The category of Compact Hausdorff Spaces

We construct the category of compact Hausdorff spaces. The type of compact Hausdorff spaces is denoted CompHaus, and it is endowed with a category instance making it a full subcategory of TopCat. The fully faithful functor CompHaus ⥤ TopCat is denoted compHausToTop.

Note: The file Mathlib/Topology/Category/Compactum.lean provides the equivalence between Compactum, which is defined as the category of algebras for the ultrafilter monad, and CompHaus. CompactumToCompHaus is the functor from Compactum to CompHaus which is proven to be an equivalence of categories in CompactumToCompHaus.isEquivalence. See Mathlib/Topology/Category/Compactum.lean for a more detailed discussion where these definitions are introduced.

Implementation

The category CompHaus is defined using the structure CompHausLike. See the file CompHausLike.Basic for more information.

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abbrev CompHaus : Type (u_1 + 1)

The category of compact Hausdorff spaces.

@[implicit_reducible]

instance CompHaus.instInhabited : Inhabited CompHaus

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instance CompHaus.instCoeSortType : CoeSort CompHaus (Type u_1)

instance CompHaus.instCompactSpaceCarrierToTopTrue {X : CompHaus} : CompactSpace ↑X.toTop

instance CompHaus.instT2SpaceCarrierToTopTrue {X : CompHaus} : T2Space ↑X.toTop

instance CompHaus.instHasPropTrue (X : Type u_1) [TopologicalSpace X] : CompHausLike.HasProp (fun (x : TopCat) => True) X

@[reducible, inline]

abbrev CompHaus.of (X : Type u_1) [TopologicalSpace X] [CompactSpace X] [T2Space X] : CompHaus

A constructor for objects of the category CompHaus, taking a type, and bundling the compact Hausdorff topology found by typeclass inference.

@[reducible, inline]

abbrev compHausToTop : CategoryTheory.Functor CompHaus TopCat

The fully faithful embedding of CompHaus in TopCat.

def stoneCechObj (X : TopCat) : CompHaus

(Implementation) The object part of the compactification functor from topological spaces to compact Hausdorff spaces.

@[simp]

theorem stoneCechObj_toTop_carrier (X : TopCat) : ↑(stoneCechObj X).toTop = StoneCech ↑X

noncomputable def stoneCechEquivalence (X : TopCat) (Y : CompHaus) : (stoneCechObj X ⟶ Y) ≃ (X ⟶ compHausToTop.obj Y)

(Implementation) The bijection of homsets to establish the reflective adjunction of compact Hausdorff spaces in topological spaces.

noncomputable def topToCompHaus : CategoryTheory.Functor TopCat CompHaus

The Stone-Cech compactification functor from topological spaces to compact Hausdorff spaces, left adjoint to the inclusion functor.

theorem topToCompHaus_obj (X : TopCat) : ↑(topToCompHaus.obj X).toTop = StoneCech ↑X

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noncomputable instance compHausToTop.reflective : CategoryTheory.Reflective compHausToTop

The category of compact Hausdorff spaces is reflective in the category of topological spaces.

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noncomputable instance compHausToTop.createsLimits : CategoryTheory.CreatesLimits compHausToTop

instance CompHaus.hasLimits : CategoryTheory.Limits.HasLimits CompHaus

instance CompHaus.hasColimits : CategoryTheory.Limits.HasColimits CompHaus

def CompHaus.limitCone {J : Type v} [CategoryTheory.SmallCategory J] (F : CategoryTheory.Functor J CompHaus) : CategoryTheory.Limits.Cone F

An explicit limit cone for a functor F : J ⥤ CompHaus, defined in terms of TopCat.limitCone.

def CompHaus.limitConeIsLimit {J : Type v} [CategoryTheory.SmallCategory J] (F : CategoryTheory.Functor J CompHaus) : CategoryTheory.Limits.IsLimit (limitCone F)

The limit cone CompHaus.limitCone F is indeed a limit cone.

theorem CompHaus.epi_iff_surjective {X Y : CompHaus} (f : X ⟶ Y) : CategoryTheory.Epi f ↔ Function.Surjective ⇑(CategoryTheory.ConcreteCategory.hom f)

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abbrev compHausLikeToCompHaus (P : TopCat → Prop) : CategoryTheory.Functor (CompHausLike P) CompHaus

Every CompHausLike admits a functor to CompHaus.