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Mathlib.Algebra.Homology.HomotopyCategory.Pretriangulated

The pretriangulated structure on the homotopy category of complexes

In this file, we define the pretriangulated structure on the homotopy category HomotopyCategory C (ComplexShape.up ℤ) of an additive category C. The distinguished triangles are the triangles that are isomorphic to the image in the homotopy category of the standard triangle K ⟶ L ⟶ mappingCone φ ⟶ K⟦(1 : ℤ)⟧ for some morphism of cochain complexes φ : K ⟶ L.

This result first appeared in the Liquid Tensor Experiment. In the LTE, the formalization followed the Stacks Project: in particular, the distinguished triangles were defined using degreewise-split short exact sequences of cochain complexes. Here, we follow the original definitions in Verdiers's thesis, I.3 (with the better sign conventions from the introduction of Brian Conrad's book Grothendieck duality and base change).

References #

The standard triangle K ⟶ L ⟶ mappingCone φ ⟶ K⟦(1 : ℤ)⟧ in CochainComplex C ℤ attached to a morphism φ : K ⟶ L. It involves φ, inr φ : L ⟶ mappingCone φ and the morphism induced by the 1-cocycle -mappingCone.fst φ.

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    @[reducible, inline]

    The (distinguished) triangle in the homotopy category that is associated to a morphism φ : K ⟶ L in the category CochainComplex C ℤ.

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      The mapping cone of the identity is contractible.

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        The morphism mappingCone φ₁ ⟶ mappingCone φ₂ that is induced by a square that is commutative up to homotopy.

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          The morphism triangleh φ₁ ⟶ triangleh φ₂ that is induced by a square that is commutative up to homotopy.

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            noncomputable def CochainComplex.mappingCone.map {C : Type u_1} [CategoryTheory.Category.{u_3, u_1} C] [CategoryTheory.Preadditive C] [CategoryTheory.Limits.HasBinaryBiproducts C] {K₁ L₁ K₂ L₂ : CochainComplex C } (φ₁ : K₁ L₁) (φ₂ : K₂ L₂) (a : K₁ K₂) (b : L₁ L₂) (comm : CategoryTheory.CategoryStruct.comp φ₁ b = CategoryTheory.CategoryStruct.comp a φ₂) :

            The morphism mappingCone φ₁ ⟶ mappingCone φ₂ that is induced by a commutative square.

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              theorem CochainComplex.mappingCone.map_comp {C : Type u_1} [CategoryTheory.Category.{u_3, u_1} C] [CategoryTheory.Preadditive C] [CategoryTheory.Limits.HasBinaryBiproducts C] {K₁ L₁ K₂ L₂ K₃ L₃ : CochainComplex C } (φ₁ : K₁ L₁) (φ₂ : K₂ L₂) (φ₃ : K₃ L₃) (a : K₁ K₂) (b : L₁ L₂) (comm : CategoryTheory.CategoryStruct.comp φ₁ b = CategoryTheory.CategoryStruct.comp a φ₂) (a' : K₂ K₃) (b' : L₂ L₃) (comm' : CategoryTheory.CategoryStruct.comp φ₂ b' = CategoryTheory.CategoryStruct.comp a' φ₃) :

              The morphism triangle φ₁ ⟶ triangle φ₂ that is induced by a commutative square.

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                theorem CochainComplex.mappingCone.triangleMap_hom₃ {C : Type u_1} [CategoryTheory.Category.{u_3, u_1} C] [CategoryTheory.Preadditive C] [CategoryTheory.Limits.HasBinaryBiproducts C] {K₁ L₁ K₂ L₂ : CochainComplex C } (φ₁ : K₁ L₁) (φ₂ : K₂ L₂) (a : K₁ K₂) (b : L₁ L₂) (comm : CategoryTheory.CategoryStruct.comp φ₁ b = CategoryTheory.CategoryStruct.comp a φ₂) :
                (CochainComplex.mappingCone.triangleMap φ₁ φ₂ a b comm).hom₃ = CochainComplex.mappingCone.map φ₁ φ₂ a b comm
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                theorem CochainComplex.mappingCone.triangleMap_hom₂ {C : Type u_1} [CategoryTheory.Category.{u_3, u_1} C] [CategoryTheory.Preadditive C] [CategoryTheory.Limits.HasBinaryBiproducts C] {K₁ L₁ K₂ L₂ : CochainComplex C } (φ₁ : K₁ L₁) (φ₂ : K₂ L₂) (a : K₁ K₂) (b : L₁ L₂) (comm : CategoryTheory.CategoryStruct.comp φ₁ b = CategoryTheory.CategoryStruct.comp a φ₂) :
                (CochainComplex.mappingCone.triangleMap φ₁ φ₂ a b comm).hom₂ = b
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                theorem CochainComplex.mappingCone.triangleMap_hom₁ {C : Type u_1} [CategoryTheory.Category.{u_3, u_1} C] [CategoryTheory.Preadditive C] [CategoryTheory.Limits.HasBinaryBiproducts C] {K₁ L₁ K₂ L₂ : CochainComplex C } (φ₁ : K₁ L₁) (φ₂ : K₂ L₂) (a : K₁ K₂) (b : L₁ L₂) (comm : CategoryTheory.CategoryStruct.comp φ₁ b = CategoryTheory.CategoryStruct.comp a φ₂) :
                (CochainComplex.mappingCone.triangleMap φ₁ φ₂ a b comm).hom₁ = a

                Given φ : K ⟶ L, K⟦(1 : ℤ)⟧ is homotopy equivalent to the mapping cone of inr φ : L ⟶ mappingCone φ.

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                  The canonical isomorphism of triangles (triangleh φ).rotate ≅ (triangleh (inr φ)).

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                    The canonical isomorphism (mappingCone φ)⟦n⟧ ≅ mappingCone (φ⟦n⟧').

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                      The canonical isomorphism (triangle φ)⟦n⟧ ≅ triangle (φ⟦n⟧').

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                        The canonical isomorphism (triangleh φ)⟦n⟧ ≅ triangleh (φ⟦n⟧').

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                          If φ : K ⟶ L is a morphism of cochain complexes in C and G : C ⥤ D is an additive functor, then the image by G of the triangle triangle φ identifies to the triangle associated to the image of φ by G.

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                            If φ : K ⟶ L is a morphism of cochain complexes in C and G : C ⥤ D is an additive functor, then the image by G of the triangle triangleh φ identifies to the triangle associated to the image of φ by G.

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