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Mathlib.Algebra.Homology.ShortComplex.LeftHomology

Left Homology of short complexes #

Given a short complex S : ShortComplex C, which consists of two composable maps f : X₁ ⟶ X₂ and g : X₂ ⟶ X₃ such that f ≫ g = 0, we shall define here the "left homology" S.leftHomology of S. For this, we introduce the notion of "left homology data". Such an h : S.LeftHomologyData consists of the data of morphisms i : K ⟶ X₂ and π : KH such that i identifies K with the kernel of g : X₂ ⟶ X₃, and that π identifies H with the cokernel of the induced map f' : X₁ ⟶ K.

When such a S.LeftHomologyData exists, we shall say that [S.HasLeftHomology] and we define S.leftHomology to be the H field of a chosen left homology data. Similarly, we define S.cycles to be the K field.

The dual notion is defined in RightHomologyData.lean. In Homology.lean, when S has two compatible left and right homology data (i.e. they give the same H up to a canonical isomorphism), we shall define [S.HasHomology] and S.homology.

A left homology data for a short complex S consists of morphisms i : K ⟶ S.X₂ and π : KH such that i identifies K to the kernel of g : S.X₂ ⟶ S.X₃, and that π identifies H to the cokernel of the induced map f' : S.X₁ ⟶ K

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    Any morphism k : A ⟶ S.X₂ that is a cycle (i.e. k ≫ S.g = 0) lifts to a morphism A ⟶ K

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      Given h : LeftHomologyData S, this is morphism S.X₁ ⟶ h.K induced by S.f : S.X₁ ⟶ S.X₂ and the fact that h.K is a kernel of S.g : S.X₂ ⟶ S.X₃.

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        When the second map S.g is zero, this is the left homology data on S given by any colimit cokernel cofork of S.f

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          When the second map S.g is zero, this is the left homology data on S given by the chosen cokernel S.f

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            When the first map S.f is zero, this is the left homology data on S given by any limit kernel fork of S.g

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              When the first map S.f is zero, this is the left homology data on S given by the chosen kernel S.g

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                When both S.f and S.g are zero, the middle object S.X₂ gives a left homology data on S

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                  Given left homology data h₁ and h₂ for two short complexes S₁ and S₂, a LeftHomologyMapData for a morphism φ : S₁ ⟶ S₂ consists of a description of the induced morphisms on the K (cycles) and H (left homology) fields of h₁ and h₂.

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                    The left homology map data associated to the zero morphism between two short complexes.

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                      @[simp]
                      theorem CategoryTheory.ShortComplex.LeftHomologyMapData.ofZeros_φK {C : Type u_1} [CategoryTheory.Category.{u_2, u_1} C] [CategoryTheory.Limits.HasZeroMorphisms C] {S₁ : CategoryTheory.ShortComplex C} {S₂ : CategoryTheory.ShortComplex C} (φ : S₁ S₂) (hf₁ : S₁.f = 0) (hg₁ : S₁.g = 0) (hf₂ : S₂.f = 0) (hg₂ : S₂.g = 0) :
                      (CategoryTheory.ShortComplex.LeftHomologyMapData.ofZeros φ hf₁ hg₁ hf₂ hg₂).φK = φ.τ₂
                      @[simp]
                      theorem CategoryTheory.ShortComplex.LeftHomologyMapData.ofZeros_φH {C : Type u_1} [CategoryTheory.Category.{u_2, u_1} C] [CategoryTheory.Limits.HasZeroMorphisms C] {S₁ : CategoryTheory.ShortComplex C} {S₂ : CategoryTheory.ShortComplex C} (φ : S₁ S₂) (hf₁ : S₁.f = 0) (hg₁ : S₁.g = 0) (hf₂ : S₂.f = 0) (hg₂ : S₂.g = 0) :
                      (CategoryTheory.ShortComplex.LeftHomologyMapData.ofZeros φ hf₁ hg₁ hf₂ hg₂).φH = φ.τ₂

                      When S₁.f, S₁.g, S₂.f and S₂.g are all zero, the action on left homology of a morphism φ : S₁ ⟶ S₂ is given by the action φ.τ₂ on the middle objects.

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                        When S₁.g and S₂.g are zero and we have chosen colimit cokernel coforks c₁ and c₂ for S₁.f and S₂.f respectively, the action on left homology of a morphism φ : S₁ ⟶ S₂ of short complexes is given by the unique morphism f : c₁.pt ⟶ c₂.pt such that φ.τ₂ ≫ c₂.π = c₁.π ≫ f.

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                          When S₁.f and S₂.f are zero and we have chosen limit kernel forks c₁ and c₂ for S₁.g and S₂.g respectively, the action on left homology of a morphism φ : S₁ ⟶ S₂ of short complexes is given by the unique morphism f : c₁.pt ⟶ c₂.pt such that c₁.ι ≫ φ.τ₂ = f ≫ c₂.ι.

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                            When both maps S.f and S.g of a short complex S are zero, this is the left homology map data (for the identity of S) which relates the left homology data ofZeros and ofIsColimitCokernelCofork.

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                              When both maps S.f and S.g of a short complex S are zero, this is the left homology map data (for the identity of S) which relates the left homology data LeftHomologyData.ofIsLimitKernelFork and ofZeros .

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                                The "boundaries" map S.X₁ ⟶ S.cycles. (Note that in this homology API, we make no use of the "image" of this morphism, which under some categorical assumptions would be a subobject of S.X₂ contained in S.cycles.)

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                                  The (unique) left homology map data associated to a morphism of short complexes that are both equipped with left homology data.

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                                    Given a morphism φ : S₁ ⟶ S₂ of short complexes and left homology data h₁ and h₂ for S₁ and S₂ respectively, this is the induced left homology map h₁.H ⟶ h₁.H.

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                                      Given a morphism φ : S₁ ⟶ S₂ of short complexes and left homology data h₁ and h₂ for S₁ and S₂ respectively, this is the induced morphism h₁.K ⟶ h₁.K on cycles.

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                                        The (left) homology map S₁.leftHomology ⟶ S₂.leftHomology induced by a morphism S₁ ⟶ S₂ of short complexes.

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                                          An isomorphism of short complexes S₁ ≅ S₂ induces an isomorphism on the H fields of left homology data of S₁ and S₂.

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                                            An isomorphism of short complexes S₁ ≅ S₂ induces an isomorphism on the K fields of left homology data of S₁ and S₂.

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                                              The isomorphism S₁.leftHomology ≅ S₂.leftHomology induced by an isomorphism of short complexes S₁ ≅ S₂.

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                                                The left homology functor ShortComplex C ⥤ C, where the left homology of a short complex S is understood as a cokernel of the obvious map S.toCycles : S.X₁ ⟶ S.cycles where S.cycles is a kernel of S.g : S.X₂ ⟶ S.X₃.

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                                                  The cycles functor ShortComplex C ⥤ C which sends a short complex S to S.cycles which is a kernel of S.g : S.X₂ ⟶ S.X₃.

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                                                    If φ : S₁ ⟶ S₂ is a morphism of short complexes such that φ.τ₁ is epi, φ.τ₂ is an iso and φ.τ₃ is mono, then a left homology data for S₁ induces a left homology data for S₂ with the same K and H fields. The inverse construction is ofEpiOfIsIsoOfMono'.

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                                                      If φ : S₁ ⟶ S₂ is a morphism of short complexes such that φ.τ₁ is epi, φ.τ₂ is an iso and φ.τ₃ is mono, then a left homology data for S₂ induces a left homology data for S₁ with the same K and H fields. The inverse construction is ofEpiOfIsIsoOfMono.

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                                                        If e : S₁ ≅ S₂ is an isomorphism of short complexes and h₁ : LeftHomologyData S₁, this is the left homology data for S₂ deduced from the isomorphism.

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                                                          This left homology map data expresses compatibilities of the left homology data constructed by LeftHomologyData.ofEpiOfIsIsoOfMono

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                                                            This left homology map data expresses compatibilities of the left homology data constructed by LeftHomologyData.ofEpiOfIsIsoOfMono'

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                                                              If a morphism of short complexes φ : S₁ ⟶ S₂ is such that φ.τ₁ is epi, φ.τ₂ is an iso, and φ.τ₃ is mono, then the induced morphism on left homology is an isomorphism.

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                                                              The following lemmas and instance gives a sufficient condition for a morphism of short complexes to induce an isomorphism on cycles.