REVIEW 5 major objections 5 minor 27 references
On finitary power monoids of linearly orderable monoids
T0 review · 5 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read For any linearly orderable monoid M, the finitary power monoid P_fin(M) is atomic exactly when M is atomic and every finite subset of M has a maximal common divisor.
desk verdict New ascent and non-ascent results for power monoids of linearly orderable monoids, but the main characterization theorem has a real proof gap that needs to be fixed. 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 engine is the sumset operation $S+T=\{s+t:s\in S,\ t\in T\}$ together with order-sensitive identities: when M is linearly ordered, $\min(S+T)=\min S+\min T$ and $\max(S+T)=\max S+\max T$ (Lemma 2.2), and cardinalities satisfy $|S+T|\ge |S|+|T|-1$ with strict growth when one summand has size at least two (Lemma 3.1). These facts make divisibility in the power monoid track minima and cardinalities, which is what makes atoms of P_fin(M) analyzable. The other load-bearing object is the maximal common divisor (MCD): Proposition 4.1 shows M is an MCD-monoid exactly when P_fin(M) is, and Lemma 4.2 builds indecomposable sets of the form $S\cup\{4\max S\}$ or $S\cup\{4\min S\}$ that are used to extract MCDs from atomic decompositions.
What would settle it
To test Theorem 4.3, one could try to exhibit a linearly orderable monoid M that is atomic and an MCD-monoid but whose power monoid contains a finite subset that is not a finite sum of atoms; no such example exists if the theorem is correct. Concretely, for the monoid (5.6) of Theorem 5.5, one could compute whether a nonempty finite subset S exists such that $S+\{(2/5,20/3),(3/7,10)\}$ is a finite sum of atoms of P_fin(M); a positive answer would contradict the paper's non-quasi-atomicity conclusion.
Extended reading notes
Core claim
Theorem 4.3 establishes that for any linearly orderable monoid M, P_fin(M) is atomic if and only if M is atomic and every nonempty finite subset of M has a maximal common divisor. The forward direction shows atomicity of the power monoid forces M to be atomic, because singletons form a divisor-closed submonoid, and also forces the maximal common divisor property by an argument using specially constructed indecomposable sets. The reverse direction transfers the MCD property from M to P_fin(M) and then shows every finite subset decomposes into atoms. Along the way the paper constructs an atomic Puiseux monoid whose power monoid is not atomic, giving an alternative proof that atomicity does not ascend in general, and it constructs a rank-2 almost atomic monoid whose power monoid is not even quasi-atomic.
Load-bearing premise
In the rank-2 construction behind Theorem 5.5, the proof assumes that every element of the monoid (5.6) that is neither 0 nor an atom is divisible by an element of D, and that the sum of two atoms stays in D+M; if that structural claim fails, the forced-coordinate argument showing that P_fin(M) is not quasi-atomic collapses.
Editorial extensions
If this is right
- For any linearly orderable M, atomicity of P_fin(M) can be read off from two properties of M itself, completely solving the ascent problem for atomicity on this class.
- The earlier Puiseux-monoid criterion becomes a special case of Theorem 4.3, which covers all cancellative torsion-free monoids regardless of rank.
- When M is an atomic MCD-monoid, P_fin(M) is not only atomic but also an MCD-monoid, so the MCD property itself ascends.
- The quasi-ACCP and almost ACCP ascend from M to P_fin(M), as do the Furstenberg, quasi-Furstenberg, almost Furstenberg, and nearly Furstenberg properties on linearly orderable monoids.
- On positive Archimedean monoids, both the finite factorization property and the TIDF property ascend; in contrast, near atomicity, almost atomicity, and quasi-atomicity do not ascend in general.
Reading between the lines
- The MCD characterization suggests a transfer principle for factorization algorithms: to decide whether a finite subset of a linearly orderable monoid is atomic in the power monoid, one could focus on solving MCD problems in the base monoid rather than searching over subset decompositions.
- The constructions of Sections 4 and 5 can be read as a recipe: adjoining elements that destroy MCDs in the base monoid destroys atomicity, and even near or quasi-atomicity, in the power monoid; this recipe may produce similar counterexamples in other ordered monoids, such as higher-rank additive submonoids of R^n.
- Question 5.6 leaves open whether there is a rank-1 torsion-free almost atomic or quasi-atomic monoid whose power monoid fails the corresponding property; a natural next attempt is to project the rank-2 example (5.6) onto a single Archimedean coordinate while preserving its D-divisibility structure.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies when atomic and divisibility properties ascend from a linearly orderable commutative monoid M to its finitary power monoid P_fin(M). Section 3 proves ascent of the quasi-ACCP and almost ACCP. Section 4 gives a proposed characterization: P_fin(M) is atomic iff M is an atomic MCD-monoid, with an auxiliary construction of an atomic Puiseux monoid that is not MCD. Section 5 constructs counterexamples showing that near atomicity does not ascend and that an almost atomic rank-2 monoid can have a power monoid that is not quasi-atomic. Section 6 proves ascent of Furstenberg-type properties and of the FFM/TIDF properties for positive Archimedean monoids, and gives a proposed linearly orderable TIDF monoid whose power monoid is not IDF.
Significance. If the main characterization (Theorem 4.3) is correct, it would generalize the Puiseux-monoid result of [13] to all linearly orderable monoids, which is a substantial contribution. The paper also supplies a useful toolkit: Lemma 2.2, Lemma 3.1, and the MCD-based arguments are clean and likely reusable. The counterexample constructions in Sections 4 and 5 address natural open ascent questions and are therefore of interest. Several proofs are self-contained and carefully structured. However, the paper's significance is currently limited by two load-bearing gaps: the forward direction of Theorem 4.3 is delegated to a mutatis mutandis argument from a Puiseux-monoid paper, and the counterexample in Theorem 6.5 contains a concrete construction error that invalidates the example as written. The remaining results appear plausible, but the overall contribution is not yet established.
major comments (5)
- [Theorem 4.3, proof of (a)⇒(c)] The proof that P_fin(M) is atomic when M is an atomic MCD-monoid is dispatched as 'follows the line of the proof of [13, Theorem 3.2] mutatis mutandis.' Since [13, Theorem 3.2] is stated for Puiseux monoids, which are rank-1 submonoids of Q_{≥0}, and the present paper explicitly allows non-Archimedean and non-positive linearly orderable monoids, the adaptation is not routine. The implication (a)⇒(c) is the substantive half of the characterization, and the paper must either prove it in full or state precisely which steps of [13] carry over and why they do not use rank 1, nonnegativity, or Archimedeanity.
- [Section 5.2, Theorem 5.5 and the paragraph before Lemma 5.4] The assertion that 'every element that is neither the identity element nor an atom is divisible by an element of D' is used in the proof of Theorem 5.5 to infer that an atom A_i of P_fin(M) contains either (0,0) or an atom of M. Lemma 5.4, as proved, only establishes that M is almost atomic, that A(M)=A∪B, and that A(M)+A(M)⊆D+M; it does not prove the quoted structural assertion. The divisibility step requires a common divisor of all elements of A_i by a single element of D, not just individual divisibility. This gap affects the conclusion that π(q_i)∈{0,1/5,1/7}, on which the contradiction in Theorem 5.5 rests.
- [Theorem 6.5, construction of M] The construction of M is internally inconsistent. Since X_n = N x_n + gp(A) and Y_n = N(x_n−y) + gp(B), the subgroup gp(⟨∪_n (X_n ∪ Y_n)⟩) contains both x_n and x_n−y for each n, hence contains y, and also contains −a and −b. As Z = Nz + gp(⟨∪_n (X_n ∪ Y_n)⟩), the monoid M contains −a and −b as well as a and b, so a and b are units. Consequently A(M) is empty (indeed M is a group), contradicting the claim that A(M)={a,b} and that each element is divisible by a or b. The later argument that {x_n, x_n−y} are atoms of P_fin(M) also collapses because in a group every finite set is invertible and there are no atoms.
- [Theorem 4.3, proof of (b)⇒(a)] The reduction 'after replacing S by a suitable subset, we can further assume that s+t≠0 for all s,t∈S' is not justified. In a linearly ordered monoid, s+t=0 implies s and t are mutual inverses and hence units; the reduction may remove elements, and it is not shown that the MCD of the reduced subset yields an MCD of the original S. This step is needed to apply Lemma 4.2 and should be proved.
- [Theorem 5.3, proof of the Claim] The statement 'these values are actually the exact multiplicities for every factorization' is asserted without proof, but it is essential for the conclusion that the coefficients are minima in each summand and for the final use of Lemma 5.2. The proof should spell out why the bounded p-adic valuations force exact multiplicities in every factorization.
minor comments (5)
- [Theorem 6.5, proof of (2)] The text says 'atoms of P dividing {z, z + y}' but the subsequent divisibility statement concerns {z, z − y}; the plus sign appears to be a typo.
- [Theorem 6.2(4)] The final sentence 'Hence we conclude that P is nearly atomic' should read 'nearly Furstenberg.'
- [Lemma 5.4(1)] The inclusion A(M) ⊆ A∪B∪D is stated, but the reason that no element of D is an atom is not given; it would be clearer to note explicitly that each (0,1/2^n) is the sum of two equal elements of D.
- [Theorem 4.3 and Section 2.6] The use of M for both the monoid and the set of singletons {{m}:m∈M} is confusing; a different symbol for the singleton submonoid would improve readability.
- [Example 4.5] In the proof that {1,4/3} has no MCD, the symbol n is reused for an index and for the exponent in 1/2^n; the argument is ultimately understandable but would benefit from a change of variables.
Circularity Check
No significant circularity; the main derivations are self-contained, though a few proof steps are delegated or asserted without full detail.
full rationale
The paper’s claimed results do not reduce by construction to their own inputs. The central characterization, Theorem 4.3, contains one substantive direction proved in the text: (b)⇒(a), showing that atomicity of P_fin(M) forces M to be an atomic MCD-monoid, using Lemma 4.2 and a decomposition of a carefully chosen finite set T into atoms. The converse direction (a)⇒(c) is not proved in the text; it is delegated with the sentence “The proof that P_fin(M) is an atomic monoid follows the line of the proof of [13, Theorem 3.2] mutatis mutandis.” This is an omitted proof and a reliance on an external preprint, and it is a real completeness concern for the strength of the characterization, but it is not circular: [13, Theorem 3.2] is a Puiseux-monoid theorem whose proof is being adapted, not an input whose conclusion is identical to the present theorem. Similarly, Corollary 6.4(1) delegates an FFM ascent argument “mutatis mutandis” to [13, Theorem 4.2], again a proof delegation rather than a circular reduction. The unproved reduction “after replacing S by a suitable subset” in Theorem 4.3(b)⇒(a) and the structural claim before Lemma 5.4 that every non-atom, non-zero element of M is divisible by an element of D are asserted details that would need justification, but neither makes a conclusion equivalent to a hypothesis. The self-citations used, [15] and [16], provide definitions and prior independent results and are not load-bearing in a circular way. Section 3 and Sections 5–6 contain direct proofs of the ascent and non-ascent results, and the counterexamples are constructed from scratch rather than by assuming the target theorem. Accordingly, no circular step is identified.
Assumptions & free parameters
assumptions (5)
- domain assumption Every linearly orderable monoid is cancellative and torsion-free, and conversely (Theorem 2.1, Levi).
- standard math Hölder's theorem: an Archimedean linearly ordered abelian group is order-isomorphic to a subgroup of (R, +).
- standard math Halter-Koch's theorem [19, Theorem 2]: a monoid is an FFM iff it is atomic and IDF.
- standard math ZFC set theory and standard arithmetic of p-adic valuations on Q.
- standard math The inequality |S+T| ≥ |S|+|T|-1 for finite subsets of a linearly ordered monoid ([9, Proposition 3.5]).
Cite this review
Pith. "Pith review of On finitary power monoids of linearly orderable monoids." pith.science (2026). https://pith.science/paper/PX6FVUY7
@misc{pith2026250103407,
author = {Pith},
title = {Pith review of: On finitary power monoids of linearly orderable monoids},
year = {2026},
howpublished = {\url{https://pith.science/paper/PX6FVUY7}},
note = {Machine review of arXiv:2501.03407}
}
abstract
A commutative monoid $M$ is called a linearly orderable monoid if there exists a total order on $M$ that is compatible with the monoid operation. The finitary power monoid of a commutative monoid $M$ is the monoid consisting of all nonempty finite subsets of $M$ under the so-called sumset. In this paper, we investigate whether certain atomic and divisibility properties ascend from linearly orderable monoids to their corresponding finitary power monoids.
Reference graph
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