Pure subrings of Du Bois singularities are Du Bois singularities
Pith reviewed 2026-05-24 11:11 UTC · model grok-4.3
The pith
Cyclically pure maps of Noetherian Q-algebras descend Du Bois singularities from the larger ring to the smaller ring.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Let R to S be a cyclically pure map of Noetherian Q-algebras. If S has Du Bois singularities, then R has Du Bois singularities. The same conclusion holds for the relevant notions in prime characteristic and mixed characteristic, and the method yields that log canonical singularities descend along such maps when K sub R is Cartier.
What carries the argument
Sheafification of the complex underline Omega zero X with respect to Grothendieck topologies, which characterizes Du Bois singularities on the schemes under study.
If this is right
- Du Bois singularities descend along cyclically pure maps of Noetherian Q-algebras.
- The descent statement extends to the corresponding notions of singularities in prime characteristic and mixed characteristic.
- Log canonical singularities descend along cyclically pure maps of rings essentially of finite type over C when K sub R is Cartier.
Where Pith is reading between the lines
- The descent may allow reduction of singularity questions to simpler subrings in arithmetic settings.
- Explicit examples such as hypersurface or quotient singularities could be checked to see the descent in action.
- Similar descent statements might hold for other classes of singularities defined by vanishing of higher direct images.
Load-bearing premise
The Grothendieck-topology sheafification characterization of Du Bois singularities applies to the Noetherian schemes considered.
What would settle it
A concrete cyclically pure map R to S of Noetherian Q-algebras in which S has Du Bois singularities but R does not.
read the original abstract
Let $R \to S$ be a cyclically pure map of Noetherian $\mathbb{Q}$-algebras. In this paper, we show that if $S$ has Du Bois singularities, then $R$ has Du Bois singularities. Our result is new even when $R \to S$ is faithfully flat. Our proof also yields interesting results in prime characteristic and in mixed characteristic. As a consequence, we show that if $R \to S$ is a cyclically pure map of rings essentially of finite type over the complex numbers $\mathbb{C}$, $S$ has log canonical type singularities, and $K_R$ is Cartier, then $R$ has log canonical singularities. Along the way, we prove a version of the key injectivity theorem of Kov\'acs and Schwede for Noetherian schemes of equal characteristic zero that have isolated non-Du Bois points. Throughout the paper, we use the characterization of the complex $\underline{\Omega}^0_X$ and of Du Bois singularities in terms of sheafification with respect to Grothendieck topologies.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper claims that for a cyclically pure map R → S of Noetherian ℚ-algebras, Du Bois singularities descend from S to R. The argument relies throughout on a characterization of the Du Bois complex Ω⁰_X via sheafification in Grothendieck topologies, establishes an isolated-non-Du-Bois-points version of the Kovács–Schwede injectivity theorem in equal characteristic zero, and derives consequences for prime/mixed characteristic and for log canonical singularities when the rings are essentially of finite type over ℂ with K_R Cartier.
Significance. If correct, the result is new even for faithfully flat maps and supplies a downward-transfer statement for Du Bois singularities that is unavailable from existing literature; the isolated-points injectivity theorem and the log-canonical consequence are additional contributions of independent interest.
major comments (1)
- [Abstract] Abstract (final paragraph) and the statement of the main theorem: the characterization of Ω⁰_X and of Du Bois singularities via Grothendieck-topology sheafification is invoked for arbitrary Noetherian ℚ-schemes. Standard references for this characterization (e.g., via the h-topology) are proved only under the hypothesis that X is essentially of finite type over ℂ. No additional argument or reference is supplied to remove this hypothesis, so the claimed generality for arbitrary Noetherian ℚ-algebras (including formal power series rings) is not justified.
Simulated Author's Rebuttal
We thank the referee for their careful reading of the manuscript and for identifying this important point about the scope of the cited characterization. We address the comment below.
read point-by-point responses
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Referee: [Abstract] Abstract (final paragraph) and the statement of the main theorem: the characterization of Ω⁰_X and of Du Bois singularities via Grothendieck-topology sheafification is invoked for arbitrary Noetherian ℚ-schemes. Standard references for this characterization (e.g., via the h-topology) are proved only under the hypothesis that X is essentially of finite type over ℂ. No additional argument or reference is supplied to remove this hypothesis, so the claimed generality for arbitrary Noetherian ℚ-algebras (including formal power series rings) is not justified.
Authors: We agree that the standard references establishing the equivalence between the Du Bois complex and its sheafification in the h-topology (or other Grothendieck topologies) are stated for schemes essentially of finite type over ℂ. The manuscript invokes this characterization for arbitrary Noetherian ℚ-schemes without supplying an additional reference or argument to justify the extension. In the revised version we will either (i) add a self-contained argument or citation showing that the relevant sheafification property holds for Noetherian ℚ-schemes in general, or (ii) restrict the main statements to the essentially finite-type-over-ℚ case while noting that the algebraic applications (including the faithfully flat case) remain new. We believe the core descent result continues to hold in the stated generality once the foundational point is clarified. revision: yes
Circularity Check
No circularity: theorem proved from independent prior results
full rationale
The paper establishes that cyclically pure maps preserve Du Bois singularities for Noetherian Q-algebras by invoking the Grothendieck-topology sheafification characterization of Ω^0_X (standard in the literature) and proving a version of the Kovács-Schwede injectivity theorem for isolated non-Du Bois points. No derivation step reduces by construction to a fitted parameter, self-definition, or load-bearing self-citation chain; the central claim has independent mathematical content beyond its inputs. The characterization is treated as an external tool rather than derived within the paper.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption The characterization of Ω^0_X and Du Bois singularities via sheafification in Grothendieck topologies holds for the Noetherian schemes considered.
Forward citations
Cited by 1 Pith paper
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Equidimensional morphisms onto splinters are pure
A Noetherian ring R is a splinter if and only if every equidimensional surjective morphism Spec(S) to Spec(R) makes R to S pure.
Reference graph
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