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Finite-length duality over a regular local base

Statement

Assume AC and DC. Let (R,m) be regular Noetherian local of dimension d and let (B,n) be a module-finite local R-algebra with the map local. For finite-length B-modules put T(M)=Ext⁡Rd(M,R) with its natural B-action. Then Ext⁡Ri(M,R)=0 for i≠d, T is exact contravariantly, M≅T(T(M)) by canonical bidual evaluation, and Ann⁡BT(M)=Ann⁡BM.

Facts & Assumptions

Given: A regular Noetherian local ring (R,m) of dimension d, a module-finite local R-algebra (B,n), and finite-length B-modules M.

[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]

cor-koszul-complex-resolves-a-regular-quotient. If M is finite free and x is M-regular, then K(x;M) is a finite free resolution of M/(x)M. (Koszul Complex Resolves A Regular Quotient)

[F4]

thm-regular-local-rings-are-domains-and-cohen-macaulay. Assume the Axiom of Choice (The Axiom of Choice). A regular local ring R of dimension d is a domain and Cohen–Macaulay. For every regular system (x1,…,xd), the tuple is R-regular and R/(x1,…,xc) is regular local of dimension d−c for all 0≤c≤d. (regular local rings are domains and cohen macaulay)

[F5]

thm-long-exact-ext-sequence-in-the-first-variable. Assume the Axiom of Dependent Choice. Let A 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)

[F6]

lem-finite-regular-base-algebra-dualizing-biduality. Assume AC and DC. Let R be a regular Noetherian ring of finite dimension d, and let B≠0 be a module-finite R-algebra. For any integer s, DB=RHom⁡R(B,R[s]) is a dualizing complex over B. (Dualizing biduality for finite algebras over a regular base)

Proof

1.1F4given

A finite-length B-module has finite length over R: the residue field extension κ(n)/κ(m) is finite because B is module-finite, and every B-composition factor is a finite-dimensional κ(m)-vector space.

2.1F3F4step 1.1

A regular system of parameters of R generates m and is a regular sequence, so the Koszul complex resolves R/m; its signed self-duality gives Ext⁡Ri(R/m,R)=0 for i≠d and Ext⁡Rd(R/m,R)=R/m.

3.1F5step 2.1

Induction on an R-composition series of M using the long exact Ext sequence in the first variable shows Ext⁡Ri(M,R)=0 for every i≠d, and that T(M)=Ext⁡Rd(M,R) is an exact contravariant functor on finite-length B-modules.

4.1F6step 3.1

The finite-base bidual lemma applied with the shift s=d identifies RHom⁡B(M,DB) with RHom⁡R(M,R[d]), which in degree zero is exactly T(M); applying the adjunction a second time gives the canonical bidual evaluation M≅T(T(M)) as B-modules.

5.1F4F6step 4.1

Multiplication by b∈B on T(M) is induced contravariantly from multiplication by b on M, so if b annihilates M it annihilates T(M), and conversely applying T again and using the bidual evaluation returns the statement for M; hence Ann⁡BT(M)=Ann⁡BM.

6.1F1F2step 5.1∎

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

Used by

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Sources