Subcategories of comma categories defined by morphism properties #
Given functors L : A ⥤ T and R : B ⥤ T and morphism properties P, Q and W
on T, A and B respectively, we define the subcategory P.Comma L R Q W of
Comma L R where
- objects are objects of
Comma L Rwith the structural morphism satisfyingP, and - morphisms are morphisms of
Comma L Rwhere the left morphism satisfiesQand the right morphism satisfiesW.
For an object X : T, this specializes to P.Over Q X which is the subcategory of Over X
where the structural morphism satisfies P and where the horizontal morphisms satisfy Q.
Common examples of the latter are e.g. the category of schemes étale (finite, affine, etc.)
over a base X. Here Q = ⊤.
Implementation details #
The object property on Comma L R induced by a morphism property.
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The object property on CostructuredArrow L X induced by a morphism property.
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The object property on StructuredArrow X R induced by a morphism property.
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The morphism property on Over X induced by a morphism property on C.
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The morphism property on Under X induced by a morphism property on C.
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The object property on Over X induced by a morphism property.
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The object property on Under X induced by a morphism property.
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P.Comma L R Q W is the subcategory of Comma L R consisting of
objects X : Comma L R where X.hom satisfies P. The morphisms are given by
morphisms in Comma L R where the left one satisfies Q and the right one satisfies W.
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A morphism in P.Comma L R Q W is a morphism in Comma L R where the left
hom satisfies Q and the right one satisfies W.
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The underlying morphism of objects in Comma L R.
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See Note [custom simps projection]
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The identity morphism of an object in P.Comma L R Q W.
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Composition of morphisms in P.Comma L R Q W.
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If i is an isomorphism in Comma L R, it is also a morphism in P.Comma L R Q W.
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Any isomorphism between objects of P.Comma L R Q W in Comma L R is also an isomorphism
in P.Comma L R Q W.
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Constructor for isomorphisms in P.Comma L R Q W from isomorphisms of the left and right
components and naturality in the forward direction.
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The forgetful functor.
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The forgetful functor from the full subcategory of Comma L R defined by P is
fully faithful.
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Weaken the conditions on all components.
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Weakening the condition on the structure morphisms is fully faithful.
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Lift a functor F : C ⥤ Comma L R to the subcategory P.Comma L R Q W under
suitable assumptions on F.
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A natural transformation L₁ ⟶ L₂ induces a functor P.Comma L₂ R Q W ⥤ P.Comma L₁ R Q W.
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The functor P.Comma L R Q W ⥤ P.Comma L R Q W induced by the identity natural transformation
on L is naturally isomorphic to the identity functor.
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The functor P.Comma L₁ R Q W ⥤ P.Comma L₃ R Q W induced by the composition of two natural
transformations l : L₁ ⟶ L₂ and l' : L₂ ⟶ L₃ is naturally isomorphic to the composition of the
two functors induced by these natural transformations.
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Two equal natural transformations L₁ ⟶ L₂ yield naturally isomorphic functors
P.Comma L₁ R Q W ⥤ P.Comma L₂ R Q W.
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A natural isomorphism L₁ ≅ L₂ induces an equivalence of categories
P.Comma L₁ R Q W ≌ P.Comma L₂ R Q W.
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A natural transformation R₁ ⟶ R₂ induces a functor P.Comma L R₁ Q W ⥤ P.Comma L R₂ Q W.
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The functor P.Comma L R Q W ⥤ P.Comma L R Q W induced by the identity natural transformation
on R is naturally isomorphic to the identity functor.
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The functor P.Comma L R₁ Q W ⥤ P.Comma L R₃ Q W induced by the composition of the natural
transformations r : R₁ ⟶ R₂ and r' : R₂ ⟶ R₃ is naturally isomorphic to the composition of the
functors induced by these natural transformations.
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Two equal natural transformations R₁ ⟶ R₂ yield naturally isomorphic functors
P.Comma L R₁ Q W ⥤ P.Comma L R₂ Q W.
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A natural isomorphism R₁ ≅ R₂ induces an equivalence of categories
P.Comma L R₁ Q W ≌ P.Comma L R₂ Q W.
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Given a morphism property P on a category C and an object X : C, this is the
subcategory of Over X defined by P where morphisms satisfy Q.
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The forgetful functor from the full subcategory of Over X defined by P to Over X.
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Occasionally useful for rewriting in the backwards direction.
Construct a morphism in P.Over Q X from a morphism in Over.X.
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Make an object of P.Over Q X from a morphism f : A ⟶ X and a proof of P f.
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Make a morphism in P.Over Q X from a morphism in T with compatibilities.
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Make an isomorphism in P.Over Q X from an isomorphism in T with compatibilities.
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The natural inclusion induced by implications of morphism properties.
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Given a morphism property P on a category C and an object X : C, this is the
subcategory of Under X defined by P where morphisms satisfy Q.
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The forgetful functor from the full subcategory of Under X defined by P to Under X.
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Occasionally useful for rewriting in the backwards direction.
Construct a morphism in P.Under Q X from a morphism in Under.X.
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Make an object of P.Under Q X from a morphism f : A ⟶ X and a proof of P f.
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Make a morphism in P.Under Q X from a morphism in T with compatibilities.
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Make an isomorphism in P.Under Q X from an isomorphism in T with compatibilities.
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Given a morphism property P on a category C and an object X : C, this is the
subcategory of CostructuredArrow F X defined by P where morphisms satisfy Q.
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Construct an object of P.CostructuredArrow Q F X from a morphism F.obj A ⟶ X.
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Construct a morphism in P.CostructuredArrow Q F X by giving a morphism on the underlying
objects of C.
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The forgetful functor from the subcategory P.CostructuredArrow Q F X.
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Reinterpreting an F-costructured arrow F.obj A ⟶ X as an arrow over X.