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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 in the permitted finite-type class.
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 all of whose members are nonempty, there exists a function with domain satisfying for all . (The Axiom of Choice)
def-dependent-choice. Let be a set and let be a binary relation on . Call entire on when The Axiom of Dependent Choice, written , is the following statement. (The axiom of dependent choice: a relation in which every element is related to something admits an -indexed chain)
def-rational-normal-surface-singularity-and-bounded-modification-h1. Assume AC and DC. A normal two-dimensional Noetherian local domain essentially of finite type over a field or complete equicharacteristic local base defines a rational singularity if for every normal integral proper modification . Bounded modification H1 means these modules have uniformly bounded -length. (Rational normal surface singularities and bounded modification cohomology)
lem-blowup-point-pushforward-vanishing. Assume the Axiom of Choice. Let be a regular surface over a field (more generally a locally Noetherian scheme of dimension two whose local rings at the center are regular of dimension two) and let be a closed point with residue field . Let be the blowup of with exceptional curve . (Pushforward and vanishing for point blowups on a surface)
lem-normal-surface-modification-leray-short-exact-sequence. Assume AC and DC. Let be a normal local domain of dimension two in the field/complete-equicharacteristic finite-type class, and normal integral modifications. Then and is injective. (The Leray sequence for normal surface modifications)
lem-normalized-point-blowups-dominate-local-normal-surface-modifications. Assume AC and DC. Let be a normal two-dimensional Noetherian local domain essentially of finite type over a field or a complete equicharacteristic Noetherian local ring. Let be a normal integral modification of , and let be an integral modification with normal . (Normalized point blowups dominate local normal surface modifications)
Proof
Every proper normal modification of is dominated by a finite sequence of normalized point blowups, and because the base is regular these are ordinary regular point blowups.
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 of the affine base is zero.
The Leray injection embeds the of the original normal modification into the of its dominating model, which is zero; since the modification was arbitrary, 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
- The Axiom of Choice
- The axiom of dependent choice: a relation in which every element is related to something admits an $\mathbb{N}$-indexed chain
- Rational normal surface singularities and bounded modification cohomology
- Pushforward and vanishing for point blowups on a surface
- The Leray sequence for normal surface modifications
- Normalized point blowups dominate local normal surface modifications
Used by
Dependency tree · two levels
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Sources
- The Stacks Project, Resolution of Surfaces, Sections 54.8–54.9: complete source arguments with local prerequisite replacements (standard reference, not scraped)