Pro-étale cohomology

1.4. Ind-étale ring maps🔗

Definition1.4.1
Group: Topological and ring-theoretic preliminaries on w-local spaces and w-local, w-strictly local and w-contractible rings, following Section 2 of Bhatt–Scholze and the Stacks Project. (136)
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Definition 1.1.1
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An R-algebra S is ind-étale if it is a filtered colimit of étale R-algebras. We say a ring homomorphism f \colon R \to S is ind-étale if S is ind-étale as an R-algebra via f. (Stacks Project, Tag 097I)

Lean code for Definition1.4.12 definitions
  • class(1 method)defined in Proetale/Algebra/IndEtale.lean
    complete
    class Algebra.IndEtale.{u} (R S : Type u) [CommRing R] [CommRing S]
      [Algebra R S] : Prop
    class Algebra.IndEtale.{u} (R S : Type u)
      [CommRing R] [CommRing S]
      [Algebra R S] : Prop
    An algebra is ind-étale if it can be written as the filtered colimit of étale
    algebras. 

    Methods

    exists_colimitPresentation :  ι x,  (_ : CategoryTheory.IsFiltered ι),  P,  (i : ι), Algebra.Etale R (P.diag.obj i)
  • defdefined in Proetale/Algebra/IndEtale.lean
    complete
    def RingHom.IndEtale.{u} {R S : Type u} [CommRing R] [CommRing S]
      (f : R →+* S) : Prop
    def RingHom.IndEtale.{u} {R S : Type u}
      [CommRing R] [CommRing S]
      (f : R →+* S) : Prop
    A ring hom is ind-étale if and only if it is an ind-étale algebra. 
Lemma1.4.2
Group: Topological and ring-theoretic preliminaries on w-local spaces and w-local, w-strictly local and w-contractible rings, following Section 2 of Bhatt–Scholze and the Stacks Project. (136)
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Definition 1.1.1
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Let A \to B and B \to C be ring maps. If A \to B and B \to C are ind-étale, then A \to C is ind-étale. (Stacks Project, Tag 0BSI)

Lean code for Lemma1.4.22 theorems
  • theoremdefined in Proetale/Algebra/IndEtale.lean
    complete
    theorem Algebra.IndEtale.trans.{u} (R S : Type u) [CommRing R] [CommRing S]
      [Algebra R S] (T : Type u) [CommRing T] [Algebra R T] [Algebra S T]
      [IsScalarTower R S T] [Algebra.IndEtale R S] [Algebra.IndEtale S T] :
      Algebra.IndEtale R T
    theorem Algebra.IndEtale.trans.{u} (R S : Type u)
      [CommRing R] [CommRing S] [Algebra R S]
      (T : Type u) [CommRing T] [Algebra R T]
      [Algebra S T] [IsScalarTower R S T]
      [Algebra.IndEtale R S]
      [Algebra.IndEtale S T] :
      Algebra.IndEtale R T
  • theoremdefined in Proetale/Algebra/IndEtale.lean
    complete
    theorem RingHom.IndEtale.comp.{u} {R S : Type u} [CommRing R] [CommRing S]
      {T : Type u} [CommRing T] {g : S →+* T} {f : R →+* S}
      (hg : g.IndEtale) (hf : f.IndEtale) : (g.comp f).IndEtale
    theorem RingHom.IndEtale.comp.{u} {R S : Type u}
      [CommRing R] [CommRing S] {T : Type u}
      [CommRing T] {g : S →+* T} {f : R →+* S}
      (hg : g.IndEtale) (hf : f.IndEtale) :
      (g.comp f).IndEtale
Lemma1.4.3
Group: Topological and ring-theoretic preliminaries on w-local spaces and w-local, w-strictly local and w-contractible rings, following Section 2 of Bhatt–Scholze and the Stacks Project. (136)
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Definition 1.1.1
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Let A \to B be an ind-étale ring map. For any ring map A \to C, the base change C \to B \otimes_A C is ind-étale. (Stacks Project, Tag 0BSH)

Lean code for Lemma1.4.31 theorem
  • theoremdefined in Proetale/Algebra/IndEtale.lean
    complete
    theorem RingHom.IndEtale.isStableUnderBaseChange.{u_1} :
      RingHom.IsStableUnderBaseChange fun {R S} [CommRing R] [CommRing S] =>
        RingHom.IndEtale
    theorem RingHom.IndEtale.isStableUnderBaseChange.{u_1} :
      RingHom.IsStableUnderBaseChange
        fun {R S} [CommRing R] [CommRing S] =>
        RingHom.IndEtale
    Ind-étale ring homomorphisms are stable under base change. 
Lemma1.4.4
Group: Topological and ring-theoretic preliminaries on w-local spaces and w-local, w-strictly local and w-contractible rings, following Section 2 of Bhatt–Scholze and the Stacks Project. (136)
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Definition 1.1.1
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Let B = \colim_i B_i be a filtered colimit of ind-étale A-algebras. Then B is an ind-étale A-algebra.

Lean code for Lemma1.4.44 theorems
  • theoremdefined in Proetale/Algebra/IndEtale.lean
    complete
    theorem Algebra.IndEtale.of_colimitPresentation.{u} (R S : Type u) [CommRing R]
      [CommRing S] [Algebra R S] {ι : Type u}
      [CategoryTheory.SmallCategory ι] [CategoryTheory.IsFiltered ι]
      (P : CategoryTheory.Limits.ColimitPresentation ι (CommAlgCat.of R S))
      (h :  (i : ι), Algebra.IndEtale R (P.diag.obj i)) :
      Algebra.IndEtale R S
    theorem Algebra.IndEtale.of_colimitPresentation.{u}
      (R S : Type u) [CommRing R] [CommRing S]
      [Algebra R S] {ι : Type u}
      [CategoryTheory.SmallCategory ι]
      [CategoryTheory.IsFiltered ι]
      (P :
        CategoryTheory.Limits.ColimitPresentation
          ι (CommAlgCat.of R S))
      (h :
         (i : ι),
          Algebra.IndEtale R
            (P.diag.obj i)) :
      Algebra.IndEtale R S
    If every stage of a filtered colimit presentation of `S` over `R` is ind-étale,
    then `S` is ind-étale over `R`. 
  • theoremdefined in Proetale/Algebra/IndEtale.lean
    complete
    theorem RingHom.IndEtale.iff_ind_indEtale.{u} {R S : Type u} [CommRing R]
      [CommRing S] (f : R →+* S) :
      f.IndEtale 
        (RingHom.toMorphismProperty fun {R S} [CommRing R] [CommRing S] =>
              RingHom.IndEtale).ind
          (CommRingCat.ofHom f)
    theorem RingHom.IndEtale.iff_ind_indEtale.{u}
      {R S : Type u} [CommRing R] [CommRing S]
      (f : R →+* S) :
      f.IndEtale 
        (RingHom.toMorphismProperty
              fun {R S} [CommRing R]
                [CommRing S] =>
              RingHom.IndEtale).ind
          (CommRingCat.ofHom f)
    Ind-étale is equivalent to ind-ind-étale. 
  • theoremdefined in Proetale/Algebra/IndEtale.lean
    complete
    theorem RingHom.IndEtale.of_isColimit.{u} {R S : CommRingCat} (f : R  S)
      (J : Type u) [CategoryTheory.SmallCategory J]
      [CategoryTheory.IsFiltered J]
      (D : CategoryTheory.Functor J CommRingCat)
      {t : (CategoryTheory.Functor.const J).obj R  D}
      {c : D  (CategoryTheory.Functor.const J).obj S}
      (hc : CategoryTheory.Limits.IsColimit { pt := S, ι := c })
      (htc :
         (i : J),
          (CommRingCat.Hom.hom (t.app i)).IndEtale 
            CategoryTheory.CategoryStruct.comp (t.app i) (c.app i) = f) :
      (CommRingCat.Hom.hom f).IndEtale
    theorem RingHom.IndEtale.of_isColimit.{u}
      {R S : CommRingCat} (f : R  S)
      (J : Type u)
      [CategoryTheory.SmallCategory J]
      [CategoryTheory.IsFiltered J]
      (D :
        CategoryTheory.Functor J CommRingCat)
      {t :
        (CategoryTheory.Functor.const J).obj
            R 
          D}
      {c :
        D 
          (CategoryTheory.Functor.const J).obj
            S}
      (hc :
        CategoryTheory.Limits.IsColimit
          { pt := S, ι := c })
      (htc :
         (i : J),
          (CommRingCat.Hom.hom
                (t.app i)).IndEtale 
            CategoryTheory.CategoryStruct.comp
                (t.app i) (c.app i) =
              f) :
      (CommRingCat.Hom.hom f).IndEtale
    A ring hom is ind-étale if it can be written as a filtered colimit of ind-étale maps. 
  • theoremdefined in Proetale/Algebra/IndEtale.lean
    complete
    theorem Algebra.IndEtale.iff_ind_indEtale.{u} {R S : Type u} [CommRing R]
      [CommRing S] [Algebra R S] :
      Algebra.IndEtale R S 
        (RingHom.toObjectProperty
              (fun {R S} [CommRing R] [CommRing S] => RingHom.IndEtale)
              R).ind
          (CommAlgCat.of R S)
    theorem Algebra.IndEtale.iff_ind_indEtale.{u}
      {R S : Type u} [CommRing R] [CommRing S]
      [Algebra R S] :
      Algebra.IndEtale R S 
        (RingHom.toObjectProperty
              (fun {R S} [CommRing R]
                  [CommRing S] =>
                RingHom.IndEtale)
              R).ind
          (CommAlgCat.of R S)
    Ind-étale algebras are equivalent to ind-ind-étale algebras. 
Proof for Lemma 1.4.4
uses 0

This follows from general theory, because étale maps are of finite presentation.

Lemma1.4.5
Group: Topological and ring-theoretic preliminaries on w-local spaces and w-local, w-strictly local and w-contractible rings, following Section 2 of Bhatt–Scholze and the Stacks Project. (136)
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Definition 1.1.1
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Definition 1.2.1
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Let A \to B be an ind-Zariski ring map. Then A \to B is ind-étale.

Lean code for Lemma1.4.52 theorems
  • theoremdefined in Proetale/Algebra/IndEtale.lean
    complete
    theorem Algebra.IndEtale.of_indZariski.{u} (R S : Type u) [CommRing R]
      [CommRing S] [Algebra R S] [Algebra.IndZariski R S] :
      Algebra.IndEtale R S
    theorem Algebra.IndEtale.of_indZariski.{u}
      (R S : Type u) [CommRing R] [CommRing S]
      [Algebra R S] [Algebra.IndZariski R S] :
      Algebra.IndEtale R S
    An ind-Zariski algebra is ind-étale, since localizations are étale. 
  • theoremdefined in Proetale/Algebra/IndEtale.lean
    complete
    theorem RingHom.IndZariski.indEtale.{u} {R S : Type u} [CommRing R] [CommRing S]
      {f : R →+* S} (hf : f.IndZariski) : f.IndEtale
    theorem RingHom.IndZariski.indEtale.{u}
      {R S : Type u} [CommRing R] [CommRing S]
      {f : R →+* S} (hf : f.IndZariski) :
      f.IndEtale
Proof for Lemma 1.4.5
uses 0

An ind-Zariski ring map is a filtered colimit of local isomorphisms. A local isomorphism is étale.