-- Mathlib/Algebra/Homology/HomotopyCategory/ShortExact.lean
lemma quasiIso_descShortComplex : QuasiIso (descShortComplex S) where
  quasiIsoAt n

Proof state:
C : Type u_1
inst✝¹ : Category.{u_2, u_1} C
inst✝ : Abelian C
S : ShortComplex (CochainComplex C ℤ)
hS : S.ShortExact
n : ℤ
φ : ((homologyFunctor C (up ℤ) 0).homologySequenceComposableArrows₅ (triangleh S.f) n (n + 1) ⋯).δlast ⟶
  (composableArrows₅ hS n (n + 1) ⋯).δlast :=
  homMk₄ ((homologyFunctorFactors C (up ℤ) n).hom.app (triangle S.f).obj₁)
    ((homologyFunctorFactors C (up ℤ) n).hom.app (triangle S.f).obj₂)
    ((homologyFunctorFactors C (up ℤ) n).hom.app (triangle S.f).obj₃ ≫
      HomologicalComplex.homologyMap (descShortComplex S) n)
    ((homologyFunctorFactors C (up ℤ) (n + 1)).hom.app (triangle S.f).obj₁)
    ((homologyFunctorFactors C (up ℤ) (n + 1)).hom.app (triangle S.f).obj₂) ⋯ ⋯ ⋯ ⋯
⊢ IsIso (HomologicalComplex.homologyMap (descShortComplex S) n)

Tactic:
have : IsIso ((homologyFunctorFactors C (up ℤ) n).hom.app (mappingCone S.f) ≫
    HomologicalComplex.homologyMap (descShortComplex S) n) := by
  apply Abelian.isIso_of_epi_of_isIso_of_isIso_of_mono
    ((homologyFunctor C (up ℤ) 0).homologySequenceComposableArrows₅_exact _
      (mappingCone_triangleh_distinguished S.f) n _ rfl).δlast
    (composableArrows₅_exact hS n _ rfl).δlast φ
  all_goals dsimp [φ]; infer_instance

Result:
C : Type u_1
inst✝¹ : Category.{u_2, u_1} C
inst✝ : Abelian C
S : ShortComplex (CochainComplex C ℤ)
hS : S.ShortExact
n : ℤ
φ : ((homologyFunctor C (up ℤ) 0).homologySequenceComposableArrows₅ (triangleh S.f) n (n + 1) ⋯).δlast ⟶
  (composableArrows₅ hS n (n + 1) ⋯).δlast :=
  homMk₄ ((homologyFunctorFactors C (up ℤ) n).hom.app (triangle S.f).obj₁)
    ((homologyFunctorFactors C (up ℤ) n).hom.app (triangle S.f).obj₂)
    ((homologyFunctorFactors C (up ℤ) n).hom.app (triangle S.f).obj₃ ≫
      HomologicalComplex.homologyMap (descShortComplex S) n)
    ((homologyFunctorFactors C (up ℤ) (n + 1)).hom.app (triangle S.f).obj₁)
    ((homologyFunctorFactors C (up ℤ) (n + 1)).hom.app (triangle S.f).obj₂) ⋯ ⋯ ⋯ ⋯
this :
  IsIso
    ((homologyFunctorFactors C (up ℤ) n).hom.app (mappingCone S.f) ≫
      HomologicalComplex.homologyMap (descShortComplex S) n)
⊢ IsIso (HomologicalComplex.homologyMap (descShortComplex S) n)

Proof state:
C : Type u_1
inst✝¹ : Category.{u_2, u_1} C
inst✝ : Abelian C
S : ShortComplex (CochainComplex C ℤ)
hS : S.ShortExact
n : ℤ
⊢ (homologyFunctorFactors C (up ℤ) n).hom.app (triangle S.f).obj₂ ≫
      (HomologicalComplex.homologyFunctor C (up ℤ) n).map (triangle S.f).mor₂ ≫
        HomologicalComplex.homologyMap (descShortComplex S) n =
    (homologyFunctorFactors C (up ℤ) n).hom.app (triangle S.f).obj₂ ≫
      (composableArrows₅ hS n (n + 1) ⋯).δlast.map' 1 2 ⋯ ⋯

Tactic:
dsimp

Result:
C : Type u_1
inst✝¹ : Category.{u_2, u_1} C
inst✝ : Abelian C
S : ShortComplex (CochainComplex C ℤ)
hS : S.ShortExact
n : ℤ
⊢ (homologyFunctorFactors C (up ℤ) n).hom.app S.X₂ ≫
      HomologicalComplex.homologyMap (inr S.f) n ≫ HomologicalComplex.homologyMap (descShortComplex S) n =
    (homologyFunctorFactors C (up ℤ) n).hom.app S.X₂ ≫ HomologicalComplex.homologyMap S.g n

Proof state:
C : Type u_1
inst✝¹ : Category.{u_2, u_1} C
inst✝ : Abelian C
S : ShortComplex (CochainComplex C ℤ)
hS : S.ShortExact
n : ℤ
⊢ ((homologyFunctor C (up ℤ) 0).homologySequenceComposableArrows₅ (triangleh S.f) n (n + 1) ⋯).δlast.map' 1 2 ⋯ ⋯ ≫
      (homologyFunctorFactors C (up ℤ) n).hom.app (triangle S.f).obj₃ ≫
        HomologicalComplex.homologyMap (descShortComplex S) n =
    (homologyFunctorFactors C (up ℤ) n).hom.app (triangle S.f).obj₂ ≫
      (composableArrows₅ hS n (n + 1) ⋯).δlast.map' 1 2 ⋯ ⋯

Tactic:
erw [(homologyFunctorFactors C (up ℤ) n).hom.naturality_assoc]

Result:
C : Type u_1
inst✝¹ : Category.{u_2, u_1} C
inst✝ : Abelian C
S : ShortComplex (CochainComplex C ℤ)
hS : S.ShortExact
n : ℤ
⊢ (homologyFunctorFactors C (up ℤ) n).hom.app (triangle S.f).obj₂ ≫
      (HomologicalComplex.homologyFunctor C (up ℤ) n).map (triangle S.f).mor₂ ≫
        HomologicalComplex.homologyMap (descShortComplex S) n =
    (homologyFunctorFactors C (up ℤ) n).hom.app (triangle S.f).obj₂ ≫
      (composableArrows₅ hS n (n + 1) ⋯).δlast.map' 1 2 ⋯ ⋯

Proof state:
C : Type u_1
inst✝¹ : Category.{u_2, u_1} C
inst✝ : Abelian C
S : ShortComplex (CochainComplex C ℤ)
hS : S.ShortExact
n : ℤ
⊢ ((homologyFunctorFactors C (up ℤ) n).hom.app (mappingCone S.f) ≫
        HomologicalComplex.homologyMap (descShortComplex S) n ≫
          hS.δ n (n + 1) ⋯ ≫ (homologyFunctorFactors C (up ℤ) (n + 1)).inv.app S.X₁) ≫
      (homologyFunctorFactors C (up ℤ) (n + 1)).hom.app S.X₁ =
    ((homologyFunctorFactors C (up ℤ) n).hom.app (mappingCone S.f) ≫
        HomologicalComplex.homologyMap (descShortComplex S) n) ≫
      hS.δ n (n + 1) ⋯

Tactic:
simp only [Functor.comp_obj, HomologicalComplex.homologyFunctor_obj, assoc,
  Iso.inv_hom_id_app, comp_id]

Result:
no goals

Proof state:
C : Type u_1
inst✝¹ : Category.{u_2, u_1} C
inst✝ : Abelian C
S : ShortComplex (CochainComplex C ℤ)
hS : S.ShortExact
n : ℤ
⊢ (homologyFunctorFactors C (up ℤ) n).hom.app S.X₂ ≫
      HomologicalComplex.homologyMap (inr S.f) n ≫ HomologicalComplex.homologyMap (descShortComplex S) n =
    (homologyFunctorFactors C (up ℤ) n).hom.app S.X₂ ≫ HomologicalComplex.homologyMap S.g n

Tactic:
rw [← HomologicalComplex.homologyMap_comp, inr_descShortComplex]

Result:
no goals