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Mathlib.AlgebraicTopology.SimplicialSet.Homology.Relative

Relative simplicial homology #

In this file, given a pair P : SSetPair of simplicial sets, we define the chain complex P.chainComplex R of P with coefficients in R (where R is an object in a preadditive category with coproducts). The homology in degree n of this complex is denoted P.homology R n: this is the relative homology of the pair, which is related to the homology of simplicial sets P.left and P.right via a long homology sequence (see the lemmas SSetPair.homology_exact₁, SSetPair.homology_exact₂ and SSetPair.homology_exact₃),

@[reducible, inline]

The bifunctor which sends R : C and a pair of simplicial sets i : X ⟶ Y (with i a monomorphism) to X.chainComplex R, which is the chain complex of X with coefficients in R (the usual one is for C := Ab and R := ℤ.).

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    @[reducible, inline]

    The bifunctor which sends R : C and a pair of simplicial sets i : X ⟶ Y (with i a monomorphism) to Y.chainComplex R, which is the chain complex of Y with coefficients in R (the usual one is for C := Ab and R := ℤ.).

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      The map X.chainComplex R ⟶ Y.chainComplex R for each pair of simplicial sets i : X ⟶ Y (with i a monomorphism), and R : C, as a natural transformation of bifunctors C ⥤ SSetPairChainComplex C ℕ.

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        The relative chain complex associated to a pair of simplicial sets, with coefficients in R : C, as a bifunctor C ⥤ SSetPair.{w} ⥤ ChainComplex C ℕ. It computes the simplicial homology of a pair of simplicial sets with coefficients in R. One can recover the ordinary relative simplicial chain complex when C := Ab and R := ℤ.

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          For any pair of simplicial sets i : X ⟶ Y (with i a monomorphism) and any R : C, this is the map from the chain complex of Y (with coefficients in R) to the chain complex of the pair, as a natural transformation of bifunctors C ⥤ SSetPairChainComplex C ℕ.

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            The relative chain complex of a pair P of simplicial sets with coefficients in R : C (e.g. C := Ab and R := ℤ.) This identifies to the cokernel of the morphism P.left.chainComplex R ⟶ P.right.chainComplex R of simplicial chain complexes.

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              The morphism of relative simplicial chain complexes induces by a morphism of pairs of simplicial sets.

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                Given a pair of simplicial sets i : X ⟶ Y (with i a monomorphism), this is the morphism from the chain complex of Y to the chain complex of the pair.

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                  Given a pair of simplicial sets i : X ⟶ Y (with i a monomorphism) and R : C (e.g. C := Ab and R := ℤ), this is the cokernel cofork expressing the chain complex SSetPair.chainComplex of the pair as a cokernel of the map X.chainComplex R ⟶ Y.chainComplex R.

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                    Given a pair of simplicial sets i : X ⟶ Y (with i a monomorphism) and R : C (e.g. C := Ab and R := ℤ), the chain complex SSetPair.chainComplex of the pair is a cokernel of the map X.chainComplex R ⟶ Y.chainComplex R.

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                      Given a pair of simplicial sets i : X ⟶ Y (with i a monomorphism), R : C (e.g. C := Ab and R := ℤ) and n : ℕ, this is the cokernel cofork expressing the chain complex SSetPair.chainComplex of the pair in degree n as a cokernel of the map (X.chainComplex R).X n ⟶ (Y.chainComplex R).X n.

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                        Given a pair of simplicial sets i : X ⟶ Y (with i a monomorphism), R : C (e.g. C := Ab and R := ℤ) and n : ℕ, the chain complex SSetPair.chainComplex of the pair in degree n is a cokernel of the map (X.chainComplex R).X n ⟶ (Y.chainComplex R).X n.

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                          Given a pair of simplicial sets corresponding to a monomorphism i : X ⟶ Y, this is the (short exact) short complex which relates the chain complex of X, of Y and of the pair.

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                            The relative simplicial homology with coefficients in R : C in degree n of a pair of simplicial sets.

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                              The morphism in relative simplicial homology that is induced by a morphism of pairs of simplicial sets.

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                                @[implicit_reducible]

                                The relative simplicial homology functor in degree n with coefficients in R : C.

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                                  Given a pair of simplicial sets corresponding to a monomorphism i : X ⟶ Y, this is the morphism from the homology of Y to the relative homology of the pair.

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                                    noncomputable def SSetPair.homologyδ {A : Type u_1} [CategoryTheory.Category.{v_1, u_1} A] [CategoryTheory.Limits.HasCoproducts A] [CategoryTheory.Abelian A] (P : SSetPair) (R : A) (n m : ) (h : m + 1 = n := by lia) :

                                    Given a pair of simplicial sets corresponding to a monomorphism i : X ⟶ Y, this is the connecting morphism from the homology of the pair to the homology of X.

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                                      theorem SSetPair.homology_exact₁ {A : Type u_1} [CategoryTheory.Category.{v_1, u_1} A] [CategoryTheory.Limits.HasCoproducts A] [CategoryTheory.Abelian A] (P : SSetPair) (R : A) (n m : ) (h : m + 1 = n := by lia) :
                                      { X₁ := P.homology R n, X₂ := P.left.homology R m, X₃ := ((CategoryTheory.Functor.id SSet).obj P.right).homology R m, f := P.homologyδ R n m h, g := SSet.homologyMap P.hom R m, zero := }.Exact

                                      Given P : SSetPair corresponding to a monomorphism i : Y ⟶ X, the sequence Hₘ₊₁(X;Y) ⟶ Hₘ(Y) ⟶ Hₘ(X) is exact.

                                      Given P : SSetPair corresponding to a monomorphism i : Y ⟶ X, the sequence Hₙ(Y) ⟶ Hₙ(X) ⟶ Hₙ(X;Y) is exact.

                                      theorem SSetPair.homology_exact₃ {A : Type u_1} [CategoryTheory.Category.{v_1, u_1} A] [CategoryTheory.Limits.HasCoproducts A] [CategoryTheory.Abelian A] (P : SSetPair) (R : A) (n m : ) (h : m + 1 = n := by lia) :
                                      { X₁ := P.right.homology R n, X₂ := P.homology R n, X₃ := P.left.homology R m, f := P.homologyπ R n, g := P.homologyδ R n m h, zero := }.Exact

                                      Given P : SSetPair corresponding to a monomorphism i : Y ⟶ X, the sequence Hₘ₊₁(X) ⟶ Hₘ₊₁(X;Y) ⟶ Hₘ(Y) is exact.