arXiv:2605.12360v3

Asymmetry of 2\ell^{2}-cohomology via skewed Følner geometry

Nachi Avraham-Re'em, Zemer Kosloff

math.GRmath.DS20F1820F6520F6937A2020J0637A4043A07

Abstract

We study the two 2\ell^{2}-Dirichlet structures on a countable group GG arising from the left and right regular actions on RG\mathbb{R}^{G}. Although the two regular representations are unitarily equivalent, their 2\ell^{2}-Dirichlet subspaces of RG\mathbb{R}^{G} need not coincide. Our main result gives a complete classification of this asymmetry for countable amenable groups: D2(G,λ)=D2(G,ρ)G is an FC-group.\mathcal{D}_{2}\left(G,λ\right)=\mathcal{D}_{2}\left(G,ρ\right)\quad\Longleftrightarrow\quad G \text{ is an FC-group}. The proof is based on a skewed Følner-geometric mechanism, called a left scheme, combining summability of left boundaries with displacement under a right translation. We develop this mechanism generally, and demonstrate it concretely in the Heisenberg group and amenable wreath products over Z\mathbb{Z}. We also show that this mechanism has a dynamical counterpart in the theory of nonsingular Bernoulli shifts: every countable amenable group that is not an FC-group admits Bernoulli schemes whose left shift is nonsingular, conservative and weakly mixing, whereas the right shift by some element is singular.

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Audited against arXiv v3

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.

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Generated August 18, 2026
01Statements2 reported findingsCorrect

The classification of equality of the left and right 2\ell^2-Dirichlet spaces for amenable groups, the nonamenable symmetric-intersection theorem, and the asymmetric Bernoulli constructions are correct.

Theorems 1.1, 1.2, and 1.4Correct

FC groups are exactly the amenable groups without left-right asymmetry

Pages 2–4 and Sections 2–5 · arXiv:2605.12360v3

For an FC group, each right difference is bounded by finitely many left differences over a conjugacy class. If an amenable group has an infinite conjugacy class, translated Følner sets form a left scheme whose normalized indicators have square-summable left boundaries but disjoint right translates; their weighted sum lies in D2(G,λ)D2(G,ρ)D_2(G,\lambda)\setminus D_2(G,\rho). A recurrent refinement and Kakutani's criterion convert the same geometry into a left-nonsingular conservative weakly mixing Bernoulli scheme with a singular right translate. The appendix establishes the claimed nonamenable symmetric intersection.

Full paper, version 3
Theorems 1.2 and 1.4Correct

FC classification and Bernoulli constructions cover the two amenable cases

Pages 3–6 and Sections 2–3 · arXiv:2605.12360v3

For FC groups the left and right regular Dirichlet spaces coincide by finite-conjugacy averaging. If an amenable group is not FC, an element with infinite conjugacy class yields the asymmetric Bernoulli scheme and hence the announced cocycle, so the alternatives exactly match the classification.

02Proofs2 reported findingsCorrect

The skewed Følner, cocycle, FC-group, Bernoulli, and nonamenable arguments are correct and complete.

Sections 2–5 and Appendix ACorrect and complete

Boundary summability and right displacement are combined correctly

Pages 4–15 · Sections 2–5 and Appendix A · arXiv:2605.12360v3

Right-translating the Følner sets preserves all left boundary ratios while conjugacy supplies disjointness from the chosen right translate. Rearrangement gives pairwise disjoint supports, so the 2\ell^2 estimates sum exactly. Truncating the constructed cocycle into (δ,1δ)(\delta,1-\delta) preserves the nonsingularity and singularity criteria, and the recurrent scheme supplies conservativity before weak mixing is invoked.

Sections 2–3Correct and complete

Følner cocycles and the Bernoulli asymmetry argument have all norm estimates

Pages 8–31 · arXiv:2605.12360v3

A suitably thinned Følner sequence makes the symmetric 2\ell^2 increments summable while keeping the cocycle unbounded. In the non-FC construction, independence computes the left differences and infinite conjugacy forces divergence on the right; truncation and monotone convergence justify passage to the infinite Bernoulli sum.

03Novelty0 reported findingsNo non-novelty findings

No non-novelty findings.

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Paper
arXiv:2605.12360v3
Authors listed
Nachi Avraham-Re'em, Zemer Kosloff
Audit date
August 18, 2026
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