Closed subspaces and labelled families descend along a gluing over a base #
The resolution of an analytic space comes with its exceptional divisor, a locally finite family of
smooth hypersurfaces with simple normal crossings ([Kol07, Definition 24]; [Kol07, Theorem 45, (3)];
[Wlo09, Theorem 2.0.1, (2)]). The sources glue the spaces only: Włodarczyk's Ṽ is "obtained by
gluing Ṽ_i along Ṽ_i ∩ Ṽ_j", and the construction "commutes with local analytic isomorphisms" by
construction [Wlo09, §4]; Kollár passes from the neighbourhoods of compact sets to their union
[Kol07, 44]. That the exceptional families of the pieces glue with the spaces is left to the reader.
This module proves the descent step for a gluing datum G : GlueOver X R π dom
(Hironaka.AnalyticSpace.Glue.Over):
- Descent of one closed subspace (
glueClosedSubspace): closed subspacesC iof the pieces whose restrictions to the gluing open subsets correspond under the transitions (CompatClosedSubspaces) descend to a closed subspace ofG.gluedOver: the ideal sheaf with the stalk idealscomap ((ιGlued i).stalkMap y) ((C i).stalkIdeal y)atιGlued i y, well defined because two representatives of one glued point are related through a transition (KGlueData.exists_of_ι_base_eq), the glue condition identifies the two legs' stalk maps, and compatibility identifies the two stalk ideals (pieceStalkIdeal_eq); its local generators are the pieces' carried through the open immersions (IsOpenImmersion.invApp). It pulls back toC ialong everyιGlued i(comap_ιGlued_glueClosedSubspace), its support is the union of the images of the pieces' supports (support_glueClosedSubspace), and it is the only closed subspace with these pull-backs (ext_of_forall_comap_ιGlued_eq,existsUnique_comap_ιGlued_eq). - The labelled family layer: families
H i : κ i → ClosedSubspace (R i)with labelslab i : κ i → Λ, injective on every piece;memberOfLabel H lab i lis the member of thei-th family carrying the labell, the empty subspace⊤when the piece has no such member.LabelledCompatasks, label by label, for the compatibility of these members: the partner of a member across an overlap is fixed by its label, and a member absent from a neighbouring piece does not meet the overlap (the⊤convention on empty members). ThenglueFamily : Λ → ClosedSubspace G.gluedOverglues label by label; it pulls back to the given members (comap_ιGlued_glueFamily_lab), has total support the union of the pieces' total supports (iUnion_support_glueFamily), is locally finite when the pieces' families are (locallyFinite_glueFamily), and is a simple-normal-crossings family (ClosedSubspace.IsSncFamily,Hironaka.AnalyticSpace.SncFamily) when the pieces' are (isSncFamily_glueFamily): the stalk clause at a glued point is the piece's, carried through the ring isomorphism of the open immersion's stalk map, the members through the point matched by the labels.
The label type is arbitrary here. In the resolution of an analytic space the pieces' exceptional
families are the traces of one family indexed by the stages of a single run of the resolution
functor on the disjoint union of the pieces, so the labelling is the identity
(Hironaka.Resolution.Analytic.Kol07Thm45.CoproductGluedFamily), and LabelledCompat comes from
the agreement of the stages across the overlaps (CoproductGluedFamilyCompat,
ExhaustionChainFamilies, ResolutionOnMembers); the gluing along an exhaustion uses the chain
form of Hironaka.Resolution.Analytic.Kol07Thm45.Glue.OverFamilyChain.
The restriction of a closed subspace Z of the i-th piece to the gluing open subset
glueOpens i j, as an ideal sheaf of the restricted piece: the pull-back along the open inclusion
(the trace on a part).
Equations
- AnalyticSpace.GlueOver.restrictGlue π dom i j Z = AnalyticSpace.QuotientSpace.comap (↑((R i).ofRestrict (AnalyticSpace.GlueOver.glueOpens X R π dom i j))) Z
Instances For
Compatibility of closed subspaces of the pieces: on every gluing open subset the transition
t i j pulls the j-th subspace back to the i-th.
Equations
- One or more equations did not get rendered due to their size.
Instances For
Two representatives of one glued point are related through the transition
(KGlueData.exists_of_ι_base_eq, Mathlib's ι_eq_iff_rel underneath).
The glue condition of the gluing over X, on the pieces: ofRestrict ≫ ιGlued i and
t i j ≫ ofRestrict ≫ ιGlued j agree on the gluing open subset.
Two stalk-specialization maps along an equality of points compose to the identity.
The ideal algebra of representative independence: in a commutative square α ≫ a = σ' ≫ β ≫ b
with a, b bijective and σ ≫ σ' = 𝟙, ideals I, J with the same image under a, b pull
back along α, β to ideals identified by σ.
The stalk ideal of the i-th subspace, read on the glued space at the image point.
Equations
- G.pieceStalkIdeal C i y = Ideal.comap (CommRingCat.Hom.hom (AlgebraicGeometry.LocallyRingedSpace.Hom.stalkMap (↑(G.ιGlued i)) y)) (Manifold.IdealSheaf.stalkIdeal (C i) y)
Instances For
Representative independence: the piece stalk ideals of two representatives of one glued point agree under the canonical identification of the stalks.
A representative (i, y) of a point of the glued space.
Instances For
The chosen representative represents: ιGlued (gluedRep z).1 sends (gluedRep z).2 to z.
The glued stalk ideal at z: the piece stalk ideal of a representative, carried to z.
Equations
- G.gluedStalkIdeal C z = Ideal.comap (CommRingCat.Hom.hom (G.gluedOver.presheaf.stalkSpecializes ⋯)) (G.pieceStalkIdeal C (G.gluedRep z).fst (G.gluedRep z).snd)
Instances For
The glued stalk ideal at the image of y is the i-th piece stalk ideal.
A germ of a section transported along an identity of open subsets is the germ of the section.
Along an open immersion, the stalk map carries the germ of the transported section (invApp) to
the germ of the section.
The glued stalk ideals have local generators: a piece's generators near the representative,
carried to the glued space through the open immersion's invApp, generate at every point of the
image neighbourhood (stalkMap_germ_invApp; the stalk map is bijective).
The glued closed subspace of compatible closed subspaces of the pieces.
Equations
- G.glueClosedSubspace C hc = Manifold.IdealSheaf.ofStalks G.gluedOver.𝒪 (G.gluedStalkIdeal C) ⋯
Instances For
The glued closed subspace pulls back along the open immersion of every piece to the given one.
The preimage of the glued support under the i-th immersion is the i-th support.
Uniqueness of descent: two closed subspaces of the glued space with the same pull-backs to every piece are equal (the pieces cover, and the stalk maps of the open immersions are isomorphisms).
Existence and uniqueness of descent: compatible closed subspaces of the pieces are the pull-backs of exactly one closed subspace of the glued space.
The support of the glued closed subspace is the union of the images of the pieces' supports.
The family layer: labelled families of closed subspaces glue #
The hypothesis is the labelled form: a label type Λ, per-piece labels lab i : κ i → Λ
injective on each piece, and for every label the members carrying it compatible across the
transitions, with the empty subspace ⊤ standing in where a piece has no member with that label
(so a member absent from a neighbouring piece must not meet the overlap: the totality clause, and
the convention on empty members).
The unit ideal sheaf (the empty closed subspace) has empty support.
The member of the i-th family carrying the label l; the empty subspace ⊤ when no member of
the piece carries l.
Equations
- AnalyticSpace.GlueOver.memberOfLabel H lab i l = if h : ∃ (k : κ i), lab i k = l then H i h.choose else ⊤
Instances For
With injective labels, the member of the label lab i k is the member k.
A label carried by no member of the piece gives the empty subspace ⊤.
The support of a label's member lies in the piece's total support.
The labels' members have the piece's total support: every member appears at its own label, and absent labels contribute the empty set.
A point in the support of a label's member lies in the support of a member carrying that label.
Labelled compatibility of families: for every label, the members carrying it are compatible
closed subspaces of the pieces (⊤ where a piece has no member with that label).
Equations
- G.LabelledCompat H lab = ∀ (l : Λ), G.CompatClosedSubspaces fun (i : ι) => AnalyticSpace.GlueOver.memberOfLabel H lab i l
Instances For
The glued family, indexed by the labels: the label l glues the members carrying it.
Equations
- G.glueFamily H lab hc l = G.glueClosedSubspace (fun (i : ι) => AnalyticSpace.GlueOver.memberOfLabel H lab i l) ⋯
Instances For
The glued member with label l pulls back along ιGlued i to the i-th piece's member of that
label (⊤ when the piece has none).
Pulled back to the i-th piece, the glued member with label lab i k is the member k.
The preimage under ιGlued i of a glued member's support is the support of the piece's member
of that label.
The support of a glued member is the union of the images of the pieces' members of that label.
The total support of the glued family is the union of the images of the pieces' total supports.
Local finiteness of the glued family, from local finiteness on every piece.
Simple normal crossings glue ([Kol07, Definition 24], the local form of the divisor with
simple normal crossings; ClosedSubspace.IsSncFamily): the glued family of labelled
simple-normal-crossings families is one: the stalk data at a glued point are the piece's, carried
through the stalk isomorphism of the open immersion; the glued members through the point are the
piece's members through its representative, matched by the labels.