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Finite-length duality over a regular local base
Statement
Assume AC and DC. Let be regular Noetherian local of dimension and let be a module-finite local -algebra with the map local. For finite-length -modules put with its natural -action. Then for , is exact contravariantly, by canonical bidual evaluation, and .
Facts & Assumptions
Given: A regular Noetherian local ring of dimension , a module-finite local -algebra , and finite-length -modules .
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)
cor-koszul-complex-resolves-a-regular-quotient. If is finite free and is -regular, then is a finite free resolution of . (Koszul Complex Resolves A Regular Quotient)
thm-regular-local-rings-are-domains-and-cohen-macaulay. Assume the Axiom of Choice (The Axiom of Choice). A regular local ring of dimension is a domain and Cohen–Macaulay. For every regular system , the tuple is -regular and is regular local of dimension for all . (regular local rings are domains and cohen macaulay)
thm-long-exact-ext-sequence-in-the-first-variable. Assume the Axiom of Dependent Choice. Let be abelian with enough projectives and enough injectives, and fix supplied projective and injective resolution data on all its objects. (The long exact Ext sequence in the first variable)
lem-finite-regular-base-algebra-dualizing-biduality. Assume AC and DC. Let be a regular Noetherian ring of finite dimension , and let be a module-finite -algebra. For any integer , is a dualizing complex over . (Dualizing biduality for finite algebras over a regular base)
Proof
A finite-length -module has finite length over : the residue field extension is finite because is module-finite, and every -composition factor is a finite-dimensional -vector space.
A regular system of parameters of generates and is a regular sequence, so the Koszul complex resolves ; its signed self-duality gives for and .
Induction on an -composition series of using the long exact Ext sequence in the first variable shows for every , and that is an exact contravariant functor on finite-length -modules.
The finite-base bidual lemma applied with the shift identifies with , which in degree zero is exactly ; applying the adjunction a second time gives the canonical bidual evaluation as -modules.
Multiplication by on is induced contravariantly from multiplication by on , so if annihilates it annihilates , and conversely applying again and using the bidual evaluation returns the statement for ; hence .
The Axiom of Choice and the Axiom of Dependent Choice are inherited from the Koszul, duality and long-exact-sequence suppliers; no general Matlis duality is asserted.
Remarks
- Exactness and concentration are proved by induction on length, so no structural theorem about finite-length modules beyond the Koszul base case is needed.
- The annihilator statement is what makes the duality usable for detecting whether an ideal acts faithfully in the applications.
Depends on
- The Axiom of Choice
- Dualizing biduality for finite algebras over a regular base
- Koszul Complex Resolves A Regular Quotient
- regular local rings are domains and cohen macaulay
- The long exact Ext sequence in the first variable
- The axiom of dependent choice: a relation in which every element is related to something admits an $\mathbb{N}$-indexed chain
Used by
Dependency tree · two levels
28 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
- The Stacks Project, Resolution of Surfaces, 54.8.8 and 54.11.6: exact imports replaced by the local argument (standard reference, not scraped)