REVIEW 7 minor 44 references
The lamplighter group Z/2 ≀ Z is polynomially Hilbert–Schmidt stable: checking O(κ^{-21}) relations at precision O(κ^{7}/M^2) forces any approximate representation to be κ-close to a true one.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-08-01 10:42 UTC pith:LH3DW2QL
load-bearing objection First explicit polynomial Hilbert-Schmidt stability bounds for the lamplighter group; the proof is sound, with a patchable exponent typo in Section 7.
Polynomial Hilbert-Schmidt stability of the lamplighter group
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
On the authors' own terms, the paper establishes Theorem 1.1: for κ > 0, with M = ⌈Cκ^{-21}⌉ and ε = cκ^7/M², any unitaries A, T on a finite-dimensional Hilbert space satisfying ∥A²−1∥_{HS} ≤ ε and ∥[A, T^{-i}AT^i]_{HS} ≤ ε for all 0 ≤ i ≤ 2M can be moved within HS distance κ of operators that form a true unitary representation of the lamplighter. Theorem 1.2 restates this as stability radius SRad(r) ⪯ r^{21} and stability rate SRate(κ) ≥ cκ^{70}, which the authors present as the first explicit upper bound on the stability radius of an infinitely presented group, answering a question in [21, Question 12.10], and note the proof also works for A ≀ Z with finite abelian A.
What carries the argument
The load-bearing mechanism is an effective tower decomposition for approximately invariant measures (Proposition 4.3). Working on the full shift X={0,1}^Z, it takes any (M,η)-invariant probability measure and returns a partition of X, up to a clopen error set of measure O(t^6(ν+δ+η)), into clopen towers of polynomial complexity; each tower is either δ-closed of height less than t or has height between t and 6t+1, with a singleton projection on its base. Around this hinge the paper uses four tools: Fourier passage from approximate representations to approximately equivariant projection-valued measures; a rounding lemma (Lemma 3.7) that turns an approximate closed projection tower into an exac
Load-bearing premise
Everything rests on Proposition 4.3: that every approximately invariant measure on {0,1}^Z admits a polynomial-complexity partition into clopen towers with the stated height-versus-closedness guarantee; if any (M,η)-invariant measure escapes such a decomposition, the proof cannot deliver the corrected representation.
What would settle it
Exhibit a family of unitaries (A, T) on growing Hilbert spaces with ∥A²−1∥_{HS} ≤ cκ^7/M² and ∥[A, T^{-i}AT^i]_{HS} ≤ cκ^7/M² for M = Cκ^{-21}, yet kept at HS distance ≥ cκ from any true lamplighter representation — such a family would disprove Theorem 1.1. A cheaper witness: construct an (M,η)-invariant measure on the shift with η ≤ cκ^{14} for which every clopen tower decomposition has a base of superpolynomial complexity in 1/κ, directly contradicting Proposition 4.3; the uniform measure on a long periodic orbit is a natural candidate to test.
If this is right
- The lamplighter group becomes the first known infinitely presented group whose stability radius admits an explicit upper bound, sharpening the earlier qualitative stability result.
- Quantitative Hilbert–Schmidt stability, previously known only for finite and abelian groups, now extends to a nilpotent-wreath family; the proof carries over to A ≀ Z for any finite abelian A.
- The proof supplies a template: any group action whose approximately invariant measures admit polynomial-complexity tower decompositions inherits polynomial HS stability bounds.
- The linear lower bound on the stability radius recorded in Appendix A shows that the true growth exponent, whatever it is, lies between 1 and 21.
Where Pith is reading between the lines
- Editorial: the polynomial tower decomposition is likely the reusable core of the method; other metabelian groups N ⋊ G with a well-behaved shift action may yield quantitative stability by constructing their own tower decompositions.
- Editorial: the exponents in Theorem 1.2 are almost certainly not optimal; a tighter marker lemma or a sharper height-versus-closedness tradeoff could lower r^{21} toward the linear lower bound shown in the appendix.
- Editorial: the same machinery might yield effective bounds for permutation stability of the lamplighter if a Hamming-norm analogue of the projection-tower rounding lemma can be found — a direction the paper explicitly leaves open.
- Editorial: the explicit error stack (ε = cκ^7/M², M = Cκ^{-21}) suggests a testable check of Proposition 4.3 by direct computation with the Bernoulli measure on the shift, where the error bound t^6(ν+δ+η) should be verifiable explicitly.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper establishes explicit polynomial bounds for Hilbert–Schmidt stability of the lamplighter group Z/2∩Z. Theorem 1.1 asserts that unitaries A,T whose defects on a^2=1 and on [A,T^{-i}AT^i], |i|≤2M, are O(κ^7/M^2), with M=⌈Cκ^{-21}⌉, are κ-close to a genuine lamplighter representation. Theorem 1.2 translates this into SRad(r)⪙r^{21} and, at radius r_κ=Cκ^{-21}, SRate≥cκ^{70}. The proof is a transparent chain: preprocessing to commuting B_i (Lemma 4.1), passage to an approximately equivariant PVM and an approximately invariant measure on the full shift (Section 4.1), a polynomial Kakutani–Rokhlin tower decomposition (Proposition 4.3) proved via periodic extensions (Lemma 6.6) and efficient marker sets from LOCAL algorithms (Propositions 5.4 and 6.8), and finally rounding of approximate projection towers (Lemma 3.7). Appendix A gives a linear lower bound on the stability radius.
Significance. If correct, the paper gives the first explicit stability-radius upper bound for an infinitely presented group, answering a question from [21]. The new dynamical tools — approximately invariant measures, polynomial tower decompositions in the presence of periodic points, and efficient marker sets — are likely to be of independent interest. The main theorem is supported by an unusually detailed error budget, and the authors also provide a claimed Lean formalization of Theorem 1.1 with a comparator file [19,20]; if the formalization is independently confirmed, this is a substantial strength. I found no circularity: the external black boxes (finite abelian stability [12,13], Linial [35], Bernshteyn [10]) do not presuppose the lamplighter bound. The issues I identified below are local and do not undermine the central argument.
minor comments (7)
- [Section 7, proof of Theorem 1.2] The proof chooses M_κ=Cκ^{-20}∙log(2/κ) and derives SRate≥cκ^{67}/log^3, while the theorem statement claims r_κ=Cκ^{-21} and SRate≥cκ^{70}. The two are compatible (the latter radius is larger and κ^{67}/log^3≥κ^{70} for small κ), but the proof should either apply Theorem 1.1 with M∼κ^{-21} to obtain the stated exponents directly, or explicitly invoke monotonicity in r and the exponent comparison. This is a bookkeeping fix, not a substantive gap.
- [Section 6.2.3, after definition of W_j] The text claims “W_j ≠ ∅ for each j≤t” and then concludes that all towers have height at least t. The F_t-independence of Z actually gives W_j=∅ for 1≤j≤t; otherwise L^t x and L^{t+j}x are two points of Z separated by a shift j≤t. The subsequent decomposition over j=t+1,…,2t+1 confirms that this is the intended statement. Please correct the displayed claim and its conclusion.
- [Lemma 6.4] Items (i) and (ii) of Lemma 6.4 are left “for the reader.” These statements are elementary, but they are used in the load-bearing covering argument of Lemma 6.6. Since the manuscript is otherwise careful about details, include short proofs or a precise citation to keep the tower decomposition fully self-contained.
- [Proof of Theorem 1.1] In the Pythagoras estimate for ‖ρ(t^{-1})-T‖^2, the displayed formula writes the norm as a sum over tower blocks plus 2μ(e), but does not explicitly account for cross terms between P_τH and E_eH. These cross terms are controlled by Claim 4.10 (the off-diagonal blocks of T have squared norm bounded by the same tower defect sums), but the manuscript should say this explicitly rather than leaving it implicit.
- [Appendix A] The computation ‖[X,Z]‖^2_HS=2 and the normalization (1/(4m))·2m appear inconsistent with the paper’s definition of normalized Hilbert–Schmidt norm on H=C^{4m}⊗C^2: with the paper’s convention one obtains a different constant (the trace of [X,Z]^*[X,Z] on C^2 is 8, so the correct constant is 2 after normalizing by dim H=8m, but the displayed intermediate formula is not the one used). The lower-bound argument itself survives with a corrected constant, but the displayed calculation should be fixed.
- [Cross-reference] Lemma 4.8 refers to “Lemma 6.9 (proved in Section 5)”, but Lemma 6.9 is stated and proved in Section 6. Update the cross-reference.
- [Reference [37]] Proposition 5.4 is attributed to P. Naryshkin via a personal-communication reference. Since the proof is included in the paper, this is not a correctness issue, but the authors may wish to replace [37] with a public source or state explicitly that the proof is self-contained, so that readers are not dependent on a personal communication.
Circularity Check
No circularity: derivation is self-contained; the Section 7 exponent mismatch is a patchable presentation issue, not a logical reduction.
full rationale
The paper's central derivation (Theorem 1.1 via Proposition 4.3, Lemma 3.7, and Lemma 4.1) does not fit any parameter to the target bound and then rename it a prediction. The approximately equivariant PVM and invariant measure are constructed from the input unitaries through a Fourier transform and finite-abelian stability, using external results [12,13]; no fitted constant is later invoked as the predicted closeness. The load-bearing dynamical input, Proposition 4.3, is proved independently through explicit tower constructions relying on Linial's LOCAL algorithm [35], Bernshteyn's correspondence [10], and Naryshkin's marker lemma [37] — none of which are authored by Dogon/Vidick or assume the lamplighter bound. The only self-citations ([17], [21], [2]) supply definitions of stability rate/radius, background, and a motivating question, not the theorem itself. The apparent exponent mismatch in Section 7 (r_kappa = C kappa^{-20} log versus Theorem 1.2's kappa^{-21}, and kappa^{67} versus kappa^{70}) is a textual parameter-inconsistency, not a circular reduction; the stated bounds are recoverable by setting M = C kappa^{-21}, as the skeptic notes, so the logical derivation remains independent of its conclusion. No equation reduces to its input by construction, and no uniqueness theorem is imported from the authors' prior work to force the choice.
Axiom & Free-Parameter Ledger
free parameters (4)
- tower height threshold t =
⌈Cκ^{-2}⌉
- closeness parameters δ, υ =
cκ^{14}
- radius parameter M and r_κ =
stated as ⌈Cκ^{-21}⌉; proof of Thm 1.2 uses ⌈Cκ^{-20}log(2/κ)⌉
- universal constants C,c =
unspecified positive constants
axioms (7)
- domain assumption Quantitative Hilbert–Schmidt stability of (Z/2)^n (Lemma 4.1, Thm 3.2 in [12])
- domain assumption Linial's deterministic LOCAL O(Δ²)-coloring algorithm (Lemma 5.3, [35])
- domain assumption Bernshteyn's correspondence between LOCAL algorithms and continuous colorings (Lemma 5.5, [10, Thm 2.13])
- standard math Mackey-machine classification of finite-dimensional irreducible representations of Z/2≀Z (Section 2.2, [8])
- ad hoc to paper Minimal periodic extension properties (Lemma 6.4(i),(ii))
- domain assumption Orbit-closing / dense periodic measures for the full shift (used in Lemma 6.6, [34,38])
- domain assumption Rounding lemma for near-involutions (Prop 1.4 in [14])
Cite this review
Pith. "Pith review of Polynomial Hilbert-Schmidt stability of the lamplighter group." pith.science (2026). https://pith.science/paper/LH3DW2QL
@misc{pith2026260720135,
author = {Pith},
title = {Pith review of: Polynomial Hilbert-Schmidt stability of the lamplighter group},
year = {2026},
howpublished = {\url{https://pith.science/paper/LH3DW2QL}},
note = {Machine review of arXiv:2607.20135}
}
read the original abstract
We establish explicit polynomial bounds on the stability rate and radius of the lamplighter group. This provides the first example of an infinitely presented group with an explicit upper bound on the stability radius, answering a question of the first author, Levit and Vigdorovich. Our approach is based on new dynamical notions, including the analysis of approximately invariant measures. We establish an effective continuous tower decomposition procedure for approximately invariant measures with presence of periodic points. To achieve polynomial bounds we appeal to recent techniques from descriptive combinatorics and distributed LOCAL algorithms.
Reference graph
Works this paper leans on
-
[1]
Tudor Achim, Alex Best, Alberto Bietti, Kevin Der, Mathïs Fédérico, Sergei Gukov, Daniel Halpern-Leistner, Kirsten Henningsgard, Yury Kudryashov, Alexander Meiburg, et al.,Aristotle: IMO-level automated theorem proving, arXiv preprint arXiv:2510.01346 (2025)
Pith/arXiv arXiv 2025
-
[2]
Akhtiamov and A
D. Akhtiamov and A. Dogon,On uniform Hilbert Schmidt stability of groups, Proc. Am. Math. Soc.150 (2022), no. 4, 1799–1809 (English)
2022
-
[3]
Bachner, A
B. Bachner, A. Dogon, and A. Lubotzky,On L1-approximation of groups, Journal of Algebra702(2026), 235–243
2026
-
[4]
Barenboim and M
L. Barenboim and M. Elkin,Distributed graph coloring: Fundamentals and recent developments, Synthesis Lectures on Distributed Computing Theory, Springer, 2013
2013
-
[5]
Becker and A
O. Becker and A. Lubotzky,Group stability and property (T), Journal of Functional Analysis278(2020), no. 1, 20, Id/No 108298
2020
-
[6]
Becker, A
O. Becker, A. Lubotzky, and A. Thom,Stability and invariant random subgroups, Duke Mathematical Journal 168(2019), no. 12, 2207–2234
2019
-
[7]
Becker and J
O. Becker and J. Mosheiff,Abelian groups are polynomially stable, International Mathematics Research No- tices2021(2021), no. 20, 15574–15632
2021
-
[8]
Bekka and P
B. Bekka and P. de la Harpe,Unitary representations of groups, duals, and characters, Mathematical Surveys and Monographs, vol. 250, American Mathematical Society, Providence, RI, 2020. MR 4249450
2020
-
[9]
Bernshteyn,Descriptive combinatorics and distributed algorithms, Notices of the American Mathematical Society69(2022), no
A. Bernshteyn,Descriptive combinatorics and distributed algorithms, Notices of the American Mathematical Society69(2022), no. 09
2022
-
[10]
Math.233 (2023), no
,Distributed algorithms, the Lovász local lemma, and descriptive combinatorics, Invent. Math.233 (2023), no. 2, 495–542
2023
-
[11]
Bradford,Groups with arbitrarily poor permutation stability, arXiv preprint arXiv:2604.14903 (2026)
H. Bradford,Groups with arbitrarily poor permutation stability, arXiv preprint arXiv:2604.14903 (2026)
Pith/arXiv arXiv 2026
-
[12]
R. Chao, B. W. Reichardt, C. Sutherland, and T. Vidick,Overlapping Qubits, 8th Innovations in Theoretical Computer Science Conference (ITCS 2017) (Dagstuhl, Germany) (Christos H. Papadimitriou, ed.), Leibniz International Proceedings in Informatics (LIPIcs), vol. 67, Schloss Dagstuhl – Leibniz-Zentrum für Informatik, 2017, pp. 48:1–48:21
2017
-
[13]
Michael Chapman, Thomas Vidick, and Henry Yuen,Efficiently stable presentations from error-correcting codes, Discrete Analysis (2026)
2026
-
[14]
De Chiffre, L
M. De Chiffre, L. Glebsky, A. Lubotzky, and A. Thom,Stability, cohomology vanishing, and nonapproximable groups, Forum Math. Sigma8(2020), 37, Id/No e18
2020
-
[15]
De Chiffre, N
M. De Chiffre, N. Ozawa, and A. Thom,Operator algebraic approach to inverse and stability theorems for amenable groups, Mathematika65(2019), no. 1, 98–118 (English)
2019
-
[16]
M. de la Salle,Spectral gap and stability for groups and non-local games, arXiv preprint arXiv:2204.07084 (2022)
arXiv 2022
-
[17]
Dogon,Stability and approximation of groups and operator algebras, 2021, M.Sc
A. Dogon,Stability and approximation of groups and operator algebras, 2021, M.Sc. thesis, published by the Hebrew University of Jerusalem
2021
-
[18]
Z.305(2023), no
,Flexible Hilbert-Schmidt stability versus hyperlinearity for property (T) groups, Math. Z.305(2023), no. 4, 20 (English), Id/No 58
2023
-
[19]
Dogon and T
A. Dogon and T. Vidick,lamplighter-comparator, GitHub repository,https://github.com/vidick/ lamplighter-comparator, 2026
2026
-
[20]
,lamplighter-lean, GitHub repository,https://github.com/vidick/lamplighter-lean, 2026
2026
-
[21]
Alon Dogon, Arie Levit, and Itamar Vigdorovich,Characters of diagonal products and hilbert-schmidt stabil- ity, arXiv preprint arXiv:2407.11608 (2024)
Pith/arXiv arXiv 2024
-
[22]
Eckhardt and T
C. Eckhardt and T. Shulman,On amenable Hilbert-Schmidt stable groups, J. Funct. Anal.285(2023), no. 3, Paper No. 109954. MR 4579918
2023
-
[23]
G. B. Folland,A course in abstract harmonic analysis, Studies in Advanced Mathematics, CRC Press, Boca Raton, FL, 1995
1995
-
[24]
Gao and S
S. Gao and S. Jackson,Countable abelian group actions and hyperfinite equivalence relations, Invent. Math. 201(2015), no. 1, 309–383. MR 3359054
2015
-
[25]
S. Gao, S. Jackson, E. Krohne, and B. Seward,Continuous combinatorics of abelian group actions, Mem. Am. Math. Soc., vol. 1573, Providence, RI: American Mathematical Society (AMS), 2025 (English). 33
2025
-
[26]
L. Glebsky,Almost commuting matrices with respect to normalized hilbert-schmidt norm, arXiv preprint arXiv:1002.3082 (2010)
Pith/arXiv arXiv 2010
-
[27]
W. T. Gowers and O. Hatami,Inverse and stability theorems for approximate representations of finite groups, Sb. Math.208(2017), no. 12, 1784–1817 (English)
2017
-
[28]
J. Grebík and Z. Vidnyánszky,From descriptive to distributed, arXiv preprint arXiv:2502.15347 (2025)
Pith/arXiv arXiv 2025
-
[29]
Hadwin and T
D. Hadwin and T. Shulman,Stability of group relations under small Hilbert–Schmidt perturbations, Journal of Functional Analysis275(2018), no. 4, 761–792
2018
-
[30]
Z. Ji, A. Natarajan, T. Vidick, J. Wright, and H. Yuen,Quantum soundness of the classical low individual degree test, arXiv preprint arXiv:2009.12982 (2020)
Pith/arXiv arXiv 2009
-
[31]
11, 131–138
,MIP*=RE, Communications of the ACM64(2021), no. 11, 131–138
2021
-
[32]
Lazarovich, A
N. Lazarovich, A. Levit, and Y . Minsky,Surface groups are flexibly stable, Journal of the European Mathem- atical Society27(2024), no. 4, 1739–1768
2024
-
[33]
Levit and A
A. Levit and A. Lubotzky,Infinitely presented permutation stable groups and invariant random subgroups of metabelian groups, Ergodic Theory Dynam. Systems42(2022), no. 6, 2028–2063. MR 4417344
2022
-
[34]
Levit and I
A. Levit and I. Vigdorovich,Characters of solvable groups, Hilbert–Schmidt stability and dense periodic measures, Mathematische Annalen (2023), 1–49
2023
-
[35]
Linial,Locality in distributed graph algorithms, SIAM Journal on Computing21(1992), no
N. Linial,Locality in distributed graph algorithms, SIAM Journal on Computing21(1992), no. 1, 193–201
1992
-
[36]
P. Naryshkin,URP, comparison, mean dimension, and sharp shift embeddability, arXiv preprint arXiv:2410.01757 (2024)
Pith/arXiv arXiv 2024
-
[37]
Petr Naryshkin, Personal communication
-
[38]
K. R. Parthasarathy,On the category of ergodic measures, Ill. J. Math.5(1961), 648–656 (English)
1961
-
[39]
Scott Schneider and Brandon Seward,Locally nilpotent groups and hyperfinite equivalence relations, arXiv preprint arXiv:1308.5853 (2013)
Pith/arXiv arXiv 2013
-
[40]
S. M. Ulam,A collection of mathematical problems, Interscience Tracts in Pure and Applied Mathematics, no. 8, Interscience Publishers, New York-London, 1960. MR 0120127
1960
-
[41]
Thomas Vidick,Almost synchronous quantum correlations, Journal of mathematical physics63(2022), no. 2
2022
-
[42]
4996–5025, EMS Press, Berlin, Germany, 2023
,MIP*=RE: A negative resolution to connes’ embedding problem and tsirelson’s problem, p. 4996–5025, EMS Press, Berlin, Germany, 2023
2023
-
[43]
von Neumann,Beweis des Ergodensatzes und des H-Theorems in der neuen Mechanik, Z
J. von Neumann,Beweis des Ergodensatzes und des H-Theorems in der neuen Mechanik, Z. Phys.57(1929), no. 1, 30–70
1929
-
[44]
R. Yaari,Density of finitely supported invariant measures for automorphisms of compact abelian groups, arXiv preprint arXiv:2507.14113 (2025). WEIZMANNINSTITUTE OFSCIENCE, ISRAEL Email address:alon.dogon@mail.huji.ac.il ÉCOLEPOLYTECHNIQUEFÉDÉRALE DELAUSANNE, SWITZERLAND ANDWEIZMANNINSTITUTE OFSCIENCE, ISRAEL Email address:thomas.vidick@epfl.ch 34
Pith/arXiv arXiv 2025
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.