Abstract

For any infinite one-relator group Γ=SwmΓ=\langle S \mid w^m\rangle, we prove that cost(Γ)=S1m\mathrm{cost}(Γ)=|S|-\frac{1}{m}. For such groups, this gives β1(2)(Γ)=cost(Γ)1β^{(2)}_1(Γ)=\mathrm{cost}(Γ) - 1, answering a special case of Gaboriau's question on the relationship between cost and first 2\ell^2-Betti number.

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Audit summary

Audited against arXiv v1

Not a correctness certificate. A “Correct” result may include yellow typos or minor formal corrections that do not affect substantive soundness. It means this audit found no unresolved substantive error under the stated criteria; it does not replace expert scrutiny or formal verification.

Current report

Detailed mathematical audit

Generated August 18, 2026
01Statements2 reported findingsCorrect

The cost formula for infinite one-relator groups, its one-relator-product extension, and the reducible-presentation corollary are correct.

Theorems 1.2 and 4.1; Corollary 4.2Correct

Cost reaches the 2\ell^2-Betti lower bound

Pages 1–2 and 10–12 · arXiv:2606.23506v1

For an embedded relator wmw^m, the positive-system construction removes asymptotically a proportion 1/m1/m of the selected generator edges while preserving connected orbit graphings, giving the upper cost bound. Howie's local-indicability and embedded-relation theorems apply to the quotient used in the construction. The Peterson–Thom relation estimate gives the matching first-2\ell^2-Betti lower bound, and Gaboriau's inequality sandwiches cost to equality. Induction over a reducible presentation yields the corollary.

Full paper, version 1
Theorem 4.1 and Corollary 4.2Correct

The edge-cutting formula iterates to reducible presentations

Section 4 · arXiv:2606.23506v1

Cutting a relator edge separates the coset process into the announced one-relator product while contributing exactly its reciprocal-exponent term to cost. The reducibility ordering lets this operation be repeated, and the infinite-group hypothesis prevents an unaccounted finite terminal component.

02Proofs2 reported findingsCorrect

The positive-system edge-cutting, coset-process, one-relator-product, and cost-comparison proofs are correct and complete.

Sections 2–4Correct and complete

The edge cutting preserves connectivity at the claimed cost

Pages 2–12 · Sections 2–4 · arXiv:2606.23506v1

Equivariant maps from the coset processes supply every coordinate of the positive system. Lemma 3.3 guarantees that along each lifted relation one retained edge reconnects any removed generator edge, and averaging the finite cyclic coordinates leaves density 1/m1/m of the relevant edges. The action remains free after adjoining a Bernoulli factor, so the graphing computes an admissible upper bound for group cost.

Sections 2–4Correct and complete

Coset-process induction and edge cutting establish both cost inequalities

Pages 8–28 · arXiv:2606.23506v1

Finite-index coset processes are coupled equivariantly, and local indicability supplies the order used to expose a removable relator edge. One graphing gives the upper bound, while the component/cycle count gives the matching lower bound; induction preserves essential freeness and yields the corollary.

03Novelty0 reported findingsNo non-novelty findings

No non-novelty findings.

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Paper
arXiv:2606.23506v1
Authors listed
Antoine Poulin, Konrad Wróbel
Audit date
August 18, 2026
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