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The local simplicial model retains actual sections on a finite-dimensional noetherian Nisnevich site
PDF p. 11: Proposition 5.5; pp. 11–12
Statement
Assume the Axiom of Choice and fixed compatible site and universe bounds. Let be a noetherian finite-dimensional scheme. Here noetherian means a quasi-compact scheme with an affine open cover by spectra of rings whose ideals are finitely generated, and finite dimension bounds lengths of strict chains of nonempty irreducible closed subsets. On the small Nisnevich site with qcqs étale -schemes, the infinity-category of ordinary covering-sieve sheaves of spaces is hypercomplete. Consequently its local injective simplicial-presheaf or simplicial-sheaf model presents that same category. Passing to that local model preserves the actual derived values and mapping classes of a sheaf of spaces, including pointed ones. No field or separatedness hypothesis is required.
Facts & Assumptions
Assume The Axiom of Choice and the compatible universes of the site supplier, including the natural numbers and larger universes for spaces and categories. Universe existence is a separate size premise. Choice is retained in the imported sheaf and local-model constructions in both proof steps.
Qcqs scheme sites and affine detection for space-valued sheafification supplies this qcqs Nisnevich site, covering pullbacks, finite affine open refinements, the separated intermediate category and the space-valued comparison input in its Fact 4. Its two-stage argument checks the affine-open intersections without assuming the base separated. We use that comparison input with the induced Nisnevich topologies, verifying the covering hypotheses below. A sheaf of spaces satisfies descent for covering sieves; ordinary means that no further hypercompletion was imposed.
Source reference 0 supplies the affine noetherian homotopy-dimension theorem, the category and its affine étale presentation. A slice over an object means objects with a map to it, with the induced mapping spaces. Homotopy dimension at most means that every -connective object has a section; -connective means its homotopy sheaves below degree vanish, with the usual local nonemptiness convention at degree zero. We use the source convention without changing its connectivity indexing.
Source reference 1 supplies the represented generators, the local homotopy-dimension criterion, its hypercompleteness consequence and the simplicial-presheaf local-model comparison. Hypercomplete means that every map inducing isomorphisms on all homotopy sheaves is inverted. Values and maps below are derived space-valued ones, not underived evaluation of an unfibrant representative.
Source reference 2 supplies the simplicial-sheaf comparison. Here simplicial sheaves are degreewise sheaves of sets. Both injective models have monomorphism cofibrations, local weak equivalences (isomorphisms on homotopy sheaves), and fibrations defined by lifting against trivial cofibrations. Write for degreewise associated sheaf and for inclusion. Model categories, fibrancy and Quillen equivalence use Model categories and Quillen adjunctions.
The spectrum of a Noetherian ring is a Noetherian topological space supplies the noetherian topology of each chosen affine chart, and Noetherian open subsets are quasi-compact supplies quasi-compactness of all its opens, under Choice. Krull dimension of a nonzero ring defines dimension of a nonzero ring by lengths of strict prime chains. A Zariski-closed subset is irreducible exactly when its radical defining ideal is prime, and then it has a unique generic point supplies their order-reversing correspondence with nonempty irreducible closed subsets of the spectrum. Empty charts may be discarded when is nonempty.
External input
Reference 0 imports only the stated affine-site results. Reference 1 imports only the categorical generation, finite local homotopy-dimension and simplicial-presheaf interfaces at its locators. Reference 2 adds the degreewise-sheafification comparison for the simplicial-sheaf model. The general site comparison and geometric closure premises are supplied by [F1]. The argument below checks these hypotheses and passes from the affine statements to this particular nonaffine site; it is a local composition, not an imported proof of the whole lemma.
Proof
Given: , its small Nisnevich site and the foundation bounds, with [A1] and [F1]–[F5].
Goal: Verify the local dimension hypothesis and retain the actual sheaf under local realization.
Choose a finite noetherian affine open cover of . By [F5], every open subset of each is quasi-compact. Every open of is therefore a finite union of quasi-compact opens obtained by intersecting it with the , and is quasi-compact. Thus intersections of affine opens are quasi-compact, so is qcqs as required by [F1]. Put when is nonempty and discard empty charts. The prime/closed-subset correspondence in [F5] identifies the ring dimension with the chart dimension. Each consequently has dimension at most : taking closures in carries a strict chain of irreducible closed subsets of to a chain of the same length, since intersecting those closures with recovers the original chain. The presheaf represented by is a sheaf: factorization through this open is unique, and holds on exactly when it holds after a covering family, since that family is jointly surjective on points. The slice site over is the small Nisnevich site of ; its objects and arrows are the same étale schemes with their structure maps, and its covering condition is unchanged. This also identifies the corresponding sheaf-topos slice: a sheaf over the represented is sent on each arrow to its homotopy fiber over that arrow, and the inverse takes the disjoint union of these fibers over the maps . Both constructions preserve covering descent. Apply the comparison input of [F1] first from affine objects to absolutely separated objects and then from separated objects to all qcqs étale objects, now with their induced Nisnevich topologies. For the first inclusion, covering arrows between affines and all their finite fiber products over an affine target are affine. Pulling back a covering sieve to an affine object and refining its sources by affine opens gives an induced Nisnevich cover. Each separated object has a finite affine open cover with affine intersections. For the second inclusion, fiber products of separated schemes are separated by [F1]; the same covering-refinement argument applies, and every qcqs object has a finite affine open cover whose intersections are quasi-compact opens of an affine, hence separated. These check all three covering and Čech-product hypotheses of that comparison. Thus this slice is computed by the affine étale -site, and hence by using [F2]. The same identification holds for slices at its represented affine étale test schemes. Theorem 2.24 therefore bounds their homotopy dimensions by .
The represented affine étale test schemes mapping into some generate all sheaves under colimits: cover any test scheme first by the inverse images of the and then by finite affine open refinements from [F1]. For completeness, an intersection in such an affine open cover is a qcqs open of an affine and hence separated by [F1]. It has a finite affine cover with affine intersections, all still mapping into the relevant . Its represented sheaf is therefore generated by these affine represented sheaves via the covering Čech colimit. The first cover's Čech colimit then generates every represented test sheaf, and represented test sheaves generate by [F3]. Here a represented sheaf means the ordinary sheafification of the represented presheaf, so this generation argument does not require a separate subcanonicity premise. Step 1.1 bounds the homotopy dimension of every slice at these generators by , so the topos is locally of homotopy dimension at most . Corollary 7.2.1.12 in [F3] makes it hypercomplete. Proposition 6.5.2.14 identifies the local simplicial-presheaf model with this ordinary sheaf category. The smallness and topology in [A1, F1] also meet [F4]. Hence gives the additional local injective simplicial-sheaf comparison. More explicitly, all objects are cofibrant; for a simplicial presheaf and a fibrant simplicial sheaf , is fibrant and as simplicial mapping objects. This enriched adjunction follows by applying degreewise sheafification's adjunction to ; the sheaf tensor is its associated sheaf. Together with the local-equivalence unit and isomorphism counit of [F4], it identifies derived mapping spaces in the two models. Thus for a simplicial presheaf representing an ordinary sheaf , a sheaf fibrant replacement has adjoint a local equivalence. Under the presheaf comparison both represent . For each test object , write for its represented presheaf. The simplicial Yoneda identity computes the derived value, since is cofibrant and fibrant; it is the same space as under the presheaf identification in [F3]. This proves the claimed preservation of actual derived values and mapping classes in both models. Pointed objects and their maps are obtained by taking the category under the terminal sheaf; the equivalence preserves that terminal object and hence preserves the pointed mapping spaces as well. If is empty, the empty covering forces every sheaf value to be contractible, and the same conclusion holds in the terminal category.
Depends on
- Qcqs scheme sites and affine detection for space-valued sheafification
- The Axiom of Choice
- Model categories and Quillen adjunctions
- The spectrum of a Noetherian ring is a Noetherian topological space
- Noetherian open subsets are quasi-compact
- Krull dimension of a nonzero ring
- A Zariski-closed subset is irreducible exactly when its radical defining ideal is prime, and then it has a unique generic point
- Jacob Lurie, Derived Algebraic Geometry XI: Descent Theorems (28 September 2011) · Proposition 1.14, pp. 5–6; Notation 2.1 and Proposition 2.8, pp. 7–9; Theorem 2.24 and Corollary 2.25, p. 16
- Jacob Lurie, Higher Topos Theory, author-hosted PDF · Lemma 5.1.5.3, p. 325; Definition 7.2.1.8, p. 720; Corollary 7.2.1.12, p. 722; Proposition 6.5.2.14, pp. 670–671
- J. F. Jardine, Local Homotopy Theory, Lecture 05 · Section 11.1, Theorem 11.1(1)–(2) and proof, printed pp. 17–19 (PDF pages 17–19)
Dependency tree · 2 levels
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External dependencies
For a noetherian ring R of Krull dimension at most d, the Nisnevich space-sheaf topos on Test_R and each slice at a represented test scheme has homotopy dimension at most d. Test_R consists of quasi-compact opens in affine etale R-schemes. The affine etale presentation gives the same topos; noetherian Nisnevich covers have the residue-field lifting description.
Hypotheses: R is a commutative noetherian ring of finite Krull dimension. Apply to the coordinate rings of a finite noetherian affine cover of X, with d=dim X. The nonaffine passage is proved locally below; it is not attributed to the affine theorem.
Reading limits: Current repair checked the live author-hosted extracted statements of Proposition 1.14, Notation 2.1, Proposition 2.8, Theorem 2.24 and Corollary 2.25, including the displayed proof of Theorem 2.24. Excision, height induction and the affine-presentation foundations are imported; no full-paper reading or independent acceptance is claimed.
Represented presheaves generate under colimits; sheafification gives represented sheaf generators. A topos generated by objects whose slices have homotopy dimension at most d is locally of that dimension and is hypercomplete. The local simplicial-presheaf model presents the hypercompletion of the ordinary space-sheaf topos.
Hypotheses: Small sites and compatible universes. Apply the finite local dimension criterion to the affine etale generators over the fixed affine opens of X. This is not a claim for arbitrary qcqs bases.
Reading limits: Current repair checked the live author-hosted extracted generation statement in Lemma 5.1.5.3 and its colimit setup, Definition 7.2.1.8, Corollary 7.2.1.12 and its displayed proof, and Proposition 6.5.2.14 with its initial presheaf/sheaf identification. The remaining foundational proofs are imported. This proposition supplies simplicial presheaves only; source reference 2 supplies simplicial sheaves.
Degreewise sheafification and inclusion give a Quillen equivalence between the local injective simplicial-presheaf and simplicial-sheaf models; the unit is a local weak equivalence and the counit is an isomorphism.
Hypotheses: A small Grothendieck site; weak equivalences are local and cofibrations are monomorphisms. No enough-points hypothesis.
Reading limits: Read Section 11.1, theorem and proof, in the live author-hosted extraction. Earlier existence and small-object results remain imported; no full-course reading or reconstruction is claimed.
Sources
- Jacob Lurie, Derived Algebraic Geometry XI: Descent Theorems (28 September 2011) · Proposition 1.14, pp. 5–6; Notation 2.1 and Proposition 2.8, pp. 7–9; Theorem 2.24 and Corollary 2.25, p. 16
- Jacob Lurie, Higher Topos Theory, author-hosted PDF · Lemma 5.1.5.3, p. 325; Definition 7.2.1.8, p. 720; Corollary 7.2.1.12, p. 722; Proposition 6.5.2.14, pp. 670–671
- J. F. Jardine, Local Homotopy Theory, Lecture 05 · Section 11.1, Theorem 11.1(1)–(2) and proof, printed pp. 17–19 (PDF pages 17–19)