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  • AI-adapted: a semantically identical restatement of literature-sourced material, modulo indexing, notation, and boundary cases adopted by the library.
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Normalizing the comparison fiber for the dimension criterion

PDF p. 4: Motivic dimension vanishing

Statement

For j≥0 and a prime field F, set Wj,F=Fj,F[j], with cochain shift Hn(C[j])=Hn+j(C). Then Wj,F satisfies every hypothesis of the dimension criterion over base ring F: it is finitary Nisnevich, cdh-acyclic, pro-cdh on Noetherian F-schemes, and has nilpotent-relative values in degrees ≤0 on every Noetherian local henselian F-algebra.

Facts & Assumptions

Given: The actual fiber Fj,F and the shift convention in the Statement.

Goal: Check the precise domain, ideal and degree hypotheses before applying the criterion.

[F1]

External geometric properties of the actual motivic comparison fiber gives finitariness, Nisnevich descent, cdh acyclicity, pro-cdh descent, and the relative bound ≤j for finitely generated nilpotent ideals.

[F2]

External dimension criterion for a cdh-acyclic Nisnevich sheaf is the criterion requiring relative bound ≤0 for Noetherian local henselian algebras. Prime-field evaluation and compatibility with the standing theory specifies the actual fiber used here.

Proof

1.1F1F2

Shift by [j] is an equivalence of the stable derived category and commutes with limits and colimits. Thus it preserves the finitary and Nisnevich properties, commutes with pointwise total fibers, and preserves the supplied degreewise pro-cdh condition: for every integer n the shifted total-fiber system has Hn=Hn+j of the original system, which is zero as a pro abelian group. This uses the explicit convention of [F1]–[F2], without asserting cartesianness in Pro⁡(D(Z)). It also commutes with cdh sheafification: shifting preserves sheaves and its inverse does likewise, so the reflective sheafification universal property identifies the two composites. Hence Wj,F retains the first geometric properties of [F1], including LcdhWj,F=0.

2.1F1F2step 1.1∎

Let A be any Noetherian local henselian F-algebra and I any nilpotent ideal. Every ideal of a Noetherian ring is finitely generated, so [F1] applies without an added ideal hypothesis. For n>0, the relative cohomology of Wj,F(A,I) in degree n is the relative cohomology of Fj,F(A,I) in degree n+j>j, hence zero. This is exactly the degree and functor-class hypothesis of [F2]; the source’s stronger arbitrary-algebra package has been used only under its actual finitely-generated-ideal condition.

Depends on

Used by · 2

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