contravariant_sequence_exact₁' — Mathlib

English

Alternative covariant exactness for the second step; the five-term short exact diagram is exact.

Русский

Альтернативная точность для второго шага ковариантной последовательности; пятиэлементный точный диаграмм точен.

LaTeX

$$$(\\mathrm{covariantSequence\\_₂'})\\;\\text{Exact}$$$

Lean4

/-- Alternative formulation of `contravariant_sequence_exact₁` -/
theorem contravariant_sequence_exact₁' :
    (ShortComplex.mk (AddCommGrpCat.ofHom (((mk₀ S.f).precomp Y (zero_add n₀))))
        (AddCommGrpCat.ofHom (hS.extClass.precomp Y h))
        (by
          ext
          dsimp
          simp only [ShortComplex.ShortExact.extClass_comp_assoc])).Exact :=
  by
  letI := HasDerivedCategory.standard C
  have :=
    (preadditiveYoneda.obj ((singleFunctor C 0).obj Y)).homologySequence_exact₃ _
      (op_distinguished _ hS.singleTriangle_distinguished) n₀ n₁ (by cutsat)
  rw [ShortComplex.ab_exact_iff_function_exact] at this ⊢
  apply
    Function.Exact.of_ladder_addEquiv_of_exact' (e₁ := Ext.homAddEquiv) (e₂ := Ext.homAddEquiv) (e₃ := Ext.homAddEquiv)
      (H := this)
  · ext; apply singleFunctor_map_comp_hom (C := C)
  · ext; dsimp; apply preadditiveYoneda_homologySequenceδ_singleTriangle_apply

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