : B) → CategoryTheory.RegularEpi (adj₁.counit.app X)] (Y : A) (X : B) : (CategoryTheory.LiftLeftAdjoint.constructLeftAdjointObj R F' adj₁ adj₂ X ⟶ Y) ≃ (X ⟶ R.obj Y)

The homset equivalence which helps show that R is a right adjoint.

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@[simp]

theorem CategoryTheory.LiftLeftAdjoint.constructLeftAdjointEquiv_apply {A : Type u₁} {B : Type u₂} {C : Type u₃} [CategoryTheory.Category.{v₁, u₁} A] [CategoryTheory.Category.{v₂, u₂} B] [CategoryTheory.Category.{v₃, u₃} C] {U : CategoryTheory.Functor B C} {F : CategoryTheory.Functor C B} (R : CategoryTheory.Functor A B) (F' : CategoryTheory.Functor C A) (adj₁ : F ⊣ U) (adj₂ : F' ⊣ R.comp U) [CategoryTheory.Limits.HasReflexiveCoequalizers A] [(X : B) → CategoryTheory.RegularEpi (adj₁.counit.app X)] (Y : A) (X : B) (a✝ : CategoryTheory.LiftLeftAdjoint.constructLeftAdjointObj R F' adj₁ adj₂ X ⟶ Y) : (CategoryTheory.LiftLeftAdjoint.constructLeftAdjointEquiv R F' adj₁ adj₂ Y X) a✝ = (CategoryTheory.Limits.Cofork.IsColimit.homIso (CategoryTheory.LiftLeftAdjoint.counitCoequalises adj₁ X) (R.obj Y)).symm ⟨(adj₁.homEquiv (U.obj X) (R.obj Y)).symm ((adj₂.homEquiv (U.obj X) Y) ↑((CategoryTheory.Limits.Cofork.IsColimit.homIso (CategoryTheory.Limits.colimit.isColimit (CategoryTheory.Limits.parallelPair (F'.map (U.map (adj₁.counit.app X))) (CategoryTheory.LiftLeftAdjoint.otherMap R F' adj₁ adj₂ X))) Y) a✝)), ⋯⟩

@[simp]

theorem CategoryTheory.LiftLeftAdjoint.constructLeftAdjointEquiv_symm_apply {A : Type u₁} {B : Type u₂} {C : Type u₃} [CategoryTheory.Category.{v₁, u₁} A] [CategoryTheory.Category.{v₂, u₂} B] [CategoryTheory.Category.{v₃, u₃} C] {U : CategoryTheory.Functor B C} {F : CategoryTheory.Functor C B} (R : CategoryTheory.Functor A B) (F' : CategoryTheory.Functor C A) (adj₁ : F ⊣ U) (adj₂ : F' ⊣ R.comp U) [CategoryTheory.Limits.HasReflexiveCoequalizers A] [(X : B) → CategoryTheory.RegularEpi (adj₁.counit.app X)] (Y : A) (X : B) (a✝ : X ⟶ R.obj Y) : (CategoryTheory.LiftLeftAdjoint.constructLeftAdjointEquiv R F' adj₁ adj₂ Y X).symm a✝ = (CategoryTheory.Limits.Cofork.IsColimit.homIso (CategoryTheory.Limits.colimit.isColimit (CategoryTheory.Limits.parallelPair (F'.map (U.map (adj₁.counit.app X))) (CategoryTheory.LiftLeftAdjoint.otherMap R F' adj₁ adj₂ X))) Y).symm ⟨(adj₂.homEquiv (U.obj X) Y).symm ((adj₁.homEquiv (U.obj X) (R.obj Y)) ↑((CategoryTheory.Limits.Cofork.IsColimit.homIso (CategoryTheory.LiftLeftAdjoint.counitCoequalises adj₁ X) (R.obj Y)) a✝)), ⋯⟩

noncomputable def CategoryTheory.LiftLeftAdjoint.constructLeftAdjoint {A : Type u₁} {B : Type u₂} {C : Type u₃} [CategoryTheory.Category.{v₁, u₁} A] [CategoryTheory.Category.{v₂, u₂} B] [CategoryTheory.Category.{v₃, u₃} C] {U : CategoryTheory.Functor B C} {F : CategoryTheory.Functor C B} (R : CategoryTheory.Functor A B) (F' : CategoryTheory.Functor C A) (adj₁ : F ⊣ U) (adj₂ : F' ⊣ R.comp U) [CategoryTheory.Limits.HasReflexiveCoequalizers A] [(X : B) → CategoryTheory.RegularEpi (adj₁.counit.app X)] : CategoryTheory.Functor B A

Construct the left adjoint to R, with object map constructLeftAdjointObj.

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theorem CategoryTheory.isRightAdjoint_triangle_lift {A : Type u₁} {B : Type u₂} {C : Type u₃} [CategoryTheory.Category.{v₁, u₁} A] [CategoryTheory.Category.{v₂, u₂} B] [CategoryTheory.Category.{v₃, u₃} C] {U : CategoryTheory.Functor B C} {F : CategoryTheory.Functor C B} (R : CategoryTheory.Functor A B) (adj₁ : F ⊣ U) [(X : B) → CategoryTheory.RegularEpi (adj₁.counit.app X)] [CategoryTheory.Limits.HasReflexiveCoequalizers A] [(R.comp U).IsRightAdjoint] : R.IsRightAdjoint

The adjoint triangle theorem: Suppose U : B ⥤ C has a left adjoint F such that each counit ε_X : FUX ⟶ X is a regular epimorphism. Then if a category A has coequalizers of reflexive pairs, then a functor R : A ⥤ B has a left adjoint if the composite R ⋙ U does.

Note the converse is true (with weaker assumptions), by Adjunction.comp. See https://ncatlab.org/nlab/show/adjoint+triangle+theorem

theorem CategoryTheory.isRightAdjoint_triangle_lift_monadic {A : Type u₁} {B : Type u₂} {C : Type u₃} [CategoryTheory.Category.{v₁, u₁} A] [CategoryTheory.Category.{v₂, u₂} B] [CategoryTheory.Category.{v₃, u₃} C] (U : CategoryTheory.Functor B C) [CategoryTheory.MonadicRightAdjoint U] {R : CategoryTheory.Functor A B} [CategoryTheory.Limits.HasReflexiveCoequalizers A] [(R.comp U).IsRightAdjoint] : R.IsRightAdjoint

If R ⋙ U has a left adjoint, the domain of R has reflexive coequalizers and U is a monadic functor, then R has a left adjoint. This is a special case of isRightAdjoint_triangle_lift which is often more useful in practice.

theorem CategoryTheory.isRightAdjoint_square_lift {A : Type u₁} {B : Type u₂} {C : Type u₃} [CategoryTheory.Category.{v₁, u₁} A] [CategoryTheory.Category.{v₂, u₂} B] [CategoryTheory.Category.{v₃, u₃} C] {D : Type u₄} [CategoryTheory.Category.{v₄, u₄} D] (Q : CategoryTheory.Functor A B) (V : CategoryTheory.Functor B D) (U : CategoryTheory.Functor A C) (R : CategoryTheory.Functor C D) (comm : U.comp R ≅ Q.comp V) [U.IsRightAdjoint] [V.IsRightAdjoint] [R.IsRightAdjoint] [(X : B) → CategoryTheory.RegularEpi ((CategoryTheory.Adjunction.ofIsRightAdjoint V).counit.app X)] [CategoryTheory.Limits.HasReflexiveCoequalizers A] : Q.IsRightAdjoint

Suppose we have a commutative square of functors

      Q
    A → B
  U ↓   ↓ V
    C → D
      R

where U has a left adjoint, A has reflexive coequalizers and V has a left adjoint such that each component of the counit is a regular epi. Then Q has a left adjoint if R has a left adjoint.

See https://ncatlab.org/nlab/show/adjoint+lifting+theorem

theorem CategoryTheory.isRightAdjoint_square_lift_monadic {A : Type u₁} {B : Type u₂} {C : Type u₃} [CategoryTheory.Category.{v₁, u₁} A] [CategoryTheory.Category.{v₂, u₂} B] [CategoryTheory.Category.{v₃, u₃} C] {D : Type u₄} [CategoryTheory.Category.{v₄, u₄} D] (Q : CategoryTheory.Functor A B) (V : CategoryTheory.Functor B D) (U : CategoryTheory.Functor A C) (R : CategoryTheory.Functor C D) (comm : U.comp R ≅ Q.comp V) [U.IsRightAdjoint] [CategoryTheory.MonadicRightAdjoint V] [R.IsRightAdjoint] [CategoryTheory.Limits.HasReflexiveCoequalizers A] : Q.IsRightAdjoint

Suppose we have a commutative square of functors

      Q
    A → B
  U ↓   ↓ V
    C → D
      R

where U has a left adjoint, A has reflexive coequalizers and V is monadic. Then Q has a left adjoint if R has a left adjoint.

See https://ncatlab.org/nlab/show/adjoint+lifting+theorem