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 #
Functor.isZero_rightDerived_obj_injective_succ: injective objects have no higher right derived functor.NatTrans.rightDerived: the natural isomorphism between right derived functors induced by natural transformation.Functor.toRightDerivedZero: the natural transformationF ⟶ F.rightDerived 0, which is an isomorphism whenFis left exact (i.e. preserves finite limits), see alsoFunctor.rightDerivedZeroIsoSelf.
TODO #
- refactor
Functor.rightDerived(andFunctor.leftDerived) when the necessary material enters mathlib: derived categories, injective/projective derivability structures, existence of derived functors from derivability structures. Eventually, we shall get a right derived functorF.rightDerivedFunctorPlus : DerivedCategory.Plus C ⥤ DerivedCategory.Plus D, andF.rightDerivedshall be redefined usingF.rightDerivedFunctorPlus.
When F : C ⥤ D is an additive functor, this is
the functor C ⥤ HomotopyCategory D (ComplexShape.up ℕ) which
sends X : C to F applied to an injective resolution of X.
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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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The right derived functors of an additive functor.
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We can compute a right derived functor using a chosen injective resolution.
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- One or more equations did not get rendered due to their size.
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The higher derived functors vanish on injective objects.
We can compute a right derived functor on a morphism using a descent of that morphism to a cochain map between chosen injective resolutions.
The natural transformation
F.rightDerivedToHomotopyCategory ⟶ G.rightDerivedToHomotopyCategory induced by
a natural transformation F ⟶ G between additive functors.
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The natural transformation between right-derived functors induced by a natural transformation.
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A component of the natural transformation between right-derived functors can be computed using a chosen injective resolution.
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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- P.toRightDerivedZero' F = ((F.mapHomologicalComplex (ComplexShape.up ℕ)).obj P.cocomplex).liftCycles (F.map (P.ι.f 0)) 1 CategoryTheory.InjectiveResolution.toRightDerivedZero'._proof_2 ⋯
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The natural transformation F ⟶ F.rightDerived 0.
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- One or more equations did not get rendered due to their size.
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The canonical isomorphism F.rightDerived 0 ≅ F when F is left exact
(i.e. preserves finite limits).