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Relative derived duality on projective space over a regular local ring

Statement

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). It respects shifts, triangles and multiplication by homogeneous sections.

Facts & Assumptions

Given: A regular Noetherian local ring R of finite dimension, the projective space P=PRN with W=OP(−N−1), the residue t on the all-minus-one Laurent monomial, and K∈DCohb(P).

[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]

lem-relative-projective-space-regular-local-base-twisted-resolution. Assume AC and DC. Let R be a regular Noetherian local ring of finite dimension d, P=PRN, and F a coherent sheaf on P. There is a finite resolution of F by finite direct sums of twists OP(m), of length at most d+N+1. Every bounded coherent complex on P is perfect and belongs to the triangulated subcategory generated by these twists. (Finite twisted resolutions over a regular local base)

[F4]

thm-cohomology-projective-space-twisting-sheaves. Assume the Axiom of Choice (The Axiom of Choice). Let A be a commutative ring with 1 (def-commutative-ring), let n≥0, let d∈Z and let X=PAn≅Proj⁡A[x0,…,xn] be relative projective space (def-relative-projective-space-standard-charts, def-polynomial-ring-on-a-family-of-indeterminates), with twisting sheaf (Cohomology of O(d) on projective space)

[F5]

def-cup-product-sheaf-cohomology. Assume the Axiom of Choice (The Axiom of Choice) and the standing smallness or supplied cofinal-denominator hypothesis of def-derived-category-of-an-abelian-category. (Cup product in sheaf cohomology)

[F6]

thm-cech-computes-qc-cohomology-separated-scheme-affine-cover. Assume the Axiom of Choice, inherited from sheaf cohomology. Let X be a quasi-compact separated scheme (def-separated-morphism-schemes), let U0,…,Ur be a finite affine open cover of X and let F be a quasi-coherent OX-module (def-quasi-coherent-module-scheme). (Cech cohomology computes quasi-coherent cohomology on a separated scheme)

[F7]

thm-hom-tensor-adjunction-for-modules. Let R be a commutative ring and let M,N,P be R-modules. There is a natural R-module isomorphism Hom⁡R(M⊗RN,P)≅Hom⁡R(M,Hom⁡R(N,P)). It sends F to the map m↦[n↦F(m⊗n)] and sends u:M→Hom⁡R(N,P) to the homomorphism determined by m⊗n↦u(m)(n). (Hom-tensor adjunction: Hom⁡R(M⊗RN,P)≅Hom⁡R(M,Hom⁡R(N,P)))

[F8]

thm-five-lemma-for-a-morphism-of-long-exact-sequences. Let ⋯→An−1→An→An+1→An+2→An+3→⋯ and ⋯→Bn−1→Bn→Bn+1→Bn+2→Bn+3→⋯ be long exact sequences in an abelian category, together with a morphism of these sequences. (Five lemma for a morphism of long exact sequences)

Proof

1.1F4F6given

The cohomology of every twist on P is computed by the Laurent-monomial complex of the standard cover, which decomposes as a direct sum over exponent vectors; a vector with some but not all negative entries gives a contractible complex, all-nonnegative vectors contribute only in degree zero and all-negative vectors only in degree N. The residue t is defined by the monomial pairing on the unique all-minus-one vector.

2.1F5F7step 1.1

For a single twist K=O(m) the monomial pairing xa⋅x−a−1↦1 identifies RΓ(P,R ⁣Hom⁡P(O(m),W[N])) with RHom⁡R(RΓ(P,O(m)),R): the finite free degree-zero cohomology is dual to the corresponding top cohomology, all other groups vanish, and the identification is compatible with polynomial multiplication by direct multiplication of Laurent monomials.

3.1F7F8step 2.1

Both sides of the asserted map are triangulated functors in K, the right-hand side being derived Hom rather than an ordinary dual; the monomial complexes above are direct sums of contractible summands and shifted finite free modules, so their derived global sections are their cohomology, and the map is a quasi-isomorphism for twists, finite sums and shifts.

4.1F3F8step 3.1

Every bounded coherent complex on P has a finite resolution by finite direct sums of twists by the twisted-resolution lemma; cones of the successive truncations and the five lemma propagate the quasi-isomorphism to all such complexes, with the same naturality.

5.1F1F2step 4.1∎

Chain evaluation and the signed total differential of the resolutions give compatibility with shifts and triangles, and the monomial calculation gives the trace normalization; for N=0 the statement reduces to ordinary perfect-complex biduality over R. The Axiom of Choice and the Axiom of Dependent Choice are retained from the resolution suppliers.

Remarks

  • The statement is relative: the dualizing object is W[N] over R and the right-hand side is derived Hom over the local ring, not a field-dual of cohomology.
  • The Laurent residue fixes the normalization of the trace; without it the identification is only up to a unit of R.

Depends on

Used by

Dependency tree · two levels

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Sources