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Relative derived duality on projective space over a regular local ring
Statement
Assume AC and DC. Let be regular Noetherian local of finite dimension and . Write . Laurent residue gives . For every , evaluation followed by gives a natural quasi-isomorphism . It respects shifts, triangles and multiplication by homogeneous sections.
Facts & Assumptions
Given: A regular Noetherian local ring of finite dimension, the projective space with , the residue on the all-minus-one Laurent monomial, and .
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)
lem-relative-projective-space-regular-local-base-twisted-resolution. Assume AC and DC. Let be a regular Noetherian local ring of finite dimension , , and a coherent sheaf on . There is a finite resolution of by finite direct sums of twists , of length at most . Every bounded coherent complex on is perfect and belongs to the triangulated subcategory generated by these twists. (Finite twisted resolutions over a regular local base)
thm-cohomology-projective-space-twisting-sheaves. Assume the Axiom of Choice (The Axiom of Choice). Let be a commutative ring with (def-commutative-ring), let , let and let 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)
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)
thm-cech-computes-qc-cohomology-separated-scheme-affine-cover. Assume the Axiom of Choice, inherited from sheaf cohomology. Let be a quasi-compact separated scheme (def-separated-morphism-schemes), let be a finite affine open cover of and let be a quasi-coherent -module (def-quasi-coherent-module-scheme). (Cech cohomology computes quasi-coherent cohomology on a separated scheme)
thm-hom-tensor-adjunction-for-modules. Let be a commutative ring and let be -modules. There is a natural -module isomorphism It sends to the map and sends to the homomorphism determined by . (Hom-tensor adjunction: )
thm-five-lemma-for-a-morphism-of-long-exact-sequences. Let and 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
The cohomology of every twist on 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 . The residue is defined by the monomial pairing on the unique all-minus-one vector.
For a single twist the monomial pairing identifies with : 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.
Both sides of the asserted map are triangulated functors in , 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.
Every bounded coherent complex on 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.
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 the statement reduces to ordinary perfect-complex biduality over . 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 over 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 .
Depends on
- The Axiom of Choice
- Finite twisted resolutions over a regular local base
- Cohomology of O(d) on projective space
- Cup product in sheaf cohomology
- Cech cohomology computes quasi-coherent cohomology on a separated scheme
- Hom-tensor adjunction: $\operatorname{Hom}_R(M\otimes_RN,P)\cong\operatorname{Hom}_R(M,\operatorname{Hom}_R(N,P))$
- Five lemma for a morphism of long exact sequences
- The axiom of dependent choice: a relation in which every element is related to something admits an $\mathbb{N}$-indexed chain
Used by
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