Pro-étale cohomology

5.3. The site🔗

Before we define the pro-étale site, we recall a few standard definitions.

Definition5.3.1
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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Lemma 5.3.2
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A jointly surjective family (f_i \colon Y_i \to X)_{i \in I} of morphisms of schemes is quasi-compact if for every affine open U of X there exist quasi-compact opens V_i in Y_i such that U = \bigcup_{i \in I} f_i(V_i).

Lemma5.3.2
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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Lemma 5.1.1
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Let (f_i \colon Y_i \to X)_{i \in I} be a jointly-surjective family of morphisms of schemes. If f_i is flat and locally of finite presentation for every i \in I, the family is quasi-compact.

Lean code for Lemma5.3.21 theorem
  • theoremdefined in Proetale/Mathlib/AlgebraicGeometry/Cover/QuasiCompact.lean
    complete
    theorem AlgebraicGeometry.QuasiCompactCover.of_flat_locallyOfFinitePresentation.{u,
        u_1}
      {S : AlgebraicGeometry.Scheme}
      {K : CategoryTheory.Precoverage AlgebraicGeometry.Scheme}
      (𝒰 : AlgebraicGeometry.Scheme.Cover K S)
      [AlgebraicGeometry.Scheme.JointlySurjective K]
      [ (i : 𝒰.I₀), AlgebraicGeometry.Flat (𝒰.f i)]
      [ (i : 𝒰.I₀),
          AlgebraicGeometry.LocallyOfFinitePresentation (𝒰.f i)] :
      AlgebraicGeometry.QuasiCompactCover 𝒰.toPreZeroHypercover
    theorem AlgebraicGeometry.QuasiCompactCover.of_flat_locallyOfFinitePresentation.{u,
        u_1}
      {S : AlgebraicGeometry.Scheme}
      {K :
        CategoryTheory.Precoverage
          AlgebraicGeometry.Scheme}
      (𝒰 : AlgebraicGeometry.Scheme.Cover K S)
      [AlgebraicGeometry.Scheme.JointlySurjective
          K]
      [ (i : 𝒰.I₀),
          AlgebraicGeometry.Flat (𝒰.f i)]
      [ (i : 𝒰.I₀),
          AlgebraicGeometry.LocallyOfFinitePresentation
            (𝒰.f i)] :
      AlgebraicGeometry.QuasiCompactCover
        𝒰.toPreZeroHypercover
    A jointly surjective family of flat morphisms locally of finite presentation is
    quasi-compact. 
Proof for Lemma 5.3.2
uses 0

In this case, every f_i is an open map, from which the claim easily follows.

Definition5.3.3
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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Definition 5.3.4
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Let \mathcal{P} be a morphism property of schemes and X be a scheme. The small \mathcal{P}-site of X is the category of X-schemes with structure morphism satisfying \mathcal{P} and with covers given by quasi-compact, jointly surjective families of morphisms satisfying \mathcal{P}.

If we let \mathcal{P} be flat morphisms of schemes, we obtain the fpqc site:

Definition5.3.4
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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Let X be a scheme. The fpqc site of X, denoted by \fpqc{X}, is the \mathcal{P}-site of X with \mathcal{P} = \text{flat}.

For us, the main relevant property of the fpqc site is that it is subcanonical:

Theorem5.3.5
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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Lemma 4.7
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The fpqc site is subcanonical, i.e., every representable presheaf is a sheaf.

Analogous to the familiar criterion for fpqc sheaves, we have a generalization for the small \mathcal{P}-site of a scheme.

Proposition5.3.6
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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Lemma 5.1.1
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Let F be a presheaf on the small \mathcal{P}-site. Then F is a sheaf if and only if it is a sheaf in the Zariski topology and satisfies the sheaf property for \{V \to U\} with V, U affine and V \to U satisfying \mathcal{P}.

We can now define the main object of this chapter:

Definition5.3.7
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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Definition 5.2.1
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Let X be a scheme. The pro-étale site of X, denoted by \proet{X}, is the \mathcal{P}-site of X with \mathcal{P} = \text{weakly étale}. (Bhatt–Scholze, Definition 4.1.1)

Despite the name pro-étale, Definition 5.3.7 does not mention pro-étale morphisms. The name is justified by Theorem 2.6.1 (see also Proposition 5.3.1.4).

Proposition5.3.8
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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The site \proet{X} is subcanonical. (Bhatt–Scholze, Lemma 4.2.4)

Proof for Proposition 5.3.8

Follows from Theorem 5.3.5, since every pro-étale sheaf is an fpqc-sheaf.

5.3.1. Affine covers🔗

Definition5.3.1.1
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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Lemma 5.3.1.2
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An object U of \proet{X} is called a pro-étale affine if it can be written as U = \lim U_i over a small cofiltered diagram i \mapsto U_i of affine schemes étale over X. The subcategory of pro-étale affines in \proet{X} is denoted by \affproet{X}. (Bhatt–Scholze, Definition 4.2.1)

Lean code for Definition5.3.1.11 definition
  • defdefined in Proetale/Topology/Comparison/Affine.lean
    complete
    def AlgebraicGeometry.Scheme.AffineProEt.{u}
      (S : AlgebraicGeometry.Scheme) : Type (u + 1)
    def AlgebraicGeometry.Scheme.AffineProEt.{u}
      (S : AlgebraicGeometry.Scheme) :
      Type (u + 1)
    The pro-étale affine site is the full subcategory of the pro-étale site where every
    object can be written as a cofiltered limit of affine étale schemes over `S`. 
Lemma5.3.1.2
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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The category \affproet{X} admits limits indexed by a connected diagram and the forgetful functor to \mathrm{Sch} / X preserves them. If X is affine, \affproet{X} has all small limits. (Bhatt–Scholze, Remark 4.2.3)

Definition5.3.1.3
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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Let X be affine. The affine pro-étale site of X is the category \affproet{X}. The covering families are the quasi-compact covers.

Lean code for Definition5.3.1.31 definition
  • defdefined in Proetale/Topology/Comparison/Affine.lean
    complete
    def AlgebraicGeometry.Scheme.AffineProEt.{u}
      (S : AlgebraicGeometry.Scheme) : Type (u + 1)
    def AlgebraicGeometry.Scheme.AffineProEt.{u}
      (S : AlgebraicGeometry.Scheme) :
      Type (u + 1)
    The pro-étale affine site is the full subcategory of the pro-étale site where every
    object can be written as a cofiltered limit of affine étale schemes over `S`. 

In other words, \affproet{X} carries the topology induced by the inclusion functor \affproet{X} \to \proet{X}. Although the objects of \proet{X} are weakly étale X-schemes, by Theorem 2.6.1 the topology of \proet{X} is still generated by objects in \affproet{X}:

Proposition5.3.1.4
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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Corollary 5.3.1.5
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The inclusion \affproet{X} \to \proet{X} is cover dense, i.e. for every object Y of \proet{X}, there exists a cover \{U_i \to Y\}_{i \in I} with U_i in \affproet{X}. (Bhatt–Scholze, Lemma 4.2.4)

Lean code for Proposition5.3.1.41 theorem
  • theoremdefined in Proetale/Topology/Comparison/Affine.lean
    complete
    theorem AlgebraicGeometry.Scheme.AffineProEt.isCoverDense_toProEt.{u}
      {S : AlgebraicGeometry.Scheme} :
      (AlgebraicGeometry.Scheme.AffineProEt.toProEt S).IsCoverDense
        (AlgebraicGeometry.Scheme.ProEt.topology S)
    theorem AlgebraicGeometry.Scheme.AffineProEt.isCoverDense_toProEt.{u}
      {S : AlgebraicGeometry.Scheme} :
      (AlgebraicGeometry.Scheme.AffineProEt.toProEt
            S).IsCoverDense
        (AlgebraicGeometry.Scheme.ProEt.topology
          S)
    The affine pro-étale site embeds densely in the pro-étale site. The key ingredient
    of the proof is the commutative algebra lemma `RingHom.WeaklyEtale.exists_indEtale_comp`. 
Proof for Proposition 5.3.1.4

Let f\colon Y \to X be a weakly étale morphism and let \{U_i \to X\} be an affine open cover of X. If we prove the lemma for each U_i and the weakly étale morphism f^{-1}(U_i) \to U_i, the lemma follows for f by taking the joint cover. Thus we may assume that X = \spec{A} is affine.

Now let \{U_i \to Y\} be an affine open cover. Again, if we prove the lemma for each U_i and the composition U_i \to Y \to X, the claim follows by joining the covers. Hence we may assume that Y = \spec{B} is affine.

We now have A \to B weakly étale, so by Theorem 2.6.1 there exists an ind-étale and faithfully flat B \to B' such that A \to B \to B' is ind-étale. The cover \{ \spec{B'} \to \spec{B} \} concludes the proof.

Corollary5.3.1.5
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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Restriction along the inclusion \affproet{X} \to \proet{X} induces an equivalence of categories \shv{\proet{X}} \to \shv{\affproet{X}}.

Lean code for Corollary5.3.1.51 theorem
  • theoremdefined in Proetale/Topology/Comparison/Affine.lean
    complete
    theorem AlgebraicGeometry.Scheme.AffineProEt.isEquivalence_sheafPushforwardContinuous_toProEt.{u,
        v_1, u_2}
      (S : AlgebraicGeometry.Scheme) {A : Type u_2}
      [CategoryTheory.Category.{v_1, u_2} A]
      [CategoryTheory.Limits.HasLimitsOfSize.{u, u + 1, v_1, u_2} A] :
      ((AlgebraicGeometry.Scheme.AffineProEt.toProEt
              S).sheafPushforwardContinuous
          A (AlgebraicGeometry.Scheme.AffineProEt.topology S)
          (AlgebraicGeometry.Scheme.ProEt.topology S)).IsEquivalence
    theorem AlgebraicGeometry.Scheme.AffineProEt.isEquivalence_sheafPushforwardContinuous_toProEt.{u,
        v_1, u_2}
      (S : AlgebraicGeometry.Scheme)
      {A : Type u_2}
      [CategoryTheory.Category.{v_1, u_2} A]
      [CategoryTheory.Limits.HasLimitsOfSize.{u,
            u + 1, v_1, u_2}
          A] :
      ((AlgebraicGeometry.Scheme.AffineProEt.toProEt
              S).sheafPushforwardContinuous
          A
          (AlgebraicGeometry.Scheme.AffineProEt.topology
            S)
          (AlgebraicGeometry.Scheme.ProEt.topology
            S)).IsEquivalence
    Restriction along inclusion of the affine pro-étale site into the pro-étale site induces an
    equivalence of categories of sheaves of `Ab.{u + 1}`, or more generally any category
    having large enough limits. 
Proof for Corollary 5.3.1.5

The functor \affproet{X} \to \proet{X} is fully faithful and makes \affproet{X} a dense subsite of \proet{X} by Proposition 5.3.1.4. Thus the claim follows from the comparison lemma for sites.

Corollary5.3.1.6
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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Restriction along the inclusion \affproet{X} \to \proet{X} commutes with sheafification.

Proof for Corollary 5.3.1.6

This follows immediately from Corollary 5.3.1.5.

We can also compare \affproet{X} to \affet{X}.

Proposition5.3.1.7
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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The canonical functor \affet{X} \to \affproet{X} induces an equivalence of categories \pro{\affet{X}} \to \affproet{X}.

Proof for Proposition 5.3.1.7

By Proposition 4.4, it suffices to show that for every U in \affet{X}, the image of U in \affproet{X} is co-finitely-presentable. This follows from the fact that étale morphisms are finitely presented.

5.3.2. Repleteness🔗

An important property of the pro-étale site is the following:

Proposition5.3.2.1
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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The category of sheaves on \proet{X} has enough weakly contractible objects. (Bhatt–Scholze, Proposition 4.2.8)

Proof for Proposition 5.3.2.1

TBA.

Corollary5.3.2.2
Group: The pro-étale site of a scheme and its basic properties, including the comparison with étale cohomology, following Sections 4 and 5 of Bhatt–Scholze. (36)
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Proposition 3.2.5
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The category of sheaves on \proet{X} is replete.