-- Mathlib/CategoryTheory/Triangulated/Functor.lean
/-- If `F : C ⥤ D` is a triangulated functor from a triangulated category, then `D`
is also triangulated if tuples of composables arrows in `D` can be lifted to `C`. -/
lemma isTriangulated_of_essSurj_mapComposableArrows_two
    (F : C ⥤ D) [F.CommShift ℤ] [F.IsTriangulated]
    [(F.mapComposableArrows 2).EssSurj] [IsTriangulated C] :
    IsTriangulated D

Proof state:
case octahedron_axiom.intro.intro.intro.intro.intro.intro.intro
C : Type u_1
D : Type u_2
inst✝¹⁵ : Category.{u_3, u_1} C
inst✝¹⁴ : Category.{u_4, u_2} D
inst✝¹³ : HasShift C ℤ
inst✝¹² : HasShift D ℤ
inst✝¹¹ : HasZeroObject C
inst✝¹⁰ : HasZeroObject D
inst✝⁹ : Preadditive C
inst✝⁸ : Preadditive D
inst✝⁷ : ∀ (n : ℤ), (shiftFunctor C n).Additive
inst✝⁶ : ∀ (n : ℤ), (shiftFunctor D n).Additive
inst✝⁵ : Pretriangulated C
inst✝⁴ : Pretriangulated D
F : C ⥤ D
inst✝³ : F.CommShift ℤ
inst✝² : F.IsTriangulated
inst✝¹ : (F.mapComposableArrows 2).EssSurj
inst✝ : IsTriangulated C
Y₁ Y₂ Y₃ Z₁₂ Z₂₃ Z₁₃ : D
u₁₂ : Y₁ ⟶ Y₂
u₂₃ : Y₂ ⟶ Y₃
u₁₃ : Y₁ ⟶ Y₃
comm : u₁₂ ≫ u₂₃ = u₁₃
v₁₂ : Y₂ ⟶ Z₁₂
w₁₂ : Z₁₂ ⟶ (shiftFunctor D 1).obj Y₁
h₁₂ : Triangle.mk u₁₂ v₁₂ w₁₂ ∈ distinguishedTriangles
v₂₃ : Y₃ ⟶ Z₂₃
w₂₃ : Z₂₃ ⟶ (shiftFunctor D 1).obj Y₂
h₂₃ : Triangle.mk u₂₃ v₂₃ w₂₃ ∈ distinguishedTriangles
v₁₃ : Y₃ ⟶ Z₁₃
w₁₃ : Z₁₃ ⟶ (shiftFunctor D 1).obj Y₁
h₁₃ : Triangle.mk u₁₃ v₁₃ w₁₃ ∈ distinguishedTriangles
X₁ X₂ X₃ : C
f : X₁ ⟶ X₂
g : X₂ ⟶ X₃
e : (F.mapComposableArrows 2).obj (ComposableArrows.mk₂ f g) ≅ ComposableArrows.mk₂ u₁₂ u₂₃
⊢ Nonempty (Octahedron comm h₁₂ h₂₃ h₁₃)

Tactic:
obtain ⟨_, _, _, h₁₂'⟩ := distinguished_cocone_triangle f

Result:
case octahedron_axiom.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
C : Type u_1
D : Type u_2
inst✝¹⁵ : Category.{u_3, u_1} C
inst✝¹⁴ : Category.{u_4, u_2} D
inst✝¹³ : HasShift C ℤ
inst✝¹² : HasShift D ℤ
inst✝¹¹ : HasZeroObject C
inst✝¹⁰ : HasZeroObject D
inst✝⁹ : Preadditive C
inst✝⁸ : Preadditive D
inst✝⁷ : ∀ (n : ℤ), (shiftFunctor C n).Additive
inst✝⁶ : ∀ (n : ℤ), (shiftFunctor D n).Additive
inst✝⁵ : Pretriangulated C
inst✝⁴ : Pretriangulated D
F : C ⥤ D
inst✝³ : F.CommShift ℤ
inst✝² : F.IsTriangulated
inst✝¹ : (F.mapComposableArrows 2).EssSurj
inst✝ : IsTriangulated C
Y₁ Y₂ Y₃ Z₁₂ Z₂₃ Z₁₃ : D
u₁₂ : Y₁ ⟶ Y₂
u₂₃ : Y₂ ⟶ Y₃
u₁₃ : Y₁ ⟶ Y₃
comm : u₁₂ ≫ u₂₃ = u₁₃
v₁₂ : Y₂ ⟶ Z₁₂
w₁₂ : Z₁₂ ⟶ (shiftFunctor D 1).obj Y₁
h₁₂ : Triangle.mk u₁₂ v₁₂ w₁₂ ∈ distinguishedTriangles
v₂₃ : Y₃ ⟶ Z₂₃
w₂₃ : Z₂₃ ⟶ (shiftFunctor D 1).obj Y₂
h₂₃ : Triangle.mk u₂₃ v₂₃ w₂₃ ∈ distinguishedTriangles
v₁₃ : Y₃ ⟶ Z₁₃
w₁₃ : Z₁₃ ⟶ (shiftFunctor D 1).obj Y₁
h₁₃ : Triangle.mk u₁₃ v₁₃ w₁₃ ∈ distinguishedTriangles
X₁ X₂ X₃ : C
f : X₁ ⟶ X₂
g : X₂ ⟶ X₃
e : (F.mapComposableArrows 2).obj (ComposableArrows.mk₂ f g) ≅ ComposableArrows.mk₂ u₁₂ u₂₃
w✝² : C
w✝¹ : X₂ ⟶ w✝²
w✝ : w✝² ⟶ (shiftFunctor C 1).obj X₁
h₁₂' : Triangle.mk f w✝¹ w✝ ∈ distinguishedTriangles
⊢ Nonempty (Octahedron comm h₁₂ h₂₃ h₁₃)

Proof state:
case octahedron_axiom
C : Type u_1
D : Type u_2
inst✝¹⁵ : Category.{u_3, u_1} C
inst✝¹⁴ : Category.{u_4, u_2} D
inst✝¹³ : HasShift C ℤ
inst✝¹² : HasShift D ℤ
inst✝¹¹ : HasZeroObject C
inst✝¹⁰ : HasZeroObject D
inst✝⁹ : Preadditive C
inst✝⁸ : Preadditive D
inst✝⁷ : ∀ (n : ℤ), (shiftFunctor C n).Additive
inst✝⁶ : ∀ (n : ℤ), (shiftFunctor D n).Additive
inst✝⁵ : Pretriangulated C
inst✝⁴ : Pretriangulated D
F : C ⥤ D
inst✝³ : F.CommShift ℤ
inst✝² : F.IsTriangulated
inst✝¹ : (F.mapComposableArrows 2).EssSurj
inst✝ : IsTriangulated C
Y₁ Y₂ Y₃ Z₁₂ Z₂₃ Z₁₃ : D
u₁₂ : Y₁ ⟶ Y₂
u₂₃ : Y₂ ⟶ Y₃
u₁₃ : Y₁ ⟶ Y₃
comm : u₁₂ ≫ u₂₃ = u₁₃
v₁₂ : Y₂ ⟶ Z₁₂
w₁₂ : Z₁₂ ⟶ (shiftFunctor D 1).obj Y₁
h₁₂ : Triangle.mk u₁₂ v₁₂ w₁₂ ∈ distinguishedTriangles
v₂₃ : Y₃ ⟶ Z₂₃
w₂₃ : Z₂₃ ⟶ (shiftFunctor D 1).obj Y₂
h₂₃ : Triangle.mk u₂₃ v₂₃ w₂₃ ∈ distinguishedTriangles
v₁₃ : Y₃ ⟶ Z₁₃
w₁₃ : Z₁₃ ⟶ (shiftFunctor D 1).obj Y₁
h₁₃ : Triangle.mk u₁₃ v₁₃ w₁₃ ∈ distinguishedTriangles
⊢ Nonempty (Octahedron comm h₁₂ h₂₃ h₁₃)

Tactic:
obtain ⟨α, ⟨e⟩⟩ : ∃ (α : ComposableArrows C 2),
    Nonempty ((F.mapComposableArrows 2).obj α ≅ ComposableArrows.mk₂ u₁₂ u₂₃) :=
  ⟨_, ⟨Functor.objObjPreimageIso _ _⟩⟩

Result:
case octahedron_axiom.intro.intro
C : Type u_1
D : Type u_2
inst✝¹⁵ : Category.{u_3, u_1} C
inst✝¹⁴ : Category.{u_4, u_2} D
inst✝¹³ : HasShift C ℤ
inst✝¹² : HasShift D ℤ
inst✝¹¹ : HasZeroObject C
inst✝¹⁰ : HasZeroObject D
inst✝⁹ : Preadditive C
inst✝⁸ : Preadditive D
inst✝⁷ : ∀ (n : ℤ), (shiftFunctor C n).Additive
inst✝⁶ : ∀ (n : ℤ), (shiftFunctor D n).Additive
inst✝⁵ : Pretriangulated C
inst✝⁴ : Pretriangulated D
F : C ⥤ D
inst✝³ : F.CommShift ℤ
inst✝² : F.IsTriangulated
inst✝¹ : (F.mapComposableArrows 2).EssSurj
inst✝ : IsTriangulated C
Y₁ Y₂ Y₃ Z₁₂ Z₂₃ Z₁₃ : D
u₁₂ : Y₁ ⟶ Y₂
u₂₃ : Y₂ ⟶ Y₃
u₁₃ : Y₁ ⟶ Y₃
comm : u₁₂ ≫ u₂₃ = u₁₃
v₁₂ : Y₂ ⟶ Z₁₂
w₁₂ : Z₁₂ ⟶ (shiftFunctor D 1).obj Y₁
h₁₂ : Triangle.mk u₁₂ v₁₂ w₁₂ ∈ distinguishedTriangles
v₂₃ : Y₃ ⟶ Z₂₃
w₂₃ : Z₂₃ ⟶ (shiftFunctor D 1).obj Y₂
h₂₃ : Triangle.mk u₂₃ v₂₃ w₂₃ ∈ distinguishedTriangles
v₁₃ : Y₃ ⟶ Z₁₃
w₁₃ : Z₁₃ ⟶ (shiftFunctor D 1).obj Y₁
h₁₃ : Triangle.mk u₁₃ v₁₃ w₁₃ ∈ distinguishedTriangles
α : ComposableArrows C 2
e : (F.mapComposableArrows 2).obj α ≅ ComposableArrows.mk₂ u₁₂ u₂₃
⊢ Nonempty (Octahedron comm h₁₂ h₂₃ h₁₃)

Proof state:
case octahedron_axiom.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro.intro
C : Type u_1
D : Type u_2
inst✝¹⁵ : Category.{u_3, u_1} C
inst✝¹⁴ : Category.{u_4, u_2} D
inst✝¹³ : HasShift C ℤ
inst✝¹² : HasShift D ℤ
inst✝¹¹ : HasZeroObject C
inst✝¹⁰ : HasZeroObject D
inst✝⁹ : Preadditive C
inst✝⁸ : Preadditive D
inst✝⁷ : ∀ (n : ℤ), (shiftFunctor C n).Additive
inst✝⁶ : ∀ (n : ℤ), (shiftFunctor D n).Additive
inst✝⁵ : Pretriangulated C
inst✝⁴ : Pretriangulated D
F : C ⥤ D
inst✝³ : F.CommShift ℤ
inst✝² : F.IsTriangulated
inst✝¹ : (F.mapComposableArrows 2).EssSurj
inst✝ : IsTriangulated C
Y₁ Y₂ Y₃ Z₁₂ Z₂₃ Z₁₃ : D
u₁₂ : Y₁ ⟶ Y₂
u₂₃ : Y₂ ⟶ Y₃
u₁₃ : Y₁ ⟶ Y₃
comm : u₁₂ ≫ u₂₃ = u₁₃
v₁₂ : Y₂ ⟶ Z₁₂
w₁₂ : Z₁₂ ⟶ (shiftFunctor D 1).obj Y₁
h₁₂ : Triangle.mk u₁₂ v₁₂ w₁₂ ∈ distinguishedTriangles
v₂₃ : Y₃ ⟶ Z₂₃
w₂₃ : Z₂₃ ⟶ (shiftFunctor D 1).obj Y₂
h₂₃ : Triangle.mk u₂₃ v₂₃ w₂₃ ∈ distinguishedTriangles
v₁₃ : Y₃ ⟶ Z₁₃
w₁₃ : Z₁₃ ⟶ (shiftFunctor D 1).obj Y₁
h₁₃ : Triangle.mk u₁₃ v₁₃ w₁₃ ∈ distinguishedTriangles
X₁ X₂ X₃ : C
f : X₁ ⟶ X₂
g : X₂ ⟶ X₃
e : (F.mapComposableArrows 2).obj (ComposableArrows.mk₂ f g) ≅ ComposableArrows.mk₂ u₁₂ u₂₃
w✝⁸ : C
w✝⁷ : X₂ ⟶ w✝⁸
w✝⁶ : w✝⁸ ⟶ (shiftFunctor C 1).obj X₁
h₁₂' : Triangle.mk f w✝⁷ w✝⁶ ∈ distinguishedTriangles
w✝⁵ : C
w✝⁴ : X₃ ⟶ w✝⁵
w✝³ : w✝⁵ ⟶ (shiftFunctor C 1).obj X₂
h₂₃' : Triangle.mk g w✝⁴ w✝³ ∈ distinguishedTriangles
w✝² : C
w✝¹ : X₃ ⟶ w✝²
w✝ : w✝² ⟶ (shiftFunctor C 1).obj X₁
h₁₃' : Triangle.mk (f ≫ g) w✝¹ w✝ ∈ distinguishedTriangles
⊢ Nonempty (Octahedron comm h₁₂ h₂₃ h₁₃)

Tactic:
exact ⟨Octahedron.ofIso (e₁ := (e.app 0).symm) (e₂ := (e.app 1).symm) (e₃ := (e.app 2).symm)
  (comm₁₂ := ComposableArrows.naturality' e.inv 0 1)
  (comm₂₃ := ComposableArrows.naturality' e.inv 1 2)
  (H := (someOctahedron rfl h₁₂' h₂₃' h₁₃').map F) ..⟩

Result:
no goals