Pro-étale cohomology

1.6. w-local rings🔗

Definition1.6.1
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 ring A is w-local if \Spec(A) is w-local. (Bhatt–Scholze, Definition 2.2.1(i))

Lean code for Definition1.6.11 definition
  • class(1 method)defined in Proetale/Algebra/WLocal.lean
    complete
    class IsWLocalRing.{u_1} (R : Type u_1) [CommSemiring R] : Prop
    class IsWLocalRing.{u_1} (R : Type u_1)
      [CommSemiring R] : Prop
    A ring is w-local if it has a w-local prime spectrum. 

    Methods

    wLocalSpace_primeSepectrum : WLocalSpace (PrimeSpectrum R)
Definition1.6.2
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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Definition 1.5.1
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XL∃∀N

A ring map f \colon A \to B between w-local rings is w-local if the induced map of w-local spaces \Spec(f) : \Spec(B) \to \Spec(A) is w-local. (Bhatt–Scholze, Definition 2.2.1(iii))

Lemma1.6.3
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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If A \to B is a surjective ring map and A is w-local, then B is w-local.

Lean code for Lemma1.6.31 theorem
  • theoremdefined in Proetale/Algebra/WLocal.lean
    complete
    theorem IsWLocalRing.of_surjective.{u_1, u_2} {R : Type u_1} {S : Type u_2}
      [CommRing R] [CommRing S] {f : R →+* S} (hf : Function.Surjective f)
      [IsWLocalRing R] : IsWLocalRing S
    theorem IsWLocalRing.of_surjective.{u_1, u_2}
      {R : Type u_1} {S : Type u_2}
      [CommRing R] [CommRing S] {f : R →+* S}
      (hf : Function.Surjective f)
      [IsWLocalRing R] : IsWLocalRing S
Proof for Lemma 1.6.3

This is Lemma 1.5.7.

Lemma1.6.4
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 A be a w-local ring. Then \Spec (A) is set theoretically the disjoint union of the spectra of the local rings at closed points.

Proof for Lemma 1.6.4
uses 0

This follows from the fact that every point specializes to a unique closed point and the spectrum of the local ring at a point is the set of all points specializing to it.

Lemma1.6.5
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 1.6.6
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L∃∀N

Let A \to B be a ring map such that

  1. A \to B identifies local rings, and

  2. \Spec (B) \to \Spec (A) is bijective.

Then A \to B is an isomorphism.

Lean code for Lemma1.6.51 theorem
  • theoremdefined in Proetale/Algebra/StalkIso.lean
    complete
    theorem RingHom.BijectiveOnStalks.bijective_of_bijective.{u, v} {R : Type u}
      {S : Type v} [CommRing R] [CommRing S] {f : R →+* S}
      (hf : f.BijectiveOnStalks)
      (hb : Function.Bijective (PrimeSpectrum.comap f)) :
      Function.Bijective f
    theorem RingHom.BijectiveOnStalks.bijective_of_bijective.{u,
        v}
      {R : Type u} {S : Type v} [CommRing R]
      [CommRing S] {f : R →+* S}
      (hf : f.BijectiveOnStalks)
      (hb :
        Function.Bijective
          (PrimeSpectrum.comap f)) :
      Function.Bijective f
    A ring homomorphism that is bijective on stalks and induces a bijection on prime spectra
    is itself bijective. 
Proof for Lemma 1.6.5

Since A \to B is flat (can be checked on stalks of B), it has going down. Thus Lemma 1.1.27 shows for any prime \q \subset B lying over \p \subset A we have B_{\q} = B_{\p B}. Since B_{\q} = A_{\p} by assumption, we see that A_{\p} = B_{\p B} for all primes \p of A. Thus A = B since being an isomorphism can be checked locally on A.

Lemma1.6.6
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 1.1.29
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used by 1L∃∀N

Let A \to B be a w-local ring map between w-local rings such that

  1. A \to B identifies local rings, and

  2. \pi_0(\Spec (B)) \to \pi_0(\Spec (A)) is bijective.

Then A \to B is an isomorphism. (Stacks Project, Tag 097E)

Lean code for Lemma1.6.61 theorem
  • theoremdefined in Proetale/Algebra/WLocal.lean
    complete
    theorem RingHom.IsWLocal.bijective_of_bijective.{u_1, u_2} {R : Type u_1}
      {S : Type u_2} [CommRing R] [CommRing S] [IsWLocalRing R]
      [IsWLocalRing S] {f : R →+* S} (hw : f.IsWLocal)
      (hs : f.BijectiveOnStalks)
      (hb : Function.Bijective .connectedComponentsMap) :
      Function.Bijective f
    theorem RingHom.IsWLocal.bijective_of_bijective.{u_1,
        u_2}
      {R : Type u_1} {S : Type u_2}
      [CommRing R] [CommRing S]
      [IsWLocalRing R] [IsWLocalRing S]
      {f : R →+* S} (hw : f.IsWLocal)
      (hs : f.BijectiveOnStalks)
      (hb :
        Function.Bijective
          .connectedComponentsMap) :
      Function.Bijective f
    A w-local ring map between w-local rings that is bijective on stalks and
    bijective on connected components is bijective. 
Proof for Lemma 1.6.6
Proof uses 3
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Lemma 1.5.16
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By Lemma 1.6.5, it suffices to show that Y = \Spec (B) \to X = \Spec (A) is bijective. Let X^c \subset X and Y^c \subset Y be the sets of closed points. By assumption Y^c maps into X^c and the induced map Y^c \to X^c is a bijection since Lemma 1.5.16 holds. By Lemma 1.6.4, we see that both X and Y, as sets, are disjoint unions of the spectra of the local rings at closed points. As A \to B identifies local rings, Y \to X is a bijection.

The goal of the remaining part of this section is to show that for every ring A, there exists a faithfully flat, ind-Zariski A-algebra A_{w} with A_w w-local. Let A be a ring.

Definition1.6.7
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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L∃∀N

Let f \in A be an element and I \subseteq A an ideal. Let S_{f, I} \subseteq A be the multiplicative subset of elements which map to invertible elements of (A/I)_{f}. We define \tildering{A}{I}{f} to be the A-algebra S_{f, I}^{-1}A and \tildeideal{I}{f} \subseteq \tildering{A}{I}{f} to be the kernel of the induced map \tildering{A}{I}{f} \to (A / I)_{f}.

Lean code for Definition1.6.73 definitions
  • defdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    def WLocalization.Generalization.submonoid.{u_1} {A : Type u_1} [CommRing A]
      (f : A) (I : Ideal A) : Submonoid A
    def WLocalization.Generalization.submonoid.{u_1}
      {A : Type u_1} [CommRing A] (f : A)
      (I : Ideal A) : Submonoid A
    The submonoid of `A` consisting of elements that become invertible in `(A / I)_f`. 
  • defdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    def WLocalization.Generalization.{u_1} {A : Type u_1} [CommRing A] (f : A)
      (I : Ideal A) : Type u_1
    def WLocalization.Generalization.{u_1}
      {A : Type u_1} [CommRing A] (f : A)
      (I : Ideal A) : Type u_1
    The localization of `A` at all elements invertible in `(A / I)_f`. 
  • defdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    def WLocalization.Generalization.ideal.{u_1} {A : Type u_1} [CommRing A]
      (f : A) (I : Ideal A) : Ideal (WLocalization.Generalization f I)
    def WLocalization.Generalization.ideal.{u_1}
      {A : Type u_1} [CommRing A] (f : A)
      (I : Ideal A) :
      Ideal (WLocalization.Generalization f I)
    The kernel of the canonical map from the generalization at `(f, I)` to `(A ⧸ I)_f`.
    This ideal defines a closed subset of the prime spectrum of the generalization at `(f, I)` that
    maps homeomorphically to `D(f) ∩ V(I)`.
    

If Z \subseteq \spec{A} is a locally closed subscheme of the form D(f) \cap V(I), we also write A_{\widetilde{Z}} instead of \tildering{A}{I}{f}. In the following, when we use the notation A_{\widetilde{Z}} there is always a canonical choice of I and f for Z. In general this notation is justified by the fact that, up to isomorphism, the ring A_{\widetilde{Z}} does not depend on the choice of f and I (Stacks Project, Tag 096V).

Lemma1.6.8
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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Definition 1.1.11
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Lemma 1.6.16
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L∃∀N

Let f \in A be an element and I \subseteq A an ideal.

  1. The map \spec{\tildering{A}{I}{f}} \to \spec{A} induces a homeomorphism onto the generalization of D(f) \cap V(I).

  2. The closed subscheme V(\tildeideal{I}{f}) induces an isomorphism onto D(f) \cap V(I).

  3. If A' is a ring with an element f' \in A' and ideal I' \subseteq A' and A \to A' is a ring map that maps D(f') \cap V(I') into D(f) \cap V(I), then there is a unique A-algebra map \tildering{A}{I}{f} \to \tildering{A'}{I'}{f'} making the obvious diagram with A \to A' commute.

In particular, every point in \spec{\tildering{A}{I}{f}} specializes to a point in V(\tildeideal{I}{f}). (Stacks Project, Tag 096V)

Lean code for Lemma1.6.85 declarations
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.Generalization.range_algebraMap_generalization.{u_1}
      {A : Type u_1} [CommRing A] (f : A) (I : Ideal A) :
      Set.range
          (PrimeSpectrum.comap
            (algebraMap A (WLocalization.Generalization f I))) =
        generalizationHull
          (WLocalization.Generalization.locClosedSubset f I)
    theorem WLocalization.Generalization.range_algebraMap_generalization.{u_1}
      {A : Type u_1} [CommRing A] (f : A)
      (I : Ideal A) :
      Set.range
          (PrimeSpectrum.comap
            (algebraMap A
              (WLocalization.Generalization f
                I))) =
        generalizationHull
          (WLocalization.Generalization.locClosedSubset
            f I)
    The image of `Spec (Generalization f I)` in `Spec A` is equal to
    the generalization hull of `D(f) ∩ V(I)`. 
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.Generalization.bijOn_algebraMap_generalization_specComap_zeroLocus_ideal.{u_1}
      {A : Type u_1} [CommRing A] (f : A) (I : Ideal A) :
      Set.BijOn
        (PrimeSpectrum.comap
          (algebraMap A (WLocalization.Generalization f I)))
        (PrimeSpectrum.zeroLocus (WLocalization.Generalization.ideal f I))
        (WLocalization.Generalization.locClosedSubset f I)
    theorem WLocalization.Generalization.bijOn_algebraMap_generalization_specComap_zeroLocus_ideal.{u_1}
      {A : Type u_1} [CommRing A] (f : A)
      (I : Ideal A) :
      Set.BijOn
        (PrimeSpectrum.comap
          (algebraMap A
            (WLocalization.Generalization f
              I)))
        (PrimeSpectrum.zeroLocus
          (WLocalization.Generalization.ideal
              f I))
        (WLocalization.Generalization.locClosedSubset
          f I)
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.Generalization.submonoid_le.{u_1} {A : Type u_1}
      [CommRing A] {f f' : A} {I I' : Ideal A}
      (h :
        WLocalization.Generalization.locClosedSubset f' I' 
          WLocalization.Generalization.locClosedSubset f I) :
      WLocalization.Generalization.submonoid f I 
        WLocalization.Generalization.submonoid f' I'
    theorem WLocalization.Generalization.submonoid_le.{u_1}
      {A : Type u_1} [CommRing A] {f f' : A}
      {I I' : Ideal A}
      (h :
        WLocalization.Generalization.locClosedSubset
            f' I' 
          WLocalization.Generalization.locClosedSubset
            f I) :
      WLocalization.Generalization.submonoid f
          I 
        WLocalization.Generalization.submonoid
          f' I'
  • defdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    def WLocalization.Generalization.map.{u_1} {A : Type u_1} [CommRing A]
      {f f' : A} {I I' : Ideal A}
      (h :
        WLocalization.Generalization.locClosedSubset f' I' 
          WLocalization.Generalization.locClosedSubset f I) :
      WLocalization.Generalization f I →ₐ[A]
        WLocalization.Generalization f' I'
    def WLocalization.Generalization.map.{u_1}
      {A : Type u_1} [CommRing A] {f f' : A}
      {I I' : Ideal A}
      (h :
        WLocalization.Generalization.locClosedSubset
            f' I' 
          WLocalization.Generalization.locClosedSubset
            f I) :
      WLocalization.Generalization f I →ₐ[A]
        WLocalization.Generalization f' I'
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.Generalization.exists_specializes_zeroLocus_ideal.{u_1}
      {A : Type u_1} [CommRing A] {f : A} (I : Ideal A)
      (x : PrimeSpectrum (WLocalization.Generalization f I)) :
      
        y 
          PrimeSpectrum.zeroLocus (WLocalization.Generalization.ideal f I),
        x  y
    theorem WLocalization.Generalization.exists_specializes_zeroLocus_ideal.{u_1}
      {A : Type u_1} [CommRing A] {f : A}
      (I : Ideal A)
      (x :
        PrimeSpectrum
          (WLocalization.Generalization f
            I)) :
      
        y 
          PrimeSpectrum.zeroLocus
            (WLocalization.Generalization.ideal
                f I),
        x  y
Proof for Lemma 1.6.8
uses 0

Let S = S_{f, I}, Z = D(f) \cap V(I) and J = \tildeideal{I}{f}.

Note that Z is homeomorphic to \spec{(A/I)_f}. Since the map \spec{S^{-1}A} \to \spec{A} is a homeomorphism onto its image, to show (1) it suffices to show that the image is the set of points of \spec{A} specializing to D(f) \cap V(I). Let \mathfrak{p} be a prime of A specializing to a \mathfrak{q} in Z, so I \subseteq \mathfrak{q} and f \not\in \mathfrak{q}. In particular, the image of every element of \mathfrak{p} in (A / I)_f is contained in \mathfrak{q} and is therefore not invertible. Conversely, let \mathfrak{p} be a prime of A which does not specialize to a point in Z, hence the image of \mathfrak{p} in (A/I)_f contains the unit element. So there exists g \in \mathfrak{p} that becomes invertible in (A / I)_f, so g \in S and \mathfrak{p} is not in the image of \spec{S^{-1} A}.

By construction, the induced map S^{-1}A \to (A / I)_f is surjective and hence induces an isomorphism S^{-1}A / J \to (A / I)_f. This shows (2).

Let g \in A. Suppose the image of g in (A' / I')_{f'} is not invertible. Then there exists a prime ideal \mathfrak{p} of (A' / I')_{f'} that contains the image of g. Hence g is in the preimage of \mathfrak{p} under A \to A', which lies in Z by assumption. Thus g is not invertible in (A / I)_f, which shows g \not\in S.

In view of Lemma 1.6.8, given Z = D(f) \cap V(I) we may view Z as a closed subscheme of \spec{A_{\widetilde{Z}}} defined by the ideal V(\tildeideal{I}{f}).

Lemma1.6.9
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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Lemma 1.6.32
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L∃∀N

Let f \in A be an element and I \subseteq A an ideal. Then \tildering{A}{I}{f} is ind-Zariski over A.

Lean code for Lemma1.6.91 theorem
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.Generalization.indZariski.{u_1} {A : Type u_1}
      [CommRing A] (f : A) (I : Ideal A) :
      Algebra.IndZariski A (WLocalization.Generalization f I)
    theorem WLocalization.Generalization.indZariski.{u_1}
      {A : Type u_1} [CommRing A] (f : A)
      (I : Ideal A) :
      Algebra.IndZariski A
        (WLocalization.Generalization f I)
Proof for Lemma 1.6.9

\tildering{A}{I}{f} = S^{-1} A for some multiplicative subset S of A.

Definition1.6.10
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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Lemma 1.6.11
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L∃∀N

Let E, F \subseteq A be subsets. We define Z(E, F) = \bigcap_{f \in E} D(f) \cap \bigcap_{f \in F} V(f).

Lean code for Definition1.6.101 definition
  • defdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    def WLocalization.stratum.{u_1} {A : Type u_1} [CommRing A]
      (E F : Finset A) : Set (PrimeSpectrum A)
    def WLocalization.stratum.{u_1} {A : Type u_1}
      [CommRing A] (E F : Finset A) :
      Set (PrimeSpectrum A)
    The single stratum `Z(E, F)` in `Spec A`. 
Lemma1.6.11
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 A be a ring, E, F \subseteq A finite subsets. Then Z(E, F) = D\left( \prod_{f \in E} f \right) \cap V\left( \sum_{f \in F} fA \right).

Lean code for Lemma1.6.111 theorem
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.stratum_eq_basicOpen_inter_zeroLocus.{u_1} {A : Type u_1}
      [CommRing A] (E F : Finset A) :
      WLocalization.stratum E F =
        (PrimeSpectrum.basicOpen (∏ f  E, f)) 
          PrimeSpectrum.zeroLocus (Ideal.span F)
    theorem WLocalization.stratum_eq_basicOpen_inter_zeroLocus.{u_1}
      {A : Type u_1} [CommRing A]
      (E F : Finset A) :
      WLocalization.stratum E F =
        (PrimeSpectrum.basicOpen
              (∏ f  E, f)) 
          PrimeSpectrum.zeroLocus
            (Ideal.span F)
Proof for Lemma 1.6.11
uses 0

This is immediate from the definitions of D and V.

Lemma1.6.12
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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Lemma 1.6.17
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L∃∀N

Let E_1, F_1, E_2, F_2 \subseteq A be subsets such that E_1 \subseteq E_2 and F_1 \subseteq F_2. Then Z(E_2, F_2) \subseteq Z(E_1, F_1).

Lean code for Lemma1.6.121 theorem
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.stratum_anti.{u_1} {A : Type u_1} [CommRing A]
      {E F E' F' : Finset A} (hEE' : E  E') (hFF' : F  F') :
      WLocalization.stratum E' F'  WLocalization.stratum E F
    theorem WLocalization.stratum_anti.{u_1}
      {A : Type u_1} [CommRing A]
      {E F E' F' : Finset A} (hEE' : E  E')
      (hFF' : F  F') :
      WLocalization.stratum E' F' 
        WLocalization.stratum E F
Proof for Lemma 1.6.12
uses 0

This is immediate from the definition of Z(-, -).

Lemma1.6.13
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 A be a ring, E \subseteq A a finite subset. Then \spec{A} = \coprod_{E = E' \coprod E''} Z(E', E'') set theoretically.

Lean code for Lemma1.6.131 theorem
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.Stratification.Index.iUnion_stratum.{u_1} {A : Type u_1}
      [CommRing A] (E : Finset A) :
       i, WLocalization.stratum i.left i.right = Set.univ
    theorem WLocalization.Stratification.Index.iUnion_stratum.{u_1}
      {A : Type u_1} [CommRing A]
      (E : Finset A) :
       i,
          WLocalization.stratum i.left
            i.right =
        Set.univ
Proof for Lemma 1.6.13
uses 0

Let x \in \spec{A} and E = E' \coprod E'' a disjoint union decomposition. Then x \in Z(E', E'') if and only if f(x) \neq 0 for all f \in E' and f(x) = 0 for all f \in E''. Hence x is contained in Z(\{ f \in E \mid f(x) \neq 0\}, \{f \in E \mid f(x) = 0\}), and this is the only set of this form in which x is contained.

Definition1.6.14
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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Definition 1.1.1
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Definition 1.6.7
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L∃∀N

Let A be a ring and E \subseteq A a finite subset. Let A_{E} = \prod_{E = E' \coprod E''} A_{\widetilde{Z(E', E'')}}. Further, we set I_E \subseteq A_E to be the product of the ideals defining Z(E', E'') in \spec{A_{\widetilde{Z(E', E'')}}}.

Lean code for Definition1.6.142 definitions
  • defdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    def WLocalization.ProdStrata.{u_1} {A : Type u_1} [CommRing A]
      (E : Finset A) : Type u_1
    def WLocalization.ProdStrata.{u_1}
      {A : Type u_1} [CommRing A]
      (E : Finset A) : Type u_1
    The product of the generalizations of `Z(E', E'')` indexed by all disjoint union decompositions
    `E = E' ⨿ E''`. 
  • defdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    def WLocalization.ProdStrata.ideal.{u_1} {A : Type u_1} [CommRing A]
      (E : Finset A) : Ideal (WLocalization.ProdStrata E)
    def WLocalization.ProdStrata.ideal.{u_1}
      {A : Type u_1} [CommRing A]
      (E : Finset A) :
      Ideal (WLocalization.ProdStrata E)
    The ideal of the stratification product, given by the product of the ideals defining
    `Z(E', E'')` in its generalization. 

Note that Definition 1.6.14 is well-defined, because for finite subsets E', E'' of A, the locally closed subset Z(E', E'') is always of the right form by Lemma 1.6.11.

Lemma1.6.15
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 1.1.1
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Let E \subseteq A be a finite subset. Then V(I_E) = \coprod_{E = E' \coprod E''} Z(E', E''). In particular, the map \spec{A_E} \to \spec{A} induces a bijection V(I_E) \to \spec{A}.

Lean code for Lemma1.6.151 theorem
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.ProdStrata.bijOn_algebraMap_specComap_zeroLocus_ideal.{u_1}
      {A : Type u_1} [CommRing A] (E : Finset A) :
      Set.BijOn
        (PrimeSpectrum.comap (algebraMap A (WLocalization.ProdStrata E)))
        (PrimeSpectrum.zeroLocus (WLocalization.ProdStrata.ideal E))
        Set.univ
    theorem WLocalization.ProdStrata.bijOn_algebraMap_specComap_zeroLocus_ideal.{u_1}
      {A : Type u_1} [CommRing A]
      (E : Finset A) :
      Set.BijOn
        (PrimeSpectrum.comap
          (algebraMap A
            (WLocalization.ProdStrata E)))
        (PrimeSpectrum.zeroLocus
          (WLocalization.ProdStrata.ideal E))
        Set.univ
Proof for Lemma 1.6.15

By definition I_E is the product of the ideals defining Z(E', E'') in \spec{A_{\widetilde{Z(E', E'')}}}. Hence the first claim. The second follows from the first, because \spec{A} = \coprod_{E = E' \coprod E''} Z(E', E'') by Lemma 1.6.13.

Lemma1.6.16
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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Definition 1.1.1
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Let E \subseteq A be a finite subset. Then every point of \spec{A_E} specializes to a point in V(I_E). In particular, V(I_E) contains all closed points of \spec{A_E}.

Lean code for Lemma1.6.162 theorems
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.ProdStrata.exists_specializes_zeroLocus_ideal.{u_1}
      {A : Type u_1} [CommRing A] {E : Finset A}
      (x : PrimeSpectrum (WLocalization.ProdStrata E)) :
       y  PrimeSpectrum.zeroLocus (WLocalization.ProdStrata.ideal E),
        x  y
    theorem WLocalization.ProdStrata.exists_specializes_zeroLocus_ideal.{u_1}
      {A : Type u_1} [CommRing A]
      {E : Finset A}
      (x :
        PrimeSpectrum
          (WLocalization.ProdStrata E)) :
      
        y 
          PrimeSpectrum.zeroLocus
            (WLocalization.ProdStrata.ideal
                E),
        x  y
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.ProdStrata.mem_zeroLocus_ideal_of_isClosed.{u_1}
      {A : Type u_1} [CommRing A] {E : Finset A}
      {x : PrimeSpectrum (WLocalization.ProdStrata E)} (hx : IsClosed {x}) :
      x  PrimeSpectrum.zeroLocus (WLocalization.ProdStrata.ideal E)
    theorem WLocalization.ProdStrata.mem_zeroLocus_ideal_of_isClosed.{u_1}
      {A : Type u_1} [CommRing A]
      {E : Finset A}
      {x :
        PrimeSpectrum
          (WLocalization.ProdStrata E)}
      (hx : IsClosed {x}) :
      x 
        PrimeSpectrum.zeroLocus
          (WLocalization.ProdStrata.ideal E)
Proof for Lemma 1.6.16

Let x \in \spec{A_E}. Then x is in \spec{A_{\widetilde{Z(E', E'')}}} for some E = E' \coprod E''. By Lemma 1.6.8, x specializes to a point in Z(E', E'') \subseteq V(I_E). If x is closed, then x specializes to itself, so x \in V(I_E).

Let A be a ring. Given finite subsets E_1, E_2 \subseteq A with E_1 \subseteq E_2, there is a canonical transition map A_{E_1} \to A_{E_2} of A-algebras: For a disjoint union decomposition E_2 = E_2' \coprod E_2'', we set E_1' = E_1 \cap E_2' and E_1'' = E_1 \cap E_2''. By Lemma 1.6.12, we obtain Z(E_2', E_2'') \subseteq Z(E_1', E_1'') and hence by Lemma 1.6.8 a unique A-algebra map A_{\widetilde{Z(E_1', E_1'')}} \to A_{\widetilde{Z(E_2', E_2'')}}.

Lemma1.6.17
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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Lemma 1.6.12
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Let E_1, E_2 \subseteq A be finite subsets with E_1 \subseteq E_2. The induced map \spec{A_{E_2}} \to \spec{A_{E_1}} maps V(I_{E_2}) into V(I_{E_1}).

Lean code for Lemma1.6.171 theorem
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.ProdStrata.mapsTo_map_specComap.{u_1} {A : Type u_1}
      [CommRing A] {E F : Finset A} (h : E  F) :
      Set.MapsTo
        (PrimeSpectrum.comap (WLocalization.ProdStrata.map h).toRingHom)
        (PrimeSpectrum.zeroLocus (WLocalization.ProdStrata.ideal F))
        (PrimeSpectrum.zeroLocus (WLocalization.ProdStrata.ideal E))
    theorem WLocalization.ProdStrata.mapsTo_map_specComap.{u_1}
      {A : Type u_1} [CommRing A]
      {E F : Finset A} (h : E  F) :
      Set.MapsTo
        (PrimeSpectrum.comap
          (WLocalization.ProdStrata.map
              h).toRingHom)
        (PrimeSpectrum.zeroLocus
          (WLocalization.ProdStrata.ideal F))
        (PrimeSpectrum.zeroLocus
          (WLocalization.ProdStrata.ideal E))
Proof for Lemma 1.6.17
uses 0

This holds because for every decomposition E_2 = E_2' \coprod E_2'' and E_1' = E_1 \cap E_2' and E_1'' = E_1 \cap E_2'', the induced map \spec{A_{\widetilde{Z(E_2', E_2'')}}} \to \spec{A_{\widetilde{Z(E_1', E_1'')}}} maps Z(E_2', E_2'') into Z(E_1', E_1'').

Definition1.6.18
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 1.6.12
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L∃∀N

Let I(A) be the partially ordered set of all finite subsets of A. We define the A-algebra A_{w} as A_w = \colim_{E \in I(A)} A_E. We denote by I_w \subseteq A_w the colimit of the ideals I_E, i.e., the union of the images of the I_E in A_w.

Lean code for Definition1.6.183 definitions
  • defdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    def WLocalization.commAlgCat.{u_1} (A : Type u_1) [CommRing A] :
      CommAlgCat A
    def WLocalization.commAlgCat.{u_1}
      (A : Type u_1) [CommRing A] :
      CommAlgCat A
    The w-localization of a ring as an object of `CommAlgCat A` is the colimit over
    the rings `A_E`. 
  • defdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    def WLocalization.{u} (A : Type u) [CommRing A] : Type u
    def WLocalization.{u} (A : Type u)
      [CommRing A] : Type u
    The w-localization of a ring. 
  • defdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    def WLocalization.ideal.{u} (A : Type u) [CommRing A] :
      Ideal (WLocalization A)
    def WLocalization.ideal.{u} (A : Type u)
      [CommRing A] : Ideal (WLocalization A)

By the following lemma, we see that V(I_w) is the inverse limit of the closed subsets V(I_E).

Lemma1.6.19
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_i, I_i)_{i \in I} be a filtered system of rings with ideals. Set A = \colim_{i \in I} A_i and I = \colim_{i \in I} I_i = \colim_{i \in I} I_i A. Then V(I) = \lim_{i \in I} V(I_i) = \bigcap_{i \in I} \text{preimage of } V(I_i) \text{ in } \spec{A}.

Lean code for Lemma1.6.192 theorems
  • theoremdefined in Proetale/Algebra/Preliminaries/Ideal.lean
    complete
    theorem zeroLocus_eq_iInter_specComap_preimage.{u_1, u_2, u_3} {ι : Type u_1}
      [CategoryTheory.Category.{u_2, u_1} ι]
      {F : CategoryTheory.Functor ι CommRingCat}
      {C : CategoryTheory.Limits.Cocone F} { : (i : ι)  Ideal (F.obj i)}
      (I : Ideal C.pt)
      (h : I =  i, Ideal.map (CommRingCat.Hom.hom (C.ι.app i)) ( i)) :
      PrimeSpectrum.zeroLocus I =
         i,
          PrimeSpectrum.comap (CommRingCat.Hom.hom (C.ι.app i)) ⁻¹'
            PrimeSpectrum.zeroLocus ( i)
    theorem zeroLocus_eq_iInter_specComap_preimage.{u_1,
        u_2, u_3}
      {ι : Type u_1}
      [CategoryTheory.Category.{u_2, u_1} ι]
      {F :
        CategoryTheory.Functor ι CommRingCat}
      {C : CategoryTheory.Limits.Cocone F}
      { : (i : ι)  Ideal (F.obj i)}
      (I : Ideal C.pt)
      (h :
        I =
           i,
            Ideal.map
              (CommRingCat.Hom.hom
                (C.ι.app i))
              ( i)) :
      PrimeSpectrum.zeroLocus I =
         i,
          PrimeSpectrum.comap
              (CommRingCat.Hom.hom
                (C.ι.app i)) ⁻¹'
            PrimeSpectrum.zeroLocus ( i)
  • theoremdefined in Proetale/Algebra/Preliminaries/Ideal.lean
    complete
    theorem specComap_preimage_zeroLocus_subset.{u_1, u_2, u_3} {ι : Type u_1}
      [CategoryTheory.Category.{u_2, u_1} ι]
      {F : CategoryTheory.Functor ι CommRingCat}
      (C : CategoryTheory.Limits.Cocone F) {i j : ι} (f : i  j)
      { : (i : ι)  Ideal (F.obj i)}
      (h :  j  Ideal.map (CommRingCat.Hom.hom (F.map f)) ( i)) :
      PrimeSpectrum.comap (CommRingCat.Hom.hom (C.ι.app i)) ⁻¹'
          PrimeSpectrum.zeroLocus ( i) 
        PrimeSpectrum.comap (CommRingCat.Hom.hom (C.ι.app j)) ⁻¹'
          PrimeSpectrum.zeroLocus ( j)
    theorem specComap_preimage_zeroLocus_subset.{u_1,
        u_2, u_3}
      {ι : Type u_1}
      [CategoryTheory.Category.{u_2, u_1} ι]
      {F :
        CategoryTheory.Functor ι CommRingCat}
      (C : CategoryTheory.Limits.Cocone F)
      {i j : ι} (f : i  j)
      { : (i : ι)  Ideal (F.obj i)}
      (h :
         j 
          Ideal.map
            (CommRingCat.Hom.hom (F.map f))
            ( i)) :
      PrimeSpectrum.comap
            (CommRingCat.Hom.hom
              (C.ι.app i)) ⁻¹'
          PrimeSpectrum.zeroLocus ( i) 
        PrimeSpectrum.comap
            (CommRingCat.Hom.hom
              (C.ι.app j)) ⁻¹'
          PrimeSpectrum.zeroLocus ( j)
Lemma1.6.20
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 1.6.23
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L∃∀N

The A-algebra A_{w} is ind-Zariski.

Lean code for Lemma1.6.201 theorem
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.indZariski.{u} (A : Type u) [CommRing A] :
      Algebra.IndZariski A (WLocalization A)
    theorem WLocalization.indZariski.{u} (A : Type u)
      [CommRing A] :
      Algebra.IndZariski A (WLocalization A)
Proof for Lemma 1.6.20
Proof uses 2
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Lemma 1.2.4
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For E \subseteq A a finite subset, the A-algebra A_E is ind-Zariski as a finite product of ind-Zariski algebras by Lemma 1.2.5. Thus A_w is a filtered colimit of ind-Zariski algebras, hence ind-Zariski by Lemma 1.2.4.

Lemma1.6.21
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 be a ring.

  1. The composition V(I_w) \to \spec{A_w} \to \spec{A} is a bijection.

  2. Every point of \spec{A_w} specializes to a unique point of V(I_w).

  3. V(I_w) is the set of closed points of \spec{A_w}.

Lean code for Lemma1.6.213 theorems, 1 incomplete
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.bijOn_algebraMap_specComap_zeroLocus_ideal.{u}
      (A : Type u) [CommRing A] :
      Set.BijOn (PrimeSpectrum.comap (algebraMap A (WLocalization A)))
        (PrimeSpectrum.zeroLocus (WLocalization.ideal A)) Set.univ
    theorem WLocalization.bijOn_algebraMap_specComap_zeroLocus_ideal.{u}
      (A : Type u) [CommRing A] :
      Set.BijOn
        (PrimeSpectrum.comap
          (algebraMap A (WLocalization A)))
        (PrimeSpectrum.zeroLocus
          (WLocalization.ideal A))
        Set.univ
    The composition `V(ideal A) ↪ Spec (WLocalization A) → Spec A` is a bijection. 
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.exists_mem_zeroLocus_ideal_specializes.{u} (A : Type u)
      [CommRing A] (x : PrimeSpectrum (WLocalization A)) :
       y  PrimeSpectrum.zeroLocus (WLocalization.ideal A), x  y
    theorem WLocalization.exists_mem_zeroLocus_ideal_specializes.{u}
      (A : Type u) [CommRing A]
      (x : PrimeSpectrum (WLocalization A)) :
      
        y 
          PrimeSpectrum.zeroLocus
            (WLocalization.ideal A),
        x  y
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    contains sorry
    theorem WLocalization.zeroLocus_ideal_eq.{u} (A : Type u) [CommRing A] :
      PrimeSpectrum.zeroLocus (WLocalization.ideal A) =
        closedPoints (PrimeSpectrum (WLocalization A))
    theorem WLocalization.zeroLocus_ideal_eq.{u}
      (A : Type u) [CommRing A] :
      PrimeSpectrum.zeroLocus
          (WLocalization.ideal A) =
        closedPoints
          (PrimeSpectrum (WLocalization A))
Proof for Lemma 1.6.21

Since V(I_w) = \lim_{E} V(I_E) (by Lemma 1.6.19) and V(I_E) \to \spec{A_E} \to \spec{A} is bijective for all E by Lemma 1.6.15, the first claim holds.

We first show that every point of \spec{A_w} specializes to a point in V(I_w). For this let y \in \spec{A_w}. For all E \subseteq A finite, denote by T_E the closure of the image of y in \spec{A_E}. By Lemma 1.6.16, the intersection T_E \cap V(I_E) is non-empty. Since both T_E and V(I_E) are closed, the intersection is closed and hence quasi-compact. Thus (\lim_E T_E) \cap V(I_w) = \lim_E (T_E \cap V(I_E)) is non-empty. Applying Lemma 1.1.8 and Lemma 1.1.9 to the singleton \{y\}, the subset \lim_E T_E is the closure of y in \spec{A_w}. (This should be a cofiltered version of IsCompact.nonempty_iInter_of_directed_nonempty_isCompact_isClosed.) Hence y specializes to a point in V(I_w).

For uniqueness, suppose y \in \spec{A_w} specializes to z, z' \in V(I_w) with z \neq z'. Denote the images of z, z' in \spec{A} by x, x'. Because V(I_w) \to \spec{A_w} \to \spec{A} is injective, x and x' are distinct. Hence we may assume there exists f \in A such that x \in D(f) and x' \in V(f). Set E = \{f\}. Then \spec{A_E} = \spec{\tildering{A}{0}{f}} \coprod \spec{\tildering{A}{(f)}{1}}. Denote the images of y, z, z' in \spec{A_E} by y_E, x_E, x'_E. Since x_E maps to x \in D(f) and x_E' maps to x' \in V(f), they lie in different components of the disjoint union decomposition above. But since \spec{A_w} \to \spec{A_E} is continuous, y_E specializes to both x_E and x'_E, which is not possible.

Finally, every closed point specializes to itself and is hence contained in V(I_{w}). Conversely, every point in V(I_{w}) specializes only to itself, so it is closed.

Corollary1.6.22
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 1.6.1
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Let A be a ring. The A-algebra A_w is w-local.

Lean code for Corollary1.6.221 theorem, incomplete
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    contains sorry
    theorem WLocalization.isWLocalRing.{u} (A : Type u) [CommRing A] :
      IsWLocalRing (WLocalization A)
    theorem WLocalization.isWLocalRing.{u}
      (A : Type u) [CommRing A] :
      IsWLocalRing (WLocalization A)
Proof for Corollary 1.6.22

Immediate from parts (2) and (3) of Lemma 1.6.21.

Lemma1.6.23
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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The A-algebra A_w is faithfully flat.

Lean code for Lemma1.6.231 theorem
  • theoremdefined in Proetale/Algebra/WLocalization/Basic.lean
    complete
    theorem WLocalization.faithfullyFlat.{u} (A : Type u) [CommRing A] :
      Module.FaithfullyFlat A (WLocalization A)
    theorem WLocalization.faithfullyFlat.{u}
      (A : Type u) [CommRing A] :
      Module.FaithfullyFlat A
        (WLocalization A)
Proof for Lemma 1.6.23
Proof uses 2
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By Lemma 1.6.20, A \to A_w is flat. Because V(I_w) surjects onto \spec{A} by Lemma 1.6.21, in particular \spec{A_w} \to \spec{A} is surjective, hence A \to A_w is faithfully flat.

Lemma1.6.24
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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Let B be an A-algebra and \mathfrak{m} a maximal ideal of A. Let \mathfrak{q} be a prime ideal of B lying over \mathfrak{m} such that \kappa(\mathfrak{q}) is algebraic over \kappa(\mathfrak{m}). Then \mathfrak{q} is maximal. (Stacks Project, Tag 00GA)

Lean code for Lemma1.6.241 theorem
  • theoremdefined in Proetale/Algebra/StalkAlgebraic.lean
    complete
    theorem Ideal.IsMaximal.of_isAlgebraic.{u_1, u_2} {A : Type u_1} {B : Type u_2}
      [CommRing A] [CommRing B] [Algebra A B] (m : Ideal A) [m.IsMaximal]
      (q : Ideal B) [q.IsPrime] [q.LiesOver m]
      [Algebra (Localization.AtPrime m) (Localization.AtPrime q)]
      [Localization.AtPrime.IsLiesOverAlgebra m q]
      [Algebra.IsAlgebraic m.ResidueField q.ResidueField] : q.IsMaximal
    theorem Ideal.IsMaximal.of_isAlgebraic.{u_1, u_2}
      {A : Type u_1} {B : Type u_2}
      [CommRing A] [CommRing B] [Algebra A B]
      (m : Ideal A) [m.IsMaximal]
      (q : Ideal B) [q.IsPrime] [q.LiesOver m]
      [Algebra (Localization.AtPrime m)
          (Localization.AtPrime q)]
      [Localization.AtPrime.IsLiesOverAlgebra
          m q]
      [Algebra.IsAlgebraic m.ResidueField
          q.ResidueField] :
      q.IsMaximal
    If `q` lies over a maximal ideal `m` and the residue field extension is algebraic, `q`
    is maximal. 
Proof for Lemma 1.6.24
uses 0

We have a factorization \kappa(\mathfrak{m}) = A / \mathfrak{m} \to B / \mathfrak{q} \to \kappa(\mathfrak{q}). Hence B / \mathfrak{q} is an intermediate subring of an algebraic field extension and therefore a field.

Lemma1.6.25
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 1.6.31
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L∃∀N

Let A be w-local and I \subseteq A an ideal. Then A_{\widetilde{V(I)}} = A_{\widetilde{(I,1)}} is w-local. If V(I) only contains closed points, then the set of closed points is V(IA_{\widetilde{V(I)}}).

Lean code for Lemma1.6.252 theorems
  • theoremdefined in Proetale/Algebra/WLocalization/Ideal.lean
    complete
    theorem isWLocalRing_generalization_one.{u} {A : Type u} [CommRing A]
      (I : Ideal A) [IsWLocalRing A] :
      IsWLocalRing (WLocalization.Generalization 1 I)
    theorem isWLocalRing_generalization_one.{u}
      {A : Type u} [CommRing A] (I : Ideal A)
      [IsWLocalRing A] :
      IsWLocalRing
        (WLocalization.Generalization 1 I)
  • theoremdefined in Proetale/Algebra/WLocalization/Ideal.lean
    complete
    theorem IsWLocalRing.zeroLocus_map_algebraMap_generalization_one_eq.{u}
      {A : Type u} [CommRing A] {I : Ideal A} [IsWLocalRing A]
      (hI : PrimeSpectrum.zeroLocus I  closedPoints (PrimeSpectrum A)) :
      PrimeSpectrum.zeroLocus
          (Ideal.map (algebraMap A (WLocalization.Generalization 1 I)) I) =
        closedPoints (PrimeSpectrum (WLocalization.Generalization 1 I))
    theorem IsWLocalRing.zeroLocus_map_algebraMap_generalization_one_eq.{u}
      {A : Type u} [CommRing A] {I : Ideal A}
      [IsWLocalRing A]
      (hI :
        PrimeSpectrum.zeroLocus I 
          closedPoints (PrimeSpectrum A)) :
      PrimeSpectrum.zeroLocus
          (Ideal.map
              (algebraMap A
                (WLocalization.Generalization
                  1 I))
              I) =
        closedPoints
          (PrimeSpectrum
            (WLocalization.Generalization 1
              I))
Proof for Lemma 1.6.25
Proof uses 3
Proof dependency previews

By Lemma 1.6.8, \spec{A_{\widetilde{V(I)}}} is homeomorphic to the generalization V(I)^g. Since \spec{A} is w-local and V(I) is closed under specialization, also V(I)^g is closed under specialization by Lemma 1.5.5. Hence \spec{A_{\widetilde{V(I)}}} \cong V(I)^g is w-local by Lemma 1.5.6.

By part (2) of Lemma 1.6.8, we may identify V(IA_{\widetilde{V(I)}}) with V(I). Since every point in \spec{A_{\widetilde{V(I)}}} \cong V(I)^g specializes to a unique point in V(I), the closed subset V(I) contains all closed points of \spec{A_{\widetilde{V(I)}}}. Hence, if V(I) only contains closed points, the closed points are exactly V(I).

Proposition1.6.26
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 1.6.2
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used by 1XL∃∀N

Let A \to B be a ring map with B w-local. There exists a unique factorization A \to A_w \to B such that A_w \to B is w-local. (Stacks Project, Tag 0977)

Definition1.6.27
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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uses 1used by 1XL∃∀N

Let f \colon A \to B be a ring map. We denote by f_w \colon A_w \to B_w the unique w-local ring map compatible with f and the maps A \to A_w and B \to B_w.

Lemma1.6.28
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 1.6.33
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L∃∀N

Let A \to B be a ring map that induces algebraic extensions on residue fields and let I \subseteq A be an ideal, such that V(I) only contains closed points. Then V(IB) only contains closed points.

Lean code for Lemma1.6.281 theorem
  • theoremdefined in Proetale/Algebra/WLocalization/Ideal.lean
    complete
    theorem zeroLocus_map_algebraMap_subset_closedPoints.{u} {A B : Type u}
      [CommRing A] [CommRing B] {I : Ideal A} [Algebra A B]
      (h :
         (m : Ideal A) (q : Ideal B) [inst : q.LiesOver m]
          [inst_1 : m.IsMaximal] [inst_2 : q.IsPrime]
          [inst_3 :
            Algebra (Localization.AtPrime m) (Localization.AtPrime q)]
          [inst_4 : Localization.AtPrime.IsLiesOverAlgebra m q],
          Algebra.IsAlgebraic m.ResidueField q.ResidueField)
      (hI : PrimeSpectrum.zeroLocus I  closedPoints (PrimeSpectrum A)) :
      PrimeSpectrum.zeroLocus (Ideal.map (algebraMap A B) I) 
        closedPoints (PrimeSpectrum B)
    theorem zeroLocus_map_algebraMap_subset_closedPoints.{u}
      {A B : Type u} [CommRing A] [CommRing B]
      {I : Ideal A} [Algebra A B]
      (h :
         (m : Ideal A) (q : Ideal B)
          [inst : q.LiesOver m]
          [inst_1 : m.IsMaximal]
          [inst_2 : q.IsPrime]
          [inst_3 :
            Algebra (Localization.AtPrime m)
              (Localization.AtPrime q)]
          [inst_4 :
            Localization.AtPrime.IsLiesOverAlgebra
              m q],
          Algebra.IsAlgebraic m.ResidueField
            q.ResidueField)
      (hI :
        PrimeSpectrum.zeroLocus I 
          closedPoints (PrimeSpectrum A)) :
      PrimeSpectrum.zeroLocus
          (Ideal.map (algebraMap A B) I) 
        closedPoints (PrimeSpectrum B)
Proof for Lemma 1.6.28

By Lemma 1.6.24, every prime of B lying above a maximal ideal of A is maximal. In particular, the closed subset V(I B) = (\spec{B} \to \spec{A})^{-1}(V(I)) only contains closed points.

Lemma1.6.29
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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uses 0used by 1L∃∀N

Let B be an A-algebra satisfying going down. If every closed point of \spec{A} has a preimage in \spec{B}, the map \spec{B} \to \spec{A} is surjective.

Lean code for Lemma1.6.291 theorem
  • theoremdefined in Proetale/Mathlib/RingTheory/Ideal/GoingDown.lean
    complete
    theorem Algebra.HasGoingDown.specComap_surjective_of_closedPoints_subset_preimage.{u_1,
        u_2}
      {R : Type u_1} {S : Type u_2} [CommRing R] [CommRing S] [Algebra R S]
      [Algebra.HasGoingDown R S]
      (h :
        closedPoints (PrimeSpectrum R) 
          Set.range (PrimeSpectrum.comap (algebraMap R S))) :
      Function.Surjective (PrimeSpectrum.comap (algebraMap R S))
    theorem Algebra.HasGoingDown.specComap_surjective_of_closedPoints_subset_preimage.{u_1,
        u_2}
      {R : Type u_1} {S : Type u_2}
      [CommRing R] [CommRing S] [Algebra R S]
      [Algebra.HasGoingDown R S]
      (h :
        closedPoints (PrimeSpectrum R) 
          Set.range
            (PrimeSpectrum.comap
              (algebraMap R S))) :
      Function.Surjective
        (PrimeSpectrum.comap (algebraMap R S))
Proof for Lemma 1.6.29
uses 0

Let \mathfrak{p} be a prime of A. Then it is contained in a maximal ideal \mathfrak{m} that is the preimage of a prime \mathfrak{q} of B. By going-down, there exists therefore a prime \mathfrak{q}' \subseteq \mathfrak{q} lying above \mathfrak{p}.

Definition1.6.30
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 1.6.7
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L∃∀N

Let A be a ring and I \subseteq A an ideal. We define the w-localization of A with respect to I to be the ring (A_w)_{\widetilde{V(IA_{w})}}, denoted by A_{w,I}.

Lean code for Definition1.6.301 definition
  • defdefined in Proetale/Algebra/WLocalization/Ideal.lean
    complete
    def Ideal.WLocalization.{u} {A : Type u} [CommRing A] (I : Ideal A) : Type u
    def Ideal.WLocalization.{u} {A : Type u}
      [CommRing A] (I : Ideal A) : Type u
    The w-localization with respect to an ideal `I`.
    
Lemma1.6.31
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 1.6.1
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used by 1L∃∀N

Let A be a ring and I \subseteq A an ideal. The ring A_{w,I} is w-local.

Lean code for Lemma1.6.312 theorems
  • theoremdefined in Proetale/Algebra/WLocalization/Ideal.lean
    complete
    theorem isWLocalRing_generalization_one.{u} {A : Type u} [CommRing A]
      (I : Ideal A) [IsWLocalRing A] :
      IsWLocalRing (WLocalization.Generalization 1 I)
    theorem isWLocalRing_generalization_one.{u}
      {A : Type u} [CommRing A] (I : Ideal A)
      [IsWLocalRing A] :
      IsWLocalRing
        (WLocalization.Generalization 1 I)
  • theoremdefined in Proetale/Algebra/WLocalization/Ideal.lean
    complete
    theorem Ideal.WLocalization.isWLocalRing.{u} {A : Type u} [CommRing A]
      {I : Ideal A} : IsWLocalRing I.WLocalization
    theorem Ideal.WLocalization.isWLocalRing.{u}
      {A : Type u} [CommRing A]
      {I : Ideal A} :
      IsWLocalRing I.WLocalization
Proof for Lemma 1.6.31
Proof uses 2
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Corollary 1.6.22
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Follows immediately from Lemma 1.6.25 and Corollary 1.6.22.

Lemma1.6.32
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 1.2.1
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Lemma 1.6.35
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L∃∀N

Let A be a ring and I \subseteq A an ideal. The ring map A \to A_{w, I} is ind-Zariski.

Lean code for Lemma1.6.321 theorem
  • theoremdefined in Proetale/Algebra/WLocalization/Ideal.lean
    complete
    theorem Ideal.WLocalization.indZariski.{u} {A : Type u} [CommRing A]
      {I : Ideal A} : Algebra.IndZariski A I.WLocalization
    theorem Ideal.WLocalization.indZariski.{u}
      {A : Type u} [CommRing A]
      {I : Ideal A} :
      Algebra.IndZariski A I.WLocalization
Proof for Lemma 1.6.32
Proof uses 3
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Lemma 1.2.6
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The map A \to A_w is ind-Zariski by Lemma 1.6.20 and the map A_w \to A_{w, I} is ind-Zariski by Lemma 1.6.9. Hence the composition A \to A_{w, I} is ind-Zariski by Lemma 1.2.6.

Lemma1.6.33
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 1.6.34
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L∃∀N

Let I \subseteq A be an ideal such that V(I) \subseteq \spec{A} only contains closed points. Then

  1. the set of closed points of \spec{A_{w, I}} is V(IA_{w, I}), and

  2. the map A / I \to A_{w, I} / I A_{w, I} is an isomorphism.

Lean code for Lemma1.6.333 theorems
  • theoremdefined in Proetale/Algebra/WLocalization/Ideal.lean
    complete
    theorem Ideal.WLocalization.zeroLocus_map_algebraMap_eq_closedPoints.{u}
      {A : Type u} [CommRing A] {I : Ideal A}
      (hI : PrimeSpectrum.zeroLocus I  closedPoints (PrimeSpectrum A)) :
      PrimeSpectrum.zeroLocus
          (Ideal.map (algebraMap A I.WLocalization) I) =
        closedPoints (PrimeSpectrum I.WLocalization)
    theorem Ideal.WLocalization.zeroLocus_map_algebraMap_eq_closedPoints.{u}
      {A : Type u} [CommRing A] {I : Ideal A}
      (hI :
        PrimeSpectrum.zeroLocus I 
          closedPoints (PrimeSpectrum A)) :
      PrimeSpectrum.zeroLocus
          (Ideal.map
              (algebraMap A I.WLocalization)
              I) =
        closedPoints
          (PrimeSpectrum I.WLocalization)
  • theoremdefined in Proetale/Algebra/WLocalization/Ideal.lean
    complete
    theorem Ideal.WLocalization.quotientMap_algebraMap_bijective.{u} {A : Type u}
      [CommRing A] (I : Ideal A)
      (hI : PrimeSpectrum.zeroLocus I  closedPoints (PrimeSpectrum A)) :
      Function.Bijective
        (Ideal.quotientMap (Ideal.map (algebraMap A I.WLocalization) I)
            (algebraMap A I.WLocalization) )
    theorem Ideal.WLocalization.quotientMap_algebraMap_bijective.{u}
      {A : Type u} [CommRing A] (I : Ideal A)
      (hI :
        PrimeSpectrum.zeroLocus I 
          closedPoints (PrimeSpectrum A)) :
      Function.Bijective
        (Ideal.quotientMap
            (Ideal.map
              (algebraMap A I.WLocalization)
              I)
            (algebraMap A I.WLocalization) )
    [Stacks Tag 097A](https://stacks.math.columbia.edu/tag/097A) ((2)(a))
  • theoremdefined in Proetale/Algebra/WLocalization/Ideal.lean
    complete
    theorem Ideal.WLocalization.bijOn_zeroLocus_map.{u} {A : Type u} [CommRing A]
      (I : Ideal A)
      (hI : PrimeSpectrum.zeroLocus I  closedPoints (PrimeSpectrum A)) :
      Set.BijOn (PrimeSpectrum.comap (algebraMap A I.WLocalization))
        (PrimeSpectrum.zeroLocus
          (Ideal.map (algebraMap A I.WLocalization) I))
        (PrimeSpectrum.zeroLocus I)
    theorem Ideal.WLocalization.bijOn_zeroLocus_map.{u}
      {A : Type u} [CommRing A] (I : Ideal A)
      (hI :
        PrimeSpectrum.zeroLocus I 
          closedPoints (PrimeSpectrum A)) :
      Set.BijOn
        (PrimeSpectrum.comap
          (algebraMap A I.WLocalization))
        (PrimeSpectrum.zeroLocus
          (Ideal.map
              (algebraMap A I.WLocalization)
              I))
        (PrimeSpectrum.zeroLocus I)

A \to A_w is ind-Zariski by Lemma 1.6.20 and therefore induces algebraic extensions of residue fields by Lemma 1.2.2. In particular, the closed subset V(I A_w) only contains closed points by Lemma 1.6.28 and the first claim follows from Lemma 1.6.25.

The map A \to A_{w, I} is ind-Zariski as a composition of ind-Zariski maps by Lemma 1.6.20 and Lemma 1.6.9. Since ind-Zariski is stable under base change, the same holds for A/I \to A_{w, I}/IA_{w, I}. Hence it identifies local rings. By part (2) of Lemma 1.6.8, V(IA_{w, I}) \to V(I A_w) is an isomorphism. Moreover V(I A_{w}) \to V(I) is a bijection by part (1) of Lemma 1.6.21 because V(I A_{w}) \subseteq V(I_w). Hence, the composition \spec{A_{w, I} / I A_{w, I}} = V(I A_{w, I}) \to V(I A_w) \to V(I) is bijective. Thus A / I \to A_{w, I} / I A_{w, I} is an isomorphism by Lemma 1.6.5.

Lemma1.6.34
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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Proposition 1.7.15
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L∃∀N

Let A be a w-local ring. Let I \subset A be an ideal cutting out the set X^c of closed points in X = \Spec (A). Let A \to B be a ring map inducing algebraic extensions on residue fields at primes. Let C be the ring B_{w, IB} constructed in Definition 1.6.30. Then

  1. B/IB \to C/IC is an isomorphism,

  2. C is w-local,

  3. the map p \colon Y = \Spec (C) \to X induced by the ring map A \to B \to C is w-local, and

  4. p^{-1}(X^c) is the set of closed points of Y, i.e. V(IC) is the set of closed points of Y.

(Stacks Project, Tag 097A, (2))

Lean code for Lemma1.6.341 theorem
  • theoremdefined in Proetale/Algebra/WLocalization/Ideal.lean
    complete
    theorem Ideal.WLocalization.algebraMap_specComap_preimage_closedPoints_eq.{u}
      {A B : Type u} [CommRing A] [CommRing B] {I : Ideal A}
      [IsWLocalRing A] [Algebra A B]
      (hI : PrimeSpectrum.zeroLocus I = closedPoints (PrimeSpectrum A))
      (h :
         (m : Ideal A) (q : Ideal B) [inst : q.LiesOver m]
          [inst_1 : m.IsMaximal] [inst_2 : q.IsPrime]
          [inst_3 :
            Algebra (Localization.AtPrime m) (Localization.AtPrime q)]
          [inst_4 : Localization.AtPrime.IsLiesOverAlgebra m q],
          Algebra.IsAlgebraic m.ResidueField q.ResidueField) :
      PrimeSpectrum.zeroLocus
          (Ideal.map
              (algebraMap A (Ideal.map (algebraMap A B) I).WLocalization)
              I) =
        closedPoints
          (PrimeSpectrum (Ideal.map (algebraMap A B) I).WLocalization)
    theorem Ideal.WLocalization.algebraMap_specComap_preimage_closedPoints_eq.{u}
      {A B : Type u} [CommRing A] [CommRing B]
      {I : Ideal A} [IsWLocalRing A]
      [Algebra A B]
      (hI :
        PrimeSpectrum.zeroLocus I =
          closedPoints (PrimeSpectrum A))
      (h :
         (m : Ideal A) (q : Ideal B)
          [inst : q.LiesOver m]
          [inst_1 : m.IsMaximal]
          [inst_2 : q.IsPrime]
          [inst_3 :
            Algebra (Localization.AtPrime m)
              (Localization.AtPrime q)]
          [inst_4 :
            Localization.AtPrime.IsLiesOverAlgebra
              m q],
          Algebra.IsAlgebraic m.ResidueField
            q.ResidueField) :
      PrimeSpectrum.zeroLocus
          (Ideal.map
              (algebraMap A
                (Ideal.map (algebraMap A B)
                    I).WLocalization)
              I) =
        closedPoints
          (PrimeSpectrum
            (Ideal.map (algebraMap A B)
                I).WLocalization)
    [Stacks Tag 097A](https://stacks.math.columbia.edu/tag/097A) ((2)(d))

Every point of V(IB) is a closed point of \spec{B} by Lemma 1.6.28. Hence we may choose C = (B_w)_{\widetilde{V(IB_w)}} as in Definition 1.6.30. Part (1) holds by Lemma 1.6.33. Part (2) holds by Lemma 1.6.31. Then the set of closed points of \spec{C} is V(IC), which is the preimage of V(I) under \spec{C} \to \spec{B} \to \spec{A}.

Lemma1.6.35
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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L∃∀N

Let A be a w-local ring. Let I \subset A be an ideal cutting out the set X^c of closed points in X = \Spec (A). Let B be a faithfully flat A-algebra. Let C be the ring B_{w, IB} constructed in Definition 1.6.30. Then the composition A \to B \to C is also faithfully flat.

Lean code for Lemma1.6.351 theorem
  • theoremdefined in Proetale/Algebra/WLocalization/Ideal.lean
    complete
    theorem Ideal.WLocalization.faithfullyFlat_map_algebraMap.{u} {A B : Type u}
      [CommRing A] [CommRing B] {I : Ideal A} [IsWLocalRing A] [Algebra A B]
      [Module.FaithfullyFlat A B]
      (hI : PrimeSpectrum.zeroLocus I = closedPoints (PrimeSpectrum A))
      (h :
         (m : Ideal A) (q : Ideal B) [inst : q.LiesOver m]
          [inst_1 : m.IsMaximal] [inst_2 : q.IsPrime]
          [inst_3 :
            Algebra (Localization.AtPrime m) (Localization.AtPrime q)]
          [inst_4 : Localization.AtPrime.IsLiesOverAlgebra m q],
          Algebra.IsAlgebraic m.ResidueField q.ResidueField) :
      Module.FaithfullyFlat A (Ideal.map (algebraMap A B) I).WLocalization
    theorem Ideal.WLocalization.faithfullyFlat_map_algebraMap.{u}
      {A B : Type u} [CommRing A] [CommRing B]
      {I : Ideal A} [IsWLocalRing A]
      [Algebra A B]
      [Module.FaithfullyFlat A B]
      (hI :
        PrimeSpectrum.zeroLocus I =
          closedPoints (PrimeSpectrum A))
      (h :
         (m : Ideal A) (q : Ideal B)
          [inst : q.LiesOver m]
          [inst_1 : m.IsMaximal]
          [inst_2 : q.IsPrime]
          [inst_3 :
            Algebra (Localization.AtPrime m)
              (Localization.AtPrime q)]
          [inst_4 :
            Localization.AtPrime.IsLiesOverAlgebra
              m q],
          Algebra.IsAlgebraic m.ResidueField
            q.ResidueField) :
      Module.FaithfullyFlat A
        (Ideal.map (algebraMap A B)
            I).WLocalization
Proof for Lemma 1.6.35
Proof uses 4
Proof dependency previews

By Lemma 1.2.3 and Lemma 1.6.32, A \to B \to C is flat, so by Lemma 1.6.29 it suffices to show every closed point of \spec{A} is in the image. But since \spec{B} \to \spec{A} is surjective, also V(IC) \cong V(IB) \to V(I) is surjective (the first map is an isomorphism by part (2) of Lemma 1.6.33).