REVIEW 2 major objections 3 minor 1 cited by
Mean dimension theory for infinite dimensional Bedford-McMullen sponges
T0 review · 2 major / 3 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read This paper proves exact, closed-form formulas for both the metric mean dimension and the mean Hausdorff dimension of every Bedford-McMullen sponge system with any number $r\ge 3$ of coordinate directions.
desk verdict New and likely correct formulas, but the lower bound for mean Hausdorff dimension rests on a false counting lemma (Lemma 3.12) and needs repair. 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 argument is carried by approximate cubes $Q_{N,M}(x_1,\ldots,x_r)$: for each level $M$, these are the sets of sponge points whose first $L_i(M)=\lfloor M\log m_1/\log m_i\rfloor$ digits in coordinate $i$ agree with those of a centre point. Each such cube has diameter on the order of $m_1^{-M}$, and the numbers of such cubes are controlled by $|\Omega|_N^{L_r(M)}\prod_{i=1}^{r-1}|\pi_i(\Omega)|_N^{L_i(M)-L_{i+1}(M)}$, which yields the metric mean dimension formula. For the mean Hausdorff dimension, the load-bearing object is the weighted topological entropy $h^a_{\mathrm{top}}$ defined by nested covers with weight vector $a_i=\log m_{r-i}/\log m_{r-i+1}$; its value is computed through the fibre counts $Z_N$ of Lemma 3.3, and the upper and lower bounds are matched by a product measure on $(\Omega|_N)^{\mathbb{N}}$ whose conditional probabilities make the masses of approximate cubes explicit.
What would settle it
Test the cube-counting lemma directly: take $r=3$ with bases $m_1=2,m_2=4,m_3=8$ and $\Omega$ the full shift, and for small $N,M$ enumerate all distinct approximate cubes $Q_{N,M}(x)$. Lemma 3.12 asserts that among any $2rN+1$ distinct cubes two are at distance at least $m_1^{-M}$; finding a counterexample would break the lower-bound proof of the mean Hausdorff formula. A second test is numerical: for a family of subshifts $\Omega$, compute the covering numbers of $(X_\Omega,d_N,\varepsilon)$ directly and compare the limiting ratios with the two closed formulas in Theorem 1.1.
Extended reading notes
Core claim
On the paper's own terms, the discovery is that both invariants of an infinite-dimensional sponge system are already determined by entropy data of the coding subshift. For a sponge system $(X_\Omega,\sigma,d)$ with bases $2\le m_1\le\cdots\le m_r$, Theorem 1.1 gives $\operatorname{mdim}_M(X_\Omega,\sigma,d)=h_{\mathrm{top}}(\Omega,\sigma)/\log m_r+\sum_{i=1}^{r-1}(1/\log m_i-1/\log m_{i+1})h_{\mathrm{top}}(\pi_i(\Omega),\sigma)$ and $\operatorname{mdim}_H(X_\Omega,\sigma,d)=h^a_{\mathrm{top}}(\{(\pi_i(\Omega),\sigma)\}_{i=1}^r,\{\tau_i\}_{i=1}^{r-1})/\log m_1$, where the weights are $a_i=\log m_{r-i}/\log m_{r-i+1}$, $\pi_i$ drops the last $r-i$ coordinates, and $\tau_i$ are the corresponding factor maps. In particular, the paper establishes for arbitrary $r$ that metric mean dimension is a linear combination of standard entropies, while mean Hausdorff dimension is governed by the weighted topological entropy of the whole projection tower. It also gives a uniform fibre-growth condition under which the two invariants coincide.
Load-bearing premise
The paper defines mean dimension for invertible dynamics only, while the sponge system is a one-sided shift that cannot be run backwards; the theorem presupposes that the same definitions and estimates extend to this case without saying so.
Editorial extensions
If this is right
- When $r=2$, formulas (1.2) and (1.3) reduce to the known carpet-system result, so the sponge formulas contain the earlier planar theory as the first case.
- If $\Omega$ has uniformly growing word complexity in the sense of (3.3), then the mean Hausdorff dimension equals the metric mean dimension; this is the dynamical analogue of the uniform-fibres case where Hausdorff and Minkowski dimensions of sponges agree.
- The variational form of the two formulas shows that the gap between mean Hausdorff dimension and metric mean dimension is exactly the gap between maximizing all projected entropies with a single measure and maximizing each projection separately.
- Since $h_{\mathrm{top}}(\pi_i(\Omega),\sigma)\le h_{\mathrm{top}}(\Omega,\sigma)$ and the coefficients $1/\log m_i-1/\log m_{i+1}$ are nonnegative, the metric mean dimension lies between $h_{\mathrm{top}}(\Omega,\sigma)/\log m_r$ and $h_{\mathrm{top}}(\Omega,\sigma)/\log m_1$.
Reading between the lines
- A testable extension the paper leaves implicit is that the same approximate-cube counts should compute metric mean dimension for graph-directed or sofic variants of sponges, provided the analogue of the fibre counts $Z_N$ is computable.
- Passing to the natural extension of the one-sided shift would likely repair the homeomorphism gap without changing the formulas, because topological entropy and the projection entropies are invariant under this passage; the paper does not discuss this.
- Equality of the two mean dimensions might hold under weaker fluctuation bounds on the fibre counts than the uniform growth condition (3.3); a numerical search over subshifts with slowly varying fibre sizes could probe how sharp that condition is.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper extends Tsukamoto's mean-dimension theory for Bedford-McMullen carpet systems to sponge systems of arbitrary dimension r≥3. For a subshift Ω and integer bases m_1≤...≤m_r, the author defines the associated subsystem X_Ω of the r-fold Hilbert cube and proves explicit formulas for the metric mean dimension and the mean Hausdorff dimension in terms of the topological entropies of the projections π_i(Ω) and of Alibabaei's weighted topological entropy. The proof proceeds by covering and separating estimates with approximate cubes, with upper and lower bounds established separately. A condition under which the two invariants coincide is also given.
Significance. If the main result is established, it is a substantial and natural extension of Tsukamoto's r=2 result: it gives closed-form, parameter-free formulas for both invariants for arbitrary r and connects the mean Hausdorff dimension of sponge systems to the weighted topological entropy and its variational principle. The metric mean dimension computation and the upper bound for the mean Hausdorff dimension are carefully executed, and the paper contains no fitted parameters or circular assumptions. However, the lower-bound proof for the mean Hausdorff dimension rests on Lemma 3.12, which is false as stated; the flaw is localized and repairable, so the contribution is likely correct after revision.
major comments (2)
- [§3.4.2, Lemma 3.12] Lemma 3.12 is false as stated, and the lower bound (3.12) therefore relies on an invalid pigeonhole argument. For the full shift Ω and any M large enough that L_q(M)≥1 for all q, choose for each q and each time coordinate t=1,...,N the partial sum ∑_{k=1}^{L_q(M)} x_{kq,t} m_q^{-k} to be either 0 or m_q^{-L_q(M)} (put all leading digits 0, or put x_{L_q,q,t}=1 and all earlier leading digits 0). This gives 2^{rN} distinct leading tuples, hence 2^{rN} distinct approximate cubes. For any two of these cubes and every q, the coordinate-wise partial sums differ by at most m_q^{-L_q(M)} in ℓ∞-norm, so inequality (3.9), which requires a difference of at least 2m_q^{-L_q(M)}, fails for every pair. Since 2rN+1≤2^{rN} for r≥3 and N≥1, the assumption of the lemma can be satisfied while its conclusion fails. The lemma is used to conclude |C_j|≤2rN in the proof of (3.12). The repair is to replace the threshold 2rN+1 by 2^{rN}+1; then |C_j|≤2^{rN}, and the existing smallness condition (3.10) still gives the desired factor (2^r m_1^s ε^{δ/2})^N<1/2, yielding ∑ D(E_j)^{log_{m1} Z_N-Nδ}>1. As written, the lower-bound proof is incomplete.
- [§2.2 and §1.2] Section 2.2 defines a TDS as a pair with T a homeomorphism, but the sponge system (X_Ω,σ) is a one-sided shift, which is not invertible. As written, mdimM(X_Ω,σ,d) and mdimH(X_Ω,σ,d) in Theorem 1.1 are therefore not covered by the definitions. The proofs only use forward Bowen metrics, and the numerical quantities are unaffected by this issue, but the paper should explicitly extend the definitions to non-invertible continuous maps using forward metrics, or state that one passes to the natural extension and that the invariants coincide.
minor comments (3)
- [§3.1, Lemma 3.1] In the proof of Lemma 3.1, 'the first equality holds' should read 'the first inequality holds'; the second displayed assertion is also an inequality, not an equality.
- [§3.3, Lemma 3.6] In the display of Lemma 3.6, the index j in u_j(⌊c_i t/L_i⌋) is undefined; it should be u_i.
- [Abstract] In the abstract, 'whose metric mean dimension and mean Hausdorff dimension does not coincide' should be 'do not coincide'.
Circularity Check
No significant circularity: the sponge mean-dimension formulas are derived from covering and combinatorial estimates, not assumed or fitted.
full rationale
The derivation chain is self-contained with respect to the stated conclusions. The metric mean dimension formula (1.2) is obtained by combining the covering and separation estimates in Lemma 3.2 with the standard reduction between Bowen metrics and finite projections in Lemma 3.1; none of these estimates assumes the formula being proved. The mean Hausdorff dimension formula (1.3) is proved separately from above and below using the weighted entropy quantity Z_N, the probability measures constructed from Z_N, Lemma 3.9, Lemma 3.11, and the external geometric measure lemma of Tsukamoto; the lower bound uses only the combinatorial control in Lemma 3.12, not the target equality. Weighted topological entropy itself is defined via open covers, and Lemma 3.3 derives the Z_N formula from that definition rather than assuming it. The paper cites external results — Alibabaei's variational principle, Tsukamoto's geometric measure lemma, and the Kenyon-Peres combinatorial lemma — as independent inputs, and these are not restatements of the present theorem. The only self-citation, [11] by the author with Rong Yuan, appears in the introduction as background on Z^d-actions and plays no load-bearing role. The special-case Lemma 3.4 explicitly works under an added uniform-word-complexity assumption and uses Theorem 1.1 to conclude equality of the two dimensions, which is a legitimate conditional application rather than circularity. The substantive concern raised by the skeptical reviewer concerns the validity of Lemma 3.12's pigeonhole claim; even if valid, that concern is about correctness of a proof step, not about circularity, because the claim is not equivalent to the theorem's input and the theorem is not assumed in proving it. Overall, no fitted parameter is renamed as a prediction, no uniqueness theorem is imported from the authors' prior work, and no central claim reduces to a self-citation chain. Hence the circularity score is 0.
Assumptions & free parameters
assumptions (5)
- domain assumption The mean dimension definitions are extended from homeomorphisms to the non-invertible one-sided shift without comment.
- standard math Standard topological entropy variational principle and entropy limits for subshifts.
- standard math Alibabaei's variational principle for weighted topological entropy, equation (2.3).
- standard math External lemmas by Kenyon-Peres, Tsukamoto, and probability lemmas 3.5, 3.6, 3.7, 3.8.
- domain assumption The alphabet sizes satisfy 2 <= m_1 <= ... <= m_r and Omega is a closed shift-invariant subshift with at least two points.
Cite this review
Pith. "Pith review of Mean dimension theory for infinite dimensional Bedford-McMullen sponges." pith.science (2026). https://pith.science/paper/3VX77ANU
@misc{pith2026241202278,
author = {Pith},
title = {Pith review of: Mean dimension theory for infinite dimensional Bedford-McMullen sponges},
year = {2026},
howpublished = {\url{https://pith.science/paper/3VX77ANU}},
note = {Machine review of arXiv:2412.02278}
}
abstract
Tsukamoto (2022) introduced the notion of Bedford-McMullen carpet system, a subsystem of $([0,1]^{\mathbb{N}}\times[0,1]^{\mathbb{N}},shift)$ whose metric mean dimension and mean Hausdorff dimension does not coincide in general. The aim of this paper is to develop the mean dimension theory for Bedford-McMullen sponge system, which is a subsystem of $(([0,1]^r)^{\mathbb{N}},shift)$ with arbitrary $3\leq r\in\mathbb{N}$. In particular, we compute the metric mean dimension and mean Hausdorff dimension of such topological dynamical systems explicitly, extending the results by Tsukamoto. The metric mean dimension is a weighted combination of the standard topological entropy, whereas the mean Hausdorff dimension is expressed in terms of weighted topological entropy. We also exhibit a special situation for which the metric mean dimension and the mean Hausdorff dimension of a sponge system coincide.
Forward citations
Cited by 1 Pith paper
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Mean Assouad dimension and spectrum, with applications to infinite dimensional fractals
Mean Assouad dimension and spectrum are defined as bi-Lipschitz invariants of dynamical systems; explicit formulas are derived for infinite-dimensional Bedford-McMullen carpets.
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