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2504.01283

The Poisson boundary of Thompson’s group T is not the circle

Martín Gilabert Vio, Cosmas Kravaris, Eduardo Silva

correctmedium confidence
Category
Not specified
Journal tier
Strong Field
Processed
Sep 28, 2025, 12:56 AM

Audit review

The uploaded paper proves that for minimal, proximal, topologically nonfree actions on S1 and any nondegenerate μ with finite entropy, the circle (S1, ν) is not the Poisson boundary, via Kaimanovich’s conditional entropy criterion and a conditional Erschler method, supported by new dynamical inputs (dominating intervals) and an “invisibility” lemma tailored to boundary conditioning. This matches Theorem A and its proof sketch in the paper and appears sound. By contrast, the model’s argument misuses an erasure map E: it asserts ℬ ⊂ σ(E) (boundary measurability through E) using a false “eventual equality” of state sequences after infinitely many block deletions, and it then incorrectly claims H(Yi|E)=HB(p) although Yi is determined by E (so H(Yi|E)=0). Hence the model’s conditional-entropy lower bound fails, and its proof is invalid. See Theorem A and the proof architecture in the paper, including the conditional entropy criterion (Theorem 2.6), the Erschler-style conditional method, and key lemmas/propositions addressing invisibility and frequency bounds.

Referee report (LaTeX)

\textbf{Recommendation:} minor revisions

\textbf{Journal Tier:} strong field

\textbf{Justification:}

The paper establishes, for finite-entropy measures on minimal, proximal, topologically nonfree circle actions, that the circle factor is strictly smaller than the Poisson boundary, answering a question for Thompson’s group T in this regime. The method—a conditional Erschler approach combined with new dynamical tools (dominating intervals) and a careful boundary-level invisibility lemma—is original and well-executed. The exposition is generally clear, with a transparent structure and helpful preliminaries; a few technical transitions could be signposted more explicitly to guide readers through the entropy argument.