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TauCeti.CategoryTheory.Exact.Conflation

The category of conflations #

For a fixed conflation class E, a conflation is not merely a proposition about a short complex: conflations and commutative diagrams between them form a category. This file realizes that category as the full subcategory of ShortComplex C on the distinguished kernel--cokernel pairs. Consequently, a morphism of conflations is exactly a ladder of two commuting squares with three vertical maps, and an isomorphism of conflations is exactly such a ladder whose vertical maps are isomorphisms.

The construction deliberately reuses Mathlib's category of short complexes. In particular, composition, identities, the preadditive structure, component functors, and the componentwise criterion for isomorphisms are inherited rather than duplicated.

Conflation-exact functors induce functors between conflation categories. Naturally isomorphic conflation-exact functors induce naturally isomorphic functors, and passage to the opposite conflation class gives the expected equivalence on conflations.

Main definitions and results #

References #

@[reducible, inline]

The category of conflations of a conflation class E. Its objects are distinguished short complexes, and its morphisms are arbitrary morphisms of the underlying short complexes.

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    The fully faithful inclusion of the category of conflations into the category of short complexes.

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      Construct a morphism of conflations from three maps making the two squares commute.

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        theorem TauCeti.ConflationClass.ConflationCategory.homMk_hom_τ₁ {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] [CategoryTheory.Preadditive C] {E : ConflationClass C} {S T : E.ConflationCategory} (τ₁ : S.obj.X₁ ⟶ T.obj.X₁) (τ₂ : S.obj.X₂ ⟶ T.obj.X₂) (τ₃ : S.obj.X₃ ⟶ T.obj.X₃) (comm₁₂ : CategoryTheory.CategoryStruct.comp τ₁ T.obj.f = CategoryTheory.CategoryStruct.comp S.obj.f τ₂ := by cat_disch) (comm₂₃ : CategoryTheory.CategoryStruct.comp τ₂ T.obj.g = CategoryTheory.CategoryStruct.comp S.obj.g τ₃ := by cat_disch) :
        (homMk τ₁ τ₂ τ₃ comm₁₂ comm₂₃).hom.τ₁ = τ₁
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        theorem TauCeti.ConflationClass.ConflationCategory.homMk_hom_τ₂ {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] [CategoryTheory.Preadditive C] {E : ConflationClass C} {S T : E.ConflationCategory} (τ₁ : S.obj.X₁ ⟶ T.obj.X₁) (τ₂ : S.obj.X₂ ⟶ T.obj.X₂) (τ₃ : S.obj.X₃ ⟶ T.obj.X₃) (comm₁₂ : CategoryTheory.CategoryStruct.comp τ₁ T.obj.f = CategoryTheory.CategoryStruct.comp S.obj.f τ₂ := by cat_disch) (comm₂₃ : CategoryTheory.CategoryStruct.comp τ₂ T.obj.g = CategoryTheory.CategoryStruct.comp S.obj.g τ₃ := by cat_disch) :
        (homMk τ₁ τ₂ τ₃ comm₁₂ comm₂₃).hom.τ₂ = τ₂
        @[simp]
        theorem TauCeti.ConflationClass.ConflationCategory.homMk_hom_τ₃ {C : Type u₁} [CategoryTheory.Category.{v₁, u₁} C] [CategoryTheory.Preadditive C] {E : ConflationClass C} {S T : E.ConflationCategory} (τ₁ : S.obj.X₁ ⟶ T.obj.X₁) (τ₂ : S.obj.X₂ ⟶ T.obj.X₂) (τ₃ : S.obj.X₃ ⟶ T.obj.X₃) (comm₁₂ : CategoryTheory.CategoryStruct.comp τ₁ T.obj.f = CategoryTheory.CategoryStruct.comp S.obj.f τ₂ := by cat_disch) (comm₂₃ : CategoryTheory.CategoryStruct.comp τ₂ T.obj.g = CategoryTheory.CategoryStruct.comp S.obj.g τ₃ := by cat_disch) :
        (homMk τ₁ τ₂ τ₃ comm₁₂ comm₂₃).hom.τ₃ = τ₃

        Construct an isomorphism of conflations from compatible isomorphisms of the three terms.

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          Taking opposites gives an equivalence from the opposite of the category of conflations to the category of conflations of the opposite conflation class.

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            The underlying short complex of the opposite of a conflation is S.unop.obj.op: its arrows are S.unop.obj.g.op and S.unop.obj.f.op, so its outer terms are exchanged.

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            The underlying short complex obtained by unopposing a conflation reverses the opposite short complex again, exchanging its outer terms.

            On morphisms, taking opposites applies ShortComplex.opMap, which reverses direction and sends τ₁ to τ₃.op.

            On morphisms, unopposing applies ShortComplex.unopMap, which reverses direction and sends τ₁ to τ₃.unop.

            A functor preserving zero morphisms and conflations induces a functor between the corresponding categories of conflations.

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              The underlying short complex of a mapped conflation is its componentwise image.

              After forgetting that its objects are conflations, the induced functor is the ordinary componentwise map on short complexes.

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                The forward component of mapCompιIso is the equality transport from the lifted object to its componentwise image.

                The mapped underlying short complex for the identity functor is the original short complex.

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                The forward component of mapIdIso is the equality transport to the original short complex.

                Mapping an underlying short complex by a composite agrees with mapping it successively.

                A natural isomorphism between functors preserving conflations induces a natural isomorphism between their functors on conflations.

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                  @[reducible, inline]

                  The category of conflations of an exact structure.

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