Pro-étale cohomology

1.5. w-local spaces🔗

Definition1.5.1
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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A topological space X is w-local if it satisfies:

  1. X is spectral,

  2. every point specializes to a unique closed point,

  3. the subspace X^c of closed points is closed.

(Bhatt–Scholze, Definition 2.1.1; Stacks Project, Tag 096A)

Lean code for Definition1.5.11 definition
  • class(extends 6, 7 methods)defined in Proetale/Topology/SpectralSpace/WLocal/Basic.lean
    complete
    class WLocalSpace.{u_1} (X : Type u_1) [TopologicalSpace X] : Prop
    class WLocalSpace.{u_1} (X : Type u_1)
      [TopologicalSpace X] : Prop
    A spectral space is w-local if every point specializes to a unique closed point
    and the set of closed points is closed.
    Note: In a spectral space, every point specializes to a closed point, so we only require
    the uniqueness.
    

    Extends

    • SpectralSpace X

    Methods

    t0 :  x y : X⦄, Inseparable x y  x = y
    Inherited from
    1. SpectralSpace
    2. T0Space
    isCompact_univ : IsCompact Set.univ
    Inherited from
    1. SpectralSpace
    2. CompactSpace
    sober :  {S : Set X}, IsIrreducible S  IsClosed S   x, IsGenericPoint x S
    Inherited from
    1. SpectralSpace
    2. QuasiSober
    inter_isCompact :  (U V : Set X), IsOpen U  IsCompact U  IsOpen V  IsCompact V  IsCompact (U  V)
    Inherited from
    1. SpectralSpace
    2. QuasiSeparatedSpace
    isTopologicalBasis : TopologicalSpace.IsTopologicalBasis {U | IsOpen U  IsCompact U}
    Inherited from
    1. SpectralSpace
    2. PrespectralSpace
    eq_of_specializes :  {x c c' : X}, IsClosed {c}  IsClosed {c'}  x  c  x  c'  c = c'
    Any two closed specializations of a point are equal. 
    isClosed_closedPoints : IsClosed (closedPoints X)
    The set of closed points is closed. 
Definition1.5.2
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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L∃∀N

Let X and Y be w-local spaces. A morphism f \colon X \to Y is w-local if it is spectral and the image of closed points f(X^c) \subseteq Y^c. (Bhatt–Scholze, Definition 2.1.1; Stacks Project, Tag 096A)

Lean code for Definition1.5.21 definition
  • structure(extends 2, 3 fields)defined in Proetale/Topology/SpectralSpace/WLocal/Basic.lean
    complete
    structure IsWLocalMap.{u_1, u_2} {X : Type u_1} {Y : Type u_2}
      [TopologicalSpace X] [TopologicalSpace Y] (f : X  Y) : Prop
    structure IsWLocalMap.{u_1, u_2} {X : Type u_1}
      {Y : Type u_2} [TopologicalSpace X]
      [TopologicalSpace Y] (f : X  Y) : Prop
    A w-local map is a spectral map that maps closed points to closed points. 

    Extends

    • IsSpectralMap f

    Fields

    isOpen_preimage :  (s : Set Y), IsOpen s  IsOpen (f ⁻¹' s)
    Inherited from
    1. IsSpectralMap
    2. Continuous
    isCompact_preimage_of_isOpen :  s : Set Y⦄, IsOpen s  IsCompact s  IsCompact (f ⁻¹' s)
    Inherited from
    1. IsSpectralMap
    closedPoints_subset_preimage_closedPoints : closedPoints X  f ⁻¹' closedPoints Y
Definition1.5.3
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 X and Y be w-local spaces. A morphism f \colon X \to Y is w-purely-local if f^{-1}(Y^c) = X^c.

Obviously, every w-purely-local morphism is w-local.

Lemma1.5.4
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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Definition 1.5.1
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Let f \colon X \to Y and g \colon Y \to Z be w-local maps between w-local spaces. Then the composition g \circ f \colon X \to Z is w-local.

Lean code for Lemma1.5.41 theorem
  • theoremdefined in Proetale/Topology/SpectralSpace/WLocal/Basic.lean
    complete
    theorem IsWLocalMap.comp.{u_1, u_2, u_3} {X : Type u_1} {Y : Type u_2}
      [TopologicalSpace X] [TopologicalSpace Y] {Z : Type u_3}
      [TopologicalSpace Z] {f : X  Y} {g : Y  Z} (hf : IsWLocalMap f)
      (hg : IsWLocalMap g) : IsWLocalMap (g  f)
    theorem IsWLocalMap.comp.{u_1, u_2, u_3}
      {X : Type u_1} {Y : Type u_2}
      [TopologicalSpace X]
      [TopologicalSpace Y] {Z : Type u_3}
      [TopologicalSpace Z] {f : X  Y}
      {g : Y  Z} (hf : IsWLocalMap f)
      (hg : IsWLocalMap g) :
      IsWLocalMap (g  f)
Lemma1.5.5
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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Lemma 1.5.15
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L∃∀N

Let X be w-local and Z \subseteq X a subset closed under specialization. Then the generalization Z^g in X is still closed under specialization.

Lean code for Lemma1.5.51 theorem
  • theoremdefined in Proetale/Topology/SpectralSpace/WLocal/Basic.lean
    complete
    theorem StableUnderSpecialization.generalizationHull_of_wLocalSpace.{u_1}
      {X : Type u_1} [TopologicalSpace X] [WLocalSpace X] {s : Set X}
      (hs : StableUnderSpecialization s) :
      StableUnderSpecialization (generalizationHull s)
    theorem StableUnderSpecialization.generalizationHull_of_wLocalSpace.{u_1}
      {X : Type u_1} [TopologicalSpace X]
      [WLocalSpace X] {s : Set X}
      (hs : StableUnderSpecialization s) :
      StableUnderSpecialization
        (generalizationHull s)
Proof for Lemma 1.5.5
uses 0

Let x \rightsquigarrow y in X with x \in Z^g. Then x specializes to some z \in Z, which in turn specializes to a unique closed point c in X because X is w-local. Because Z is stable under specialization, c \in Z. Also y specializes to a unique closed point c' in X and by uniqueness we have c' = c \in Z. Hence y \in Z^g.

Lemma1.5.6
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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Lemma 1.5.7
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L∃∀N

Let X be w-local and Z \subseteq X a subset closed under specialization. If Z is spectral, it is w-local and the inclusion Z \to X maps closed points to closed points.

Lean code for Lemma1.5.61 theorem
  • theoremdefined in Proetale/Topology/SpectralSpace/WLocal/Basic.lean
    complete
    theorem Topology.IsEmbedding.wLocalSpace_of_stableUnderSpecialization_range.{u_1,
        u_2}
      {X : Type u_1} {Y : Type u_2} [TopologicalSpace X]
      [TopologicalSpace Y] {f : X  Y} (hf : Topology.IsEmbedding f)
      (h : StableUnderSpecialization (Set.range f)) [SpectralSpace X]
      [WLocalSpace Y] : WLocalSpace X
    theorem Topology.IsEmbedding.wLocalSpace_of_stableUnderSpecialization_range.{u_1,
        u_2}
      {X : Type u_1} {Y : Type u_2}
      [TopologicalSpace X]
      [TopologicalSpace Y] {f : X  Y}
      (hf : Topology.IsEmbedding f)
      (h :
        StableUnderSpecialization
          (Set.range f))
      [SpectralSpace X] [WLocalSpace Y] :
      WLocalSpace X
Proof for Lemma 1.5.6

Since Z is closed under specializations, we have Z^c = X^c \cap Z. In particular Z^c is closed in Z and Z \to X preserves closed points. By Lemma 1.1.10, specializations in Z are the same as specializations in X. Since Z is closed under specializations, every point in Z specializes to a unique closed point in X and therefore in Z.

Lemma1.5.7
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 1.5.1
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Let X be a w-local space. Then every closed subspace Y \subseteq X is w-local. Moreover, the inclusion map Y \to X is w-local. (Stacks Project, Tag 096B)

Lean code for Lemma1.5.71 theorem
  • theoremdefined in Proetale/Topology/SpectralSpace/WLocal/Basic.lean
    complete
    theorem Topology.IsClosedEmbedding.wLocalSpace.{u_1, u_2} {X : Type u_1}
      {Y : Type u_2} [TopologicalSpace X] [TopologicalSpace Y] {f : X  Y}
      (hf : Topology.IsClosedEmbedding f) [WLocalSpace Y] : WLocalSpace X
    theorem Topology.IsClosedEmbedding.wLocalSpace.{u_1,
        u_2}
      {X : Type u_1} {Y : Type u_2}
      [TopologicalSpace X]
      [TopologicalSpace Y] {f : X  Y}
      (hf : Topology.IsClosedEmbedding f)
      [WLocalSpace Y] : WLocalSpace X
Proof for Lemma 1.5.7

This follows from Lemma 1.5.6, because closed subsets are stable under specialization. The inclusion Y \to X is spectral as a closed embedding and hence w-local by the lemma.

Lemma1.5.8
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 X be a profinite topological space and x, y \in X be distinct. Then there exist U, V \subseteq X open and closed such that X = U \coprod V and x \in U, y \in V.

Lean code for Lemma1.5.81 theorem
  • theoremdefined in Mathlib/Topology/Separation/Profinite.lean
    complete
    theorem instTotallySeparatedSpaceOfTotallyDisconnectedSpace.{u_3} {H : Type u_3}
      [TopologicalSpace H] [LocallyCompactSpace H] [T2Space H]
      [TotallyDisconnectedSpace H] : TotallySeparatedSpace H
    theorem instTotallySeparatedSpaceOfTotallyDisconnectedSpace.{u_3}
      {H : Type u_3} [TopologicalSpace H]
      [LocallyCompactSpace H] [T2Space H]
      [TotallyDisconnectedSpace H] :
      TotallySeparatedSpace H
    A totally disconnected compact Hausdorff space is totally separated. 
Proof for Lemma 1.5.8
uses 0

Write X = \lim_{i \in I} X_i as a directed limit of finite discrete spaces. Let \pi_i \colon X \to X_i be the projection maps. A base of the open sets of X is given by the preimages \pi_i^{-1}(\{z\}) for z \in X_i and i \in I. Every open set in this base is also closed, because its complement is a finite union of such sets.

Choose an open subset in this base containing x but not y, say U = \pi_{i_0}^{-1}(\{z_0\}) for some i_0 \in I and z_0 \in X_{i_0}. Then V = X - U is also open and closed, and we have X = U \coprod V with x \in U and y \in V.

Lemma1.5.9
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 X be a spectral space such that every point is closed. Then X is profinite. (Stacks Project, Tag 0905, (6) ⟹ (1))

Lean code for Lemma1.5.91 theorem
  • theoremdefined in Proetale/Topology/SpectralSpace/Constructible.lean
    complete
    theorem SpectralSpace.totallySeparatedSpace.{u_1} {X : Type u_1}
      [TopologicalSpace X] [SpectralSpace X] [T1Space X] :
      TotallySeparatedSpace X
    theorem SpectralSpace.totallySeparatedSpace.{u_1}
      {X : Type u_1} [TopologicalSpace X]
      [SpectralSpace X] [T1Space X] :
      TotallySeparatedSpace X
    [Stacks Tag 0905](https://stacks.math.columbia.edu/tag/0905) ((6) → (1))
Proof for Lemma 1.5.9

Since every point is closed, there are no nontrivial specializations between points. By part (2) of Lemma 1.5.14, all constructible subsets of X are closed. In particular, every compact open subset is also closed. Since X is spectral, thus T0, for any two points there is a quasi-compact open (also closed) containing exactly one of them. Thus X is profinite by isTotallyDisconnected_of_isClopen_set.

Lemma1.5.10
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 X be a w-local space. Then the set of closed points X^c of X is profinite.

Lean code for Lemma1.5.101 theorem
  • theoremdefined in Proetale/Topology/SpectralSpace/WLocal/ClosedPoints.lean
    complete
    theorem WLocalSpace.CompactSpace_closedPoints.{u_1} {X : Type u_1}
      [TopologicalSpace X] [WLocalSpace X] : CompactSpace (closedPoints X)
    theorem WLocalSpace.CompactSpace_closedPoints.{u_1}
      {X : Type u_1} [TopologicalSpace X]
      [WLocalSpace X] :
      CompactSpace (closedPoints X)
Proof for Lemma 1.5.10

It is a closed subspace of a spectral space, hence spectral. Therefore it is profinite by Lemma 1.5.9.

Lemma1.5.11
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 X be a spectral space and E \subseteq X a quasi-compact subset. Then the generalization E^g is an intersection of quasi-compact open subsets. (Stacks Project, Tag 0A31, (3) ⟹ (4))

Lean code for Lemma1.5.111 theorem
  • theoremdefined in Proetale/Mathlib/Topology/Spectral/Basic.lean
    complete
    theorem generalizationHull.eq_sInter_of_isCompact.{u_1} {X : Type u_1}
      [TopologicalSpace X] [SpectralSpace X] {s : Set X}
      (hs : IsCompact s) :
       S  {U | IsOpen U  IsCompact U}, generalizationHull s = ⋂₀ S
    theorem generalizationHull.eq_sInter_of_isCompact.{u_1}
      {X : Type u_1} [TopologicalSpace X]
      [SpectralSpace X] {s : Set X}
      (hs : IsCompact s) :
       S  {U | IsOpen U  IsCompact U},
        generalizationHull s = ⋂₀ S
Proof for Lemma 1.5.11
uses 0

Let X be a spectral space and let W denote E^g. Since every point of W specializes to a point of E we see that an open of W which contains E is equal to W. Hence since E is quasi-compact, so is W. If x \in X, x \notin W, then Z = \overline{\{x\}} is disjoint from W. Since W is quasi-compact we can find a quasi-compact open U (since there is an open basis of quasi-compact opens of X and finitely many of them suffice to cover W) with W \subset U and U \cap Z = \emptyset. We conclude that W is an intersection of quasi-compact open subsets.

Definition1.5.12
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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Lemma 1.5.13
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L∃∀N

Let X be a topological space. The constructible topology on X is the topology with subbase of opens the sets U and X - U for all quasi-compact opens U of X. (Stacks Project, Tag 08YF)

Lean code for Definition1.5.121 definition
  • defdefined in Mathlib/Topology/Spectral/ConstructibleTopology.lean
    complete
    def constructibleTopology.{u_2} (X : Type u_2) [TopologicalSpace X] :
      TopologicalSpace X
    def constructibleTopology.{u_2} (X : Type u_2)
      [TopologicalSpace X] :
      TopologicalSpace X
    The constructible topology on a topological space `X` has as a subbasis
    the open and compact sets of `X` and their complements. 

In other words, a subset of X is open in the constructible topology if and only if it is a union of finite intersections of sets of the form U or X - U for a quasi-compact open U.

Lemma1.5.13
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 X be a spectral space. Then X is quasi-compact in the constructible topology. (Stacks Project, Tag 0901)

Lean code for Lemma1.5.131 theorem
  • theoremdefined in Mathlib/Topology/Spectral/ConstructibleTopology.lean
    complete
    theorem compactSpace_withConstructibleTopology.{u_1} {X : Type u_1}
      [TopologicalSpace X] [CompactSpace X] [QuasiSober X]
      [PrespectralSpace X] [QuasiSeparatedSpace X] :
      CompactSpace (WithConstructibleTopology X)
    theorem compactSpace_withConstructibleTopology.{u_1}
      {X : Type u_1} [TopologicalSpace X]
      [CompactSpace X] [QuasiSober X]
      [PrespectralSpace X]
      [QuasiSeparatedSpace X] :
      CompactSpace
        (WithConstructibleTopology X)
    If `X` is quasi-separated, quasi-sober, prespectral and quasi-compact, then `X`
    is still quasi-compact in the constructible topology. This holds in particular for spectral
    spaces. 
Proof for Lemma 1.5.13
uses 0

Let \mathfrak{B} be the set of subsets of X that are quasi-compact open or the complement of a quasi-compact open. By definition of the constructible topology and the Alexander subbasis theorem, it suffices to show that every covering X = \bigcup_{i \in I} B_i with B_i \in \mathfrak{B} has a finite subcover. Since for every B \in \mathfrak{B} also X - B \in \mathfrak{B}, it suffices to show: If \{B_i\}_{i \in I} is a family of elements of \mathfrak{B} such that all finite subfamilies have non-empty intersection, then \bigcap_{i \in I} B_i is non-empty.

Let \mathcal{S} be the set of families in \mathfrak{B} that are pairwise unequal, have non-empty finite intersections and empty intersection. For the sake of contradiction, suppose \mathcal{S} is non-empty. If we equip \mathcal{S} with the ordering by subset inclusion, Zorn's Lemma yields a maximal family \{B_i\}_{i \in I} \in \mathcal{S}.

Let I' \subseteq I be the indices such that B_i is closed and set Z = \bigcap_{i \in I'} B_i. This is a closed and non-empty subset of X, because X is quasi-compact and all finite intersections are non-empty by assumption. Note that for any i_1, \ldots, i_n \in I the intersection \bigcap_{a=1}^n B_{i_a} is quasi-compact, hence Z \cap \bigcap_{a=1}^n B_{i_a} is non-empty by the same argument.

Suppose Z = Z' \cup Z'' is reducible with Z', Z'' \subsetneq Z closed. Because X is a spectral space, there exist quasi-compact opens U', U'' \subseteq X with U' \cap Z' non-empty, U'' \cap Z'' non-empty and U' \cap Z'' = \emptyset = U'' \cap Z'. Then set B' = X - U' \in \mathfrak{B} and B'' = X - U'' \in \mathfrak{B}. We claim that \{B_i\}_{i \in I} with B' or B'' added is still in \mathcal{S}. Suppose this is not the case. Then there exist finitely many indices i_a and j_b in I such that B' \cap \bigcap_{a=1}^n B_{i_a} = \emptyset = B'' \cap \bigcap_{b=1}^m B_{j_b}. By construction, Z \subseteq B' \cup B'', so in particular Z \cap \bigcap_{a=1}^n B_{i_a} \cap \bigcap_{b=1}^m B_{j_b} = \emptyset, which is a contradiction. Hence \{B_i\}_{i \in I} is not maximal, also a contradiction. So Z is irreducible. But for every i \in I - I', the set B_i is open and hence the non-empty open Z \cap B_i of Z contains the unique generic point of Z. Hence \bigcap_{i \in I} B_i = Z \cap \bigcap_{i \in I - I'} B_i is non-empty, which is again a contradiction.

Lemma1.5.14
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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Lemma 1.5.9
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L∃∀N

Let X be a spectral space and E \subseteq X a subset closed in the constructible topology.

  1. If x \in \overline{E}, then x is the specialization of a point in E.

  2. If E is stable under specialization, then E is closed.

(Stacks Project, Tag 0903)

Lean code for Lemma1.5.142 theorems
  • theoremdefined in Proetale/Topology/SpectralSpace/Constructible.lean
    complete
    theorem exists_specializes_of_isClosed_constructibleTopology_of_mem_closure.{u_1}
      {X : Type u_1} [TopologicalSpace X] [SpectralSpace X] {s : Set X}
      (hs : IsClosed s) {x : X} (hx : x  closure s) :  y  s, y  x
    theorem exists_specializes_of_isClosed_constructibleTopology_of_mem_closure.{u_1}
      {X : Type u_1} [TopologicalSpace X]
      [SpectralSpace X] {s : Set X}
      (hs : IsClosed s) {x : X}
      (hx : x  closure s) :  y  s, y  x
    If `s` is closed in the constructible topology and `x` is in the closure of `s`, then
    it is the specialization of a point in `s`. 
  • theoremdefined in Proetale/Topology/SpectralSpace/Constructible.lean
    complete
    theorem IsClosed.of_isClosed_constructibleTopology.{u_1} {X : Type u_1}
      [TopologicalSpace X] [SpectralSpace X] {s : Set X} (hs : IsClosed s)
      (h : StableUnderSpecialization s) : IsClosed s
    theorem IsClosed.of_isClosed_constructibleTopology.{u_1}
      {X : Type u_1} [TopologicalSpace X]
      [SpectralSpace X] {s : Set X}
      (hs : IsClosed s)
      (h : StableUnderSpecialization s) :
      IsClosed s
    If `s` is closed in the constructible topology and stable under specialization, it is closed. 
Proof for Lemma 1.5.14

It suffices to show the first claim. Let x \in \overline{E} and let \{U_i\}_{i \in I} be the set of quasi-compact open neighbourhoods of x. Since X is spectral, these form a neighbourhood basis of x, so \bigcap_{i \in I} U_i is the set of generalizations of x. It hence suffices to show that \bigcap_{i \in I} U_i \cap E is non-empty. Indeed, as X is quasi-separated, any finite intersection of the U_i is another one and since x \in \overline{E}, the intersection U_i \cap E is non-empty for all i. Since U_i \cap E is closed in the constructible topology for all i and X is quasi-compact in the constructible topology by Lemma 1.5.13, the intersection \bigcap_{i \in I} U_i \cap E is non-empty.

Lemma1.5.15
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 1.1.11
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Let X be a w-local space and Z \subseteq X a closed subset. Then Z^g is closed.

Lean code for Lemma1.5.151 theorem
  • theoremdefined in Proetale/Topology/SpectralSpace/WLocal/Basic.lean
    complete
    theorem isClosed_generalizationHull_of_wLocalSpace.{u_1} {X : Type u_1}
      [TopologicalSpace X] [WLocalSpace X] {s : Set X} (hs : IsClosed s) :
      IsClosed (generalizationHull s)
    theorem isClosed_generalizationHull_of_wLocalSpace.{u_1}
      {X : Type u_1} [TopologicalSpace X]
      [WLocalSpace X] {s : Set X}
      (hs : IsClosed s) :
      IsClosed (generalizationHull s)
Proof for Lemma 1.5.15
Proof uses 3
Proof dependency previews

Since Z is closed, it is quasi-compact. Hence by Lemma 1.5.11, we see that Z^g is closed in the constructible topology as an intersection of quasi-compact opens. Since Z is stable under specialization, the same holds for Z^g by Lemma 1.5.5. Thus by Lemma 1.5.14, the claim follows.

Lemma1.5.16
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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uses 1
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L∃∀N

Let X be a w-local space. Then the composition X^c \to X \to \pi_0(X) is a homeomorphism, where X^c denotes the closed points of X. (Stacks Project, Tag 0968)

Lean code for Lemma1.5.161 theorem
  • theoremdefined in Proetale/Topology/SpectralSpace/WLocal/ConnectedComponents.lean
    complete
    theorem WLocalSpace.isHomeomorph_connectedComponents_closedPoints.{u_1}
      (X : Type u_1) [TopologicalSpace X] [WLocalSpace X] :
      IsHomeomorph (ConnectedComponents.mk  Subtype.val)
    theorem WLocalSpace.isHomeomorph_connectedComponents_closedPoints.{u_1}
      (X : Type u_1) [TopologicalSpace X]
      [WLocalSpace X] :
      IsHomeomorph
        (ConnectedComponents.mk  Subtype.val)
    If `X` is w-local, the composition `closedPoints X → X → ConnectedComponents X` is
    a homeomorphism. 
Proof for Lemma 1.5.16

Since X^c is quasi-compact and \pi_0(X) is Hausdorff by Lemma 1.1.17, it suffices to show the map is bijective. To show injectivity let x, y \in X^c be distinct. Since X^c is profinite (Lemma 1.5.10), there exist open and closed subsets U_0, V_0 \subseteq X^c such that X^c = U_0 \coprod V_0 by Lemma 1.5.8. Let U = U_0^g and V = V_0^g. Because X is w-local, we get X = U \coprod V as sets. By Lemma 1.5.15, both U and V are closed and therefore also both open. Hence x and y lie in distinct connected components of X, so the images in \pi_0(X) are distinct.

Since every connected component is closed, it is quasi-compact and hence contains a closed point. Thus the map is surjective.

Lemma1.5.17
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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used by 1L∃∀N

Let X be a spectral space. Let \begin{CD} Y @>>> T \\ @VVV @VVV \\ X @>>> \pi_0(X) \end{CD} be a cartesian diagram in the category of topological spaces with T profinite. If moreover X is w-local, then Y is w-local, Y \to X is w-local, and the set of closed points of Y is the inverse image of the set of closed points of X. (Stacks Project, Tag 096C, second part)

Lean code for Lemma1.5.173 theorems, 3 incomplete
  • theoremdefined in Proetale/Topology/SpectralSpace/WLocal/Pullback.lean
    contains sorry
    theorem ConnectedComponents.wlocalSpace_of_isPullback.{u} {X Y T : Type u}
      [TopologicalSpace X] [WLocalSpace X] [TopologicalSpace Y]
      [TopologicalSpace T] [CompactSpace T] [T2Space T]
      [TotallyDisconnectedSpace T] {f : C(Y, X)} {g : C(Y, T)}
      {i : C(T, ConnectedComponents X)}
      (pb :
        CategoryTheory.IsPullback (TopCat.ofHom g) (TopCat.ofHom f)
          (TopCat.ofHom i)
          (TopCat.ofHom
            { toFun := ConnectedComponents.mk, continuous_toFun :=  })) :
      WLocalSpace Y
    theorem ConnectedComponents.wlocalSpace_of_isPullback.{u}
      {X Y T : Type u} [TopologicalSpace X]
      [WLocalSpace X] [TopologicalSpace Y]
      [TopologicalSpace T] [CompactSpace T]
      [T2Space T] [TotallyDisconnectedSpace T]
      {f : C(Y, X)} {g : C(Y, T)}
      {i : C(T, ConnectedComponents X)}
      (pb :
        CategoryTheory.IsPullback
          (TopCat.ofHom g) (TopCat.ofHom f)
          (TopCat.ofHom i)
          (TopCat.ofHom
            { toFun := ConnectedComponents.mk,
              continuous_toFun :=  })) :
      WLocalSpace Y
    [Stacks Tag 096C](https://stacks.math.columbia.edu/tag/096C) (second part)
  • theoremdefined in Proetale/Topology/SpectralSpace/WLocal/Pullback.lean
    contains sorry
    theorem ConnectedComponents.isWLocalMap_of_isPullback.{u} {X Y T : Type u}
      [TopologicalSpace X] [WLocalSpace X] [TopologicalSpace Y]
      [TopologicalSpace T] [CompactSpace T] [T2Space T]
      [TotallyDisconnectedSpace T] {f : C(Y, X)} {g : C(Y, T)}
      {i : C(T, ConnectedComponents X)}
      (pb :
        CategoryTheory.IsPullback (TopCat.ofHom g) (TopCat.ofHom f)
          (TopCat.ofHom i)
          (TopCat.ofHom
            { toFun := ConnectedComponents.mk, continuous_toFun :=  })) :
      IsWLocalMap f
    theorem ConnectedComponents.isWLocalMap_of_isPullback.{u}
      {X Y T : Type u} [TopologicalSpace X]
      [WLocalSpace X] [TopologicalSpace Y]
      [TopologicalSpace T] [CompactSpace T]
      [T2Space T] [TotallyDisconnectedSpace T]
      {f : C(Y, X)} {g : C(Y, T)}
      {i : C(T, ConnectedComponents X)}
      (pb :
        CategoryTheory.IsPullback
          (TopCat.ofHom g) (TopCat.ofHom f)
          (TopCat.ofHom i)
          (TopCat.ofHom
            { toFun := ConnectedComponents.mk,
              continuous_toFun :=  })) :
      IsWLocalMap f
    [Stacks Tag 096C](https://stacks.math.columbia.edu/tag/096C) (second part)
  • theoremdefined in Proetale/Topology/SpectralSpace/WLocal/Pullback.lean
    contains sorry
    theorem ConnectedComponents.preimage_closedPoints_eq_closedPoints_of_isPullback.{u}
      {X Y T : Type u} [TopologicalSpace X] [WLocalSpace X]
      [TopologicalSpace Y] [TopologicalSpace T] [CompactSpace T] [T2Space T]
      [TotallyDisconnectedSpace T] {f : C(Y, X)} {g : C(Y, T)}
      {i : C(T, ConnectedComponents X)}
      (pb :
        CategoryTheory.IsPullback (TopCat.ofHom g) (TopCat.ofHom f)
          (TopCat.ofHom i)
          (TopCat.ofHom
            { toFun := ConnectedComponents.mk, continuous_toFun :=  })) :
      f ⁻¹' closedPoints X = closedPoints Y
    theorem ConnectedComponents.preimage_closedPoints_eq_closedPoints_of_isPullback.{u}
      {X Y T : Type u} [TopologicalSpace X]
      [WLocalSpace X] [TopologicalSpace Y]
      [TopologicalSpace T] [CompactSpace T]
      [T2Space T] [TotallyDisconnectedSpace T]
      {f : C(Y, X)} {g : C(Y, T)}
      {i : C(T, ConnectedComponents X)}
      (pb :
        CategoryTheory.IsPullback
          (TopCat.ofHom g) (TopCat.ofHom f)
          (TopCat.ofHom i)
          (TopCat.ofHom
            { toFun := ConnectedComponents.mk,
              continuous_toFun :=  })) :
      f ⁻¹' closedPoints X = closedPoints Y
    [Stacks Tag 096C](https://stacks.math.columbia.edu/tag/096C) (second part)
Proof for Lemma 1.5.17
Proof uses 3
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Lemma 1.1.24
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We know that Y is spectral and T = \pi_0(Y) by Lemma 1.1.26. Assume X is w-local and let X^c \subset X be its closed points. The inverse image Y^c \subset Y of X^c maps bijectively onto T, since X^c \to \pi_0(X) is a bijection by Lemma 1.5.16. (Note that we do not yet know whether Y^c is the set of closed points of Y.) Moreover, Y^c is quasi-compact as a closed subset of the spectral space Y. Hence Y^c \to \pi_0(Y) = T is a homeomorphism by isHomeomorph_iff_continuous_bijective (Stacks Project, Tag 08YE). It follows that all points of Y^c are closed in Y: if some y \in Y^c specialized to a distinct closed point y' (which also falls in Y^c since Y^c is closed), then both would map to the same point in T, contradicting the bijectivity of Y^c \to T.

Conversely, if y \in Y is a closed point, then it is closed in its fibre over T, and its image x \in X is closed in the corresponding (homeomorphic) fibre of X \to \pi_0(X). Since a point in a w-local space is closed if and only if it is closed in its connected component (which is closed), we have x \in X^c, so y \in Y^c. Thus Y^c is exactly the set of closed points of Y. Moreover, since Y^c \cong T, each connected component contains a unique closed point, and for each y \in Y^c the set of generalizations of y is precisely the fibre of Y \to \pi_0(Y) (as any generalization of y lies in the same connected component with y). The lemma follows.