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Mathlib.CategoryTheory.Triangulated.TStructure.TruncLTGE

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.

theorem CategoryTheory.Triangulated.TStructure.triangle_map_ext {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) {T T' : Pretriangulated.Triangle C} {f₁ f₂ : T T'} (hT : T Pretriangulated.distinguishedTriangles) (hT' : T' Pretriangulated.distinguishedTriangles) (a b : ) (h₀ : t.IsLE T.obj₁ a) (h₁ : t.IsGE T'.obj₃ b) (H : f₁.hom₂ = f₂.hom₂ := by cat_disch) (hab : a b := by lia) :
f₁ = f₂

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.

theorem CategoryTheory.Triangulated.TStructure.triangle_map_exists {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) {T T' : Pretriangulated.Triangle C} (hT : T Pretriangulated.distinguishedTriangles) (hT' : T' Pretriangulated.distinguishedTriangles) (φ : T.obj₂ T'.obj₂) (a b : ) (h₀ : t.IsLE T.obj₁ a) (h₁' : t.IsGE T'.obj₃ b) (h : a < b := by lia) :
∃ (f : T T'), f.hom₂ = φ

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.

theorem CategoryTheory.Triangulated.TStructure.triangle_iso_exists {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) {T T' : Pretriangulated.Triangle C} (hT : T Pretriangulated.distinguishedTriangles) (hT' : T' Pretriangulated.distinguishedTriangles) (e : T.obj₂ T'.obj₂) (a b : ) (h₀ : t.IsLE T.obj₁ a) (h₁ : t.IsGE T.obj₃ b) (h₀' : t.IsLE T'.obj₁ a) (h₁' : t.IsGE T'.obj₃ b) (h : a < b := by lia) :
∃ (e' : T T'), e'.hom.hom₂ = e.hom

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.

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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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          theorem CategoryTheory.Triangulated.TStructure.isLE_truncLT_obj {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) (X : C) (a b : ) (hn : a b + 1 := by lia) :
          t.IsLE ((t.truncLT a).obj X) b
          theorem CategoryTheory.Triangulated.TStructure.isGE_truncGE_obj {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) (X : C) (a b : ) (hn : b a := by lia) :
          t.IsGE ((t.truncGE a).obj X) b

          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.

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              @[simp]
              theorem CategoryTheory.Triangulated.TStructure.triangleLTGE_map_hom₁ {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) (n : ) {X✝ Y✝ : C} (φ : X✝ Y✝) :
              ((t.triangleLTGE n).map φ).hom₁ = (t.truncLT n).map φ
              @[simp]
              theorem CategoryTheory.Triangulated.TStructure.triangleLTGE_map_hom₂ {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) (n : ) {X✝ Y✝ : C} (φ : X✝ Y✝) :
              ((t.triangleLTGE n).map φ).hom₂ = φ
              @[simp]
              theorem CategoryTheory.Triangulated.TStructure.triangleLTGE_map_hom₃ {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) (n : ) {X✝ Y✝ : C} (φ : X✝ Y✝) :
              ((t.triangleLTGE n).map φ).hom₃ = (t.truncGE n).map φ

              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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                    theorem CategoryTheory.Triangulated.TStructure.isLE_iff_isIso_truncLTι_app {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) (n₀ n₁ : ) (h : n₀ + 1 = n₁) (X : C) :
                    t.IsLE X n₀ IsIso ((t.truncLTι n₁).app X)
                    theorem CategoryTheory.Triangulated.TStructure.isLE_iff_isZero_truncGE_obj {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) (n₀ n₁ : ) (h : n₀ + 1 = n₁) (X : C) :
                    t.IsLE X n₀ Limits.IsZero ((t.truncGE n₁).obj X)
                    theorem CategoryTheory.Triangulated.TStructure.isZero_truncGE_obj_of_isLE {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) (n₀ n₁ : ) (h : n₀ + 1 = n₁) (X : C) [t.IsLE X n₀] :
                    theorem CategoryTheory.Triangulated.TStructure.from_truncGE_obj_ext {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) {n : } {X Y : C} {f₁ f₂ : (t.truncGE n).obj X Y} (h : CategoryStruct.comp ((t.truncGEπ n).app X) f₁ = CategoryStruct.comp ((t.truncGEπ n).app X) f₂) [t.IsGE Y n] :
                    f₁ = f₂
                    theorem CategoryTheory.Triangulated.TStructure.to_truncLT_obj_ext {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) {n : } {Y X : C} {f₁ f₂ : Y (t.truncLT n).obj X} (h : CategoryStruct.comp f₁ ((t.truncLTι n).app X) = CategoryStruct.comp f₂ ((t.truncLTι n).app X)) [t.IsLE Y (n - 1)] :
                    f₁ = f₂
                    theorem CategoryTheory.Triangulated.TStructure.liftTruncLT_aux {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) {X Y : C} (f : X Y) (n₀ n₁ : ) (h : n₀ + 1 = n₁) [t.IsLE X n₀] :
                    ∃ (f' : X (t.truncLT n₁).obj Y), f = CategoryStruct.comp f' ((t.truncLTι n₁).app Y)
                    noncomputable def CategoryTheory.Triangulated.TStructure.liftTruncLT {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) {X Y : C} (f : X Y) (n₀ n₁ : ) (h : n₀ + 1 = n₁) [t.IsLE X n₀] :
                    X (t.truncLT n₁).obj Y

                    Constructor for morphisms to (t.truncLT n₁).obj Y.

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                      @[simp]
                      theorem CategoryTheory.Triangulated.TStructure.liftTruncLT_ι {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) {X Y : C} (f : X Y) (n₀ n₁ : ) (h : n₀ + 1 = n₁) [t.IsLE X n₀] :
                      CategoryStruct.comp (t.liftTruncLT f n₀ n₁ h) ((t.truncLTι n₁).app Y) = f
                      @[simp]
                      theorem CategoryTheory.Triangulated.TStructure.liftTruncLT_ι_assoc {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) {X Y : C} (f : X Y) (n₀ n₁ : ) (h : n₀ + 1 = n₁) [t.IsLE X n₀] {Z : C} (h✝ : Y Z) :
                      theorem CategoryTheory.Triangulated.TStructure.descTruncGE_aux {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) {X Y : C} (f : X Y) (n : ) [t.IsGE Y n] :
                      ∃ (f' : (t.truncGE n).obj X Y), f = CategoryStruct.comp ((t.truncGEπ n).app X) f'
                      noncomputable def CategoryTheory.Triangulated.TStructure.descTruncGE {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) {X Y : C} (f : X Y) (n : ) [t.IsGE Y n] :
                      (t.truncGE n).obj X Y

                      Constructor for morphisms from (t.truncGE n).obj X.

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                        theorem CategoryTheory.Triangulated.TStructure.isLE_iff_orthogonal {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) (n₀ n₁ : ) (h : n₀ + 1 = n₁) (X : C) :
                        t.IsLE X n₀ ∀ (Y : C) (f : X Y), t.IsGE Y n₁f = 0
                        theorem CategoryTheory.Triangulated.TStructure.isGE_iff_orthogonal {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) (n₀ n₁ : ) (h : n₀ + 1 = n₁) (X : C) :
                        t.IsGE X n₁ ∀ (Y : C) (f : Y X), t.IsLE Y n₀f = 0
                        theorem CategoryTheory.Triangulated.TStructure.isIso_truncGE_map_iff {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) {Y Z : C} (g : Y Z) (n₀ n₁ : ) (hn : n₀ + 1 = n₁) :
                        IsIso ((t.truncGE n₁).map g) ∃ (X : C) (f : X Y) (h : (t.truncGE n₁).obj Z (shiftFunctor C 1).obj X) (_ : Pretriangulated.Triangle.mk f (CategoryStruct.comp g ((t.truncGEπ n₁).app Z)) h Pretriangulated.distinguishedTriangles), t.IsLE X n₀
                        @[reducible, inline]

                        The composition t.truncLT b ⋙ t.truncGE a.

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                          @[reducible, inline]

                          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.

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                                  @[simp]
                                  theorem CategoryTheory.Triangulated.TStructure.triangleLTLTGELT_map_hom₃ {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) (a b : ) (h : a b) {X✝ Y✝ : C} (φ : X✝ Y✝) :
                                  ((t.triangleLTLTGELT a b h).map φ).hom₃ = (t.truncGE a).map ((t.truncLT b).map φ)
                                  @[simp]
                                  theorem CategoryTheory.Triangulated.TStructure.triangleLTLTGELT_map_hom₁ {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) (a b : ) (h : a b) {X✝ Y✝ : C} (φ : X✝ Y✝) :
                                  ((t.triangleLTLTGELT a b h).map φ).hom₁ = (t.truncLT a).map φ
                                  @[simp]
                                  theorem CategoryTheory.Triangulated.TStructure.triangleLTLTGELT_map_hom₂ {C : Type u} [Category.{v, u} C] [Preadditive C] [Limits.HasZeroObject C] [HasShift C ] [∀ (n : ), (shiftFunctor C n).Additive] [Pretriangulated C] (t : TStructure C) (a b : ) (h : a b) {X✝ Y✝ : C} (φ : X✝ Y✝) :
                                  ((t.triangleLTLTGELT a b h).map φ).hom₂ = (t.truncLT b).map φ

                                  The natural transformation t.truncGELT a b ≅ t.truncLTGE a b.

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