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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The (distinguished) triangle in the homotopy category that is associated to
a morphism φ : K ⟶ L
in the category CochainComplex C ℤ
.
Equations
- CochainComplex.mappingCone.triangleh φ = (HomotopyCategory.quotient C (ComplexShape.up ℤ)).mapTriangle.obj (CochainComplex.mappingCone.triangle φ)
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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.
Equations
- CochainComplex.mappingCone.triangleMap φ₁ φ₂ a b comm = { hom₁ := a, hom₂ := b, hom₃ := CochainComplex.mappingCone.map φ₁ φ₂ a b comm, comm₁ := comm, comm₂ := ⋯, comm₃ := ⋯ }
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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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Auxiliary definition for rotateTrianglehIso
.
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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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A triangle in HomotopyCategory C (ComplexShape.up ℤ)
is distinguished if it is isomorphic to
the triangle CochainComplex.mappingCone.triangleh φ
for some morphism of cochain
complexes φ
.
Equations
- HomotopyCategory.Pretriangulated.distinguishedTriangles C T = ∃ (X : CochainComplex C ℤ) (Y : CochainComplex C ℤ) (φ : X ⟶ Y), Nonempty (T ≅ CochainComplex.mappingCone.triangleh φ)
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Equations
- ⋯ = ⋯