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Mathlib.Algebra.Homology.ShortComplex.PreservesHomology

Functors which preserves homology #

If F : C ⥤ D is a functor between categories with zero morphisms, we shall say that F preserves homology when F preserves both kernels and cokernels. This typeclass is named [F.PreservesHomology], and is automatically satisfied when F preserves both finite limits and finite colimits.

If S : ShortComplex C and [F.PreservesHomology], then there is an isomorphism S.mapHomologyIso F : (S.map F).homology ≅ F.obj S.homology, which is part of the natural isomorphism homologyFunctorIso F between the functors F.mapShortComplex ⋙ homologyFunctor D and homologyFunctor C ⋙ F.

A functor preserves homology when it preserves both kernels and cokernels.

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    A left homology data h of a short complex S is preserved by a functor F is F preserves the kernel of S.g : S.X₂ ⟶ S.X₃ and the cokernel of h.f' : S.X₁ ⟶ h.K.

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      When a left homology data h of a short complex S is preserved by a functor F, this is the induced left homology data h.map F for the short complex S.map F.

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        A right homology data h of a short complex S is preserved by a functor F is F preserves the cokernel of S.f : S.X₁ ⟶ S.X₂ and the kernel of h.g' : h.Q ⟶ S.X₃.

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          When a right homology data h of a short complex S is preserved by a functor F, this is the induced right homology data h.map F for the short complex S.map F.

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            When a homology data h of a short complex S is such that both h.left and h.right are preserved by a functor F, this is the induced homology data h.map F for the short complex S.map F.

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              Given a homology map data ψ : HomologyMapData φ h₁ h₂ such that h₁.left, h₁.right, h₂.left and h₂.right are all preserved by a functor F, this is the induced homology map data for the morphism F.mapShortComplex.map φ.

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                A functor preserves the left homology of a short complex S if it preserves all the left homology data of S.

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                  A functor preserves the right homology of a short complex S if it preserves all the right homology data of S.

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                    When a functor F preserves the left homology of a short complex S, this is the canonical isomorphism (S.map F).cycles ≅ F.obj S.cycles.

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                      When a functor F preserves the left homology of a short complex S, this is the canonical isomorphism (S.map F).leftHomology ≅ F.obj S.leftHomology.

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                        When a functor F preserves the right homology of a short complex S, this is the canonical isomorphism (S.map F).opcycles ≅ F.obj S.opcycles.

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                          When a functor F preserves the right homology of a short complex S, this is the canonical isomorphism (S.map F).rightHomology ≅ F.obj S.rightHomology.

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                            If a short complex S is such that S.f = 0 and that the kernel of S.g is preserved by a functor F, then F preserves the left homology of S.

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                              If a short complex S is such that S.g = 0 and that the cokernel of S.f is preserved by a functor F, then F preserves the right homology of S.

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                                If a short complex S is such that S.g = 0 and that the cokernel of S.f is preserved by a functor F, then F preserves the left homology of S.

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                                  If a short complex S is such that S.f = 0 and that the kernel of S.g is preserved by a functor F, then F preserves the right homology of S.

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