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Regular local surfaces have rational modification cohomology

Statement

Assume AC and DC. A regular two-dimensional local domain in the permitted finite-type class defines a rational singularity.

Facts & Assumptions

Given: A regular two-dimensional local domain A in the permitted finite-type class.

[F1]

def-axiom-of-choice. The Axiom of Choice (AC) is the following statement. > Every family of nonempty sets has a choice function > (def-choice-function). Written out: for every set F all of whose members are nonempty, there exists a function g with domain F satisfying g(S)∈S for all S∈F. (The Axiom of Choice)

[F2]

def-dependent-choice. Let X be a set and let R⊆X×X be a binary relation on X. Call R entire on X when for every x∈X there is y∈X with xRy. The Axiom of Dependent Choice, written DC, is the following statement. (The axiom of dependent choice: a relation in which every element is related to something admits an N-indexed chain)

[F3]

def-rational-normal-surface-singularity-and-bounded-modification-h1. Assume AC and DC. A normal two-dimensional Noetherian local domain A essentially of finite type over a field or complete equicharacteristic local base defines a rational singularity if H1(Y,OY)=0 for every normal integral proper modification Y→Spec⁡A. Bounded modification H1 means these modules have uniformly bounded A-length. (Rational normal surface singularities and bounded modification cohomology)

[F4]

lem-blowup-point-pushforward-vanishing. Assume the Axiom of Choice. Let S be a regular surface over a field k (more generally a locally Noetherian scheme of dimension two whose local rings at the center are regular of dimension two) and let p be a closed point with residue field κ(p). Let π ⁣:S′→S be the blowup of p with exceptional curve E. (Pushforward and vanishing for point blowups on a surface)

[F5]

lem-normal-surface-modification-leray-short-exact-sequence. Assume AC and DC. Let A be a normal local domain of dimension two in the field/complete-equicharacteristic finite-type class, and X′→gX→Spec⁡A normal integral modifications. Then g∗OX′=OX and H1(X,OX)→H1(X′,OX′) is injective. (The Leray sequence for normal surface modifications)

[F6]

lem-normalized-point-blowups-dominate-local-normal-surface-modifications. Assume AC and DC. Let A be a normal two-dimensional Noetherian local domain essentially of finite type over a field or a complete equicharacteristic Noetherian local ring. Let S be a normal integral modification of Spec⁡A, and let Y→S be an integral modification with normal Y. (Normalized point blowups dominate local normal surface modifications)

Proof

1.1F3F6given

Every proper normal modification of Spec⁡A is dominated by a finite sequence of normalized point blowups, and because the base is regular these are ordinary regular point blowups.

2.1F4step 1.1

The point-blowup pushforward theorem, in its arbitrary-Noetherian regular-center form, makes every higher direct image of the structure sheaf under a point blowup vanish; hence the higher direct images of the structure sheaf under the composite of the sequence vanish, and H1 of the affine base is zero.

3.1F1F2F3F5step 2.1∎

The Leray injection embeds the H1 of the original normal modification into the H1 of its dominating model, which is zero; since the modification was arbitrary, A defines a rational singularity. The Axiom of Choice and the Axiom of Dependent Choice are inherited from the cited suppliers.

Remarks

  • Rationality of a regular local surface is thus reduced to the vanishing of higher direct images of a point blowup.
  • No classification of resolutions is used, only domination by point blowups.

Depends on

Used by

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Sources