arXiv:2601.09553v3

The non-ergodic Host-Kra-Ziegler structure theorem for Zd\mathbb{Z}^d-actions via measurable selections

Asgar Jamneshan, Simon Machado

math.DS37A35

Abstract

We establish a non-ergodic version of the Host-Kra-Ziegler structure theorem for measure-preserving Zd\mathbb{Z}^d-actions. Our argument reduces the non-ergodic case to the ergodic theorem (for d2d\ge 2 due to Candela and Szegedy) via a measurable selection procedure. We also establish a non-ergodic vertical nilcharacter version of our main result. The non-ergodic version of the Host-Kra-Ziegler structure theorem is a key input in the companion paper by the second author and Fraczyk classifying point processes (i.e. random subsets) of Zd\mathbb{Z}^d whose law is invariant under the group ASLd(Z)\mathrm{ASL}_d(\mathbb{Z}) of affine transformations.

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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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Detailed mathematical audit

Generated August 18, 2026
01Statements2 reported findingsCorrect

The nonergodic Host–Kra–Ziegler structure theorem for Zd\mathbb Z^d-actions and the spanning theorem by nonergodic vertical nilcharacters are supported.

Theorem 1.2Correct

Nonergodic Host–Kra factors are inverse limits of nilsystem bundles

Pages 2 and 8–17 · Section 3 · arXiv:2601.09553v3

Ergodic decomposition reduces almost every fiber to the classical ergodic structure theorem. Encoding factor maps by graph joinings makes the admissible-fiber relation Borel, so measurable uniformization selects approximating nilfactors simultaneously. Finite joins remain ergodic subnilmanifolds of finite products and hence are nilsystem bundles of step at most kk. The dense countable family of functions is approximated with error tending to zero, which proves that the increasing join has the full Host–Kra sigma-algebra and gives the inverse limit.

Theorem 1.3Correct

Spanning by nonergodic vertical nilcharacters

Pages 3 and 18–20 · Section 4 · arXiv:2601.09553v3

On each fixed nilmanifold fiber, the compact vertical group A=Nk/(NkΓ)A=N_k/(N_k\cap\Gamma) acts unitarily. Fourier projections onto its characters are mutually orthogonal and their closed span is all of L2L^2; otherwise the continuous positive-definite correlation function would have every Fourier coefficient zero, including its value at the identity. Applying this fiberwise on the countable bundle partition and then passing through the inverse system proves the stated totality.

02Proofs2 reported findingsCorrect

The measurable-selection, finite-join, inverse-limit, and Fourier-decomposition arguments are correct and complete. The measurable trivialization claim is compressed, but the needed version follows from the stated countability and uniformization inputs.

Lemmas 3.1–3.8Correct and complete

Measurable reconstruction and exhaustion of the factor

Pages 9–17 · arXiv:2601.09553v3

Disintegration makes the conditional Dirac criterion Borel, invariance is checked on the countable group Zd\mathbb Z^d, and ergodicity is expressed using a countable dense family and mean averages. Jankov–von Neumann selection is sufficient after completing the base measure; although Claim 3.6 phrases the assignments as Borel, universal measurability followed by modification on a null set gives exactly the measurable bundle used in the proof. Lemma 3.7 transfers the fiberwise approximation globally, and the orthogonality argument in Lemma 3.8 proves exhaustion without assuming the selected factors are nested.

Section 4Correct and complete

Vertical Fourier decomposition

Pages 18–20 · arXiv:2601.09553v3

The projection PχP_\chi is the standard character projection for the compact abelian vertical group. Vanishing of every projection forces all Fourier coefficients of aUaF,Fa\mapsto\langle U_aF,F\rangle to vanish; density of characters in C(A)C(A) then gives F22=0\|F\|_2^2=0. Countability of bundle components and nilmanifold character groups supplies the claimed global closed span.

03Novelty0 reported findingsNo non-novelty findings

No non-novelty findings.

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Paper
arXiv:2601.09553v3
Authors listed
Asgar Jamneshan, Simon Machado
Audit date
August 18, 2026
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