REVIEW 4 major objections 4 minor 17 references
Some asymptotic results on $p$-lengths of factorizations for numerical semigroups and arithmetical congruence monoids
T0 review · 4 major / 4 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read Extremal p-lengths of factorizations in numerical semigroups are eventually quasipolynomial for p = 0, 1, 2, and ∞, with explicit periods and leading coefficients.
desk verdict Genuinely new p-length invariant with mostly clean asymptotics; the ℓ^m_2 proof has a real gap and the examples need repair, but this deserves refereeing. 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 pair of extremal functions ℓ^m_p(n) and ℓ^M_p(n) on the factorization set Z(n) = {z ∈ Z≥0^k : z1g1 + ··· + zkgk = n}, together with three structural identities that turn the discrete optimization into a quasipolynomial: the Apéry-set formulas ℓ^M_∞(n) = (n - a_i)/g1 and ℓ^m_∞(n) = (n + a_i)/g, where a_i is the element of Ap(S;g1) or Ap(S;g) in the appropriate residue class; the translation property for the Euclidean norm, that adding (g1,...,gk) to a minimizer preserves minimality, which yields ℓ^m_2(n+N) - ℓ^m_2(n) = 2n + N; and the one-variable reduction for p ≥ 2 that the maximum p-length is achieved by the factorization with maximal first coordinate. For ACMs the load-bearing tool is a classification of the atoms of M_{4,6} (an atom divides 2^a 5^b 7^c only in restricted forms), which permits counting distinct atoms in factorizations of x^n.
What would settle it
Take the numerical semigroup S = <2,3> and compute, for each n up to a large bound (say $10^{5}$), the unique integer solution (x,y) to 2x + 3y = n minimizing $x^{2}$ + $y^{2}$. If there are infinitely many n with the minimizing solution having x < 0 or y < 0, or if the second difference of the true minimum ℓ^m_2(n) ever fails to equal 2($2^{2}$ + $3^{2}$) = 26 for large n, then the claimed quasipolynomiality with period 13 is false. For a semigroup with three generators, the analogous check is whether the integer minimizer of $x1^{2}$ + $x2^{2}$ + $x3^{2}$ subject to g1x1 + g2x2 + g3x3 = n eventually has all coordinates nonnegative.
Extended reading notes
Core claim
The central discovery is that extremal p-lengths are asymptotically rigid for numerical semigroups: for each p ∈ {0, 1, 2, ∞}, both the minimal and the maximal p-length functions ℓ^m_p(n) and ℓ^M_p(n) are eventually quasipolynomial, meaning that for all n beyond a finite threshold they equal a polynomial in n whose coefficients depend periodically on n. The paper determines the full set of attributes: ℓ^M_p has degree p, period g1, and leading coefficient 1/g1^p for p ≥ 1 (degree 0 and period 1 for p = 0), while ℓ^m_1 has degree 1, period gk, and leading coefficient 1/gk, ℓ^m_2 has degree 2, period N = $g1^{2}$ + ··· + $gk^{2}$, and leading coefficient 1/N, and ℓ^m_∞ has degree 1, period g = g1 + ··· + gk, and leading coefficient 1/g. The proof for p = ∞ uses Apéry sets to determine the constant term, and the proof for p = 2 uses a translation identity that reduces the problem to second differences. For arithmetical congruence monoids, the paper does not attempt quasipolynomial formulas but instead establishes explicit growth rates for the singular monoid M_{4,6}.
Load-bearing premise
The proof that the minimum 2-length is eventually quasipolynomial assumes, without proof, that for all sufficiently large n there is a factorization of n that minimizes the Euclidean norm over all integer solutions to the same linear equation; if that largeness condition ever fails, the quasipolynomiality of ℓ^m_2 rests on nothing.
Editorial extensions
If this is right
- For any numerical semigroup, the value of the extremal p-length for every sufficiently large n is given by an explicit quasipolynomial, so the invariant is effectively computable rather than merely asymptotic.
- The period and leading coefficient of each p-length function constitute new arithmetic invariants of the semigroup, such as N = g1^2 + ··· + gk^2 for the minimum 2-length and the Apéry elements for the maximum ∞-length.
- The p = ∞ formulas tie extremal factorization lengths directly to the semigroup's Apéry structure, giving a clean geometric meaning to the periodic constant terms.
- The ACM results show that the maximum number of distinct atoms in a factorization of x^n can grow like n^{1/2} or n^{2/3} depending on the base element, so the growth rate is a discriminating invariant of the base element even within a single monoid.
- The Θ(n) lower bound for minimal 1- and ∞-lengths in M_{4,6} confirms that singular ACMs can mimic the linear growth of regular ACMs despite their unusual factorization behavior.
Reading between the lines
- The unproved largeness step in Theorem 2.10 could likely be supplied by a lattice-geometry argument: the Euclidean minimizer over all integer solutions converges to the ray in direction (g1,...,gk) as n grows, so for all n beyond a bound depending only on the generator set, the minimizer lies in the nonnegative orthant; if that holds, the second-difference proof of quasipolynomiality is complete.
- The special role of p = 2 may be explained by the fact that the ℓ_2-norm is the only ℓ_p-norm whose sublevel sets are rational ellipsoids, making the translation identity (z + (g1,...,gk)) preserve the norm difference in a linear way; p ≥ 3 norms lack such an identity, which is consistent with the paper's example that ℓ^m_3 for S = <2,3> involves a floor of an irrational multiple.
- The growth exponents 1/2 and 2/3 for ℓ^M_0(x^n) in M_{4,6} are reminiscent of the divisor function's behavior in short intervals, suggesting a possible connection between factorization counting in singular ACMs and analytic number theory; examining other singular ACMs might yield other rational exponents.
- The good/evil atom dichotomy used for M_{4,6} could be adapted to other finite-atomic ACMs to obtain similar Θ(n^α) bounds, potentially answering Question 3.8 for a larger class.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces p-length invariants for factorizations in commutative monoids, defined as the ℓ_p-norm of the exponent vector, and studies their extremal values for large elements of numerical semigroups and arithmetical congruence monoids (ACMs). For numerical semigroups, the paper claims that the minimal and maximal p-length functions are eventually quasipolynomial for p ∈ {0,1,2,∞}, with the degrees, periods, and leading coefficients listed in Table 1; it also claims by example that for p ≥ 3 the minimal p-length need not be eventually quasipolynomial. For ACMs, the paper establishes linear growth of some p-length functions along powers x^n and exhibits two singular ACMs where the maximum 0-length grows as n^{1/2} and n^{2/3}.
Significance. If the results are correct, the paper introduces a genuinely new family of factorization invariants and gives clean, explicit asymptotics for numerical semigroups and for several ACMs. The table of quasipolynomial attributes is a useful contribution, and the ACM examples displaying sublinear growth are striking and well-motivated. The paper also benefits from using explicit external benchmarks for the p=0 and p=1 cases rather than fitting parameters, and many of the Section 2 arguments are simple and checkable. However, several load-bearing steps currently contain gaps or explicit computational errors, so the results cannot be accepted in their present form.
major comments (4)
- [Theorem 2.10 (Section 2)] The proof of Theorem 2.10 is incomplete at the step asserting that one may choose n ∈ S large enough so that some z ∈ Z(n) minimizes ℓ2(·) over all integer solutions to g1 x1 + ... + gk xk = n. This is the load-bearing point: it must show that for all sufficiently large n, and in every residue class modulo N = g1^2 + ... + gk^2, a global integer minimizer can be taken with nonnegative coordinates. Proposition 2.9 only propagates an existing integer minimizer from n to n+N; it does not establish the existence of such a minimizer for all large n. The sentence comparing the smallest coordinate of the minimizer for (2.3) with the smallest coordinate of the minimizer for (2.3) is garbled and does not prove a uniform threshold. Without this step, the recurrence ℓ^m_2(n+N) − ℓ^m_2(n) = 2n + N, and hence the degree-2 quasipolynomiality asserted in Table 1, is unsupported. A repair is available, for example by showing that every integer minimizer lies within bounded distance of the real minimizer (n/N)(g1,...,gk), but the proof as written needs a substantial addition.
- [Example 2.11 (Section 2)] The displayed closed form for the ℓ^m_3(n) minimizer in S = <2,3> is incorrect. Solving the Lagrange multiplier condition for minimizing z1^3 + z2^3 subject to 2z1 + 3z2 = n gives z1 = n(3√6 − 4)/19 (up to the appropriate integer rounding), not (−8 ± n√130)/19. The qualitative conclusion may be salvageable with the corrected formula, but as written the example is false. Moreover, the phrase 'similar computation to the proof of Proposition 2.9' is misleading: over all integer solutions the cubic is unbounded below along the kernel direction (−3,2), so the minimization must be restricted to the nonnegative solution set Z(n).
- [Proposition 3.6 (Section 3)] For x = 40 = 2^3·5, the two displayed factorizations of (2^3·5)^n contain only n total factors of 2, not 3n; they factor 10^n rather than 40^n. The exponent on the atom 2^2 should be (3n−k−1)/2 in the first case and (3n−k−2)/2 in the second. In addition, the second construction has final 5-exponent n−k^2−1, which is negative at the endpoint n = k^2 of the claimed interval. The Θ(n^{1/2}) claim may be true, but the proof as written is invalid and needs correction.
- [Proposition 3.7 (Section 3)] The lower-bound construction for x = 70 contains negative exponents: the factor (2·5^{2i+1}7^{2a−2i−1}) has exponent 2a−2i−1 = −1 when i = a. The subsequent assertion that 'we may choose c appropriately so that ... for some n ≤ k^3' is not justified. Since this construction is the entire lower bound for the Θ(n^{2/3}) claim, the proof must be rewritten with a valid choice of exponents and an explicit verification that c is a nonnegative integer and that the resulting n lies in the claimed range.
minor comments (4)
- [Section 1 and Section 2] The definition of ℓ0(z) via z_i^0 is ambiguous because 0^0 is undefined; the authors should define ℓ0(z) as the number of nonzero coordinates of z.
- [Lemma 2.7] The proof assumes that g2 is the smallest among g2,...,gk; this should be stated explicitly, for instance by assuming the generators are ordered g1 < g2 < ... < gk.
- [Theorem 2.6 proof] The line 'ℓm∞(n+ai) = l∞(qg) = q' contains a typo; it should be 'ℓm∞(qg) = q'.
- [Theorem 3.5 proof] The chain 'a(Q′+R′) ≤ a(Q+R+Q′+R′) = 1/n a(b+c) = (P+P′)(b+c)' appears to contain a typo; it should read '= a n (b+c) = (P+P′)(b+c)'. Also, the derived inequality 2P′ < P actually yields m < k, so the stated m ≤ 2k is weaker than what the argument gives.
Circularity Check
No significant circularity: the new asymptotic results are derived from first principles, and the only substantive weakness (the largeness claim in Theorem 2.10) is an unproved proof step, not a circular reduction.
full rationale
The paper's new results (Theorems 2.4, 2.6, 2.8, 2.10, and 3.5-3.7) are obtained by direct arguments using Apéry sets, integer-optimization comparisons, and explicit atom characterizations; they are not fitted to the quantities they predict. The p=0 and p=1 entries in Table 1 are explicitly quoted from prior published work ([2], [7]) rather than re-derived, and the cited theorems are parameter-free results with stated assumptions that do not include the target asymptotics. Although some cited authors overlap with the present authors, this is ordinary reliance on prior literature, not a self-citation chain that forces the conclusion. Lemma 3.4 is imported from [6] and [5, Example 4.10], but it is an external published characterization of atoms of M4,6, not a restatement of the paper's own asymptotic claims. The genuine weakness is in the proof of Theorem 2.10: the sentence 'the smallest coordinate of the integer solution to (2.3) minimizing ℓ2(·) is strictly larger than the smallest coordinate of the integer solution to (2.3) minimizing ℓ2(·)' is garbled, and the assertion that one may choose n large enough so that some z in Z(n) minimizes ℓ2 over all integer solutions is not fully justified. This is a correctness gap in proving quasipolynomiality of ℓ^m_2, not a circular step: Proposition 2.9 supplies a real shift mechanism, and the claimed period N and leading coefficient 1/N are not assumed in the hypotheses. No derivation in the paper reduces to its own input by construction.
Assumptions & free parameters
assumptions (4)
- standard math Standard properties of numerical semigroups and Apéry sets (e.g., Ap(S;g) has exactly g elements, one per residue class modulo g)
- domain assumption Generators are ordered g1 < g2 < ... < gk, with g1 the multiplicity
- domain assumption Convention 0^0 = 0 for ℓ_0(z), so ℓ_0 counts nonzero coordinates
- domain assumption The atom characterization of M_{4,6} in Lemma 3.4 is correct (cited from [6] and [5])
Cite this review
Pith. "Pith review of Some asymptotic results on $p$-lengths of factorizations for numerical semigroups and arithmetical congruence monoids." pith.science (2026). https://pith.science/paper/VF7IPPSL
@misc{pith2026241117010,
author = {Pith},
title = {Pith review of: Some asymptotic results on $p$-lengths of factorizations for numerical semigroups and arithmetical congruence monoids},
year = {2026},
howpublished = {\url{https://pith.science/paper/VF7IPPSL}},
note = {Machine review of arXiv:2411.17010}
}
abstract
A factorization of an element $x$ in a monoid $(M, \cdot)$ is an expression of the form $x = u_1^{z_1} \cdots u_k^{z_k}$ for irreducible elements $u_1, \ldots, u_k \in M$, and the length of such a factorization is $z_1 + \cdots + z_k$. We introduce the notion of $p$-length, a generalized notion of factorization length obtained from the $\ell_p$-norm of the sequence $(z_1, \ldots, z_k)$, and present asymptotic results on extremal $p$-lengths of factorizations for large elements of numerical semigroups (additive submonoids of $\mathbb Z_{\ge 0}$) and arithmetical congruence monoids (certain multiplicative submonoids of $\mathbb Z_{\ge 1}$). Our results, inspired by analogous results for classical factorization length, demonstrate the types of combinatorial statements one may hope to obtain for sufficiently nice monoids, as well as the subtlety such asymptotic questions can have for general monoids.
Reference graph
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