The core of a category #
The core of a category C is the (non-full) subcategory of C consisting of all objects,
and all isomorphisms. We construct it as a CategoryTheory.Groupoid.
CategoryTheory.Core.inclusion : Core C ⥤ C gives the faithful inclusion into the original
category.
Any functor F from a groupoid G into C factors through CategoryTheory.Core C,
but this is not functorial with respect to F.
The core of a category C is the groupoid whose morphisms are all the isomorphisms of C.
- of : C
The object of the base category underlying an object in
Core C.
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The core of a category is naturally included in the category.
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- CategoryTheory.Core.inclusion C = { obj := CategoryTheory.Core.of, map := fun {X Y : CategoryTheory.Core C} (f : X ⟶ Y) => f.iso.hom, map_id := ⋯, map_comp := ⋯ }
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A functor from a groupoid to a category C factors through the core of C.
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We can functorially associate to any functor from a groupoid to the core of a category C,
a functor from the groupoid to C, simply by composing with the embedding Core C ⥤ C.
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A functor C ⥤ D induces a functor Core C ⥤ Core D.
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The core of the identity functor is the identity functor on the cores.
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The core of the composition of F and G is the composition of the cores.
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- F.coreComp G = CategoryTheory.Iso.refl (F.comp G).core
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The natural isomorphism
F.core
Core C ⥤ Core D
inclusion C ‖ ‖ inclusion D
V V
C ⥤ D
F
thought of as pseudonaturality of inclusion,
when viewing Core as a pseudofunctor.
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A natural isomorphism of functors induces a natural isomorphism between their cores.
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- α.core = CategoryTheory.NatIso.ofComponents (fun (x : CategoryTheory.Core C) => (CategoryTheory.Groupoid.isoEquivHom (F.core.obj x) (G.core.obj x)).symm { iso := α.app x.of }) ⋯
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The functor functorToCore (F ⋙ H) factors through functortoCore H.
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- CategoryTheory.Core.functorToCoreCompLeftIso H F = CategoryTheory.NatIso.ofComponents (fun (x : G') => CategoryTheory.Iso.refl ((CategoryTheory.Core.functorToCore (F.comp H)).obj x)) ⋯
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The functor functorToCore (H ⋙ F) factors through functorToCore H.
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The functor functorToCore (𝟭 G) is a section of inclusion G.
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The functor functorToCore (inclusion C) is isomorphic to the identity on Core C.
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Equivalent categories have equivalent cores.
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Taking the core of a functor is functorial if we discard non-invertible natural transformations.
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ofEquivFunctor m lifts a type-level EquivFunctor
to a categorical functor Core (Type u₁) ⥤ Core (Type u₂).
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