REVIEW 2 major objections 5 minor 1 cited by
Trace ideals, conductors, and ideals of finite (phantom) projective dimension
T0 review · 2 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read Trace ideals of big Cohen–Macaulay modules avoid ideals of finite projective dimension and parameter ideals in complete local rings; a two-dimensional Cohen–Macaulay domain refutes the conductor question.
desk verdict Strong Section 3, a fixable c/d swap in the counterexample, and an unpublished Hochster-Yao dependency—worth reviewing but not ready as is. 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 proof replaces the Cohen–Macaulay hypotheses in earlier trace-ideal arguments with a dualizing complex $D$. The canonical module is $\omega \simeq H^0(D)$, and the key orthogonality statement, Proposition 3.6, is that $H_i(G\otimes^{\mathbf L}_R \operatorname{RHom}_R(N,D))=0$ for $i>0$ whenever $G$ is a bounded complex of free modules satisfying the standard conditions on rank and height (the rank of the $i$-th differential equals the alternating sum of the free ranks, and the ideal of maximal minors has height at least $i$); Lemma 3.8 adds the nonvanishing $H^0(\operatorname{RHom}_R(M,D))\neq 0$ for a big Cohen–Macaulay module $M$. These two facts turn an exactness argument into the desired non-containment. A separate mechanism powers the Frobenius-side result, Theorem 2.4: a cited embedding theorem embeds a module of finite projective dimension into a direct sum of quotients by an $R$-regular sequence, after which a tight-closure contradiction using Frobenius powers shows that a non-zero $F$-ideal cannot sit inside the ideal.
What would settle it
Compute the conductor of the localization at $(x,y,z,w)$ of $R=K[x,y,z,w]/(x^3z-y^2,\,x^3w^4-yz,\,w^4y-z^2)$; if any conductor element lies outside $(x,w)R$, the claimed counterexample to the conductor question fails.
Extended reading notes
Core claim
The paper's central claim is a non-containment principle for trace ideals: if $R$ is a complete local ring with a canonical module $\omega$, $M$ is a big Cohen–Macaulay module, and $I$ is a proper ideal such that $R/I$ is presented by a bounded complex of free modules satisfying the standard conditions on rank and height, then $\operatorname{tr}_\omega(M)\nsubseteq I\omega$; in particular this covers ideals of finite projective dimension and parameter ideals (Corollary 3.10). When $R$ is quasi-Gorenstein the conclusion upgrades to $\operatorname{tr}_R(M)\nsubseteq I$, and for a complete local domain the conductor is therefore never contained in such an ideal. In prime characteristic the parameter test submodule $\tau(\omega)$ is shown to equal $\operatorname{tr}_\omega(R^+)$, so $\tau(\omega)\nsubseteq I\omega$ whenever $R/I$ has finite phantom projective dimension, and the parameter test ideal itself avoids $I$ in the quasi-Gorenstein case (Corollary 3.11). The paper also presents two explicit rings: one shows that a parameter test ideal of a reduced, equidimensional, complete, two-dimensional Cohen–Macaulay ring need not be an $F$-ideal, and the other, a two-dimensional Cohen–Macaulay local domain, has its conductor contained in the parameter ideal $(x,w)R$, giving a negative answer to the conductor question.
Load-bearing premise
A theorem quoted from a preliminary manuscript says that every module with a finite free resolution embeds, in a controlled way, into a direct sum of quotients by regular sequences; the paper's main F-ideal non-containment proof assumes that theorem is valid exactly as cited, while the counterexample is self-contained.
Editorial extensions
If this is right
- In every complete local ring with a canonical module, the trace of any big Cohen–Macaulay module is not contained in $I\omega$ when $I$ has finite projective dimension, is a parameter ideal, or more generally $R/I$ has a free complex satisfying the standard conditions on rank and height.
- In quasi-Gorenstein complete local rings, the trace ideal itself, and hence the conductor in the domain case, is not contained in any such $I$.
- In a complete local domain of prime characteristic, the parameter test submodule $\tau(\omega)$ is not contained in $I\omega$ whenever $R/I$ has finite phantom projective dimension; in the quasi-Gorenstein case the parameter test ideal itself is not contained in $I$.
- The conductor question is answered negatively: a two-dimensional analytically unramified Cohen–Macaulay local domain has its conductor contained in the parameter ideal $(x,w)$, and in positive characteristic its parameter test ideal is likewise contained in $(x,w)$.
- The parameter test ideal of a reduced equidimensional complete two-dimensional Cohen–Macaulay ring need not be an $F$-ideal, so the earlier Gorenstein argument cannot extend to all Cohen–Macaulay rings without extra hypotheses.
Reading between the lines
- One could test whether the $F$-ideal non-containment of Theorem 2.4 extends to every excellent reduced local ring of prime characteristic, since the proof's test-element step appears to work once the embedding theorem is granted.
- The counterexample suggests that the real boundary for conductor non-containment may be quasi-Gorensteinness rather than Cohen–Macaulayness; searching among Cohen–Macaulay local rings of minimal multiplicity for which conductors are or are not contained in parameter ideals would sharpen that line.
- Because $\tau(\omega)=\operatorname{tr}_\omega(R^+)$, any construction of other big Cohen–Macaulay algebras with controlled trace could transfer the non-containment theorem to mixed characteristic, where Frobenius and tight closure are unavailable.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies containment of parameter test ideals, conductors, F-ideals, and trace ideals in ideals whose quotient has finite (phantom) projective dimension. It proves several non-containment results, most notably Theorem 2.4 (non-zero F-ideals are not contained in ideals of finite projective dimension under Cohen-Macaulay or excellent equidimensional reduced hypotheses), Corollary 2.6, and the derived-category Theorem 3.9/Corollary 3.10 (trace ideals of big Cohen-Macaulay modules avoid such ideals in complete local rings, with consequences for conductors and parameter test ideals). It also constructs two families of examples: Example 2.8, where the parameter test ideal is not an F-ideal, and Example 2.9, a two-dimensional Cohen-Macaulay local domain whose conductor is contained in a parameter ideal, answering negatively a question of Huneke-Swanson.
Significance. If the results are correct, they substantially extend earlier non-containment theorems of Smith, Dey-Dutta, and Asgharzadeh from Gorenstein or Cohen-Macaulay rings to larger classes, and they give a negative answer to the Huneke-Swanson question with a self-contained two-dimensional example. The use of standard conditions on rank and height in place of finite projective dimension is a conceptually valuable generalization. The paper also contains a self-contained counterexample to [31, Proposition 4.5] (Example 2.8). However, the central Section 2 argument depends on an unpublished 'Preliminary Version' of Hochster-Yao, and several computational claims are delegated to unshown Macaulay2 code, which limits verifiability.
major comments (2)
- [Theorem 2.4 proof, use of [15]] The proof of Theorem 2.4 depends essentially on [15, Theorem 2.3], which is cited only as a 'Preliminary Version.' This theorem supplies the embedding R/I into a direct sum of quotients by a regular sequence on which the entire contradiction argument rests, and Theorem 2.4 in turn feeds Corollary 2.6. Without access to the statement (or proof) of [15, Theorem 2.3], the main non-containment theorem of Section 2 is not checkable by readers. Please quote the theorem explicitly, provide a proof in an appendix, or update to a published reference.
- [Example 2.8, final paragraph; Remark 2.10(3)] Several claims are assigned to Macaulay2 without accompanying code or a reproducible transcript: that the parameter test ideal of R is (x,y,z)R in Example 2.8, and that for a≤2 or b≤2 the conductor and parameter test ideal of S and R_m are not contained in a parameter ideal in Remark 2.10(3). These claims are not load-bearing for the Huneke-Swanson counterexample in Example 2.9, but they are presented as part of the evidence for the paper's main examples. Please supply the Macaulay2 code or replace these assertions with self-contained arguments.
minor comments (5)
- [Example 2.9, Claim 3] The definitions c = min{n : 2a ≤ 3n} and d = min{n : 2b ≤ 3n} are correct as written: they give 3c ≥ 2a and 3d ≥ 2b, so the displayed integrality computations are consistent. A concern that these definitions are swapped does not appear to be supported by the manuscript text.
- [Example 2.9, opening] The maximal ideal is written m = (x,y,z,v,w)R, but the ring involves only the variables x,y,z,w; this should read (x,y,z,w)R.
- [Theorem 2.4 proof] The notation 'Jr^p_{ij}' is not defined; it denotes the ideal generated by elements z r_{ij}^p with z in J, and should be introduced for clarity.
- [Example 2.9, Claim 3] The phrase 'Due to symmetry' in the argument that D = E = 0 is terse, since the defining equations are not literally symmetric under exchanging x and w along with a and b. Expanding the analogous coefficient argument would improve readability.
- [Proposition 3.6(1), proof] The reduction to the finite-length case is compressed: after localizing at the chosen minimal prime p, the argument that the resulting homology modules have finite length should be stated more explicitly, as it is a key step in the general case.
Circularity Check
No circularity: the non-containment theorems are derived from independent external results, with no fitted parameters or self-referential definitions.
full rationale
The paper does not exhibit any of the enumerated circularity patterns. The main results are proved from external ingredients rather than from their own conclusions. Theorem 2.4 invokes the Hochster–Yao embedding theorem [15, Theorem 2.3] and standard tight-closure results; this is an independent structural theorem, not an assertion equivalent to the F-ideal non-containment being proved. Corollary 3.10 is derived through the dualizing-complex orthogonality of Proposition 3.6, Lemma 3.8, and the Dey–Dutta strategy; the trace ideal tr_ω(M) is defined as a sum of homomorphic images and Iω is an arbitrary proper module, so the non-containment conclusion is not built into the definition. There are no fitted parameters or data-driven 'predictions': no quantity is adjusted to a subset of data and then reported as a prediction. There is no load-bearing self-citation: the author cites no prior work of his own, and the only preliminary-version citation ([15]) is to Hochster–Yao, not to the present authors. The paper explicitly notes in Remark 3.2 that the standard conditions are treated as independent hypotheses and that some supporting facts are not even required, which further reduces any concern that the conclusion is being assumed. The caveats that exist are correctness or verification risks, not circularity: [15] is a Preliminary Version, Smith's Proposition 4.5 gap is explicitly acknowledged in Remark 2.10(4), and some claims in Remark 2.10(3) rely on Macaulay2 computations. In particular, the alleged swap of c and d in Example 2.9 is not present in the full text: the paper defines c by 2a ≤ 3n and d by 2b ≤ 3n, matching the displayed cube computations, so the example's internal consistency is a separate mathematical question. None of these issues makes a derivation reduce to its own inputs.
Assumptions & free parameters
assumptions (5)
- domain assumption Hochster-Yao embedding theorem: for a local ring R and an R-module of finite projective dimension, there is a regular sequence x_1,...,x_s and an exact sequence 0 -> R/I -> direct sum of (R/(x_1,...,x_i))^{n_i} -> N -> 0.
- domain assumption Existence of big Cohen-Macaulay algebras: over a complete local domain, the absolute integral closure R+ is a big Cohen-Macaulay R-module (Andre and others).
- standard math Stably phantom acyclic complexes satisfy standard conditions on rank and height [11, Theorem 9.8]; existence of test elements [13, Theorem 6.1(a)].
- standard math Karpilovsky's irreducibility criterion for X^n - a over fraction fields [20, Chapter 8, Theorem 1.6].
- domain assumption Macaulay2 computations: the parameter test ideal of Example 2.8 equals (x,y,z)R; conductor and test ideal facts in Remark 2.10(3) are accepted as correct without shipped code.
Cite this review
Pith. "Pith review of Trace ideals, conductors, and ideals of finite (phantom) projective dimension." pith.science (2026). https://pith.science/paper/EIQ7EAOP
@misc{pith2026250103442,
author = {Pith},
title = {Pith review of: Trace ideals, conductors, and ideals of finite (phantom) projective dimension},
year = {2026},
howpublished = {\url{https://pith.science/paper/EIQ7EAOP}},
note = {Machine review of arXiv:2501.03442}
}
abstract
In this paper, we consider whether parameter test ideals, conductors, $F$-ideals, and trace ideals are contained in an ideal whose quotient ring has finite phantom projective dimension (for example, ideals generated by a system of parameters or ideals with finite projective dimension). One of the main results asserts that such inclusions do not exist in quasi-Gorenstein complete local domains. We also provide examples of Cohen-Macaulay local rings with good properties where such inclusions occur, thus answering negatively a question of Huneke-Swanson.
Forward citations
Cited by 1 Pith paper
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When do pseudo-Gorenstein rings become Gorenstein?
A pseudo-Gorenstein graded ring becomes Gorenstein when the trace ideal of its canonical module contains a length-two regular sequence in the initial degree, with applications to nearly and almost Gorenstein rings.
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