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Projective coherent duality over a regular local base

Statement

Assume AC and DC. Let R be regular Noetherian local of dimension d, let X be projective over R, and fix i:X↪P=PRN. Put DX=i!(OP(−N−1)[N+d]). Then DX is a dualizing complex on X, with coherent biduality. For every bounded coherent complex K on X, trace/evaluation gives RΓ(X,R ⁣HomX(K,DX))≅RHom⁡R(RΓ(X,K),R[d]). The pairing, not just its vector-space shadow, is canonical for the supplied embedding.

Facts & Assumptions

Given: A regular Noetherian local ring R of dimension d, a projective R-scheme X with a fixed closed immersion i ⁣:X↪P=PRN, the complex DX=i!(OP(−N−1)[N+d]), and a bounded coherent complex K on X.

[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-dualizing-complex-on-projective-cm-scheme. A dualizing complex on a Noetherian scheme X is an object DX∈DCohb(X) such that, locally on affine open neighborhoods U=Spec⁡B, its corresponding complex has finite injective dimension over B and the homothety map B→RHom⁡B(DX∣U,DX∣U) is an isomorphism. (Dualizing complexes and the normalized dualizing sheaf on a projective CM scheme)

[F3]

lem-relative-projective-space-derived-duality-regular-local-base. Assume AC and DC. Let R be regular Noetherian local of finite dimension and P=PRN. Write W=OP(−N−1). Laurent residue gives t:RΓ(P,W[N])→R. For every K∈DCohb(P), evaluation followed by t gives a natural quasi-isomorphism RΓ(P,R ⁣HomP(K,W[N]))≅RHom⁡R(RΓ(P,K),R). (Relative derived duality on projective space over a regular local ring)

[F4]

lem-finite-closed-immersion-derived-coinduction-adjunction. Assume AC. For a finite homomorphism A→B of Noetherian rings and G∈D+(A), the complex f!G=RHom⁡A(B,G) has its natural B-action and is right adjoint to restriction of scalars. (Derived adjunction for finite rings and closed immersions)

[F5]

lem-regular-quotient-dualizing-complex-and-biduality. Assume AC. Let A be a regular Noetherian ring of finite dimension n, let L be an invertible A-module, and let B=A/I be nonzero. For any integer s, DB=RHom⁡A(B,L[s]) is a dualizing complex over B. (Dualizing complexes and coherent biduality for regular-ring quotients)

[F6]

thm-localisation-and-polynomial-extension-of-regular-rings. Assume the Axiom of Choice (The Axiom of Choice). Localizations and finite polynomial extensions of a commutative regular Noetherian ring are regular. Regularity can equivalently be tested at maximal ideals. For every nonzero such ring, gldim⁡R=dim⁡R, allowing infinity. (localisation and polynomial extension of regular rings)

[F7]

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)

Proof

1.1F5F6given

On an affine chart Spec⁡A of P the ring A is a regular finite-dimensional polynomial ring over R and X is cut out by an ideal I with X∩Spec⁡A=Spec⁡A/I; the ambient twist is invertible, so the regular-quotient biduality supplier applies over A and shows that i! produces a local dualizing complex with coherent biduality.

2.1F4F5step 1.1

The coinduction adjunction for the closed immersion uses the natural quotient action and evaluation at one, so the local chart complexes and their biduality structures glue to a global dualizing complex DX on X.

3.1F3step 2.1

Uniform finite twist resolutions over P bound the coherence degrees of i! applied to the finite twists, so DX is bounded with coherent cohomology.

4.1F4step 3.1

Closed-immersion internal Hom adjunction identifies i∗R ⁣Hom⁡X(K,DX) with R ⁣Hom⁡P(i∗K,OP(−N−1)[N+d]), and closed-immersion cohomology comparison identifies the global sections of the two sides.

5.1F1F2F3F7step 4.1∎

Applying the relative projective-space duality, shifted by d, gives the natural quasi-isomorphism RΓ(X,R ⁣Hom⁡X(K,DX))≅RHom⁡R(RΓ(X,K),R[d]); its trace is the coinduction counit followed by the Laurent residue, so it is the actual compatible composition pairing. Coherent biduality holds locally by step 1.1 and hence globally, and the Axiom of Choice and the Axiom of Dependent Choice are inherited from the cited suppliers.

Remarks

  • The dualizing complex is defined from the fixed embedding; the statement is canonical for that embedding, not independent of it.
  • The concentration of this complex into a single dualizing module on a normal surface modification is not asserted here.

Depends on

Used by

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Sources