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associated graded ring of a regular local ring
Statement
Assume the Axiom of Choice (The Axiom of Choice). If is regular local of dimension , any cotangent basis induces a graded isomorphism . Conversely, if the associated graded ring of a nonzero Noetherian local ring is isomorphic as a graded -algebra to with standard grading, then is regular of dimension .
Facts & Assumptions
Given: The objects and hypotheses in the statement. We work with the Axiom of Choice; cited dependent-choice and resolution-existence hypotheses are retained.
associated graded polynomial surjection: Let be nonzero Noetherian local and let lift a basis of . There is a surjective graded -algebra map , determined by , with every variable of degree one.
regular local graded surjection has zero kernel: Assume the Axiom of Choice. For the graded map defined by a cotangent basis in a nonzero Noetherian local ring, if , then .
The degree of the Hilbert-Samuel polynomial equals the dimension of the support: Assume the Axiom of Choice. Let be a Noetherian local ring, let be a finite -module, and let be an ideal of definition for . Then the Hilbert-Samuel polynomial has degree
Proof
In a regular local ring the cotangent dimension equals . The polynomial map is surjective and has zero kernel, hence is the claimed isomorphism.
Conversely, the degree-one component of a supplied graded isomorphism has dimension , so . Its cumulative graded dimensions are , also when , where the count is one. These are the lengths of . The Hilbert–Samuel dimension theorem gives , proving regularity.
Depends on
Used by
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- regular local hilbert samuel multiplicity one Corollary
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- The monomial part, the non-monomial part and the companion ideal Definition
- A holomorphic extension of a rational map on a product of smooth complex curves is algebraic Lemma
- A marked ideal is equivalent to its powers Lemma
- A point blowup lowers pairwise contact order by one and separates transverse branches Lemma
- Addition and multiplication of marked ideals Lemma
- An automorphism of the completed local ring matching two tangent directions preserves the homogenization Lemma
- Blowing up a multiple point separates pairwise transverse components Lemma
- Coprime tangent cones force a power of the maximal ideal into the local ideal Lemma
- Graded Nakayama and the Hesselink regularity comparison Lemma
- Iterated derivative ideals preserve support in the safe characteristic range Lemma
- Order and simultaneous normal crossings are preserved by smooth morphisms Lemma
- Point blowups of regular surfaces stay regular, with rational exceptional curves at two-dimensional local rings Lemma
- regular local domain induction Lemma
- The truncated multiplication map is injective exactly when the tangent cones are coprime Lemma
- Total transform equals strict transform plus multiplicity times the exceptional divisor Lemma
- Fixed-point schemes and centralizers of linearly reductive actions Theorem
- Point blowups of regular surfaces stay regular, with rational exceptional fibre over the residue field Theorem
Dependency tree · two levels
14 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- Proposition 25.6, p.67 (standard reference, not scraped)