Resolution of Singularities in Characteristic Zero

4.3. Analytic manifolds and analytic spaces🔗

Lean names in this section are shown relative to AnalyticSpace, Manifold, AnalyticManifold, TopologicalSpace, Topology, Filter, Set.

The theorems of this chapter are the analytic resolution theorems of the library: Hironaka's principalization on a real-analytic manifold, Bierstone–Milman's principalization with the Jacobian condition, Włodarczyk's functorial principalization on a real- or complex-analytic manifold, Włodarczyk's embedded desingularization of a closed subspace of a real- or complex-analytic manifold, the resolution of real- and complex-analytic spaces, and its form for a single reduced complex space.

On a manifold that is not compact no finite succession of blow-ups can be expected to resolve everything, and Hironaka states his global analytic theorems I′(n), II′(N), I″(n) and II″(N) for locally finite successions indexed by a countable well-ordered set. The library uses instead the form of Włodarczyk's locally finite principalization [Wlo09, Theorem 2.0.3]: a proper analytic map \sigma \colon \tilde M \to M which, over an open neighbourhood U_K of every compact K \subseteq M, is a finite succession of blow-ups, the successions for different compacts being compatible (Definition 3.2.2.2). Over a compact manifold, K = M gives one finite succession.

The objects of the manifold statements are analytic manifolds (Definition 3.1.1.1), locally finitely generated ideal sheaves of analytic functions (Definition 3.1.3.3), finite successions of monoidal transformations with their weak transforms, boundaries and strict transforms (Definition 3.2.1.1, Definition 3.2.1.3), and simple normal crossings boundaries (Definition 3.1.5.1), described in the part on analytic geometry. The section of each theorem states, before the theorem, the properties it asserts; a property asserted by several theorems is stated at the first of them in this chapter.

4.3.1. Main Theorem II″(N), compatible-family form🔗

Main Theorem II″(N) principalizes an ideal sheaf on a real-analytic manifold over every compact subset. Its conditions on boundaries and orders are used again by Bierstone–Milman's principalization and by Włodarczyk's theorems.

Hironaka's conditions on boundaries and orders.

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Hironaka's conditions on boundaries and orders, for a finite succession S of an analytic manifold M with centres C_i; they are the analytic forms of those of the algebraic theorems (Definition 4.2.1.1, Definition 4.2.3.1):

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Let X be a real-analytic manifold, E_0 an ideal sheaf with only normal crossings (the reduced divisor of a simple normal crossings family of smooth hypersurfaces of X), and J_0 an ideal sheaf with nonzero stalks. Then there are a real-analytic manifold \tilde X and a proper analytic map \sigma \colon \tilde X \to X which, over an open neighbourhood U_K of every compact K \subseteq X and compatibly across compacts, is a finite succession of monoidal transformations U_K = U_0 \leftarrow U_1 \leftarrow \cdots \leftarrow U_r with centres D_i, such that for every compact K:

  1. every centre D_i is non-singular;

  2. the order of the weak transform J_i of J_0|_{U_K} is a positive constant along D_i;

  3. the boundary E_i (E_0|_{U_K} and then E_{i+1} = \mathrm{red}(\sigma_{i+1}^{-1}(E_i) \cup \sigma_{i+1}^{-1}(D_i))) has only normal crossings with D_i;

  4. E_r has only normal crossings and J_r = \mathcal{O}_{U_r}.

The conclusion is that of Hironaka's Main Theorem II′(N) (p. 156). This is the real-analytic counterpart of Main Theorem II(N) (Theorem 4.2.2.1): principalize a nonzero ideal sheaf by blowing up non-singular centres along which the order of the weak transform is a positive constant, keeping the boundary in simple normal crossings. The constant of (2) may depend on K and i. For J_0 = \mathcal{O}_X condition (2) leaves every centre empty, and \sigma is then an isomorphism.

Condition (2) is HasConstantPositiveOrderAlongCenters and condition (3) with the first half of (4) HasSncBoundaries (Definition 4.3.1.1).

Lean code for Theorem4.3.1.2●1 theorem
Proof for Theorem 4.3.1.2
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The hypothesis hE₀ provides coordinates E \cong \mathbb{R}^n and a simple normal crossings hypersurface family whose reduced ideal sheaf is E_0. Transporting X, J_0 and the family along the coordinates gives a triple on the standard model, to which the order-reduction algorithm is applied: the real-analytic form of the blow-up sequence functors of [Kol07, Theorem 103] and, for marked ideals, [Kol07, Theorem 107], assembled functorially over the exhaustion of the manifold by compacts. This yields a compatible family with proper blow-down satisfying the conditions on the model, and each condition is carried back along the coordinates.

4.3.2. Real-analytic principalization with the Jacobian condition🔗

Theorem4.3.2.1source[BM97, Theorem 1.10] ↗
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Let M be a real-analytic manifold modelled on a finite-dimensional space and I an ideal sheaf with nonzero stalks. Then there is a proper analytic map \sigma \colon \tilde M \to M which, over a neighbourhood U_K of every compact K and compatibly across compacts, is a finite succession of blow-ups U_K = U_0 \leftarrow \cdots \leftarrow U_r with non-singular centres, such that

  • for every compact K, the exceptional divisors E_i (the boundaries started at the empty boundary) have simple normal crossings with the centres, the last one E_r has simple normal crossings, and the weak transform of I|_{U_K} on U_r is the unit ideal;

  • the pull-back \sigma^* I is a normal-crossings divisor;

  • the product of the Jacobian ideal of \sigma with \sigma^* I is a normal-crossings divisor;

  • \sigma is an analytic isomorphism over the complement of the support of I.

The Jacobian condition is what the change of variables in an integral needs: in local coordinates on \tilde M both I \circ \sigma and the Jacobian determinant of \sigma are monomials times units. The conditions on the successions are stated over each compact; the normal-crossings, Jacobian and isomorphism conditions, being local on \tilde M, are stated on \sigma itself. For I = \mathcal{O}_M the support of I is empty, so \sigma is an isomorphism.

The condition on the exceptional divisors is HasSncBoundaries ⊤ (Definition 4.3.1.1). The additional vocabulary is the Jacobian ideal, analytic isomorphisms over a set and normal-crossings divisors (Definition 3.1.6.1, Definition 3.1.5.1).

Lean code for Theorem4.3.2.1●1 theorem
Proof for Theorem 4.3.2.1
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As for Theorem 4.3.1.2: the model space is identified with \mathbb{R}^n, I is transported to the model, where it forms a triple with the empty boundary family, and the order-reduction algorithm yields a compatible family whose blow-down is proper, whose successions satisfy the conditions of Main Theorem II″(N) for the empty boundary, and whose last weak transform is the unit ideal. The normal-crossings, Jacobian and isomorphism conditions are proved for that family and carried back along the identification.

4.3.3. Functorial principalization on analytic manifolds🔗

Włodarczyk's functorial principalization assigns a compatible family of finite successions (Definition 3.2.2.2) to every ideal sheaf on every manifold, modelled on any finite-dimensional space, by one rule. It asserts of each value that it is a locally finite principalization, and of the rule that it commutes with local analytic isomorphisms, over the compact sets and globally (every local analytic isomorphism lifts to the spaces of the values), and with closed embeddings of analytic manifolds, over the compact sets.

Locally finite principalization.

Definition4.3.3.1source[Wlo09, Theorem 2.0.3 (1)–(3)] ↗
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I.IsLocallyFinitelyPrincipalizedBy F, for an ideal sheaf I on M and a compatible family F with map \mathrm{prin}_I, says that F is a locally finite principalization of I in Włodarczyk's sense. Write F.seq K for the succession over the neighbourhood U_K of a compact K, with composite \sigma_K and exceptional divisors E_i. Then \mathrm{prin}_I is proper; every U_K has compact closure; the centres are smooth; (F.seq K).HasSncBoundaries ⊤ (Definition 4.3.1.1); the total transform \sigma_K^*(I|_{U_K}) is near every point the ideal of an effective combination of the components of E_r (IsMulBoundaryMonomial with the unit ideal as its first factor, Definition 3.1.5.2); the total transform \mathrm{prin}_I^*(I) is a normal-crossings divisor; and \mathrm{prin}_I is an isomorphism off the support of I [Wlo09, Theorem 2.0.3 (1)–(3); Lemma 4.0.3]. It is the analogue on manifolds of Triple.IsPrincipalizedBy on schemes (Definition 4.2.4.2), with properness and the compact closures of the U_K added and each condition stated over every compact.

Lean code for Definition4.3.3.1●1 definition
  • structure(7 fields)defined in Hironaka/Manifold/Resolution/Defs.lean
    complete
    structure IdealSheaf.IsLocallyFinitelyPrincipalizedBy {𝕜 : Type} [RCLike 𝕜]
      {E : Type u_1} [NormedAddCommGroup E] [NormedSpace 𝕜 E]
      {M : AnalyticManifold 𝕜 E} (I : M.IdealSheaf)
      (F : M.ExtensionCompatibleFamily) : Prop
    structure IdealSheaf.IsLocallyFinitelyPrincipalizedBy
      {𝕜 : Type} [RCLike 𝕜] {E : Type u_1}
      [NormedAddCommGroup E] [NormedSpace 𝕜 E]
      {M : AnalyticManifold 𝕜 E}
      (I : M.IdealSheaf)
      (F : M.ExtensionCompatibleFamily) : Prop

    The compatible family F is a locally finite principalization of the ideal sheaf I on M [Wlo09, Theorem 2.0.3 (1)–(3)]: with prin_I = F.map : M̃ → M, and over the neighbourhood U_K of a compact K ⊆ M the succession F.seq K : U_K = U_0 ← U_1 ← ⋯ ← U_r with composite σ_K : U_r → U_K and exceptional divisors E_i (the boundaries (F.seq K).boundarySeq ⊤ i started at the empty boundary ⊤): prin_I is proper; every U_K has compact closure; the centres C_i are smooth; every E_i has simple normal crossings with C_i and E_r has simple normal crossings; the total transform σ_K^*(I|_{U_K}) is, near every point, the ideal of an effective combination of the components of E_r through it; the total transform prin_I^*(I) on M̃ is a normal-crossings divisor; and prin_I is an isomorphism over the complement of the support of I. The compatibility of the successions over two compacts is a field of ExtensionCompatibleFamily. The analogue on manifolds of Triple.IsPrincipalizedBy on schemes, whose fields are Kollár's clauses (1)–(3) for a single blow-up sequence.

    isProperMap : IsProperMap ⇑F.map

    prin_I : M̃ → M is proper (the preamble of Theorem 2.0.3).

    isCompact_closure_nhd : ∀ (K : Compacts M.carrier), IsCompact (closure ↑(F.nhd K))

    Every neighbourhood U_K has compact closure. Then g(U_K) lies in a compact for every analytic map g out of M, so that the commutation with local analytic isomorphisms (IdealSheafPair.CommutesWithLocalAnalyticIsomorphisms) applies to every compact of M.

    isNonsingular_center : ∀ (K : Compacts M.carrier) (i : Fin (F.seq K).length), ((F.seq K).center i).IsNonsingular

    (1) The centres C_i are smooth (automatic for every finite succession, FiniteSuccession.center_isNonsingular).

    hasSncBoundaries : ∀ (K : Compacts M.carrier), (F.seq K).HasSncBoundaries ⊤

    (2) Every exceptional divisor E_i has simple normal crossings with the centre C_i, and E_r has simple normal crossings.

    isMulBoundaryMonomial_pullback : ∀ (K : Compacts M.carrier),
      IdealSheaf.IsMulBoundaryMonomial (IdealSheaf.pullback ⇑(F.seq K).composite ⋯ (I.restrict (F.nhd K))) ⊤
        ((F.seq K).boundarySeq ⊤ (Fin.last (F.seq K).length))

    (3) The total transform σ_K^*(I|_{U_K}) is the ideal of a divisor Ẽ_U which near every point is an effective combination of the components of E_r (IsMulBoundaryMonomial with the unit ideal ⊤ as its first factor).

    isNormalCrossingsDivisor_pullback : IdealSheaf.IsNormalCrossingsDivisor (IdealSheaf.pullback ⇑F.map ⋯ I)

    (3) on M̃: the total transform prin_I^*(I) is a normal-crossings divisor.

    isIsoOver_compl_support : F.map.IsIsoOver (IdealSheaf.support I)ᶜ

    prin_I is an isomorphism over the complement of the support of I ([Wlo09, Lemma 4.0.3]; [Kol07, Theorem 35 (3)]).

The inputs: pairs of a manifold and an ideal sheaf.

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A pair (M, I) is an analytic manifold M over \mathbb{K}, modelled on a finite-dimensional space E, with an ideal sheaf I on M with nonzero stalks: the input of Włodarczyk's principalization, Kollár's triple (X, I, \emptyset) with an empty boundary (Definition 4.2.4.1). The model space is part of the input, so that one assignment covers the manifolds of every dimension. T.pullback g hg is the pair (N, g^* I) over a local analytic isomorphism g \colon N \to M of manifolds modelled on one space, the source of the morphism of pairs over g [Wlo09, Theorem 3.5.1].

Lean code for Definition4.3.3.2●2 definitions

Compatible family assignments.

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A type C of inputs has underlying manifolds when every input has an underlying analytic manifold over \mathbb{K} on a finite-dimensional model space (HasUnderlyingManifold, the counterpart of HasUnderlyingScheme, Definition 2.4.4.1); the pairs are such a type. A compatible family assignment on C (CompatibleFamilyAssignment) is a compatible family of finite successions of blow-ups (Definition 3.2.2.2) of the underlying manifold of every input: Włodarczyk's \mathrm{prin} and \mathrm{res} as rules on all pairs, the counterpart on manifolds of a blow-up sequence functor [Kol07, Definition 31]. Its properties are the clauses of the functorial theorems.

Lean code for Definition4.3.3.3●3 definitions
  • class(5 methods)defined in Hironaka/Manifold/Resolution/Defs.lean
    complete
    class HasUnderlyingManifold (C : Type w) (𝕜 : outParam Type) [RCLike 𝕜] :
      Type (max w (u + 1) (v + 1))
    class HasUnderlyingManifold (C : Type w)
      (𝕜 : outParam Type) [RCLike 𝕜] :
      Type (max w (u + 1) (v + 1))

    A type C of inputs each with an underlying analytic manifold over 𝕜, modelled on a finite-dimensional space (the analogue of HasUnderlyingScheme for the algebraic inputs).

    model : C → Type v

    The model space of the underlying manifold of an input.

    normedAddCommGroup : (T : C) → NormedAddCommGroup (HasUnderlyingManifold.model T)
    normedSpace : (T : C) → NormedSpace 𝕜 (HasUnderlyingManifold.model T)
    finiteDimensional : ∀ (T : C), FiniteDimensional 𝕜 (HasUnderlyingManifold.model T)
    manifold : (T : C) → AnalyticManifold 𝕜 (HasUnderlyingManifold.model T)

    The underlying manifold of an input.

  • complete
    abbrev CompatibleFamilyAssignment {𝕜 : Type} [RCLike 𝕜] (C : Type w)
      [HasUnderlyingManifold C 𝕜] : Type (max (max (u + 1) v) w) :=
      (T : C) → (HasUnderlyingManifold.manifold T).ExtensionCompatibleFamily
    abbrev CompatibleFamilyAssignment {𝕜 : Type}
      [RCLike 𝕜] (C : Type w)
      [HasUnderlyingManifold C 𝕜] :
      Type (max (max (u + 1) v) w) :=
      (T : C) →
        (HasUnderlyingManifold.manifold
            T).ExtensionCompatibleFamily

    A compatible family assignment on the inputs C: a compatible family of finite successions of blow-ups (ExtensionCompatibleFamily) of the underlying manifold of every input — Włodarczyk's prin and res as rules on all pairs, the counterpart on manifolds of a blow-up sequence functor (BlowUpSequenceFunctor, [Kol07, Definition 31]). Its properties are the clauses of the functorial theorems.

  • def IdealSheafPair.hasUnderlyingManifold {𝕜 : Type} [RCLike 𝕜] :
      HasUnderlyingManifold (IdealSheafPair 𝕜) 𝕜
    def IdealSheafPair.hasUnderlyingManifold
      {𝕜 : Type} [RCLike 𝕜] :
      HasUnderlyingManifold (IdealSheafPair 𝕜)
        𝕜

The push-forward of an ideal sheaf along a closed embedding.

Definition4.3.3.4source[Kol07, 34.3] ↗
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I.IsPushforwardAlong J τ, for ideal sheaves I on M and J on N and an analytic map \tau \colon N \to M between analytic manifolds modelled on E' and on E: \tau is a closed embedding of analytic manifolds, that is an analytic embedding (Mathlib's IsSmoothEmbedding at \omega: an analytic immersion, of the form u \mapsto (u, 0) in suitable charts near every point, which is a homeomorphism onto its image) with closed image; the ideal of its image is contained in I (at every point, the vanishing ideal of \tau(N), Definition 3.1.3.4, lies in the stalk of I); and J = \tau^* I. So N carries the analytic structure induced on the closed subset \tau(N) (every analytic map with values in \tau(N) factors analytically through \tau; for a finite-dimensional model of M, \tau(N) is a closed submanifold, Definition 3.1.4.1), and the closed subspace of I lies in \tau(N) and is the image of the closed subspace of J: Kollár's "closed embedding of smooth schemes" and his hypothesis \mathcal{O}_X / I_X = j_*(\mathcal{O}_Y / I_Y) on the ideal sheaves in his commutation with closed embeddings, for an empty boundary; the embedding of Włodarczyk's "embeddings of ambient varieties".

Lean code for Definition4.3.3.4●1 definition

Commuting with local analytic isomorphisms.

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IdealSheafPair.CommutesWithLocalAnalyticIsomorphisms P, for a compatible family assignment P on the pairs: the last sentence of [Wlo09, Theorem 2.0.3] for local analytic isomorphisms, in two forms. Over every pair of compact sets: for a local analytic isomorphism g \colon N \to M of manifolds modelled on one space, the pair (N, g^* I) and compacts K' \subseteq N, K \subseteq M with g(U_{K'}) \subseteq U_K, the succession of P(N, g^* I) over U_{K'} is the pull-back along g of the succession of P(M, I) over U_K, up to blow-ups with empty centre (Definition 3.2.4.1), the form of [Wlo09, Theorem 3.5.1 (2)]. Globally: writing \mathrm{prin}_I \colon \tilde M \to M and \mathrm{prin}_J \colon \tilde N \to N for the maps of P(M, I) and P(N, J), J = g^* I, there is an analytic map \tilde g \colon \tilde N \to \tilde M with \mathrm{prin}_I \circ \tilde g = g \circ \mathrm{prin}_J which is a local analytic isomorphism, maps the fibre of \mathrm{prin}_J over every point y bijectively onto the fibre of \mathrm{prin}_I over g(y), and is the only continuous map \tilde N \to \tilde M over g: the "natural lifting" of [Wlo09, Theorem 2.0.1 (3)], whose fibre bijection is the identification of \tilde N with the fibre product N \times_M \tilde M stated pointwise, and whose uniqueness makes the lift of a composite the composite of the lifts. The two forms are bundled as the two conditions of Kollár's 34.1 are in CommutesWithSmoothMorphisms (Definition 4.2.4.3).

Lean code for Definition4.3.3.5●1 definition
  • structure(2 fields)defined in Hironaka/Manifold/Resolution/Defs.lean
    complete
    structure IdealSheafPair.CommutesWithLocalAnalyticIsomorphisms {𝕜 : Type}
      [RCLike 𝕜] (P : CompatibleFamilyAssignment (IdealSheafPair 𝕜)) : Prop
    structure IdealSheafPair.CommutesWithLocalAnalyticIsomorphisms
      {𝕜 : Type} [RCLike 𝕜]
      (P :
        CompatibleFamilyAssignment
          (IdealSheafPair 𝕜)) :
      Prop

    A compatible family assignment P on the pairs (M, I) commutes with local analytic isomorphisms, in two forms. Over every pair of compact sets: for a local analytic isomorphism g : N → M of manifolds modelled on one space, the family P(N, g^*I) is the pull-back of P(M, I) along g (ExtensionCompatibleFamily.IsPullbackAlong): for compacts K' ⊆ N, K ⊆ M with g(U_{K'}) ⊆ U_K, the succession of P(N, g^*I) over U_{K'} is the pull-back along g of the succession of P(M, I) over U_K, up to blow-ups with empty centre. Globally: writing prin_I : M̃ → M and prin_J : Ñ → N for the maps of P(M, I) and P(N, J), J = g^*I, there is an analytic map g̃ : Ñ → M̃ over g (prin_I ∘ g̃ = g ∘ prin_J) which is a local analytic isomorphism, maps the fibre of prin_J over every y ∈ N bijectively onto the fibre of prin_I over g y, and is the only continuous map Ñ → M̃ over g; the fibre bijection with the local isomorphism is the identification of Ñ with the fibre product N ×_M M̃, stated without forming it.

    isPullbackAlong : ∀ (T : IdealSheafPair 𝕜) {N : AnalyticManifold 𝕜 T.E} (g : AnalyticMap N T.M)
      (hg : IsLocalDiffeomorph 𝓘(𝕜, T.E) 𝓘(𝕜, T.E) ⊤ ⇑g), (P (T.pullback g hg)).IsPullbackAlong (P T) g

    The family P(N, g^*I) is the pull-back of P(M, I) along g, over every pair of compacts.

    exists_lift : ∀ (T : IdealSheafPair 𝕜) {N : AnalyticManifold 𝕜 T.E} (g : AnalyticMap N T.M)
      (hg : IsLocalDiffeomorph 𝓘(𝕜, T.E) 𝓘(𝕜, T.E) ⊤ ⇑g),
      ∃ g',
        (∀ (q : (P (T.pullback g hg)).space.carrier), (P T).map (g' q) = g ((P (T.pullback g hg)).map q)) ∧
          IsLocalDiffeomorph 𝓘(𝕜, T.E) 𝓘(𝕜, T.E) ⊤ ⇑g' ∧
          ∀ (y : N.carrier), BijOn (⇑g') (⇑(P (T.pullback g hg)).map ⁻¹' {y}) (⇑(P T).map ⁻¹' {g y}) ∧
          ∀ (φ : (P (T.pullback g hg)).space.carrier → (P T).space.carrier),
            Continuous φ →
              (∀ (q : (P (T.pullback g hg)).space.carrier), (P T).map (φ q) = g ((P (T.pullback g hg)).map q)) → φ = ⇑g'

    There is an analytic map g̃ : Ñ → M̃ over g, a local analytic isomorphism bijective between the fibres over y and over g y, and the only continuous map over g.

Commuting with closed embeddings.

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IdealSheafPair.CommutesWithClosedEmbeddings P, for a compatible family assignment P on the pairs: for pairs (M, I) and (N, J) and a closed embedding of analytic manifolds \tau \colon N \to M along which I is the push-forward of J (Definition 4.3.3.4), and compacts K' \subseteq N, K \subseteq M with \tau(U_{K'}) \subseteq U_K, the succession of P(M, I) over U_K is the push-forward along \tau of the succession of P(N, J) over U_{K'}, up to blow-ups whose centres lie outside the part over \tau(U_{K'}) (Definition 3.2.5.1). This is the rest of the last sentence of [Wlo09, Theorem 2.0.3], "commutes with … embeddings of ambient varieties", read as Kollár's commutation with closed embeddings [Kol07, Theorem 35 (5)] in the form of [Kol07, 34.3], B(X, I_X, E) = j_* B(Y, I_Y, E|_Y) for an empty E, over every pair of compact sets; the counterpart on manifolds of CommutesWithClosedEmbeddingsOfEmptyBoundary (Definition 4.2.4.5).

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Let \mathbb{K} be \mathbb{R} or \mathbb{C}. There is a compatible family assignment P on the pairs (M, I) of an analytic manifold M over \mathbb{K}, modelled on a finite-dimensional space, and an ideal sheaf I on M with nonzero stalks (Definition 4.3.3.2, Definition 4.3.3.3) — to each pair a map \mathrm{prin}_I \colon \tilde M \to M which over an open neighbourhood U_K of every compact K \subseteq M is, compatibly across compacts, a finite succession of blow-ups U_K = U_0 \leftarrow U_1 \leftarrow \cdots \leftarrow U_r with composite \sigma_K — such that:

  • every value P(M, I) is a locally finite principalization of I (Definition 4.3.3.1): with exceptional divisors E_i, \mathrm{prin}_I is proper, every U_K has compact closure, and (1) the centres are smooth; (2) every E_i has simple normal crossings with the centre at its stage, and E_r has simple normal crossings; (3) the total transform \sigma_K^*(I|_{U_K}) is near every point the ideal of an effective combination of the components of E_r, and the total transform \mathrm{prin}_I^*(I) is a normal-crossings divisor; moreover \mathrm{prin}_I is an isomorphism off the support of I;

  • P commutes with local analytic isomorphisms (Definition 4.3.3.5): for a local analytic isomorphism g \colon N \to M of manifolds modelled on one space and the pair (N, g^* I), over compacts K' \subseteq N, K \subseteq M with g(U_{K'}) \subseteq U_K the succession of P(N, g^* I) over U_{K'} is the pull-back along g of the succession of P(M, I) over U_K, up to blow-ups with empty centre; and there is a unique continuous map \tilde g \colon \tilde N \to \tilde M over g between the spaces of P(N, g^* I) and P(M, I), a local analytic isomorphism mapping every fibre of \mathrm{prin}_{g^*I} bijectively onto the fibre of \mathrm{prin}_I over the image point, so that \tilde N = N \times_M \tilde M;

  • P commutes with closed embeddings (Definition 4.3.3.6): for pairs (M, I) and (N, J) and a closed embedding of analytic manifolds \tau \colon N \to M along which I is the push-forward of J (Definition 4.3.3.4), and compacts K' \subseteq N, K \subseteq M with \tau(U_{K'}) \subseteq U_K, the succession of P(M, I) over U_K is the push-forward along \tau of the succession of P(N, J) over U_{K'}, up to blow-ups whose centres lie outside the part over \tau(U_{K'}).

Włodarczyk's (4), that the factorization over a larger compact restricts to the factorization over a smaller one, is the compatibility of the family. This is the analytic counterpart of Kollár's functorial principalization (Theorem 4.2.4.6) for an empty boundary: the principalization is chosen for all pairs at once, by one rule, compatibly with local analytic isomorphisms and with closed embeddings. The three properties are IdealSheaf.IsLocallyFinitelyPrincipalizedBy, IdealSheafPair.CommutesWithLocalAnalyticIsomorphisms and IdealSheafPair.CommutesWithClosedEmbeddings, the second through the pull-back of a succession up to empty blow-ups (Definition 3.2.4.1) and the third through the push-forward along a closed embedding (Definition 3.2.5.1).

Lean code for Theorem4.3.3.7●1 theorem
  • theorem exists_functorial_principalization (𝕜 : Type) [RCLike 𝕜] :
      ∃ P,
        (∀ (T : IdealSheafPair 𝕜),
            T.I.IsLocallyFinitelyPrincipalizedBy (P T)) ∧
          IdealSheafPair.CommutesWithLocalAnalyticIsomorphisms P ∧
          IdealSheafPair.CommutesWithClosedEmbeddings P
    theorem exists_functorial_principalization
      (𝕜 : Type) [RCLike 𝕜] :
      ∃ P,
        (∀ (T : IdealSheafPair 𝕜),
            T.I.IsLocallyFinitelyPrincipalizedBy
              (P T)) ∧
          IdealSheafPair.CommutesWithLocalAnalyticIsomorphisms
            P ∧
          IdealSheafPair.CommutesWithClosedEmbeddings
            P

    Włodarczyk's functorial locally finite principalization [Wlo09, Theorem 2.0.3], over 𝕜 = ℝ or 𝕜 = ℂ, the analytic counterpart of Kollár's functorial principalization [Kol07, Theorem 35] for an empty boundary. There is a compatible family assignment P on the pairs (M, I) of an analytic manifold M over 𝕜, modelled on a finite-dimensional space, and an ideal sheaf I on M with nonzero stalks (CompatibleFamilyAssignment (IdealSheafPair 𝕜)) — to each pair an analytic map prin_I : M̃ → M which over an open neighbourhood U_K of every compact K ⊆ M splits, compatibly across compacts, into a finite succession of blow-ups U_K = U_0 ← U_1 ← ⋯ ← U_r with smooth centres C_i ⊆ U_i and composite σ_K : U_r → U_K — such that

    • every value P(M, I) is a locally finite principalization of I (IdealSheaf.IsLocallyFinitelyPrincipalizedBy): with exceptional divisors E_i (the boundaries started at the empty boundary ⊤), prin_I is proper and every U_K has compact closure; (1) every centre C_i is smooth; (2) every E_i has simple normal crossings with C_i, and E_r has simple normal crossings (FiniteSuccession.HasSncBoundaries); (3) the total transform σ_K^*(I|_{U_K}) is, near every point, the ideal of an effective combination of the components of E_r through it (IdealSheaf.IsMulBoundaryMonomial, with the unit ideal ⊤ as its first factor), and the total transform prin_I^*(I) on M̃ is a normal-crossings divisor (IdealSheaf.IsNormalCrossingsDivisor); and prin_I is an isomorphism over the complement of the support of I (AnalyticMap.IsIsoOver);

    • P commutes with local analytic isomorphisms (IdealSheafPair.CommutesWithLocalAnalyticIsomorphisms): for a local analytic isomorphism g : N → M of manifolds modelled on one space and the pair (N, g^*I) (IdealSheafPair.pullback), over every pair of compacts K' ⊆ N, K ⊆ M with g(U_{K'}) ⊆ U_K the succession of P(N, g^*I) over U_{K'} is the pull-back along g of the succession of P(M, I) over U_K, up to blow-ups with empty centre (FiniteSuccession.IsPullbackUpToEmptyAlong; [Wlo09, Theorem 3.5.1 (2)]); and there is a unique continuous map g̃ : Ñ → M̃ over g between the spaces of P(N, g^*I) and P(M, I), which is analytic, a local analytic isomorphism, and maps every fibre of prin_{g^*I} bijectively onto the fibre of prin_I over the image point, so that Ñ is the fibre product N ×_M M̃;

    • P commutes with closed embeddings (IdealSheafPair.CommutesWithClosedEmbeddings): for pairs (M, I) and (N, J) and a closed embedding of analytic manifolds τ : N → M (an analytic embedding with closed image) along which I is the push-forward of J — the ideal of the closed submanifold τ(N) lies in I, and J = τ^*I (IdealSheaf.IsPushforwardAlong) — and compacts K' ⊆ N, K ⊆ M with τ(U_{K'}) ⊆ U_K, the succession of P(M, I) over U_K is the push-forward along τ of the succession of P(N, J) over U_{K'}, up to blow-ups whose centres lie outside the part over τ(U_{K'}) (FiniteSuccession.IsPushforwardUpToEmptyAlong; [Kol07, 34.3]).

    Włodarczyk's (4), that the factorization over a larger compact restricts to the factorization over a smaller one, is the compatibility of the family: the succession over K ⊆ K', restricted to U_K, is an extension of the succession over K, the same blow-ups with isomorphisms interspersed (ExtensionCompatibleFamily.isExtensionOf_restrict; [Wlo09, Definition 3.2.6]). Centres of codimension one are admitted, as in [Wlo09, Remark (1) after Theorem 2.0.3]: the blow-up of such a centre is an isomorphism, and the centre becomes a component of the exceptional divisor.

    Source statement: [Wlo09, Theorem 2.0.3, p. 35]

    Source statement: [Kol07, Theorem 35] (analytic manifolds, empty boundary; item 44)

Proof for Theorem 4.3.3.7
uses 0

The model space is identified with \mathbb{K}^n. On a manifold modelled on \mathbb{K}^n the value at I is Kollár's order reduction of the marked ideal (M, I, \emptyset, 1) [Kol07, 72], over \mathbb{R} or \mathbb{C} alike: the marked family of the concrete order-reduction tower at the mark 1 ([Kol07, Theorem 107], the three-step construction of [Kol07, 111]), over the relatively compact opens, glued along the exhaustion of the manifold. Over every relatively compact open its value is a smooth blow-up sequence of order at least 1 for (I, \emptyset) whose marked transform has order less than 1 at the end; so its exceptional divisors are simple normal crossings families having simple normal crossings with the centres, which gives (1) and (2), the total transform of I is Kollár's monomial in the final exceptional divisor [Kol07, 72], which gives (3), and the centres lie over the support of I, which gives the isomorphism off the support. The marked family commutes with local analytic isomorphisms [Kol07, Theorem 107 (2)]: the value of g^* I over a relatively compact open of N is the pull-back of the value of I over its image with the empty blow-ups erased, and by the compatibility of the family the pull-back up to empty blow-ups of the value over any larger relatively compact open. On a manifold modelled on E the value is the value on the re-modelled manifold carried back along the identification, with every condition and the pull-back relation; the assignment on the pairs reads this construction at the model space, the manifold and the ideal sheaf of each pair. The lift \tilde g is glued from the pull-back relation: over the neighbourhood U_{K'} of a compact of N, the compact closure of g(U_{K'}) lies in some U_K, the relation supplies a local analytic isomorphism between the last stages over g, bijective on the fibres, and through the identifications of the last stages with the preimages of U_{K'} and U_K it is a map from a piece of \tilde N to \tilde M over g; two such maps agree on an overlap, because a map over g is determined on the dense set over which the composite blow-down of U_K is injective and \tilde M is Hausdorff, so the pieces glue, and the same argument gives the uniqueness.

The commutation with closed embeddings is Kollár's Claim 71.2 [Kol07, 72]: the marked family at the mark 1 in dimension n commutes with closed embeddings of empty divisor through the marked family in dimension n - s. For a hypersurface this is [Kol07, Theorem 107 (3)] at the mark 1, which the three-step construction satisfies because an ideal containing the ideal of a hypersurface has order at most 1, so that its marked order reduction at the mark 1 is a single round of the order reduction of [Kol07, Theorem 103], whose commutation with closed embeddings is [Kol07, Theorem 103 (3)]; the general codimension follows along a flag of hypersurfaces [Kol07, 108]. On the standard models a closed embedding \tau \colon N \to M identifies N with the bundled closed submanifold \tau(N), by the universal property of the embedding, which fixes the dimension of N; the value over U_K is the push-forward of the value over the trace of U_K on \tau(N), the value over U_{K'} is the pull-back up to empty blow-ups of that value through the identification, and the embeddings of the relation are the inclusions of the stages of the push-forward after the lifts of the pull-back, which are bijective on the fibres over U_{K'}. The relation is carried back from the re-modelled manifolds along the identifications of the two models.

4.3.4. Embedded desingularization of analytic spaces🔗

Włodarczyk's embedded desingularization of a closed subspace of a manifold is stated, like his principalization, over every compact subset, and as an assignment on all pairs of a manifold and a closed subspace. It asserts that the succession over every compact desingularizes the subspace, and that the assignment commutes with local analytic isomorphisms.

Embedded desingularization of a subspace.

Definition4.3.4.1source[Wlo09, Theorem 2.0.2] ↗
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For a finite succession S of an analytic manifold M with centres C_i, and the closed subspace Y with ideal sheaf I, I.IsDesingularizedBy S says, with composite \sigma, exceptional divisors E_i and strict transforms Y_i, \tilde Y = Y_r: the centres are smooth; S.HasSncBoundaries ⊤ (Definition 4.3.1.1); no point of a centre lies over a point of \operatorname{Reg}(Y); the support of E_r is the preimage of \operatorname{Sing}(Y) = Y \setminus \operatorname{Reg}(Y); \tilde Y is smooth and IsSncBoundaryTransversalTo holds for E_r and \tilde Y; and \sigma^*(I_Y) = I_{\tilde Y} \cdot I_{\tilde E} (IsMulBoundaryMonomial, Definition 3.1.5.2) [Wlo09, Theorem 2.0.2 (1)–(3), (6)]. It is the analogue on manifolds of EmbeddedPair.IsDesingularizedBy on schemes (Definition 4.2.10.2).

Lean code for Definition4.3.4.1●1 definition

Locally finite embedded desingularization.

Definition4.3.4.2source[Wlo09, Theorem 2.0.2] ↗
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I.IsLocallyFinitelyDesingularizedBy F, for the ideal sheaf I of a closed subspace Y of M and a compatible family F with map \mathrm{res}_{Y,M}, says that F is a locally finite embedded desingularization of Y in Włodarczyk's sense: \mathrm{res}_{Y,M} is proper, it is an isomorphism over the complement of the singular locus \operatorname{Sing}(Y) = Y \setminus \operatorname{Reg}(Y), and for every compact K the succession F.seq K over the neighbourhood U_K desingularizes Y \cap U_K (I.IsDesingularizedBy, Definition 4.3.4.1), has no empty centre and has every centre of codimension at least two, or of codimension one inside the exceptional divisor of its stage, and \tilde M carries an exceptional divisor E and a strict transform \tilde Y gluing the last exceptional divisors and strict transforms of the successions: over every compact K, read on the last stage U_r of F.seq K through its identification F.toSpace K with \mathrm{res}_{Y,M}^{-1}(U_K), E is E_r and \tilde Y the strict transform Y_r of Y \cap U_K (Definition 3.2.1.3). Then E is the reduced ideal sheaf of a simple normal crossings family of hypersurfaces, with which \tilde Y has simple normal crossings, transversally (Definition 3.1.5.2), and \mathrm{res}_{Y,M}^*(I_Y) = I_{\tilde Y} \cdot I_{\tilde E} with \tilde E near every point an effective combination of the components of E. His clause (5) is the compatibility of the family (Definition 3.2.2.2).

Lean code for Definition4.3.4.2●1 definition

The inputs: embedded pairs.

Definition4.3.4.3source[Wlo09, Theorem 2.0.2] ↗
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An embedded pair (M, Y) is an analytic manifold M over \mathbb{K}, modelled on a finite-dimensional space, with a reduced closed analytic subspace Y \subseteq M, given by its ideal sheaf: the input of Włodarczyk's embedded desingularization, the counterpart on manifolds of the algebraic embedded pair (Definition 4.2.10.1), with the model space part of the input as for Definition 4.3.3.2; the embedded pairs have underlying manifolds (Definition 4.3.3.3). T.pullback g hg is the embedded pair (N, g^{-1}(Y)) over a local analytic isomorphism g \colon N \to M of manifolds modelled on one space; its subspace is reduced since the germ maps of g are ring isomorphisms.

Lean code for Definition4.3.4.3●3 definitions

Commuting with local analytic isomorphisms.

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EmbeddedPair.CommutesWithLocalAnalyticIsomorphisms R, for a compatible family assignment R on the embedded pairs: for a local analytic isomorphism g \colon N \to M of manifolds modelled on one space, the embedded pair (N, g^{-1}(Y)) and compacts K' \subseteq N, K \subseteq M with g(U_{K'}) \subseteq U_K, the succession of R(N, g^{-1}(Y)) over U_{K'} is the pull-back along g of the succession of R(M, Y) over U_K, up to blow-ups with empty centre (Definition 3.2.4.1): the first half of [Wlo09, Theorem 2.0.2 (4)], read as [Wlo09, Theorem 3.5.1 (2)], the per-compact form of Definition 4.3.3.5 (the lift of the local isomorphism is not stated for the embedded desingularization).

Lean code for Definition4.3.4.4●1 definition

Commuting with closed embeddings of ambient manifolds.

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EmbeddedPair.CommutesWithClosedEmbeddings R, for a compatible family assignment R on the embedded pairs: for embedded pairs (M, Y) and (N, Y') and a closed embedding of analytic manifolds \tau \colon N \to M along which the ideal sheaf of Y is the push-forward of that of Y' (Definition 4.3.3.4: \tau(N) a closed submanifold containing Y, and Y' = \tau^{-1}(Y)), such that no connected component of N lies in Y' (the ideal sheaf of Y' has nonzero stalks, Kollár's 0 \neq I_Y), and compacts K' \subseteq N, K \subseteq M with \tau(U_{K'}) \subseteq U_K, the succession of R(M, Y) over U_K is the push-forward along \tau of the succession of R(N, Y') over U_{K'}, up to blow-ups whose centres lie outside the part over \tau(U_{K'}) (Definition 3.2.5.1): the second half of [Wlo09, Theorem 2.0.2 (4)], "embeddings of ambient varieties", read as [Kol07, 34.3] as Definition 4.3.3.6 reads the last sentence of Theorem 2.0.3.

Lean code for Definition4.3.4.5●1 definition
Theorem4.3.4.6source[Wlo09, Theorem 2.0.2] ↗
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Let \mathbb{K} be \mathbb{R} or \mathbb{C}. There is a compatible family assignment R on the embedded pairs (M, Y) of an analytic manifold M over \mathbb{K}, modelled on a finite-dimensional space, and a reduced closed analytic subspace Y \subseteq M with ideal sheaf I_Y (Definition 4.3.4.3, Definition 4.3.3.3) — to each pair a map \mathrm{res}_{Y,M} \colon \tilde M \to M which over an open neighbourhood U_K of every compact K \subseteq M is, compatibly across compacts, a finite succession of blow-ups U_K = U_0 \leftarrow U_1 \leftarrow \cdots \leftarrow U_r with smooth closed centres C_i and composite \sigma_K \colon U_r \to U_K — such that:

  • every value R(M, Y) is a locally finite embedded desingularization of Y (Definition 4.3.4.2): \mathrm{res}_{Y,M} is proper and an isomorphism over the complement of the singular locus \operatorname{Sing}(Y) = Y \setminus \operatorname{Reg}(Y), and for every compact K, with exceptional divisors E_i (the boundaries started at the empty boundary), strict transforms Y_i of Y \cap U_K and \tilde Y = Y_r: every centre C_i is smooth; (1) every E_i has simple normal crossings with C_i, and E_r has simple normal crossings; (2) no point of a centre C_i lies over a point of \operatorname{Reg}(Y); the support of E_r is the preimage \sigma_K^{-1}(\operatorname{Sing} Y); (3) \tilde Y is smooth and has simple normal crossings with E_r, no component of E_r containing \tilde Y near a point; (6) \sigma_K^*(I_Y) = I_{\tilde Y} \cdot I_{\tilde E}, with \tilde E near every point an effective combination of the components of E_r; and \tilde M carries an exceptional divisor E and a strict transform \tilde Y: E is the reduced ideal sheaf of a simple normal crossings family of hypersurfaces, with which \tilde Y has simple normal crossings, transversally, \mathrm{res}_{Y,M}^*(I_Y) = I_{\tilde Y} \cdot I_{\tilde E} with \tilde E near every point an effective combination of the components of E, and over every compact K, read on U_r \cong \mathrm{res}_{Y,M}^{-1}(U_K), E is E_r and \tilde Y is Y_r; no centre of any succession is empty, and every centre has codimension at least two, or has codimension one and lies in the exceptional divisor of its stage;

  • (4) R commutes with local analytic isomorphisms (Definition 4.3.4.4): for a local analytic isomorphism g \colon N \to M of manifolds modelled on one space, the embedded pair (N, g^{-1}(Y)) and compacts K' \subseteq N, K \subseteq M with g(U_{K'}) \subseteq U_K, the succession of R(N, g^{-1}(Y)) over U_{K'} is the pull-back along g of the succession of R(M, Y) over U_K, up to blow-ups with empty centre;

  • (4) R commutes with closed embeddings of ambient manifolds (Definition 4.3.4.5): for embedded pairs (M, Y) and (N, Y') and a closed embedding of analytic manifolds \tau \colon N \to M along which I_Y is the push-forward of I_{Y'} (\tau(N) a closed submanifold containing Y, and Y' = \tau^{-1}(Y)), such that no connected component of N lies in Y', and compacts K' \subseteq N, K \subseteq M with \tau(U_{K'}) \subseteq U_K, the succession of R(M, Y) over U_K is the push-forward along \tau of the succession of R(N, Y') over U_{K'}, up to blow-ups whose centres lie outside the part over \tau(U_{K'}).

Włodarczyk's (5), that the factorization over a larger compact restricts to the factorization over a smaller one, is the compatibility of the family: the succession over K \subseteq K', restricted to U_K, is an extension of the succession over K [Wlo09, Definition 3.2.6]. This is the analytic counterpart of Włodarczyk's algebraic embedded desingularization (Theorem 4.2.10.4), in the locally finite form of Theorem 4.3.1.2, chosen for all embedded pairs at once, by one rule, compatibly with local analytic isomorphisms and with closed embeddings of ambient manifolds.

The three properties are IdealSheaf.IsLocallyFinitelyDesingularizedBy (Definition 4.3.4.2), whose conditions on the successions are IdealSheaf.IsDesingularizedBy (Definition 4.3.4.1), built on the strict transforms of a closed subspace (Definition 3.2.1.3), the simple locus \operatorname{Reg}(Y) and reducedness (Definition 3.1.3.3), and the transversal simple normal crossings and monomial factorization predicates (Definition 3.1.5.2), in which it also states its conditions on the exceptional divisor and the strict transform on \tilde M; EmbeddedPair.CommutesWithLocalAnalyticIsomorphisms (Definition 4.3.4.4), through the pull-back of a succession up to empty blow-ups (Definition 3.2.4.1); and EmbeddedPair.CommutesWithClosedEmbeddings (Definition 4.3.4.5), through the push-forward along a closed embedding (Definition 3.2.5.1).

Lean code for Theorem4.3.4.6●1 theorem
  • theorem exists_embeddedDesingularization (𝕜 : Type) [RCLike 𝕜] :
      ∃ R,
        (∀ (T : EmbeddedPair 𝕜),
            T.Y.IsLocallyFinitelyDesingularizedBy (R T)) ∧
          EmbeddedPair.CommutesWithLocalAnalyticIsomorphisms R ∧
          EmbeddedPair.CommutesWithClosedEmbeddings R
    theorem exists_embeddedDesingularization
      (𝕜 : Type) [RCLike 𝕜] :
      ∃ R,
        (∀ (T : EmbeddedPair 𝕜),
            T.Y.IsLocallyFinitelyDesingularizedBy
              (R T)) ∧
          EmbeddedPair.CommutesWithLocalAnalyticIsomorphisms
            R ∧
          EmbeddedPair.CommutesWithClosedEmbeddings
            R

    Włodarczyk's locally finite embedded desingularization [Wlo09, Theorem 2.0.2], over 𝕜 = ℝ or 𝕜 = ℂ, with its commutation with local analytic isomorphisms and with closed embeddings of ambient manifolds. There is a compatible family assignment R on the embedded pairs (M, Y) of an analytic manifold M over 𝕜, modelled on a finite-dimensional space, and a reduced closed analytic subspace Y ⊆ M with ideal sheaf I_Y (CompatibleFamilyAssignment (EmbeddedPair 𝕜)) — to each pair an analytic map res_{Y,M} : M̃ → M which over an open neighbourhood U_K of every compact K ⊆ M splits, compatibly across compacts, into a finite succession of blow-ups U_K = U_0 ← U_1 ← ⋯ ← U_r with smooth closed centres C_i ⊆ U_i and composite σ_K : U_r → U_K — such that

    • every value R(M, Y) is a locally finite embedded desingularization of Y (IdealSheaf.IsLocallyFinitelyDesingularizedBy): res_{Y,M} is proper and an isomorphism off the singular locus Sing(Y) = Y ∖ Reg(Y); over every compact the succession satisfies (1)–(3) and (6) (IdealSheaf.IsDesingularizedBy), has no empty centre, and has every centre of codimension at least two or of codimension one inside the exceptional divisor of its stage; and M̃ carries a divisor E and a strict transform Ỹ restricting to E_r and Y_r over every compact, with (1), (3) and (6) on M̃;

    • (4) R commutes with local analytic isomorphisms (EmbeddedPair.CommutesWithLocalAnalyticIsomorphisms): for a local analytic isomorphism g : N → M of manifolds modelled on one space and compacts K' ⊆ N, K ⊆ M with g(U_{K'}) ⊆ U_K, the succession of R(N, g^{-1}(Y)) (EmbeddedPair.pullback) over U_{K'} is the pull-back along g of the succession of R(M, Y) over U_K, up to blow-ups with empty centre (FiniteSuccession.IsPullbackUpToEmptyAlong; [Wlo09, Theorem 3.5.1 (2)]);

    • (4) R commutes with closed embeddings of ambient manifolds (EmbeddedPair.CommutesWithClosedEmbeddings): for a closed embedding of analytic manifolds τ : N → M (an analytic embedding with closed image) along which I_Y is the push-forward of I_{Y'} (IdealSheaf.IsPushforwardAlong: τ(N) contains Y, and Y' = τ^{-1}(Y)), no connected component of N lying in Y', and compacts K' ⊆ N, K ⊆ M with τ(U_{K'}) ⊆ U_K, the succession of R(M, Y) over U_K is the push-forward along τ of that of R(N, Y') over U_{K'}, up to blow-ups whose centres lie outside the part over τ(U_{K'}) (FiniteSuccession.IsPushforwardUpToEmptyAlong; [Kol07, 34.3]).

    Włodarczyk's (5), that the factorization over a larger compact restricts to the factorization over a smaller one, is the compatibility of the family: the succession over K ⊆ K', restricted to U_K, is an extension of that over K (ExtensionCompatibleFamily.isExtensionOf_restrict; [Wlo09, Definition 3.2.6]).

    Source statement: [Wlo09, Theorem 2.0.2]

Proof for Theorem 4.3.4.6
uses 0

The model space is identified with \mathbb{K}^n and I is transported to the model, where, made the unit ideal on the connected components of M on which it vanishes (a union of components, by the identity theorem), it forms a triple with the empty boundary family. Włodarczyk's modification of the resolution algorithm for the marked ideal (M, I, \emptyset, 1) (the proof of [Wlo09, Theorem 7.4.1]: the algorithm is stopped as soon as the controlled transform is the ideal of a smooth hypersurface of maximal contact), run on the real- or complex-analytic form of the blow-up sequence functors of [Kol07, Theorem 103] and [Kol07, Theorem 107], gives an embedded desingularization functor over the relatively compact opens of the model, which is glued along the exhaustion of the manifold to a compatible family with a proper blow-down. Its centres lie over Y by the order condition of the algorithm, hence over \operatorname{Sing}(Y) by (2), so the blow-down is an isomorphism over the complement of \operatorname{Sing}(Y); and the support of E_r is the preimage of \operatorname{Sing}(Y), because off E_r the composite blow-down is an isomorphism near the point, carrying the smooth \tilde Y onto Y. Over the components where I vanishes nothing is blown up, the strict transform is the component itself and the exceptional divisor is empty, so every condition holds for I as it does for the modified ideal sheaf. The exceptional families of the pieces glue to a locally finite simple normal crossings family on the glued space, each member born on the piece where it first appears and continued along the embeddings of the pieces, which carry exceptional families to exceptional families; E is its reduced ideal sheaf, and \tilde Y is the saturation of the total transform \mathrm{res}_{Y,M}^*(I) by E, which on each piece is the last strict transform [BM97, Proposition 3.13], the strict transform being, at a point of the last stage, the saturation of the total transform by the exceptional ideal: near a point of \tilde Y the exceptional ideal is a product of coordinates and the ideal of \tilde Y a prime generated by coordinates not among them, so the saturation changes nothing. The conditions on E and \tilde Y are read on the pieces through the inclusions, which are local analytic isomorphisms. The functor commutes with local analytic isomorphisms [Kol07, 34.1]: the value of g^* I over a relatively compact open is the pull-back of the value of I over its image with the empty blow-ups erased, and by the compatibility of the family the pull-back up to empty blow-ups of the value over any larger relatively compact open. The commutation with closed embeddings of ambient manifolds is that of the functor over the relatively compact opens, through the functor of the lower dimension at the restriction of I to the submanifold, which is reduced when I is and contains the ideal of the submanifold: on the standard models the embedding identifies N with the bundled submanifold, the value over U_K is the push-forward of the value over the trace of U_K on \tau(N), and the value over U_{K'} is the pull-back up to empty blow-ups of that value through the identification, as for the principalization; the modification of I on its zero components is invisible along \tau(N), since J has nonzero stalks. Each condition and the two relations are carried back along the identifications of the models. A centre of codimension at most one lies over \operatorname{Sing}(Y) and has simple normal crossings with the exceptional divisor of its stage; off that divisor the composite blow-down is a local isomorphism and would carry the centre, a smooth hypersurface or an open set, into \operatorname{Sing}(Y), which contains neither: the singular locus of a reduced subspace contains no smooth hypersurface germ, since the stalk of the ideal sheaf at a point of such a germ would lie in every power of its equation, hence vanish by Krull's intersection theorem; so the centre lies in the exceptional divisor and has codimension one.

4.3.5. Resolution of analytic spaces🔗

The resolution of analytic spaces is a resolution assignment (Definition 3.3.4.1), a morphism \Pi_X \colon R(X) \to X for every analytic space X, here on the reduced spaces. It asserts of the values that they are strong resolutions onto the closure of the simple locus and locally composites of blow-ups of an ambient manifold, and of the assignment that it lifts isomorphisms between open subspaces of inputs and local analytic isomorphisms between inputs. The resolution of a single reduced complex space asserts the first two properties again.

Strong resolutions of analytic spaces.

Definition4.3.5.1source[Kol07, (2) and (3)] ↗
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For a morphism \pi \colon R \to X of K-analytic spaces:

  • π.IsResolution: R is non-singular, \pi is proper, and \pi is bimeromorphic (Kollár: birational [Kol07, (2)]; [Wlo09, Theorem 2.0.1]), read onto its image: an isomorphism over an open subset U \subseteq X whose preimage is dense in R. Kollár asks for \pi projective; only the properness that projectivity implies is recorded. The density is asked of the preimage and not of U: over \mathbb{R} the simple locus of a reduced space need not be dense, and then no resolution is an isomorphism over a dense open subset of X.

  • π.IsStrongResolution [Kol07, (3)], conditions (1)–(3) of Kollár's Theorem 45: a resolution that is an isomorphism over the simple locus X.regularLocus and such that \pi^{-1}(\operatorname{Sing} X) is the support of a closed subspace E with E.IsSncBoundary (Definition 3.3.3.3).

  • π.IsStrongResolutionOntoClosureRegularLocus: a strong resolution whose image is the closure of the simple locus of X, conditions (1)–(3) of Theorem 45 with the image clause. Over \mathbb{C} the simple locus of a reduced space is dense and the clause is the surjectivity of \pi; over \mathbb{R} it need not be dense, and \pi misses the part of X away from the closure of the simple points.

Lean code for Definition4.3.5.1●3 definitions
  • structure(3 fields)defined in Hironaka/AnalyticSpace/Resolution/Defs.lean
    complete
    structure Hom.IsResolution {K : Type} [RCLike K] {X R : AnalyticSpace K}
      (π : R ⟶ X) : Prop
    structure Hom.IsResolution {K : Type} [RCLike K]
      {X R : AnalyticSpace K} (π : R ⟶ X) :
      Prop

    A morphism π : R → X of K-analytic spaces is a resolution of X: R is non-singular, π is proper, and π is bimeromorphic (Kollár: birational [Kol07, (2)]; [Wlo09, Theorem 2.0.1]), read onto its image: an isomorphism over an open subset U ⊆ X whose preimage is dense in R. Kollár asks for π projective; only the properness that projectivity implies is recorded. The density is asked of the preimage and not of U: over ℝ the simple locus of a reduced space need not be dense ([Hir64, Introduction]), and then no resolution is an isomorphism over a dense open subset of X.

    isNonsingular : R.IsNonsingular

    R is non-singular.

    isProperMap : IsProperMap ↑π

    π is proper [Kol07, (2)] (Kollár: projective).

    exists_isIsoOver_dense_preimage : ∃ U, IsOpen U ∧ Hom.IsIsoOver π U ∧ Dense (↑π ⁻¹' U)

    π is an isomorphism over an open subset of X whose preimage is dense in R.

  • structure(extends 1, 5 fields)defined in Hironaka/AnalyticSpace/Resolution/Defs.lean
    complete
    structure Hom.IsStrongResolution {K : Type} [RCLike K] {X R : AnalyticSpace K}
      (π : R ⟶ X) : Prop
    structure Hom.IsStrongResolution {K : Type}
      [RCLike K] {X R : AnalyticSpace K}
      (π : R ⟶ X) : Prop

    A morphism π : R → X of K-analytic spaces is a strong resolution of X [Kol07, (3)], clauses (1)–(3) of [Kol07, Theorem 45]: a resolution (non-singular, proper, bimeromorphic onto its image) that is an isomorphism over the simple locus X.regularLocus (Kollár's smooth locus X^{ns}), and such that π⁻¹(X.singularLocus) is the support of a simple normal crossing divisor of R: a closed subspace which is the reduced divisor of a locally finite family of closed smooth hypersurfaces with simple normal crossings (ClosedSubspace.IsSncBoundary; [Kol07, Definition 24]).

    isNonsingular : R.IsNonsingular
    Inherited from
    1. AnalyticSpace.Hom.IsResolution
    isProperMap : IsProperMap ↑π
    Inherited from
    1. AnalyticSpace.Hom.IsResolution
    exists_isIsoOver_dense_preimage : ∃ U, IsOpen U ∧ Hom.IsIsoOver π U ∧ Dense (↑π ⁻¹' U)
    Inherited from
    1. AnalyticSpace.Hom.IsResolution
    isIsoOver_regularLocus : Hom.IsIsoOver π X.regularLocus

    π is an isomorphism over the simple locus X^{ns}.

    exists_isSncBoundary_preimage : ∃ E, E.IsSncBoundary ∧ IdealSheaf.support E = ↑π ⁻¹' X.singularLocus

    π⁻¹(Sing X) is the support of a simple normal crossing divisor.

  • structure(extends 2, 6 fields)defined in Hironaka/AnalyticSpace/Resolution/Defs.lean
    complete
    structure Hom.IsStrongResolutionOntoClosureRegularLocus {K : Type} [RCLike K]
      {X R : AnalyticSpace K} (π : R ⟶ X) : Prop
    structure Hom.IsStrongResolutionOntoClosureRegularLocus
      {K : Type} [RCLike K]
      {X R : AnalyticSpace K} (π : R ⟶ X) :
      Prop

    A morphism π : R → X of K-analytic spaces is a strong resolution onto the closure of the simple locus of X: a strong resolution (Hom.IsStrongResolution: non-singular, proper, bimeromorphic onto its image, an isomorphism over the simple locus X.regularLocus, and π⁻¹(Sing X) the support of a simple normal crossing divisor — clauses (1)–(3) of [Kol07, Theorem 45]) whose image is the closure of the simple locus. Over ℂ the simple locus of a reduced space is dense and the clause says that π is surjective; over ℝ it need not be dense ([Hir64, Introduction]: the subspace x² + y² = 0 of ℝ² is reduced with support the origin and no simple point), and then π misses the part of X away from the closure of the simple points.

    isNonsingular : R.IsNonsingular
    Inherited from
    1. AnalyticSpace.Hom.IsStrongResolution
    2. AnalyticSpace.Hom.IsResolution
    isProperMap : IsProperMap ↑π
    Inherited from
    1. AnalyticSpace.Hom.IsStrongResolution
    2. AnalyticSpace.Hom.IsResolution
    exists_isIsoOver_dense_preimage : ∃ U, IsOpen U ∧ Hom.IsIsoOver π U ∧ Dense (↑π ⁻¹' U)
    Inherited from
    1. AnalyticSpace.Hom.IsStrongResolution
    2. AnalyticSpace.Hom.IsResolution
    isIsoOver_regularLocus : Hom.IsIsoOver π X.regularLocus
    Inherited from
    1. AnalyticSpace.Hom.IsStrongResolution
    exists_isSncBoundary_preimage : ∃ E, E.IsSncBoundary ∧ IdealSheaf.support E = ↑π ⁻¹' X.singularLocus
    Inherited from
    1. AnalyticSpace.Hom.IsStrongResolution
    range_eq : range ↑π = closure X.regularLocus

    The image of π is the closure of the simple locus of X.

Local factorization through ambient blow-ups.

Definition4.3.5.2source[Kol07, Theorem 45 (4)] ↗
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Condition (4) of the resolution theorem describes \Pi_X locally as a composite of blow-ups of an ambient manifold restricted to strict transforms. The ambient is a disjoint union A = \bigsqcup_i G_i of countably many open subsets of K^n (AnalyticManifold.sigmaOpens, built on the charted space structure of a disjoint union, ChartedSpace.sigma, ChartedSpace.sigmaOfNonempty). An ideal sheaf J on A cuts out the closed subspace \operatorname{Sp}(A)/J (IdealSheaf.toAnalyticSpace, with its morphism toAnalyticSpaceι), and overPiece is its part over G_i.

An ambient blow-up factorization f.AmbientBlowUpFactorization U of f \colon Y \to X over an open U (Hom.AmbientBlowUpFactorization) consists of finitely many open subspaces U_i covering U, closed embeddings of the X|_{U_i} into open subsets G_i \subseteq K^n, a finite succession of blow-ups of the ambient A with smooth centres, the strict transforms of \bigsqcup X|_{U_i}, the last one a closed submanifold, and isomorphisms over each U_i identifying f^{-1}(U_i) \to U_i with the restricted composite [Kol07, Warning 23]; [Wlo09, Theorem 2.0.2]. f.IsLocallyFiniteAmbientBlowUpComposite asks for such a factorization over every relatively compact open U \subseteq X.

Lean code for Definition4.3.5.2●8 definitions

Lifting local analytic isomorphisms.

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A morphism \varphi \colon X' \to X of analytic spaces is a local analytic isomorphism (Hom.IsLocalIso) when every point of X' has an open neighbourhood U' which \varphi maps isomorphically onto an open subspace X|_V: open immersions, isomorphisms, covering maps and the morphism \coprod U_i \to X from the disjoint union of an open cover, injective or not. A morphism \psi \colon A \to B is the only morphism over \varphi \colon X \to Y, along p \colon A \to X and q \colon B \to Y, when the morphisms \psi' with q \circ \psi' = \varphi \circ p are exactly \psi. A resolution assignment R on inputs C (Definition 3.3.4.1) lifts local analytic isomorphisms (R.LiftsLocalIsomorphisms) in two forms: every isomorphism \varphi \colon X|_U \to Y|_V between open subspaces of two inputs lifts to exactly one morphism \psi \colon R(X)|_{\Pi_X^{-1} U} \to R(Y)|_{\Pi_Y^{-1} V} over \varphi, an isomorphism; and every local analytic isomorphism \varphi \colon X' \to X between inputs lifts to exactly one morphism \psi \colon R(X') \to R(X) with \Pi_X \circ \psi = \varphi \circ \Pi_{X'}, a local analytic isomorphism. Over every open U' which \varphi maps isomorphically onto an open subspace, the lift of \varphi restricts to the lift of that isomorphism, since the restriction lies over it and the lift of the first form is the only morphism over it. Because the lifts are unique, they respect identities and composition: R is a functor on the groupoid of isomorphisms between open subspaces of inputs and on the local analytic isomorphisms between inputs. The first form stands for condition (5) of Kollár's Theorem 45, "R commutes with smooth K-morphisms", for the étale morphisms that are injective, and the second is [Wlo09, Theorem 2.0.1 (3)] and the case of étale morphisms of Kollár's (5); the two are bundled as the two conditions of Kollár's 34.1 are in CommutesWithSmoothMorphisms (Definition 4.2.4.3). Smooth morphisms that are not étale are not lifted. For strong resolutions, the second form follows from the first by gluing the local lifts over the opens on which \varphi is injective, the gluing being compatible on the overlaps because a morphism out of a non-singular space is determined on a dense open subset.

Lean code for Definition4.3.5.3●2 definitions

The inputs: reduced spaces.

Definition4.3.5.4source[Kol07, Theorem 45] ↗
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The reduced K-analytic spaces, as a full subcategory of the K-analytic spaces (ReducedSpace, the counterpart of Definition 4.2.6.1), are the inputs of the resolution of analytic spaces. Kollár states the theorem for every separable analytic space; his notion of resolution is stated for varieties, and his proofs are for reduced spaces, extended to reducible ones. A non-reduced space X is resolved through its reduction: the resolution of X_{\mathrm{red}} followed by the closed immersion X_{\mathrm{red}} \to X.

Lean code for Definition4.3.5.4●1 definition
Theorem4.3.5.5source[Kol07, Theorem 45] ↗
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For K = \mathbb{R} or \mathbb{C} there is a resolution assignment X \mapsto (\Pi_X \colon R(X) \to X) on the reduced K-analytic spaces (Definition 4.3.5.4, Definition 3.3.4.1) such that for every input X:

  1. R(X) is non-singular;

  2. \Pi_X is an isomorphism over the simple points of X;

  3. \Pi_X^{-1}(\operatorname{Sing} X) is the support of a simple normal crossings divisor;

  4. \Pi_X is proper and bimeromorphic onto its image, and the image of \Pi_X is the closure of the simple points; over every relatively compact open subset of X, \Pi_X is a finite composite of blow-ups of smooth centres of an ambient manifold restricted to strict transforms;

  5. every analytic isomorphism between open subspaces of two inputs lifts to exactly one morphism of the resolutions over it, an isomorphism, and every local analytic isomorphism \varphi \colon X' \to X between inputs lifts to exactly one morphism R(X') \to R(X) over it, itself a local analytic isomorphism, which restricts over every open on which \varphi is injective to the lift of the isomorphism of open subspaces.

This is resolution of singularities for analytic spaces, functorial for local analytic isomorphisms: the lifts are unique, so they respect identities and composition, and the resolution is local on the space. Kollár's condition (5) asks more, the commutation with all smooth morphisms; the lifting of local analytic isomorphisms is its case of étale morphisms and is [Wlo09, Theorem 2.0.1 (3)]. Conditions (1)–(3), with properness, bimeromorphy and the image clause, are Hom.IsStrongResolutionOntoClosureRegularLocus (Definition 4.3.5.1), condition (4) is Hom.IsLocallyFiniteAmbientBlowUpComposite (Definition 4.3.5.2), and condition 5 is ResolutionAssignment.LiftsLocalIsomorphisms (Definition 4.3.5.3). The bimeromorphy follows from (2) and (3): the preimage of the simple locus is the complement of the support of a simple normal crossings divisor, hence dense.

Analytic spaces, their morphisms, simple points and closed subspaces are in the chapter Analytic spaces: Definition 3.3.3.1, Definition 3.3.3.2, Definition 3.3.3.3, Definition 3.3.4.1.

Lean code for Theorem4.3.5.5●1 theorem
  • theorem exists_functorial_resolution (K : Type) [RCLike K] :
      ∃ R,
        (∀ (X : ReducedSpace K),
            Hom.IsStrongResolutionOntoClosureRegularLocus (R.map X) ∧
              Hom.IsLocallyFiniteAmbientBlowUpComposite (R.map X)) ∧
          R.LiftsLocalIsomorphisms
    theorem exists_functorial_resolution (K : Type)
      [RCLike K] :
      ∃ R,
        (∀ (X : ReducedSpace K),
            Hom.IsStrongResolutionOntoClosureRegularLocus
                (R.map X) ∧
              Hom.IsLocallyFiniteAmbientBlowUpComposite
                (R.map X)) ∧
          R.LiftsLocalIsomorphisms

    Resolution of analytic spaces [Kol07, Theorem 45]; [Wlo09, Theorem 2.0.1], over K = ℝ or K = ℂ. There is a resolution assignment X ↦ (Π_X : R(X) → X) on the reduced K-analytic spaces (ResolutionAssignment (ReducedSpace K)) such that

    • every Π_X is a strong resolution of X onto the closure of its simple locus (Hom.IsStrongResolutionOntoClosureRegularLocus): Π_X is proper and bimeromorphic onto its image, (1) R(X) is non-singular, (2) Π_X is an isomorphism over the simple locus X.regularLocus, (3) Π_X⁻¹(X.singularLocus) is the support of a simple normal crossing divisor of R(X), and the image of Π_X is the closure of X.regularLocus (in neither source; see the relation to the source below);

    • (4) over every relatively compact open subset of X the morphism Π_X is the composite of a finite sequence of blow-ups of smooth centres of an ambient manifold, restricted to strict transforms (Hom.IsLocallyFiniteAmbientBlowUpComposite);

    • (5) R lifts local analytic isomorphisms (ResolutionAssignment.LiftsLocalIsomorphisms): every isomorphism between open subspaces of two inputs lifts to exactly one morphism between their preimages in the resolutions over it, an isomorphism; and every local analytic isomorphism φ : X' → X between inputs (Hom.IsLocalIso: locally an isomorphism onto an open subspace, injective or not) lifts to exactly one morphism R(X') → R(X) over it, itself a local analytic isomorphism ([Wlo09, Theorem 2.0.1 (3)]). Over every open on which φ is injective, the lift of φ restricts to the lift of the isomorphism of open subspaces (ResolutionAssignment.LiftsLocalIsomorphisms.lift_restrict).

    Kollár's (4), "Π_X is projective over any compact subset of X", is split between the properness of Hom.IsResolution and the factorization clause, whose ambient manifold is the disjoint union of finitely many opens of Kⁿ, one per piece of a finite open cover of the relatively compact open, with the identification with Π_X given piecewise. The non-singularity of R(X) is repeated by (3), since ClosedSubspace.IsSncBoundary asks for a regular system of parameters at every point of R(X).

    Source statement: [Kol07, Theorem 45]

    Source statement: [Wlo09, Theorem 2.0.1]

Proof for Theorem 4.3.5.5
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An embedded desingularization functor for closed subspaces of manifolds (the order-reduction algorithm applied to the ideal of the subspace in every dimension, functorial under local analytic isomorphisms and compatible with closed embeddings of the ambient manifolds) is applied to local embeddings of the reduced space into manifolds. The resolutions of the pieces are independent of the embedding chosen and glue over the exhaustion of the space by relatively compact opens to R(X); the conditions are read off the properties of the functor, the lifting of condition (5) from its functoriality. The lift of an isomorphism between open subspaces is the only morphism over it, because two morphisms out of a non-singular open part of R(X) over the same morphism agree over the simple locus, where the resolutions are isomorphisms, a dense open subset. The lifting of local analytic isomorphisms follows: the lifts over the opens on which \varphi is injective agree on the overlaps by the same rigidity, and glue.

4.3.6. Resolution of a reduced complex-analytic space🔗

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Let X be a reduced complex-analytic space. Then there is a morphism \pi \colon R \to X such that

  1. R is non-singular;

  2. \pi is an isomorphism over the simple points of X;

  3. \pi^{-1}(\operatorname{Sing} X) is the support of a simple normal crossings divisor;

  4. \pi is proper and bimeromorphic onto its image and, over every relatively compact open subset of X, a finite composite of blow-ups of smooth centres of an ambient manifold restricted to strict transforms;

  5. \pi is surjective.

This is resolution of singularities for a single reduced complex space, without the functoriality of the resolution of analytic spaces (Theorem 4.3.5.5). Conditions (1)–(3), with properness and bimeromorphy, are Hom.IsStrongResolution (Definition 4.3.5.1), and (4) is Hom.IsLocallyFiniteAmbientBlowUpComposite (Definition 4.3.5.2).

Lean code for Theorem4.3.6.1●1 theorem
Proof for Theorem 4.3.6.1
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The specialization of Theorem 4.3.5.5 to K = \mathbb{C}: its conditions (1)–(4) are taken as they stand, its condition on the image, \operatorname{range} \pi = \overline{X^{ns}}, becomes surjectivity because the simple locus of a reduced complex space is dense, and the lifting condition (5) is dropped.