Truncations for a t-structure #
Let t be a t-structure on a (pre)triangulated category C.
In this file, for any n : ℤ, we construct truncation functors t.truncLT n : C ⥤ C,
t.truncGE n : C ⥤ C and natural transformations t.truncLTι n : t.truncLT n ⟶ 𝟭 C,
t.truncGEπ n : 𝟭 C ⟶ t.truncGE n and
t.truncGEδLT n : t.truncGE n ⟶ t.truncLT n ⋙ shiftFunctor C (1 : ℤ) which are
part of a distinguished triangle
(t.truncLT n).obj X ⟶ X ⟶ (t.truncGE n).obj X ⟶ ((t.truncLT n).obj X)⟦1⟧ for any X : C,
with (t.truncLT n).obj X < n and (t.truncGE n).obj X ≥ n.
We obtain various properties of these truncation functors.
Two morphisms T ⟶ T' between distinguished triangles must coincide when
they coincide on the middle object, and there are integers a ≤ b such that
for a t-structure, we have T.obj₁ ≤ a and T'.obj₃ ≥ b.
If a < b, then a morphism T.obj₂ ⟶ T'.obj₂ extends to a morphism T ⟶ T'
of distinguished triangles when for a t-structure T.obj₁ ≤ a and T'.obj₃ ≥ b.
If a < b, then an isomorphism T.obj₂ ≅ T'.obj₂ extends to an isomorphism T ≅ T'
of distinguished triangles when for a t-structure, both T.obj₁ and T'.obj₁ are ≤ a and
both T.obj₃ and T'.obj₃ are ≥ b.
The private definitions in the namespace TStructure.TruncAux are part of the
implementation of the truncation functors truncLT, truncGE and the
distinguished triangles they fit in.
Given a t-structure t on a pretriangulated category C and n : ℤ, this
is the < n-truncation functor. See also the natural transformation truncLTι.
Equations
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The natural transformation t.truncLT n ⟶ 𝟭 C when t is a t-structure
on a category C and n : ℤ.
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Given a t-structure t on a pretriangulated category C and n : ℤ, this
is the ≥ n-truncation functor. See also the natural transformation truncGEπ.
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The natural transformation 𝟭 C ⟶ t.truncGE n when t is a t-structure
on a category C and n : ℤ.
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The connecting morphism t.truncGE n ⟶ t.truncLT n ⋙ shiftFunctor C (1 : ℤ)
when t is a t-structure on a pretriangulated category and n : ℤ.
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The distinguished triangle (t.truncLT n).obj A ⟶ A ⟶ (t.truncGE n).obj A ⟶ ...
as a functor C ⥤ Triangle C when t is a t-structure on a pretriangulated
category C and n : ℤ.
Equations
- t.triangleLTGE n = CategoryTheory.Pretriangulated.Triangle.functorMk (t.truncLTι n) (t.truncGEπ n) (t.truncGEδLT n)
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The natural transformation t.truncLT a ⟶ t.truncLT b when a ≤ b.
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The natural transformation t.truncGE a ⟶ t.truncGE b when a ≤ b.
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The natural transformation t.triangleLTGE a ⟶ t.triangleLTGE b
when a ≤ b.
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Constructor for morphisms to (t.truncLT n₁).obj Y.
Equations
- t.liftTruncLT f n₀ n₁ h = ⋯.choose
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Constructor for morphisms from (t.truncGE n).obj X.
Equations
- t.descTruncGE f n = ⋯.choose
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The composition t.truncLT b ⋙ t.truncGE a.
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The composition t.truncGE b ⋙ t.truncLT a.
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The natural transformation t.truncGELT a b ⟶ t.truncLTGE a b
(which is an isomorphism, see truncGELTIsoLTGE.)
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The connecting homomorphism
(t.truncGELT a b).obj X ⟶ ((t.truncLT a).obj X)⟦1⟧,
as a natural transformation.
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The functorial (distinguished) triangle
(t.truncLT a).obj X ⟶ (t.truncLT b).obj X ⟶ (t.truncGELT a b).obj X ⟶ ...
when a ≤ b.
Equations
- t.triangleLTLTGELT a b h = CategoryTheory.Pretriangulated.Triangle.functorMk (t.natTransTruncLTOfLE a b h) ((t.truncLT b).whiskerLeft (t.truncGEπ a)) (t.truncGELTδLT a b)
Instances For
The natural transformation t.truncGELT a b ≅ t.truncLTGE a b.
Equations
- t.truncGELTIsoLTGE a b = CategoryTheory.asIso (t.truncGELTToLTGE a b)