Documentation

Mathlib.CategoryTheory.Sites.Plus

The plus construction for presheaves. #

This file contains the construction of P⁺, for a presheaf P : Cᵒᵖ ⥤ D where C is endowed with a grothendieck topology J.

See https://stacks.math.columbia.edu/tag/00W1 for details.

The diagram whose colimit defines the values of plus.

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def CategoryTheory.GrothendieckTopology.diagramPullback {C : Type u} [Category.{v, u} C] (J : GrothendieckTopology C) {D : Type w} [Category.{w', w} D] [∀ (P : Functor Cᵒᵖ D) (X : C) (S : J.Cover X), Limits.HasMultiequalizer (S.index P)] (P : Functor Cᵒᵖ D) {X Y : C} (f : X Y) :
J.diagram P Y (J.pullback f).op.comp (J.diagram P X)

A helper definition used to define the morphisms for plus.

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def CategoryTheory.GrothendieckTopology.diagramNatTrans {C : Type u} [Category.{v, u} C] (J : GrothendieckTopology C) {D : Type w} [Category.{w', w} D] [∀ (P : Functor Cᵒᵖ D) (X : C) (S : J.Cover X), Limits.HasMultiequalizer (S.index P)] {P Q : Functor Cᵒᵖ D} (η : P Q) (X : C) :
J.diagram P X J.diagram Q X

A natural transformation P ⟶ Q induces a natural transformation between diagrams whose colimits define the values of plus.

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@[simp]

J.diagram P, as a functor in P.

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theorem CategoryTheory.GrothendieckTopology.diagramFunctor_map {C : Type u} [Category.{v, u} C] (J : GrothendieckTopology C) (D : Type w) [Category.{w', w} D] [∀ (P : Functor Cᵒᵖ D) (X : C) (S : J.Cover X), Limits.HasMultiequalizer (S.index P)] (X : C) {X✝ Y✝ : Functor Cᵒᵖ D} (η : X✝ Y✝) :
@[simp]

The plus construction, associating a presheaf to any presheaf. See plusFunctor below for a functorial version.

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def CategoryTheory.GrothendieckTopology.plusMap {C : Type u} [Category.{v, u} C] (J : GrothendieckTopology C) {D : Type w} [Category.{w', w} D] [∀ (P : Functor Cᵒᵖ D) (X : C) (S : J.Cover X), Limits.HasMultiequalizer (S.index P)] [∀ (X : C), Limits.HasColimitsOfShape (J.Cover X)ᵒᵖ D] {P Q : Functor Cᵒᵖ D} (η : P Q) :

An auxiliary definition used in plus below.

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theorem CategoryTheory.GrothendieckTopology.plusMap_comp {C : Type u} [Category.{v, u} C] (J : GrothendieckTopology C) {D : Type w} [Category.{w', w} D] [∀ (P : Functor Cᵒᵖ D) (X : C) (S : J.Cover X), Limits.HasMultiequalizer (S.index P)] [∀ (X : C), Limits.HasColimitsOfShape (J.Cover X)ᵒᵖ D] {P Q R : Functor Cᵒᵖ D} (η : P Q) (γ : Q R) :

The plus construction, a functor sending P to J.plusObj P.

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theorem CategoryTheory.GrothendieckTopology.plusFunctor_map {C : Type u} [Category.{v, u} C] (J : GrothendieckTopology C) (D : Type w) [Category.{w', w} D] [∀ (P : Functor Cᵒᵖ D) (X : C) (S : J.Cover X), Limits.HasMultiequalizer (S.index P)] [∀ (X : C), Limits.HasColimitsOfShape (J.Cover X)ᵒᵖ D] {X✝ Y✝ : Functor Cᵒᵖ D} (η : X✝ Y✝) :
(J.plusFunctor D).map η = J.plusMap η

The canonical map from P to J.plusObj P. See toPlusNatTrans for a functorial version.

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The natural transformation from the identity functor to plus.

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(P ⟶ P⁺)⁺ = P⁺ ⟶ P⁺⁺

The natural isomorphism between P and P⁺ when P is a sheaf.

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def CategoryTheory.GrothendieckTopology.plusLift {C : Type u} [Category.{v, u} C] (J : GrothendieckTopology C) {D : Type w} [Category.{w', w} D] [∀ (P : Functor Cᵒᵖ D) (X : C) (S : J.Cover X), Limits.HasMultiequalizer (S.index P)] [∀ (X : C), Limits.HasColimitsOfShape (J.Cover X)ᵒᵖ D] {P Q : Functor Cᵒᵖ D} (η : P Q) (hQ : Presheaf.IsSheaf J Q) :
J.plusObj P Q

Lift a morphism P ⟶ Q to P⁺ ⟶ Q when Q is a sheaf.

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theorem CategoryTheory.GrothendieckTopology.toPlus_plusLift {C : Type u} [Category.{v, u} C] (J : GrothendieckTopology C) {D : Type w} [Category.{w', w} D] [∀ (P : Functor Cᵒᵖ D) (X : C) (S : J.Cover X), Limits.HasMultiequalizer (S.index P)] [∀ (X : C), Limits.HasColimitsOfShape (J.Cover X)ᵒᵖ D] {P Q : Functor Cᵒᵖ D} (η : P Q) (hQ : Presheaf.IsSheaf J Q) :
theorem CategoryTheory.GrothendieckTopology.plusLift_unique {C : Type u} [Category.{v, u} C] (J : GrothendieckTopology C) {D : Type w} [Category.{w', w} D] [∀ (P : Functor Cᵒᵖ D) (X : C) (S : J.Cover X), Limits.HasMultiequalizer (S.index P)] [∀ (X : C), Limits.HasColimitsOfShape (J.Cover X)ᵒᵖ D] {P Q : Functor Cᵒᵖ D} (η : P Q) (hQ : Presheaf.IsSheaf J Q) (γ : J.plusObj P Q) ( : CategoryStruct.comp (J.toPlus P) γ = η) :
γ = J.plusLift η hQ
theorem CategoryTheory.GrothendieckTopology.plus_hom_ext {C : Type u} [Category.{v, u} C] (J : GrothendieckTopology C) {D : Type w} [Category.{w', w} D] [∀ (P : Functor Cᵒᵖ D) (X : C) (S : J.Cover X), Limits.HasMultiequalizer (S.index P)] [∀ (X : C), Limits.HasColimitsOfShape (J.Cover X)ᵒᵖ D] {P Q : Functor Cᵒᵖ D} (η γ : J.plusObj P Q) (hQ : Presheaf.IsSheaf J Q) (h : CategoryStruct.comp (J.toPlus P) η = CategoryStruct.comp (J.toPlus P) γ) :
η = γ
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theorem CategoryTheory.GrothendieckTopology.plusMap_plusLift {C : Type u} [Category.{v, u} C] (J : GrothendieckTopology C) {D : Type w} [Category.{w', w} D] [∀ (P : Functor Cᵒᵖ D) (X : C) (S : J.Cover X), Limits.HasMultiequalizer (S.index P)] [∀ (X : C), Limits.HasColimitsOfShape (J.Cover X)ᵒᵖ D] {P Q R : Functor Cᵒᵖ D} (η : P Q) (γ : Q R) (hR : Presheaf.IsSheaf J R) :