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Canonical resolutions commute with embeddings of ambient smooth schemes

Statement

Assume AC (The Axiom of Choice).

Let K have characteristic zero (Field) and let φ ⁣:X↪X+ be a closed immersion of smooth K-schemes of finite type (Closed immersions of schemes) and let I⊆OX be a coherent ideal sheaf that is not identically zero on any irreducible component of X, with I+ the inverse-image ideal of φ∗I under the quotient map OX+→φ∗OX. For the marked ideals (I,∅,1) on X and (I+,∅,1) on X+ the canonical resolutions are related, at points x∈supp⁡(I,1), by inv⁡I+(x)=(0,1,0,0;0,1,0,0;… ;0,1,0,0;inv⁡I(x)),νI+(x)=νI(x),ρI+(x)=ρI(x), with (0,1,0,0) repeated k times, where k is the local codimension of the smooth immersion at x, and resolving (X+,I+,∅,1) is locally equivalent to resolving (X,I,∅,1) (Canonical resolutions with invariants of a marked ideal).

Facts & Assumptions

Given: A characteristic-zero field K, a closed embedding φ ⁣:X↪X′ of smooth finite-type K-schemes, and a marked ideal (I,∅,1) on X generically nonzero on every component. Let I′ be the inverse image of φ∗I under OX′→φ∗OX; locally the ideal of X is generated by regular parameters u1,…,uk∈I′.

[A1]

The Axiom of Choice: AC is assumed for the canonical-resolution consumer clauses and their cited AC-dependent construction suppliers.

[F1]

Closed immersions of schemes, Marked ideals and their support: the quotient map to φ∗OX defines I′ as the ideal of the same closed subscheme of X viewed inside X′. Locally it is generated by the equations u1,…,uk of X and lifts of generators of I. It is coherent because the affine smooth coordinate rings are Noetherian, its restriction to X is I, and its support for mark 1 is the image of supp⁡(I,1).

[F2]

Marked ideals of maximal order, tangent directions and transversality to the exceptional divisors, Giraud's tangent-direction lemma, The supports of a multiple test blow-up stay inside the strict transforms of a hypersurface of maximal contact: the sections u1,…,uk are tangent directions of (I′,1) at points of X, and along the canonical resolution the supports are contained in the strict transforms of the hypersurfaces V(ui).

[F3]

Canonical resolution of marked ideals: the canonical resolution of a marked ideal is built by the algorithm of Steps 1–2, whose reductions depend only on intrinsic data (supports, orders, derivative ideals and the invariants).

Proof

1.1A1F1F2F3

The invariant of the embedded ideal. Running Steps 2a and 1bb of the algorithm for (I′,∅,1) at a point x∈X: in Step 2a the ideal is of maximal order with ord⁡N=1, so the companion ideal is I′ itself and the invariant receives the first coordinate 1; in Step 1bb (non-boundary case s(x)=0) the resolution passes to a hypersurface of maximal contact. Passing successively to the tangent directions u1,…,uk, each passage adjoins the source pair (1,0), encoded as the four rational coordinates (0,1,0,0) under [F3], to the invariant by [F2], and after k passages one arrives at the restriction to X, where the invariant of (I,1) appears. Hence inv⁡I′(x)=(0,1,0,0;… ;0,1,0,0;inv⁡I(x)) with (0,1,0,0) repeated k times, and analogously νI′(x)=νI(x) and ρI′(x)=ρI(x).

2.1A1F2F3step 1.1∎

The resolutions correspond. By [F3] the centers of the resolution of (I′,1) are the maximal loci of the invariants computed in step 1.1; because the first 4k encoded coordinates of inv⁡I′ are constant on the image of the resolution of (I,1), the maximal locus over X is the image of the maximal locus of inv⁡I, and blowing it up in X′ restricts to blowing it up in X. Iterating, the canonical resolution of (I′,1) restricts over X to the canonical resolution of (I,1), with the invariant comparison of step 1.1 (the prefixed constant pairs are removed on restriction).

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