The ideal-theoretic strict transforms along pull-backs and along the deletion of empty blow-ups #
The independence of the local resolution of a piece from the chosen embedding
[Kol07, Theorem 36, proof] compares the final ideal-theoretic strict transforms
(FiniteSuccession.strictTransformSubspaceSeq) along the values of the embedded
desingularization functor for two embeddings. The comparison transports them along the sequences
that the functoriality clause of IsEmbeddedDesing (CommutesWithLocalIsos, the commutation with
smooth morphisms of [Kol07, 34.1]) produces: the pull-back along a local analytic isomorphism g
restricted onto its image, with the empty blow-ups deleted. This module proves the ideal-level
counterparts of the corresponding transports of the set-level strict transforms and of the weak
transforms:
- (A)
saturationStalk_congr,strictTransformSubspace_congr: the strict transformstrictTransformSubspacedepends on the witnesseshY : IsClosedSubmanifold ψ Y c,h : IsBlowUp ψ Y c πonly through the SETYand the blow-downπ(the ideal sheaf of a closed submanifold depends only on the set,IsClosedSubmanifold.idealSheaf_congr). A succession's witnessesisClosedSubmanifold_center icarry the chosen chart and codimension of its monoidality witness, so the constructor form of the recursion (cons_strictTransformSubspaceSeqAux_succ) is notrflas it is for the weak transform (whose centre argument is the ideal sheaf itself); (A) bridges the two. - (B) The pull-back of a blow-up sequence along a smooth morphism [Kol07, Definition 30.1] at
the ideal level: along the lift
liftStepof a SURJECTIVE local analytic isomorphismh : N → Mto the blowings-up, the strict transform pulls back to the strict transform of the pull-back (strictTransformSubspace_comap_liftStep), then along a whole list (BlowUpSequence.pullback) in the ℕ-indexed,Fin-indexed and last-stage forms (strictTransformSubspaceSeqAux_pullback,strictTransformSubspaceSeq_pullback,strictTransformSubspaceSeq_last_pullbackLiftLast). The saturation stalks correspond under the bijective germ maps of the lift (saturationStalk_comap_liftStep:Ideal.map_colon_pow_of_bijective,totalTransform_comap_liftStep,exceptionalIdealSheaf_comap_liftStep); the two saturations take the same branch of the case distinction instrictTransformSubspace(hasLocalGenerators_saturationStalk_comap_liftStep_iff: local generators pull back,hasLocalGenerators_pullback, and descend along a surjective local isomorphism,HasLocalGenerators.of_map_germMap_of_surjective). The surjectivity ofhis a hypothesis of the formalization forced by that sheaf-wide case distinction — a non-surjectivehsees the branch onNwhileMmay take the other branch off the image; the sources' statement is pointwise, and the map produced by the functoriality clause (restrictMap g U' (imageOpens g hg U')) is surjective. - (C) Deleting the empty blow-ups [Kol07, 34.1]: the final strict transform along the cleaned
list
L.eraseEmptyis the pull-back along the isomorphismeraseEmptyLast⁻¹of the last stages of the one alongL(strictTransformSubspaceSeq_last_eraseEmpty, the counterpart ofweakTransformSeq_last_eraseEmpty): an empty blowing-up is an isomorphism whose exceptional ideal sheaf is the unit ideal, so its strict transform is the pull-back (strictTransformSubspace_blowUpπ_of_eq_empty); the two saturations are then isomorphic sheaves and take one branch. - (D)
strictTransformSubspaceSeq_last_pullback_eraseEmpty: (B) at the last stage composed with (C), the exact right-hand side that the functoriality clause produces. - (B′) The surjectivity of
hin (B) and (D) was needed only for the coincidence of the branches; since every saturation has local generators (saturationStalk_hasLocalGenerators,SaturationFiniteType.lean), the condition of the case distinction is a theorem for every centre, blow-up and ideal sheaf (strictTransformSubspace_eq_ofStalks), so the identities hold along ANY local analytic isomorphism, the open inclusions of Kollár's proof included. The unconditional formsstrictTransformSubspace_comap_liftStep',strictTransformSubspaceSeqAux_pullback',strictTransformSubspaceSeq_pullback',strictTransformSubspaceSeq_last_pullbackLiftLast',strictTransformSubspaceSeq_last_pullback_eraseEmpty'stand beside the surjective ones.
The strict transform of a closed subspace is the saturation of its total transform by the
exceptional divisor [BM97, §3, Proposition 3.13]. Not in the sources; bookkeeping on the pull-back
and deletion transports of the Functor modules and on the saturation stalks.
(A) The strict transform depends on its witnesses only through the set #
(A) The saturation stalks depend on the witnesses only through the centre set and the
blow-down: the total transform depends only on π, the exceptional ideal sheaf only on the set
(IsClosedSubmanifold.idealSheaf_congr).
(A) The strict transform strictTransformSubspace depends on the witnesses only through the
centre set and the blow-down.
(B′) The strict transform always takes its ofStalks branch #
(B′) The strict transform strictTransformSubspace ALWAYS takes its ofStalks branch: the
condition of its case distinction, that the saturation stalks have local generators, is a theorem
for every centre, blow-up and ideal sheaf (saturationStalk_hasLocalGenerators).
(B) One step: the strict transform along the lift of a local isomorphism #
(B) at the stalks: the saturation stalks of the pulled-back ideal sheaf along the pulled-back
blowing-up [Kol07, Definition 30.1] are the images of the saturation stalks under the bijective
germ maps of the lift liftStep — colons by powers transport along a ring isomorphism
(Ideal.map_colon_pow_of_bijective), the total transform and the exceptional ideal sheaf pull back
(totalTransform_comap_liftStep, exceptionalIdealSheaf_comap_liftStep). No surjectivity is
needed here.
(B) For a SURJECTIVE h the two saturations take the same branch of the case distinction in
strictTransformSubspace: local generators pull back along the lift
(hasLocalGenerators_pullback) and descend along the surjective local isomorphism liftStep
(HasLocalGenerators.of_map_germMap_of_surjective).
(B) The pull-back of a blow-up sequence [Kol07, Definition 30.1] at the ideal level, one
step: along the lift of a SURJECTIVE local analytic isomorphism the strict transform pulls back to
the strict transform of the pull-back — in the then branches by the stalk identity
saturationStalk_comap_liftStep, in the else branches by ⊤ pulling back to ⊤, the branches
coinciding by hasLocalGenerators_saturationStalk_comap_liftStep_iff.
(B′) strictTransformSubspace_comap_liftStep WITHOUT surjectivity: both sides take the
ofStalks branch (strictTransformSubspace_eq_ofStalks), and the stalk identity
saturationStalk_comap_liftStep never needed surjectivity.
(B) for any analytic G between the blowings-up lying over h with S = h⁻¹(Y): such a
G is the lift (heq_liftStep_of_forall); the counterpart of weakTransform_comap_of_square.
The constructor form of the recursion of the ideal-theoretic strict transforms #
The recursion of the ideal-theoretic strict transforms on the constructor form, at the level
of the ℕ-indexed auxiliary recursion (the counterpart of cons_weakTransformSeqAux_succ); the
witnesses of the cons — the chosen chart and codimension of the monoidality witness of the first
step, on the cosupport of hY.idealSheaf — are bridged to hY itself by
strictTransformSubspace_congr.
(B) Along a list: the strict transforms of the pull-back by a surjective local #
isomorphism
(B) along a list [Kol07, Definition 30.1]: the ideal-theoretic strict transforms along
h^* L, for a SURJECTIVE h, are the pull-backs along the lifts of the strict transforms along L
(strictTransformSubspace_comap_liftStep at each step; the counterpart of
boundarySeqAux_pullback).
(B), Fin-indexed (the counterpart of boundarySeq_pullback).
(B′) strictTransformSubspaceSeqAux_pullback WITHOUT surjectivity.
(B′) strictTransformSubspaceSeq_pullback WITHOUT surjectivity.
(B′) strictTransformSubspaceSeq_last_pullbackLiftLast WITHOUT surjectivity: (B) at the last
stage along ANY local analytic isomorphism — the open inclusions of [Kol07, Theorem 36, proof]
included.
(B) at the last stage, along the bundled last-stage lift pullbackLiftLast (the counterpart
of strictTransformSeq_last_pullbackLiftLast).
(C) Along the deletion of the empty blow-ups #
(C), one step: the strict transform along the blowing-up of an EMPTY centre is the
pull-back — the exceptional ideal sheaf is the unit ideal (idealSheaf_eq_top_of_eq_empty), so
every colon is the total transform's stalk, which has local generators; the counterpart of
weakTransform_blowUpπ_of_eq_empty.
(C) transport along an equality of lists (the counterpart of
weakTransformSeq_last_stageOfEq).
(C) transport along a diffeomorphism and its lift to the last stages: the strict transform at
the last stage of the transported list Z.map φ is the pull-back along (mapLast φ)⁻¹ of the
strict transform of φ^* J at the last stage of Z (strictTransformSubspace_comap_of_square at
each step; the counterpart of weakTransformSeq_last_map).
(C) Deleting the empty blow-ups [Kol07, 34.1], the ideal-level counterpart of
strictTransformSeq_last_eraseEmpty and weakTransformSeq_last_eraseEmpty: the strict transform
of I at the last stage of the cleaned list is the pull-back along eraseEmptyLast⁻¹ of the one
at the last stage of the list — a kept step contributes the same strict transform on both sides,
a deleted step the pull-back along the empty blowing-up
(strictTransformSubspace_blowUpπ_of_eq_empty), which the transport of the cleaned tail along
Bl_∅ M ≃ M absorbs (strictTransformSubspaceSeq_last_map).
(D) = (B) + (C): the shape the functoriality clause CommutesWithLocalIsos produces #
(D) The final strict transform along (h^* L).eraseEmpty — the right-hand side of the
functoriality clause of IsEmbeddedDesing — is the pull-back of the final strict transform along
L, through the last-stage lift and eraseEmptyLast⁻¹; h SURJECTIVE (the clause's
restrictMap g U' (imageOpens g hg U') is).
(D′) strictTransformSubspaceSeq_last_pullback_eraseEmpty WITHOUT surjectivity: the value of
the functoriality clause at ANY local analytic isomorphism — an OPEN INCLUSION W' ↪ W in
particular — has its final strict transform pulled back from the one over the source, through the
last-stage lift and eraseEmptyLast⁻¹.