The category of "structured arrows" #
For T : C ⥤ D, a T-structured arrow with source S : D
is just a morphism S ⟶ T.obj Y, for some Y : C.
These form a category with morphisms g : Y ⟶ Y' making the obvious diagram commute.
We prove that 𝟙 (T.obj Y) is the initial object in T-structured objects with source T.obj Y.
The category of T-structured arrows with domain S : D (here T : C ⥤ D),
has as its objects D-morphisms of the form S ⟶ T Y, for some Y : C,
and morphisms C-morphisms Y ⟶ Y' making the obvious triangle commute.
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See through the type synonym StructuredArrow S T = Comma _ _.
The obvious projection functor from structured arrows.
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Construct a structured arrow from a morphism.
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To construct a morphism of structured arrows, we need a morphism of the objects underlying the target, and to check that the triangle commutes.
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Given a structured arrow X ⟶ T(Y), and an arrow Y ⟶ Y', we can construct a morphism of
structured arrows given by (X ⟶ T(Y)) ⟶ (X ⟶ T(Y) ⟶ T(Y')).
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Variant of homMk' where both objects are applications of mk.
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To construct an isomorphism of structured arrows, we need an isomorphism of the objects underlying the target, and to check that the triangle commutes.
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The converse of this is true with additional assumptions, see mono_iff_mono_right.
Eta rule for structured arrows. Prefer StructuredArrow.eta for rewriting, since equality of
objects tends to cause problems.
Eta rule for structured arrows.
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A morphism between source objects S ⟶ S'
contravariantly induces a functor between structured arrows,
StructuredArrow S' T ⥤ StructuredArrow S T.
Ideally this would be described as a 2-functor from D
(promoted to a 2-category with equations as 2-morphisms)
to Cat.
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An isomorphism S ≅ S' induces an equivalence StructuredArrow S T ≌ StructuredArrow S' T.
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A natural isomorphism T ≅ T' induces an equivalence
StructuredArrow S T ≌ StructuredArrow S T'.
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The identity structured arrow is initial.
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The functor (S, F ⋙ G) ⥤ (S, G).
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If F is an equivalence, then so is the functor (S, F ⋙ G) ⥤ (S, G).
The functor (S, F) ⥤ (G(S), F ⋙ G).
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If G is fully faithful, then post S F G : (S, F) ⥤ (G(S), F ⋙ G) is an equivalence.
The functor StructuredArrow L R ⥤ StructuredArrow L' R' that is deduced from
a natural transformation R ⋙ G ⟶ F ⋙ R' and a morphism L' ⟶ G.obj L.
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The composition of two applications of map₂ is naturally isomorphic to a single such one.
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StructuredArrow.post is a special case of StructuredArrow.map₂ up to natural isomorphism.
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StructuredArrow.map is a special case of StructuredArrow.map₂ up to natural isomorphism.
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StructuredArrow.pre is a special case of StructuredArrow.map₂ up to natural isomorphism.
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A structured arrow is called universal if it is initial.
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The family of morphisms out of a universal arrow.
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Any structured arrow factors through a universal arrow.
Any structured arrow factors through a universal arrow.
Two morphisms out of a universal T-structured arrow are equal if their image under T are
equal after precomposing the universal arrow.
The category of S-costructured arrows with target T : D (here S : C ⥤ D),
has as its objects D-morphisms of the form S Y ⟶ T, for some Y : C,
and morphisms C-morphisms Y ⟶ Y' making the obvious triangle commute.
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The obvious projection functor from costructured arrows.
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Construct a costructured arrow from a morphism.
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To construct a morphism of costructured arrows, we need a morphism of the objects underlying the source, and to check that the triangle commutes.
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Given a costructured arrow S(Y) ⟶ X, and an arrow Y' ⟶ Y', we can construct a morphism of
costructured arrows given by (S(Y) ⟶ X) ⟶ (S(Y') ⟶ S(Y) ⟶ X).
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Variant of homMk' where both objects are applications of mk.
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To construct an isomorphism of costructured arrows, we need an isomorphism of the objects underlying the source, and to check that the triangle commutes.
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The converse of this is true with additional assumptions, see epi_iff_epi_left.
Eta rule for costructured arrows. Prefer CostructuredArrow.eta for rewriting, as equality of
objects tends to cause problems.
Eta rule for costructured arrows.
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A morphism between target objects T ⟶ T'
covariantly induces a functor between costructured arrows,
CostructuredArrow S T ⥤ CostructuredArrow S T'.
Ideally this would be described as a 2-functor from D
(promoted to a 2-category with equations as 2-morphisms)
to Cat.
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An isomorphism T ≅ T' induces an equivalence
CostructuredArrow S T ≌ CostructuredArrow S T'.
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A natural isomorphism S ≅ S' induces an equivalence
CostrucutredArrow S T ≌ CostructuredArrow S' T.
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The identity costructured arrow is terminal.
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The functor (F ⋙ G, S) ⥤ (G, S).
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If F is an equivalence, then so is the functor (F ⋙ G, S) ⥤ (G, S).
The functor (F, S) ⥤ (F ⋙ G, G(S)).
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If G is fully faithful, then post F G S : (F, S) ⥤ (F ⋙ G, G(S)) is an equivalence.
The functor CostructuredArrow S T ⥤ CostructuredArrow U V that is deduced from
a natural transformation F ⋙ U ⟶ S ⋙ G and a morphism G.obj T ⟶ V
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CostructuredArrow.post is a special case of CostructuredArrow.map₂ up to natural
isomorphism.
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A costructured arrow is called universal if it is terminal.
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The family of morphisms into a universal arrow.
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Any costructured arrow factors through a universal arrow.
Any costructured arrow factors through a universal arrow.
Two morphisms into a universal S-costructured arrow are equal if their image under S are
equal after postcomposing the universal arrow.
Given X : D and F : C ⥤ D, to upgrade a functor G : E ⥤ C to a functor
E ⥤ StructuredArrow X F, it suffices to provide maps X ⟶ F.obj (G.obj Y) for all Y making
the obvious triangles involving all F.map (G.map g) commute.
This is of course the same as providing a cone over F ⋙ G with cone point X, see
Functor.toStructuredArrowIsoToStructuredArrow.
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Upgrading a functor E ⥤ C to a functor E ⥤ StructuredArrow X F and composing with the
forgetful functor StructuredArrow X F ⥤ C recovers the original functor.
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Given F : C ⥤ D and X : D, to upgrade a functor G : E ⥤ C to a functor
E ⥤ CostructuredArrow F X, it suffices to provide maps F.obj (G.obj Y) ⟶ X for all Y
making the obvious triangles involving all F.map (G.map g) commute.
This is of course the same as providing a cocone over F ⋙ G with cocone point X, see
Functor.toCostructuredArrowIsoToCostructuredArrow.
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Upgrading a functor E ⥤ C to a functor E ⥤ CostructuredArrow F X and composing with the
forgetful functor CostructuredArrow F X ⥤ C recovers the original functor.
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For a functor F : C ⥤ D and an object d : D, we obtain a contravariant functor from the
category of structured arrows d ⟶ F.obj c to the category of costructured arrows
F.op.obj c ⟶ (op d).
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For a functor F : C ⥤ D and an object d : D, we obtain a contravariant functor from the
category of structured arrows op d ⟶ F.op.obj c to the category of costructured arrows
F.obj c ⟶ d.
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For a functor F : C ⥤ D and an object d : D, we obtain a contravariant functor from the
category of costructured arrows F.obj c ⟶ d to the category of structured arrows
op d ⟶ F.op.obj c.
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For a functor F : C ⥤ D and an object d : D, we obtain a contravariant functor from the
category of costructured arrows F.op.obj c ⟶ op d to the category of structured arrows
d ⟶ F.obj c.
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For a functor F : C ⥤ D and an object d : D, the category of structured arrows d ⟶ F.obj c
is contravariantly equivalent to the category of costructured arrows F.op.obj c ⟶ op d.
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For a functor F : C ⥤ D and an object d : D, the category of costructured arrows
F.obj c ⟶ d is contravariantly equivalent to the category of structured arrows
op d ⟶ F.op.obj c.
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The functor establishing the equivalence StructuredArrow.preEquivalence.
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The inverse functor establishing the equivalence StructuredArrow.preEquivalence.
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A structured arrow category on a StructuredArrow.pre e F G functor is equivalent to the
structured arrow category on F
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The functor StructuredArrow d T ⥤ StructuredArrow e (T ⋙ S) that u : e ⟶ S.obj d
induces via StructuredArrow.map₂ can be expressed up to isomorphism by
StructuredArrow.preEquivalence and StructuredArrow.proj.
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The functor establishing the equivalence CostructuredArrow.preEquivalence.
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The inverse functor establishing the equivalence CostructuredArrow.preEquivalence.
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A costructured arrow category on a CostructuredArrow.pre F G e functor is equivalent to the
costructured arrow category on F
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The functor CostructuredArrow T d ⥤ CostructuredArrow (T ⋙ S) e that u : S.obj d ⟶ e
induces via CostructuredArrow.map₂ can be expressed up to isomorphism by
CostructuredArrow.preEquivalence and CostructuredArrow.proj.
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Implementation; see StructuredArrow.prodEquivalence.
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Implementation; see StructuredArrow.prodEquivalence.
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The natural equivalence
StructuredArrow (S, S') (T.prod T') ≌ StructuredArrow S T × StructuredArrow S' T'.
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Implementation; see CostructuredArrow.prodEquivalence.
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Implementation; see CostructuredArrow.prodEquivalence.
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The natural equivalence
CostructuredArrow (S.prod S') (T, T') ≌ CostructuredArrow S T × CostructuredArrow S' T'.