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2504.17914

Étale equivalence relations with certain prescribed torsion in their homology

Michael Francesco Ala, Hung-Chang Liao, Aaron Tikuisis

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

Audit review

The paper gives a careful, self-contained construction on a single Cantor path space using a split Bratteli diagram and a specially controlled partial homeomorphism, proving an isomorphism S(R) ≅ {(0,0)} ∪ ((D_+\{0}) ⊕ (E/Z)) and hence H0(R) ≅ D ⊕ (E/Z) (Theorem 6.4), with all étaleness checks handled via explicit bisection criteria and ‘recipe’ conditions; it also discusses approximately inner flip. The candidate solution attains the same invariants by a product-and-stripes construction that combines an AF tail-equivalence part realizing D with a torsion gadget realizing E/Z, and then shows the same semigroup and homology identifications. However, the model does not address minimality (prominent in the paper’s abstract) nor does it verify the étale join condition with the same care; still, on the core invariants S(R) and H0(R), the constructions are compatible and correct, albeit by different methods.

Referee report (LaTeX)

\textbf{Recommendation:} minor revisions

\textbf{Journal Tier:} strong field

\textbf{Justification:}

The paper successfully delivers a systematic construction of étale equivalence relations whose zeroth homology realizes D ⊕ E with precise control of the type semigroup. The étaleness of the join is handled carefully via a general criterion, and the main semigroup/homology isomorphisms are proved with attention to combinatorial details on cylinders. The approximately inner flip discussion is an additional asset. Clarifying where minimality is guaranteed in the general construction (not only in examples) would align the body with the abstract more tightly.