Recognition: unknown
mathbb{G}_m-cohomology of p-adic Stein spaces
Pith reviewed 2026-05-08 15:37 UTC · model grok-4.3
The pith
The étale G_m-cohomology of p-adic rigid analytic Stein spaces is computed by filtering through principal units.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We compute the étale G_m-cohomology of some p-adic rigid analytic Stein spaces. The computation is done by considering the filtration induced by the subgroup of principal units U=1+ m O^+ of G_m. We then determine the U-cohomology via methods from p-adic Hodge theory (passage to the pro-étale site, comparison theorems with p-adic cohomologies), while the G_m/U-cohomology is obtained using Kummer exact sequences. In particular, our formula applies to the case of Drinfeld upper-half space.
What carries the argument
The filtration induced by the principal units subgroup U of G_m, which permits separate treatment of U-cohomology by p-adic Hodge theory and G_m/U-cohomology by Kummer sequences.
If this is right
- The formula gives the cohomology groups for the Drinfeld upper half-space.
- U-cohomology is accessible through pro-étale site passage and p-adic cohomology comparisons.
- G_m/U-cohomology follows from Kummer exact sequences.
- The method covers a range of p-adic rigid analytic Stein spaces.
Where Pith is reading between the lines
- This provides a method that could be tested on additional Stein spaces to verify the general applicability.
- The separation of the multiplicative group into units and quotient parts may connect to computations in other p-adic cohomology theories.
Load-bearing premise
The filtration by the principal units subgroup U allows the U-cohomology to be determined via p-adic Hodge theory methods and the G_m/U-cohomology via Kummer exact sequences.
What would settle it
A mismatch between the predicted cohomology groups from the formula and a direct calculation of the étale G_m-cohomology groups on the Drinfeld upper half-space.
read the original abstract
We compute the \'etale $\mathbb{G}_m$-cohomology of some $p$-adic rigid analytic Stein spaces. The computation is done by considering the filtration induced by the subgroup of principal units $U=1+ \mathfrak{m} \mathcal{O}^+$ of $\mathbb{G}_m$. We then determine the $U$-cohomology via methods from $p$-adic Hodge theory (passage to the pro-\'etale site, comparison theorems with $p$-adic cohomologies), while the $\mathbb{G}_m/U$-cohomology is obtained using Kummer exact sequences. In particular, our formula applies to the case of Drinfeld upper-half space.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper computes the étale G_m-cohomology of certain p-adic rigid analytic Stein spaces. It proceeds by filtering G_m via the subgroup U of principal units, determining the U-cohomology through p-adic Hodge theory (pro-étale site and comparison theorems with p-adic cohomologies) and the G_m/U-cohomology via Kummer exact sequences. The resulting formula is applied in particular to the Drinfeld upper half space.
Significance. If the comparison theorems extend to the non-proper Stein setting, the explicit formulas would give concrete descriptions of G_m-cohomology for key examples in p-adic rigid geometry, including the Drinfeld upper half space. This would strengthen the toolkit for relating étale cohomology to de Rham and crystalline data in non-proper cases.
major comments (1)
- [U-cohomology via pro-étale site and p-adic Hodge comparisons] The central step identifying H^*(X, U) via pro-étale comparisons with p-adic cohomologies (abstract and the section on U-cohomology) is load-bearing but lacks explicit justification for non-proper Stein spaces. Standard pro-étale-to-de Rham or crystalline comparisons are typically stated for proper smooth rigid spaces over O_C; the manuscript does not cite a reduction, compact-support argument, or theorem covering the Stein case (e.g., Drinfeld upper half space).
minor comments (1)
- [Abstract] The abstract refers to 'some p-adic rigid analytic Stein spaces' without an explicit list; adding a sentence naming the spaces treated (beyond the Drinfeld example) would improve clarity.
Simulated Author's Rebuttal
We thank the referee for the positive evaluation of the paper's significance and for identifying the need for clearer justification of the key comparison step. We address the major comment below and will revise the manuscript accordingly.
read point-by-point responses
-
Referee: [U-cohomology via pro-étale site and p-adic Hodge comparisons] The central step identifying H^*(X, U) via pro-étale comparisons with p-adic cohomologies (abstract and the section on U-cohomology) is load-bearing but lacks explicit justification for non-proper Stein spaces. Standard pro-étale-to-de Rham or crystalline comparisons are typically stated for proper smooth rigid spaces over O_C; the manuscript does not cite a reduction, compact-support argument, or theorem covering the Stein case (e.g., Drinfeld upper half space).
Authors: We agree that the current manuscript does not provide an explicit reduction or citation justifying the pro-étale comparisons for non-proper Stein spaces, even though the pro-étale site and the underlying p-adic Hodge comparison theorems are formulated in a way that permits passage to direct limits over affinoid subdomains. Stein spaces are exhausted by increasing unions of affinoids, and the relevant cohomology groups commute with these direct limits in the pro-étale topology. In the revised version we will add a short subsection (or appendix) that spells out this reduction, citing the relevant statements from the literature on pro-étale cohomology that allow the extension to the Stein case, together with a brief verification that the Drinfeld upper half space satisfies the necessary hypotheses. This will make the load-bearing step fully rigorous. revision: yes
Circularity Check
No circularity; computation chains to external p-adic Hodge and Kummer methods
full rationale
The derivation splits Gm-cohomology via the U-filtration, then invokes standard pro-étale comparisons and Kummer sequences for the pieces. These are presented as established external tools rather than results derived inside the paper. No self-definitional equations, fitted parameters renamed as predictions, or load-bearing self-citations appear in the provided abstract or summary. The approach is therefore self-contained against external benchmarks, yielding only a minor score for routine citation of prior theory.
Axiom & Free-Parameter Ledger
axioms (2)
- standard math Standard comparison theorems between étale cohomology and p-adic cohomologies on the pro-étale site
- standard math Kummer exact sequences for the quotient Gm/U
Reference graph
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