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Mathlib.CategoryTheory.Galois.Prorepresentability

Pro-Representability of fiber functors #

We show that any fiber functor is pro-representable, i.e. there exists a pro-object X : I ⥤ C such that F is naturally isomorphic to the colimit of X ⋙ coyoneda.

From this we deduce the canonical isomorphism of Aut F with the limit over the automorphism groups of all Galois objects.

Main definitions #

Main results #

Implementation details #

The pro-representability statement and the isomorphism of Aut F with the limit over the automorphism groups of all Galois objects naturally forces F to take values in FintypeCat.{u₂} where u₂ is the Hom-universe of C. Since this is used to show that Aut F acts transitively on F.obj X for connected X, we a priori only obtain this result for the mentioned specialized universe setup. To obtain the result for F taking values in an arbitrary FintypeCat.{w}, we postcompose with an equivalence FintypeCat.{w} ≌ FintypeCat.{u₂} and apply the specialized result.

In the following the section Specialized is reserved for the setup where F takes values in FintypeCat.{u₂} and the section General contains results holding for F taking values in an arbitrary FintypeCat.{w}.

References #

A pointed Galois object is a Galois object with a fixed point of its fiber.

  • obj : C

    The underlying object of C.

  • pt : (F.obj self.obj)

    An element of the fiber of obj.

  • isGalois : IsGalois self.obj

    obj is Galois.

The type of homomorphisms between two pointed Galois objects. This is a homomorphism of the underlying objects of C that maps the distinguished points to each other.

  • val : A.obj B.obj

    The underlying homomorphism of C.

  • comp : F.map self.val A.pt = B.pt

    The distinguished point of A is mapped to the distinguished point of B.

The canonical functor from pointed Galois objects to C.

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F ⋙ FintypeCat.incl as a cocone over (can F).op ⋙ coyoneda. This is a colimit cocone (see PreGaloisCategory.isColimìt)

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cocone F is a colimit cocone, i.e. F is pro-represented by incl F.

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The diagram sending each pointed Galois object to its automorphism group as an object of C.

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The canonical projection from AutGalois F to the C-automorphism group of each pointed Galois object.

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theorem CategoryTheory.PreGaloisCategory.AutGalois.ext {C : Type u₁} [Category.{u₂, u₁} C] [GaloisCategory C] (F : Functor C FintypeCat) {f g : AutGalois F} (h : ∀ (A : PointedGaloisObject F), (π F A) f = (π F A) g) :
f = g

Equality of elements of AutGalois F can be checked on the projections on each pointed Galois object.

autGalois.π is surjective for every pointed Galois object.

Isomorphism between Aut F and AutGalois F #

In this section we establish the isomorphism between the automorphism group of F and the limit over the automorphism groups of all Galois objects.

We first establish the isomorphism between End F and AutGalois F, from which we deduce that End F is a group, hence End F = Aut F. The isomorphism is built in multiple steps:

The endomorphisms of F are isomorphic to the limit over the fibers of F on all Galois objects.

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Functorial isomorphism Aut A ≅ F.obj A for Galois objects A.

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The equivalence between endomorphisms of F and the limit over the automorphism groups of all Galois objects.

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The monoid isomorphism between endomorphisms of F and the (multiplicative opposite of the) limit of automorphism groups of all Galois objects.

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Any endomorphism of a fiber functor is a unit.

Any endomorphism of a fiber functor is an isomorphism.

The automorphism group of F is multiplicatively isomorphic to (the multiplicative opposite of) the limit over the automorphism groups of the Galois objects.

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theorem CategoryTheory.PreGaloisCategory.autMulEquivAutGalois_π {C : Type u₁} [Category.{u₂, u₁} C] [GaloisCategory C] (F : Functor C FintypeCat) [FiberFunctor F] (f : Aut F) (A : C) [IsGalois A] (a : (F.obj A)) :
F.map ((AutGalois.π F { obj := A, pt := a, isGalois := }) (MulOpposite.unop ((autMulEquivAutGalois F) f))).hom a = f.hom.app A a
@[simp]
theorem CategoryTheory.PreGaloisCategory.autMulEquivAutGalois_symm_app {C : Type u₁} [Category.{u₂, u₁} C] [GaloisCategory C] (F : Functor C FintypeCat) [FiberFunctor F] (x : AutGalois F) (A : C) [IsGalois A] (a : (F.obj A)) :
((autMulEquivAutGalois F).symm { unop' := x }).hom.app A a = F.map ((AutGalois.π F { obj := A, pt := a, isGalois := }) x).hom a

The Aut F action on the fiber of a Galois object is transitive. See pretransitive_of_isConnected for the same result for connected objects.

The Aut F action on the fiber of a connected object is transitive.