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2103.15505

Veelike actions and the MCG of a mixing SFT

Ville Salo

correctmedium confidence
Category
Not specified
Journal tier
Specialist/Solid
Processed
Sep 28, 2025, 12:56 AM

Audit review

The paper proves the embeddings V → MCG(X) and 2V → MCG(Y) by first establishing faithful veelike actions (Theorems 3–6) and then encoding them into orientation-preserving homeomorphisms of the suspension via carefully defined tile rewrites and anchored isotopies (Theorems 7–8) . In contrast, the model attempts to lift arbitrary base homeomorphisms H:X→X (or Y→Y) to the mapping torus by [x,t]↦[H(x),t]. This lift is ill-defined on the suspension unless H commutes with the shift σ, because the quotient identifies (x,t)∼(σx,t−1) and one needs Hσ=σH to respect the equivalence relation; the proposed H (acting nontrivially only on a clopen workbench W) does not commute with σ. The injectivity claim in Lemma B is also invalid: along an isotopy, the image of a given point need not remain on the same flow orbit, so the “integer shift amount” argument does not go through. The paper’s method aligns with the definition of MCG(X) and its flow-based framework , while the model’s construction does not define a legitimate mapping class group element.

Referee report (LaTeX)

\textbf{Recommendation:} minor revisions

\textbf{Journal Tier:} specialist/solid

\textbf{Justification:}

The paper delivers a conceptually clear embedding of V and 2V into mapping class groups of explicit mixing SFTs via veelike actions and flow encodings. The argument is sound and situates the soficity question for these mapping class groups within major open problems. The presentation is succinct; modest elaborations on the suspension rewrites and the isotopy argument would further aid readers.