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Mathlib.CategoryTheory.Abelian.RightDerived

Right-derived functors #

We define the right-derived functors F.rightDerived n : C ⥤ D for any additive functor F out of a category with injective resolutions.

We first define a functor F.rightDerivedToHomotopyCategory : C ⥤ HomotopyCategory D (ComplexShape.up ℕ) which is injectiveResolutions C ⋙ F.mapHomotopyCategory _. We show that if X : C and I : InjectiveResolution X, then F.rightDerivedToHomotopyCategory.obj X identifies to the image in the homotopy category of the functor F applied objectwise to I.cocomplex (this isomorphism is I.isoRightDerivedToHomotopyCategoryObj F).

Then, the right-derived functors F.rightDerived n : C ⥤ D are obtained by composing F.rightDerivedToHomotopyCategory with the homology functors on the homotopy category.

Similarly we define natural transformations between right-derived functors coming from natural transformations between the original additive functors, and show how to compute the components.

Main results #

If I : InjectiveResolution Z and F : C ⥤ D is an additive functor, this is an isomorphism between F.rightDerivedToHomotopyCategory.obj X and the complex obtained by applying F to I.cocomplex.

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    We can compute a right derived functor using a chosen injective resolution.

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      If P : InjectiveResolution X and F is an additive functor, this is the canonical morphism from F.obj X to the cycles in degree 0 of (F.mapHomologicalComplex _).obj P.cocomplex.

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        The natural transformation F ⟶ F.rightDerived 0.

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