Abstract

This paper is devoted to the study of Bowen's dimensional entropy on subsets for actions of amenable groups. We prove three main results. (1) First, topological conditional entropy is characterized by the dimensional entropy of stable sets (Theorem 1.1), answering a question of Dou, Wang and the second author of the present paper raised in [Fund. Math., 2025]. We remark that our Theorem 1.1 is the first characterization of topological conditional entropy via Bowen's dimensional entropy of stable sets even for Z\mathbb{Z}-actions. (2) Second, we establish a dimensional entropy inequality for factor maps (Theorem 1.2). It relates dimensional entropy of a set to that of its image and topological entropy of fibres, and may be viewed as the dimensional-entropy counterpart of the factor-map inequality for packing topological entropy due to Dou, Zheng, and Zhou proved as Theorem 1.4 in [Ergodic Theory Dynam. Systems, 2023]. (3) Third, the relative topological entropy of a factor map is determined by the dimensional entropy of the fibres (Theorem 1.3). Notably, our proof of this formula (Theorem 1.3) is purely topological, in contrast to the recent measure-theoretic approach of Dou, Wang and Zhou based on relative Shannon--McMillan--Breiman theorems. These results (Theorem 1.2 and 1.3) not only generalize the work of Oprocha and the second author of the present paper [Nonlinearity, 2011] from single transformations to amenable group actions, but also provide a purely topological and self-contained proof of a fibre entropy characterization recently obtained through measure-theoretic arguments.

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Generated August 18, 2026
01Statements2 reported findingsCorrect

The characterizations of conditional and relative entropy by Bowen dimensional entropy of stable sets and fibres, and the factor-map inequality for arbitrary subsets, are correct for the stated tempered Følner sequences.

Theorems 1.1–1.3Correct

Stable-set and fibre dimensional entropy recover the relative invariants

Pages 4–5 and Sections 3–5 · arXiv:2606.28681v1

The Lindenstrauss covering lemma converts local Bowen covers into global covers with controlled overlap. Applied to stable sets it gives the conditional-entropy upper bound, while the known topological stable-set formula and monotonicity give equality. For a factor map, fibre covers and image covers combine additively in the Carathéodory exponent, yielding Theorem 1.2; uniformization over the compact base then identifies relative entropy with the supremum of fibre dimensional entropies.

Full paper, version 1
Theorems 1.2–1.3Correct

Factor and fibre inequalities give the announced relative entropy formula

Pages 5–7 and Sections 4–5 · arXiv:2606.28681v1

Følner-set covers of the base and of individual fibres combine with the correct dimensional exponents for the upper bound. The reverse inequality localizes a high-entropy separated set to one fibre, giving equality after optimizing over scales and preserving the amenable-group normalization.

02Proofs2 reported findingsCorrect

The combinatorial covering and Carathéodory-dimension arguments are correct and complete.

Sections 3–5Correct and complete

Tempered covering families give the required uniform bounds

Pages 11–22 · Sections 3–5 · arXiv:2606.28681v1

The growth condition Fn/logn|F_n|/\log n\to\infty makes the exceptional covering sums converge, temperedness supplies the bounded-overlap subfamily, and enlarging FnF_n by a fixed finite set preserves both properties. Compactness turns pointwise fibre covers into finitely many uniform ones, and all scale limits are taken in the order required by the definitions.

Sections 3–5Correct and complete

Stable-set localization and uniform fibre coverings close all three theorems

Pages 10–30 · arXiv:2606.28681v1

Tempered Følner tilings convert local stable-set complexity to global separated-set estimates. For factors, fibre covers are chosen uniformly after compactness and multiplied with base covers; the converse extracts a single fibre carrying the required exponential multiplicity. Boundary and tiling errors vanish after normalization.

03Novelty0 reported findingsNo non-novelty findings

No non-novelty findings.

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Paper
arXiv:2606.28681v1
Authors listed
Xinyao He, Guohua Zhang
Audit date
August 18, 2026
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