The separation step of the marked order reduction: the invariant and the separating ideal #
The second step of Kollár's proof of the marked order reduction theorem [Kol07, Theorem 107]
separates the cosupport of (I, m) from the cosupport of the nonmonomial part N(I)
[Kol07, 111, Step 2]. With s the maximal order of N(I) along cosupp(I, m) = {x | ord_x I ≥ m},
one applies the order reduction BO_{n, ms} [Kol07, Theorem 68] to the single ideal
N(I)^m + I^s, by the observation
ord_Z J₁ ≥ s and ord_Z J₂ ≥ m ⇔ ord_Z (J₁^m + J₂^s) ≥ ms,
so that every smooth blow-up sequence of order ms for N(I)^m + I^s is one of order s for
N(I) and one of order m for I; then s has dropped and one continues with s − 1, until
s = 0, i.e. cosupp(I, m) ∩ cosupp N(I) = ∅.
On an analytic manifold the rounds of this descent are links of a chain over shrinking relatively
compact opens (ChainState), and Kollár's s is carried as an invariant of the state rather
than read as a supremum: SepOrdLe S m s says ord N(𝓘) ≤ s at every point of cosupp(𝓘, m).
The descent starts at the bound s = t − 1 with which the first step exits (NonmonomialOrdLe)
and runs through s = t − 1, …, 1; a round at a bound above the actual separation order is empty
up to the deletion of empty blow-ups [Kol07, 32]. This module provides:
- the observation along a smooth centre (
IdealSheaf.le_ordAlongIdeal_pow_add_pow_iff), from the characterisation of the order along a centre by the pointwise orders at the nearby points of the centre (IdealSheaf.le_ordAlongIdeal_iff_eventually_le_ord) and the pointwise formIdealSheaf.le_ord_pow_add_pow_iff; and the additivity of pull-back,IdealSheaf.pullback_add; - the transform identity modulo
N,NonmonomialTransformIdentityMod: the predicate, which the assembly takes as a parameter and whichnonmonomialTransformIdentityMod_inhabitantproves; - the separation invariant
SepOrdLe, the two exit facts ats = 0, the separating ideal and triplesepIdealAtandsepTripleAtat an explicits, the order boundord_sepIdealAt_le, and the classSepClass m sof triples whose separating triple lies in the domain ofBO_{n, ms}; - the order reduction read at the separating triple,
sepFunctor, a family functor onSepClass m swhich commutes with local analytic isomorphisms and is indifferent to empty boundary members, so that the transport lemmas for values at induced triples apply to the links of the separation step.
The scheme-theoretic counterparts are Hironaka.BMO.sepTriple and
AlgebraicGeometry.Scheme.IdealSheafData.leOrdAlong_pow_sup_pow_iff.
The observation of the separation step along a smooth centre #
The order of I along a smooth centre Y at a point a ∈ Y is ≥ p iff the pointwise order
of I is ≥ p at all points of Y near a. Forwards: the order along Y is locally constant
on Y (isLocallyConstant_ordAlong) and bounds the pointwise order from below
(ordAlong_le_ord); backwards: stalkIdeal_le_pow_of_eventually_le_ord.
The observation of [Kol07, 111, Step 2] along a smooth centre: for m, s ≥ 1 and a ∈ Y,
ms ≤ ord_{Y,a} (J₁^m + J₂^s) iff s ≤ ord_{Y,a} J₁ and m ≤ ord_{Y,a} J₂. It is the pointwise
statement IdealSheaf.le_ord_pow_add_pow_iff at the points of Y near a, through the
characterisation le_ordAlongIdeal_iff_eventually_le_ord.
The pull-back of a sum of ideal sheaves is the sum of the pull-backs (stalkwise
Ideal.map_sup).
The transform identity modulo N, as a predicate #
The transform identity modulo N along a list of centres: for a list of order ≥ m for
(𝓘, m) and of order ≥ s for (N(𝓘), s), with m ≥ 1, no bound on the order of N(𝓘) and no
relation between m and s, the nonmonomial part of the marked transform of (𝓘, m) is the
nonmonomial part of the marked transform of (N(𝓘), s) at every stage, with respect to the
transformed boundary. This is the observation at the end of [Kol07, 111, Step 1] (the two
birational transforms differ by a product of powers of the exceptional divisors, hence only in their
monomial part) read modulo the monomial part. The descent of the separation step
uses it at s < m, where the identity at the exact order (NonmonomialTransformIdentity, which
needs m ≤ d) does not apply. The assembly takes the predicate as a parameter;
nonmonomialTransformIdentityMod_inhabitant proves it.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The separation invariant and the separating triple at an explicit s #
The separation invariant: ord N(𝓘) ≤ s at every point where ord 𝓘 ≥ m. It carries
Kollár's s, "the maximum order of N(I) along cosupp(I, m)" [Kol07, 111, Step 2], as an upper
bound rather than as a supremum. SepOrdLe S m 0 is the disjointness
cosupp(𝓘, m) ∩ cosupp N(𝓘) = ∅ (sepOrdLe_zero_iff).
Equations
- Hironaka.Manifold.BMO.SepOrdLe S m s = ∀ (x : ↑↧M.carrier), ↑m ≤ Manifold.IdealSheaf.ord S.I x → Manifold.IdealSheaf.ord (Hironaka.Manifold.BMO.nonmonomialTriple S).I x ≤ ↑s
Instances For
The separation bound 0 is the disjointness of the cosupports: no point of cosupp(𝓘, m)
lies in cosupp N(𝓘) [Kol07, 111, Step 2].
At the exit of the separation step, ord 𝓘 < m at every point of cosupp N(𝓘); hence the
centre of any further blow-up, which lies in cosupp(𝓘, m), misses cosupp N(𝓘)
[Kol07, 111, Step 2].
Off cosupp N(𝓘) the stalk of 𝓘 is the stalk of its monomial part: 𝓘 = M(𝓘) · N(𝓘)
(monomialPart_mul_nonmonomialPart) and N(𝓘)_x = 𝒪_x there. This is the pointwise form of
"replace X by X ∖ cosupp N(I) and thus assume that I = M(I)" [Kol07, 111, Step 2].
The separating ideal N(𝓘)^m + 𝓘^s of [Kol07, 111, Step 2] at an explicit s; the sum is
the sum of ideal sheaves, stalkwise the join.
Equations
- Hironaka.Manifold.BMO.sepIdealAt S m s = (Hironaka.Manifold.BMO.nonmonomialTriple S).I ^ m + S.I ^ s
Instances For
The separating ideal is nonzero at every point: its summand N(𝓘)^m is nonzero in the stalk,
which is a domain (for m = 0 the summand is the unit ideal).
The separating triple (M, N(𝓘)^m + 𝓘^s, E) of [Kol07, 111, Step 2] at an explicit s:
the manifold and the boundary of S, with the separating ideal.
Equations
- Hironaka.Manifold.BMO.sepTripleAt S m s = { I := Hironaka.Manifold.BMO.sepIdealAt S m s, isNonzeroEverywhere := ⋯, F := S.F, isSnc := ⋯ }
Instances For
Under the separation invariant at s ≥ 1 (and m ≥ 1) the separating ideal has order at most
ms at every point, so that the separating triple lies in the domain of BO_{n, ms} (Kollár's
"which has order ≥ ms" [Kol07, 111, Step 2] is the lower bound at the centres; this is the upper
bound the order reduction theorem needs). The order of a sum is the minimum of the orders and the
order of a power is the multiple (the cosupport identities of [Kol07, Definition 59 (3), (4)], read
on orders); on
cosupp(𝓘, m) the bound comes from ord N(𝓘) ≤ s, off it from ord 𝓘 < m.
The separating triple of a triple whose boundary is replaced by an empty extension is the
separating triple with the boundary replaced: the nonmonomial part ignores empty members
(nonmonomialTriple_eq_of_isEmptyExtension). This is the input of the indifference property of
the round.
The exit bound of the first step, ord N(𝓘) ≤ d everywhere, gives the separation invariant at
d: the separation step starts at the bound the first step exits with (stepAChainState_bound),
with no new supremum.
The class of triples whose separating triple at (m, s) lies in the domain of BO_{n, ms}
[Kol07, Theorem 68]: ms ≥ 1, order at most ms everywhere, finitely many nonempty boundary
members. It is the domain of the round of the separation step.
Equations
- Hironaka.Manifold.BMO.SepClass m s T = Manifold.AnalyticTriple.BOClass (m * s) (Hironaka.Manifold.BMO.sepTripleAt T m s)
Instances For
A triple of the marked class which satisfies the separation invariant at s ≥ 1 lies in
SepClass m s: the mark ms ≥ 1, the order bound ord_sepIdealAt_le, and the finiteness of the
nonempty members from BMOClass.
The class is indifferent to empty boundary members: deleting or adding empty members keeps the
separating ideal (sepTripleAt_eq_of_isEmptyExtension) and the finiteness of the nonempty
members.
The class of the round and the order-reduction family read at the separating triple #
The separating triple of a pull-back is the pull-back of the separating triple — the nonmonomial
part commutes with pull-back (NonmonomialComap), and pull-back is additive and multiplicative
(pullback_add, pullback_pow).
The class is closed under pull-back along local analytic isomorphisms (sepTripleAt_pullback,
boClass_of_isPullbackOf).
The order reduction BO_{n, ms} read at the separating triple, as a family functor on
SepClass m s: the value of bo at (M, N(𝓘)^m + 𝓘^s, E), re-typed as a family for (M, 𝓘, E)
(CompatibleFamily.changeTriple; the triple is not part of the data of a compatible family).
This is "we apply order reduction to the ideal N(I)^m + I^s" [Kol07, 111, Step 2]. The link of
the separation step is ChainState.linkWith at its value on the induced triple, so the transport
lemmas for values at induced triples (AnalyticFamilyFunctor.inducedValue_rel) apply.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The family read at the separating triple commutes with local analytic isomorphisms
([Kol07, Theorem 103 (2)] for the composite): the separating triple of a pull-back is the
pull-back of the separating triple (sepTripleAt_pullback), and bo commutes.
The family read at the separating triple is indifferent to empty boundary members (the
counterpart, for boundary members, of [Kol07, 32]): the separating triple of the triple with the
replaced boundary is
the separating triple with the boundary replaced (sepTripleAt_eq_of_isEmptyExtension), and bo
is indifferent.