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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A helper definition used to define the morphisms for plus.
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A natural transformation P ⟶ Q induces a natural transformation
between diagrams whose colimits define the values of plus.
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J.diagram P, as a functor in P.
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The plus construction, associating a presheaf to any presheaf.
See plusFunctor below for a functorial version.
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An auxiliary definition used in plus below.
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- J.plusMap η = { app := fun (X : Cᵒᵖ) => CategoryTheory.Limits.colimMap (J.diagramNatTrans η (Opposite.unop X)), naturality := ⋯ }
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The plus construction, a functor sending P to J.plusObj P.
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- J.plusFunctor D = { obj := fun (P : CategoryTheory.Functor Cᵒᵖ D) => J.plusObj P, map := fun {X Y : CategoryTheory.Functor Cᵒᵖ D} (η : X ⟶ Y) => J.plusMap η, map_id := ⋯, map_comp := ⋯ }
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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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- J.toPlusNatTrans D = { app := fun (P : CategoryTheory.Functor Cᵒᵖ D) => J.toPlus P, naturality := ⋯ }
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(P ⟶ P⁺)⁺ = P⁺ ⟶ P⁺⁺
The natural isomorphism between P and P⁺ when P is a sheaf.
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- J.isoToPlus P hP = CategoryTheory.asIso (J.toPlus P)
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Lift a morphism P ⟶ Q to P⁺ ⟶ Q when Q is a sheaf.