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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][verdier1996] (with the better sign conventions from the introduction of [Brian Conrad's book Grothendieck duality and base change][conrad2000]).

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

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

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                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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                        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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