REVIEW 3 major objections 5 minor 44 references
Deligne 1-motives with torsion and \'etale motives
T0 review · 3 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read This paper proves that the motivic t-structure, which truncates étale motives into past and future parts, restricts to the category of 1-motives with integral coefficients over any Q-scheme or Dedekind scheme, with heart the abelian…
desk verdict A credible and nontrivial extension of the motivic t-structure to integral 1-motives over base schemes, held back by a reduction to prior work that the reader has to take on faith. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The machinery is the motivic t-structure on Ind-1-motives, generated by the images of Deligne 1-motives under the functor Φ from Deligne motives to étale Voevodsky motives, together with the structural results on Deligne 1-motives with torsion: generic-fiber full faithfulness over normal schemes and the ℓ-adic Tate-module criterion for good reduction. These reduce the comparison to the regular case, where the proof distinguishes smooth 1-motives in the heart using the motivic Picard functor ω1 and degree estimates for the localization triangle of an open immersion. Once the regular case is established, the general case follows by a p-local reduction, one prime at a time, borrowed from the earlier 0-motive construction.
What would settle it
A concrete way to test the claim is to take a non-regular Q-scheme S and an integral 1-motive M such that the truncation τ≤0(M) computed in the Ind-1-motive category is a genuine Ind-1-motive but not an object of DM1(S,Z). Equivalently, one can look for a Deligne 1-motive over the generic point of a normal Q-scheme whose ℓ-adic Tate modules all extend to local systems but which itself fails to extend over a codimension-one point; either example would contradict Corollary 5.4 or Proposition 2.10.
Extended reading notes
Core claim
The central claim, Corollary 5.4, is that for S a Q-scheme or a Dedekind scheme and Λ a localization of Z, the motivic t-structure induces a t-structure on the category DM1(S,Λ) of 1-motives, compatible with pullbacks and with the ℓ-adic realization functors. For regular S and coefficients in Z′, the comparison functor ΦS gives an equivalence between the category of Deligne 1-motives with torsion and the intersection of smooth 1-motives with the heart of the t-structure, so the heart is abelian. Two structural facts about Deligne 1-motives with torsion carry the proof: over a connected normal scheme they are determined by their generic fiber, and a generic Deligne 1-motive extends to the whole scheme exactly when all its ℓ-adic Tate modules extend. The main theorem is therefore presented as a reduction of the integral problem to the rational case plus a new integral comparison in the regular setting.
Load-bearing premise
The proof assumes that the reduction from an arbitrary Q-scheme or Dedekind scheme to the regular case, a step written down for 0-motives in an earlier paper, works unchanged for integral 1-motives; that reduction is cited rather than proved here.
Editorial extensions
If this is right
- Every integral 1-motive over a Q-scheme or Dedekind scheme is bounded in the motivic t-structure, so truncation functors are available for 1-motives integrally, not just rationally.
- The heart of the t-structure on smooth 1-motives over a regular base is the abelian category of Deligne 1-motives with torsion, giving an integral analogue of the field-case equivalence.
- The ℓ-adic realization functors become t-exact on DM1(S,Λ), so truncations of a 1-motive are detected on each ℓ-adic realization.
- The t-structure is compatible with pullback, so base-change maps preserve non-positive and non-negative 1-motives.
- Since Deligne 1-motives with torsion form a Serre subcategory of generic-fiber Deligne 1-motives when the good-reduction hypothesis holds, the category of such motives over the base is abelian.
Reading between the lines
- Extending beyond the paper, the good-reduction criterion suggests that integral 1-motives over a normal base are determined by their generic fiber plus finitely many ℓ-adic unramifiedness conditions, a Hasse-principle-type statement not spelled out in the paper.
- Extending beyond the paper, a direct p-local proof for 1-motives that avoids citing the 0-motive reduction would make Theorem 5.1 self-contained; the current proof cites that reduction.
- Extending beyond the paper, whether the t-structure is compatible with the six operations, such as pushforwards and tensor products, is left open; the rational theory suggests such compatibility should hold.
- Extending beyond the paper, over a base where the good-reduction hypothesis fails, such as a smooth surface over a field of positive characteristic, Proposition 2.10 predicts the heart is no longer abelian, and a counterexample there would mark the precise limit of the construction.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces Deligne 1-motives with torsion over a noetherian base scheme as a Gabriel-Zisman localization of effective 1-motives, and proves that this localization is a 1-category. Over a connected normal scheme it establishes full faithfulness of the generic-point restriction functor for coefficients in a flat Z′-algebra, together with a good-reduction criterion in terms of ℓ-adic Tate modules, from which it deduces that MD1(S,Λ) is abelian when Λ is a localization of Z′. It then defines a motivic t-structure on the category of Ind-1-motives and proves compatibility with pullbacks, localizations, and ℓ-adic realizations, and shows that the functor ΦS from Deligne 1-motives to étale motives lands in the heart. The main theorem (Corollary 5.4) asserts that, for S a Q-scheme or a Dedekind scheme and Λ a localization of Z, this t-structure restricts to DM1(S,Λ) and is compatible with pullback and ℓ-adic realization.
Significance. If the main theorem is correct, it is a significant advance: it provides an integral motivic t-structure on relative 1-motives over bases that need not be regular or of finite type over a field, extending the field-coefficient results of [BVK16], the rational-coefficient results of [Leh19b, Leh19a, Vai19], and the author's earlier work on integral Artin motives. The paper also contains independent contributions: the proof that Deligne 1-motives with torsion form an abelian category after inverting residue characteristic exponents, and a good-reduction criterion generalizing work of Haas. Many local steps are proved in detail (e.g., Theorem 1.5, Theorem 2.8, Proposition 4.12, Lemma 5.3), and the paper is carefully structured. The main caveat is that the passage from the regular case to arbitrary Q-schemes in Corollary 5.4 is not fully proved; this is the load-bearing step that the rest of the report addresses.
major comments (3)
- [Section 5, proof of Corollary 5.4] The proof of the main theorem relies on the assertion 'exactly as in the proof of [Rui22, Theorem 4.2.7], we can reduce to the case when ...' without carrying out the reduction. This is load-bearing: Theorem 5.1, the only place where the motivic t-structure is actually shown to restrict to smooth 1-motives, assumes S is regular, and the entire novelty of Corollary 5.4 beyond the Dedekind case is the treatment of singular Q-schemes. The analogous reduction for 0-motives in [Rui22] uses structure that is not established here for integral 1-motives: smooth 0-motives are exactly lisse sheaves, the ordinary t-structure on D_lisse is explicit, and p-geometric objects are controlled through those identifications. For integral 1-motives over a singular base, the manuscript does not prove that every 1-motive is glued from smooth 1-motives over regular strata, nor that τ≤0 preserves p-geometric Ind-1-motives for the 1-motive generator class, nor that the non-excellent case is covered (the statement allows arbitrary noetherian finite-dimensional Q-schemes, while [Rui22] assumes excellent schemes allowing resolution by alterations). Without a written proof of this reduction, Corollary 5.4 is not established for singular Q-schemes.
- [Section 4, Corollary 4.16] The statement of Corollary 4.16 claims that ΦS lands in the heart of the motivic t-structure for an arbitrary scheme S and arbitrary flat Z-algebra Λ, but its proof invokes Corollary 4.13, which assumes S is excellent and allows resolution of singularities by alterations. No justification is given for why these hypotheses can be dropped, or for why they hold in the applications of Corollary 4.16 (e.g., in the proof of Corollary 5.4, where the reduced scheme is not explicitly shown to be regular). If Corollary 4.16 is needed for the main theorem, its hypotheses and proof must be aligned; otherwise the statement should be restricted to the cases actually used.
- [Section 4, Remark 4.14] Remark 4.14 asserts that the method of [Leh19a] works with Λ⊗Q coefficients for any flat Z-algebra Λ 'by direct inspection' and invites the reader to check. This is not a proof, and it is particularly problematic because several statements in the paper (e.g., Theorem 3.5 and Corollary 4.16) are formulated for arbitrary flat Z-algebras. Since the main theorem only needs Λ to be a localization of Z, the paper would be cleaner if these statements were restricted accordingly, or if a proof of the asserted extension were supplied.
minor comments (5)
- [Abstract and Introduction] There are a few typographical glitches, such as 'on1-motives' in the abstract and 'finial section' in the introduction; these should be corrected.
- [Section 2, proof of Proposition 2.10] The parenthetical 'for any ℓ that is invertible on S (and thus any non-invertible prime in Z′)' appears to have the primes reversed; it should say 'any prime ℓ≠p' or 'any prime invertible in Z′'.
- [Section 2, proof of Theorem 2.8] The reference '[Ray70, Collaire IX.1.4]' should presumably read 'Corollaire IX.1.4'.
- [Section 3, proof of Lemma 3.7] The claim that the assumptions on cohomological dimension in [Ayo14, Proposition 11.1] can be removed by the rigidity theorem is stated without proof; please provide a reference for this removal or a short argument.
- [Section 5, proof of Theorem 5.1] In the proof that the kernel of a map of Deligne 1-motives is again a Deligne 1-motive, the exactness of the functor between the abelian categories is asserted; a brief explanation of why the family i_x^* is exact and conservative (using Lemma 4.15 and Proposition 4.12) would improve readability.
Circularity Check
Singular-base case of main theorem rests on a one-line reduction to the author's own [Rui22]; otherwise the derivation is substantively self-contained.
-
self citation load bearing
[Corollary 5.4, proof (Section 5)]
"exactly as in the proof of [Rui22, Theorem 4.2.7], we can reduce to the case when 1. p is invertible on S or S is of characteristic p. 2. M belongs to DMsm1(S, Λ)."
This is the sole step that extends the main theorem from the regular case proved in Theorem 5.1 to arbitrary Q-schemes, which is the genuinely new singular-base content of Corollary 5.4. The reduction is not proved for integral 1-motives; it is imported from the author's earlier [Rui22], which treats 0-motives over excellent schemes allowing resolution by alterations. The paper gives no argument that p-geometric Ind-1-motives, the class DMsm1, or the t-structure restriction behave as required for 1-motive generators on singular bases. The main theorem for singular Q-schemes therefore reduces to an unverified self-citation rather than to a derivation contained in this paper.
full rationale
The core of the paper is not circular by construction. The motivic t-structure on Ind-1-motives is generated by images of Deligne 1-motives (Definition 4.1), but the nontrivial content lies in proving that these generators land in the heart (Corollary 4.16), that smooth 1-motives in the heart are exactly Deligne 1-motives (Theorem 5.1), and that the t-structure restricts to DM1(S,Λ) (Corollary 5.4). Those proofs use independent ingredients: the field-case t-exactness from BVK16, rigidity, the Néron–Ogg–Shafarevich criterion and Hypothesis 2.3, and Lemma 5.3. No fitted parameter is renamed as a prediction, and no known result is repackaged as new. The single load-bearing self-citation is the reduction in the proof of Corollary 5.4, where the singular-Q-scheme case is dispatched by copying [Rui22, Theorem 4.2.7] without checking that the p-geometric reduction works for integral 1-motives. This is a self-citation carrying the central new case, but it is not a definitional tautology, so the score is moderate rather than high.
Assumptions & free parameters
assumptions (5)
- domain assumption Rigidity theorem of Bachmann-Hoyois (BH21, Cor. 3.2): for ℓ invertible on S, ρ! is an equivalence between ℓ-torsion étale sheaves and ℓ-torsion étale motives.
- domain assumption Hypothesis 2.3: an abelian variety over the generic point of a connected normal scheme with ℓ-adic Tate module unramified on S[1/ℓ] extends to an abelian scheme.
- domain assumption Continuity of étale motives with integral coefficients, adapting CD16, Theorem 6.3.9.
- ad hoc to paper Lehalleur's rational-coefficient 1-motive machinery extends to Λ⊗Q coefficients for any flat Z-algebra Λ.
- standard math Voevodsky's cancellation theorem for étale motives at the level used in Lemma 3.7 and Construction 4.6.
Cite this review
Pith. "Pith review of Deligne 1-motives with torsion and \'etale motives." pith.science (2026). https://pith.science/paper/Q5NUXLXI
@misc{pith2026250610637,
author = {Pith},
title = {Pith review of: Deligne 1-motives with torsion and \'etale motives},
year = {2026},
howpublished = {\url{https://pith.science/paper/Q5NUXLXI}},
note = {Machine review of arXiv:2506.10637}
}
read the original abstract
We construct the motivic t-structure on 1-motives with integral coefficients over a scheme of characteristic zero or a Dedekind scheme. When we invert the residue characteristic exponents of the base, this t-structure induces a t-structure on the category of smooth 1-motives whose heart is the category of Deligne 1-motives with torsion which we prove to be abelian. This relies on the fact that over a normal scheme, Deligne 1-motives with torsion are determined by their fiber on the generic point and on an explicit description of good reduction Deligne 1-motives with torsion.
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