Documentation

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 #

TODO #

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.

Equations
Instances For

    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.

    Equations
    Instances For

      The right derived functors of an additive functor.

      Equations
      Instances For

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

        Equations
        • One or more equations did not get rendered due to their size.
        Instances For

          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.

          noncomputable def CategoryTheory.NatTrans.rightDerived {C : Type u} [Category.{v, u} C] {D : Type u_1} [Category.{u_2, u_1} D] [Abelian C] [HasInjectiveResolutions C] [Abelian D] {F G : Functor C D} [F.Additive] [G.Additive] (α : F G) (n : ) :

          The natural transformation between right-derived functors induced by a natural transformation.

          Equations
          Instances For

            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.

            Equations
            Instances For

              The natural transformation F ⟶ F.rightDerived 0.

              Equations
              • One or more equations did not get rendered due to their size.
              Instances For

                The canonical isomorphism F.rightDerived 0 ≅ F when F is left exact (i.e. preserves finite limits).

                Equations
                Instances For