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 #
TauCeti.ConflationClass.ConflationCategory: the full subcategory of distinguished short complexes.TauCeti.ConflationClass.ConflationCategory.homMkand.isoMk: constructors for maps and isomorphisms of conflations from their three components.TauCeti.ConflationClass.ConflationCategory.isIso_iff: a map of conflations is an isomorphism exactly when all three components are isomorphisms.TauCeti.ConflationClass.ConflationCategory.map: the functor induced by a functor preserving conflations, withTauCeti.ExactStructure.ConflationCategory.mapas its exact-structure specialization.TauCeti.ConflationClass.ConflationCategory.mapIdIsoand.mapCompIso: identity and composition coherence for the induced functor.TauCeti.ConflationClass.ConflationCategory.mapNatIso: natural-isomorphism invariance of the induced functor.TauCeti.ConflationClass.ConflationCategory.opEquivalence: the equivalenceE.ConflationCategoryᵒᵖ ≌ E.op.ConflationCategory.
References #
- Theo Bühler, Exact categories, Expositiones Mathematicae 28 (2010), 1--69, https://arxiv.org/abs/0811.1480, Sections 2 and 5.
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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The left-object functor on the category of conflations.
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The middle-object functor on the category of conflations.
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The right-object functor on the category of conflations.
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The first arrow of every conflation, as a natural transformation.
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The second arrow of every conflation, as a natural transformation.
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The composite of the natural transformations π₁Toπ₂ E and π₂Toπ₃ E between the
component functors is zero.
The composite of the natural transformations π₁Toπ₂ E and π₂Toπ₃ E between the
component functors is zero.
The first arrow of a conflation is a monomorphism.
The second arrow of a conflation is an epimorphism.
Construct a morphism of conflations from three maps making the two squares commute.
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- TauCeti.ConflationClass.ConflationCategory.homMk τ₁ τ₂ τ₃ comm₁₂ comm₂₃ = CategoryTheory.ObjectProperty.homMk (CategoryTheory.ShortComplex.homMk τ₁ τ₂ τ₃ comm₁₂ comm₂₃)
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Construct an isomorphism of conflations from compatible isomorphisms of the three terms.
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A morphism of conflations is an isomorphism exactly when its three components are isomorphisms.
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.
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 the first component, taking opposites reverses direction and sends τ₁ to τ₃.op.
On the middle component, taking opposites sends τ₂ to τ₂.op.
On the third component, taking opposites reverses direction and sends τ₃ to τ₁.op.
On morphisms, unopposing applies ShortComplex.unopMap, which reverses direction and sends
τ₁ to τ₃.unop.
On the first component, unopposing reverses direction and sends τ₁ to τ₃.unop.
On the middle component, unopposing sends τ₂ to τ₂.unop.
On the third component, unopposing reverses direction and sends τ₃ to τ₁.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 inverse component of mapCompιIso is the reverse equality transport.
The functor on conflations induced by the identity functor is naturally isomorphic to the identity functor.
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The mapped underlying short complex for the identity functor is the original short complex.
The forward component of mapIdIso is the equality transport to the original short complex.
The inverse component of mapIdIso is the reverse equality transport.
Mapping conflations by a composite is naturally isomorphic to mapping successively.
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Mapping an underlying short complex by a composite agrees with mapping it successively.
The forward component of mapCompIso is the equality transport between the two mapped
short complexes.
The inverse component of mapCompIso is the reverse equality transport.
A natural isomorphism between functors preserving conflations induces a natural isomorphism between their functors on conflations.
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The τ₁ component of (mapNatIso F G hF hG e).hom at S is e.hom at S.obj.X₁,
transported along the identifications map_obj_obj_X₁ for F and G.
The τ₂ component of (mapNatIso F G hF hG e).hom at S is e.hom at S.obj.X₂,
transported along the identifications map_obj_obj_X₂ for F and G.
The τ₃ component of (mapNatIso F G hF hG e).hom at S is e.hom at S.obj.X₃,
transported along the identifications map_obj_obj_X₃ for F and G.
The τ₁ component of (mapNatIso F G hF hG e).inv at S is e.inv at S.obj.X₁,
transported along the identifications map_obj_obj_X₁ for G and F.
The τ₂ component of (mapNatIso F G hF hG e).inv at S is e.inv at S.obj.X₂,
transported along the identifications map_obj_obj_X₂ for G and F.
The τ₃ component of (mapNatIso F G hF hG e).inv at S is e.inv at S.obj.X₃,
transported along the identifications map_obj_obj_X₃ for G and F.
The category of conflations of an exact structure.
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A conflation-exact functor induces a functor between the corresponding categories of conflations.
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After forgetting that its objects are conflations, the induced functor is the ordinary componentwise map on short complexes.
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The functor on conflations induced by the identity functor is naturally isomorphic to the identity functor.
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Mapping conflations by a composite is naturally isomorphic to mapping successively.
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A natural isomorphism between conflation-exact functors induces a natural isomorphism between their functors on conflations.