REVIEW 3 major objections 5 minor 49 references
Towers and clubs
T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read Club guessing produces long ascending or descending towers in every ideal extending the nonstationary ideal over a successor cardinal.
desk verdict Matet's tower theorems are a useful generalization but Theorem 3.9 has a genuine cardinality gap that needs fixing before the centerpiece holds. 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 carrier of the argument is the cofinal club-guessing principle $\clubsuit^{\mathrm{cof},-}_{\kappa}[J]$, which supplies small sets $B^i_\delta\subseteq\delta$ for $i<\delta$ such that for every $W\in[\kappa]^\kappa$, the collection of $\delta$ where some $B^i_\delta$ has cofinal intersection with $W$ is $J^+$. Lemma 3.8 converts these into $\nu$-sized pieces $A^i_\delta$ of $\kappa\times\kappa$ that capture $f\upharpoonright Z$ for cofinal $Z\subseteq\delta$. Towers are sequences $\langle A_\alpha:\alpha<\tau\rangle$ of $J^+$ sets whose successive differences lie in a prescribed family $Y$; an ascending $(J,I_\kappa)$-tower means each new part is almost disjoint from the union of earlier parts, and a descending $(J,J)$-tower means the leftover part stays $J^+$. The proof tracks the guessed sets $S_\alpha$ and the crossing sets $S_{\alpha\beta}$, and uses $\mathrm{Depth}(\kappa^\kappa)$, the least length of an unavoidable increasing chain in $(\kappa^\kappa,<^{*})$, as the ceiling for $\tau$.
What would settle it
Build a model with $\kappa=\nu^+$, a $\kappa$-complete ideal $J$ extending the nonstationary ideal, $\clubsuit^{\mathrm{cof},-}_{\kappa}[J]$, and a regular $\tau<\mathrm{Depth}(\kappa^\kappa)$ for which $J$ is $\tau$-saturated; then neither tower of length $\tau$ can exist and Theorem 3.9 fails. Concretely, inspect the proof's choice of $T$ and $\theta$: if every $J^+$-set carries guesses of size at least $\tau$, the recursion $g:\theta^+\to\tau$ cannot be defined, so exhibiting such an ideal with $\mathrm{Depth}(\kappa^\kappa)>\tau$ would pinpoint the gap.
Extended reading notes
Core claim
The paper's central claim is a uniform transfer: cofinal club guessing converts any strictly $<^{*}$-increasing chain of functions in $\kappa^\kappa$ of length $\tau$ into either an ascending $(J,I_\kappa)$-tower or a descending $(J,J)$-tower of length $\tau$, for any $\kappa$-complete ideal $J$ extending the nonstationary ideal. Theorem 3.9 states this for $\kappa=\nu^+$ under $\clubsuit^{\mathrm{cof},-}_{\kappa}[J]$ and every regular $\tau<\mathrm{Depth}(\kappa^\kappa)$. The proof first turns club guesses into sets $A^i_\delta\subseteq\kappa\times\kappa$ that capture restrictions of functions to cofinal subsets of $\delta$, then defines $J^+$ sets $S_\alpha$ where a function is guessed and $S_{\alpha\beta}$ where $\kappa$-truncations of two functions cross. Assuming neither tower exists, a diagonal recursion on those sets produces a contradiction. The paper derives corollaries for ideals extending $NS_\kappa|E^\kappa_\theta$ under cardinal arithmetic assumptions, for ideals with generalized-club guessing, and for stationary reflection.
Load-bearing premise
The proof of Theorem 3.9 secretly assumes that the guessed sets have size smaller than the tower length $\tau$; the stated hypotheses only bound them by $\nu$, and no argument is given that this smaller bound holds.
Editorial extensions
If this is right
- Under $\kappa=\nu^+$ and $\clubsuit^{\mathrm{cof},-}_{\kappa}[J]$, every regular $\tau<\mathrm{Depth}(\kappa^\kappa)$ is witnessed by a tower, so $J$ is not $I_\kappa$-$\tau$-saturated (by Observation 2.29).
- For ideals extending $NS_\kappa|E^\kappa_\theta$ with $\mathrm{cf}(\nu)\neq\theta$, Fact 3.12 supplies the club guessing and Proposition 3.14 yields the same tower dichotomy without extra hypotheses.
- The non-normal case is covered, so ideals that are not subnormal still admit long towers whenever the relevant club principle holds.
- Guessing generalized clubs (Theorem 4.7) and full stationary reflection (Theorem 5.12) each produce the same ascending-or-descending tower conclusion.
- The slow-train propositions show that $\clubsuit^{\mathrm{cof},-}_{\kappa}[J]$ together with $2^{<\kappa}=\kappa$ upgrades to diamond $\lozenge_\kappa[J]$, recovering and extending an earlier diamond theorem.
Reading between the lines
- If the missing assumption that the guessed sets have size below $\tau$ can be proved from the stated hypotheses, then club guessing and $\mathrm{Depth}(\kappa^\kappa)$ alone control non-saturation; if not, the theorem likely needs an explicit size bound on the guesses.
- The same construction might yield ascending $(J,J)$-towers or descending $(J,I_\kappa)$-towers by symmetry, which would answer the paper's Questions 6.1 and 6.2 affirmatively under club guessing.
- The bound $\tau<\mathrm{Depth}(\kappa^\kappa)$ may be improvable to $\tau<\mathrm{Depth}(C_\kappa)$, since Fact 2.47 shows the two depths differ by at most one; testing this would show whether the tower length is really tied to the function space.
- A natural test is whether $\clubsuit^{\mathrm{cof},-}_{\kappa}[J]$ alone yields a tower of length $\nu^+$ even when $\mathrm{Depth}(\kappa^\kappa)=\nu^+$, which would isolate whether the depth parameter is essential.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies club principles (diamond, club, and their variants) and their connection to the non-saturation of ideals extending the nonstationary ideal on a regular uncountable cardinal κ. The main new result is Theorem 3.9, which asserts that if κ = ν^+, J is a κ-complete ideal on κ satisfying ♣cof,−_κ[J], and τ is a regular cardinal less than Depth(κ^κ), then there exists either an ascending (J,I_κ)-tower of length τ or a descending (J,J)-tower of length τ. The paper also revisits several known results, proves variants, and extends results on generalized club guessing and on Gitik–Shelah style non-saturation.
Significance. If Theorem 3.9 and its corollaries (notably Proposition 3.14) are correct, they provide a uniform framework for deriving tower non-saturation of ideals from weak club principles, covering ideals that are not necessarily normal. The paper is also useful as a compendium of related principles and contains many small observations. However, the proof of the central theorem contains a gap (see Major Comment 1), so the significance of the paper is conditional on repairing that argument.
major comments (3)
- [§3.1, Theorem 3.9 proof] The recursive construction of g : θ^+ → τ \(γ+1) implicitly requires θ < τ. Since τ is regular, the supremum of fewer than τ ordinals below τ is below τ; for ζ < θ^+ this requires θ^+ ≤ τ, i.e. θ < τ. The hypotheses only give θ < ν, with no relation between ν and τ (τ < Depth(κ^κ) may be larger or smaller than ν). Concretely, take κ = ω_{ω+1}, ν = ω_ω, τ = ω, and θ = ω_1; at step ζ = ω the supremum sup{g(ξ)^* : ξ < ω} is already ω = τ, so g(ω) cannot be defined. The proof therefore depends on an unstated cardinality assumption that is load-bearing for Theorem 3.9, and Proposition 3.14 inherits the gap.
- [§4, Theorem 4.7 proof] The same issue appears in the proof of Theorem 4.7: the function g : θ^+ → τ \(γ+1) is defined with θ < σ, but nothing ensures θ < τ. Since τ is regular and θ^+ may exceed τ, the recursion can break before reaching θ^+. Thus Theorem 4.7 is not proved as stated.
- [§3.3, Proposition 3.15] The proof of Proposition 3.15(i) begins with the sentence 'We modify the proof (which we do not understand) of Theorem 3.5 in [36]'. This is an explicit admission that the author cannot vouch for the correctness of the argument being modified. Since Proposition 3.15 is used in Proposition 3.17 and in the proof of Fact 2.18, this is a missing-support concern in a chain leading to a stated proof. The author should either supply a fully understood proof or explicitly mark the result as dependent on [36].
minor comments (5)
- [Abstract] The abstract contains the typo 'n onsaturation'; it should read 'non-saturation'.
- [Definition 2.21] The definition of ♣_κ[J] contains an extra closing parenthesis: '♣κ[J])' should be '♣κ[J]'.
- [Observation 2.27 proof] In the proof, 'fix i < j < κ' should be 'fix i < j < ρ', since the almost disjoint family is indexed by ρ.
- [Theorem 4.7, Claim 4] The text 'fg(ξ(j)' appears to be a typo for 'f_{g(ξ)}(j)', and the following inequality has mismatched parentheses, making the claim hard to read.
- [Various proofs] Several proofs are only sketched or left to the reader, including the descending case in Theorem 2.31, part (i) of Theorem 4.7, and part (ii) of Observation 4.8; at least brief indications would improve accessibility.
Circularity Check
No significant circularity: Theorem 3.9 and the related tower results are proved from stated hypotheses by internal arguments, and cited prior work is used as independent background lemmas rather than as a substitute for the conclusions.
full rationale
The paper's central new result, Theorem 3.9, derives from club-cofinality guessing and tau < Depth(kappa^kappa) the existence of either an ascending (J,I_kappa)-tower or a descending (J,J)-tower of length tau. The proof is a genuine contradiction argument: it assumes the absence of both towers, uses Lemma 3.8 to obtain the sets A^i_delta, defines the sets S_alpha and S_alphabeta, and then constructs g:theta^+ -> tau\ (gamma+1] from the assumption that no descending tower exists. The tower is produced from the guessing data and the ideal J; the conclusion is not assumed in the hypotheses or in the cited facts. The paper does cite several results from the author's own prior work (e.g., Facts 2.10, 2.12, 2.13, 2.16, 5.16, 5.18), but these are background facts about density numbers, diamond, and good points. None of them states or presupposes the tower theorems proved here, and several are jointly attributed to external authors such as Jensen, Shelah, Kojman, and Cummings, Foreman, and Magidor. A possible concern is that the recursive definition of g in Theorem 3.9 requires theta < tau for the successor step (sup{g(xi)^* : xi < zeta}) + 1 to remain below the regular cardinal tau, while the hypotheses only give theta < nu and tau < Depth(kappa^kappa). That is a cardinality gap in the proof, not a circularity: it concerns the size of the guessing sets |A^i_delta| = theta, not an equivalence between input and output. No fitted parameter is renamed as a prediction, no uniqueness theorem is imported from the author's prior work to force a choice, and no known result is relabeled as a new principle. The derivation chain is therefore self-contained apart from standard external lemmas.
Assumptions & free parameters
assumptions (10)
- standard math ZFC (standard axioms of set theory) is the background theory.
- standard math Fact 2.10: various computations of d(tau,sigma) and m(tau,sigma) from [21],[27],[29].
- standard math Fact 2.12: Shelah's Strong Hypothesis equivalence from [31].
- standard math Fact 2.13: Shelah's bounds on d(tau,sigma) from [45],[27].
- standard math Fact 2.35: characterization of b_kappa via closed unbounded sets from [1].
- standard math Fact 2.47: inequalities relating b_kappa, Depth(C_kappa), and Depth(kappa^kappa) from [46].
- standard math Fact 2.55: Shelah's PCF result on Depth(C_rho^+) from [46].
- standard math Fact 3.12: Rinot's club-guessing lemma from [37].
- standard math Fact 5.11: Krueger's full reflection equivalence from [23].
- standard math Facts 5.16 to 5.18: PCF and good points from [4],[28],[26].
Cite this review
Pith. "Pith review of Towers and clubs." pith.science (2026). https://pith.science/paper/IF7OCWWY
@misc{pith2026190805336,
author = {Pith},
title = {Pith review of: Towers and clubs},
year = {2026},
howpublished = {\url{https://pith.science/paper/IF7OCWWY}},
note = {Machine review of arXiv:1908.05336}
}
abstract
We revisit several results concerning club principles and nonsaturation of the nonstationary ideal, attempting to improve them in various ways. So we typically deal with a (non necessarily normal) ideal $J$ extending the nonstationary ideal on a regular uncountable (non necessarily successor) cardinal $\kappa$, our goal being to witness the nonsaturation of $J$ by the existence of towers (of length possibly greater than $\kappa^+$).
Reference graph
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