REVIEW 2 major objections 5 minor 31 references
The generalized phase retrieval problem over compact groups
T0 review · 2 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read Phase retrieval is a special case of recovering matrices from their Gram matrices; the second moment fixes signals up to a product of unitary groups, and low-dimensional semialgebraic priors make that recovery unique up to sign.
desk verdict Useful survey of the authors' own second-moment program, but the genuinely new content is a conjecture plus numerics, and Corollary III.6 contains a definite sign error that overstates the cryo-EM uniqueness threshold. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the second moment viewed as a $G$-equivariant endomorphism of the signal space $V$. Schur's lemma makes this endomorphism block-diagonal over the isotypic decomposition $V=\bigoplus_{\ell=1}^L V_\ell^{\oplus R_\ell}$, with each block a scalar multiple of the identity, and a direct trace computation shows those scalars are exactly the inner products defining the Gram matrices $X_\ell^* X_\ell$; this is what turns the measurement into a Gram-matrix tuple. The uniqueness results then rest on a transversality statement: in an orthogonal representation $V$ of a compact Lie group $H$, a $GL(V)$-generic semialgebraic set $M$ of dimension $M$ is transverse to the $H$-orbits, in the sense that the orbit of a generic point meets $M$ only at $\pm x$ when $K=\dim V-\max_x\dim Hx$ exceeds $M$, and only at $\pm x$ for all points when $K>2M$. The algorithm side is carried by the Procrustes projection, which replaces the classical 'match the measured magnitudes' projection by an orthogonal matching of a current estimate to the Gram-matrix constraint.
What would settle it
Take a fixed semialgebraic prior $M$ of dimension $m<K$, for instance a specific low-dimensional subspace or a union of subspaces, and numerically compute the second-moment map $\Psi(x)=(X_1^*X_1,\ldots,X_L^*X_L)$ restricted to $M$; finding two points $x,y\in M$ with $y\neq \pm x$ and $\Psi(x)=\Psi(y)$ would contradict the all-vectors claim of Corollary III.4. For the generic claim, the same search over random linear translates $A(M)$ would settle whether the $GL(V)$-generic condition delivers the promised uniqueness.
Extended reading notes
Core claim
On the paper's own terms, the discovery is that the generalized phase retrieval problem over a compact group $G$ is the problem of lifting a tuple of Gram matrices $X_\ell^* X_\ell$ back to the matrices $X_\ell$, where the missing data are unitary matrices rather than phases. Theorems II.1 and Corollary II.2 show that the second moment of the MRA observation model is a $G$-invariant element of $\operatorname{Hom}(V,V)$; Schur's lemma forces such an element to act as a scalar multiple of the identity between copies of each irreducible representation, and a trace calculation identifies those scalars with the entries of the Gram matrices. Consequently the second moment determines $x$ only up to $H=\prod_{\ell=1}^L U(N_\ell)$, and the remaining work is to pin down the unitaries. The paper's main new tool is a transversality theorem for semialgebraic sets: for a $GL(V)$-generic prior $M$ of dimension $M$, if $K>M$ then a generic point of $M$ has its $H$-orbit meet $M$ only at $\pm x$, yielding uniqueness up to sign from the second moment; if $K>2M$, this holds for every point. The same machinery gives explicit thresholds for phase retrieval and cryo-EM, and numerical experiments on a linear prior support the conjecture that the recovery map is bi-Lipschitz.
Load-bearing premise
The load-bearing premise is that the semialgebraic prior $M$ is $GL(V)$-generic, and for cryo-EM that $R\ge 2L+1$; a fixed natural prior (exact sparsity with known support, or a trained generative model) is not proven to satisfy this genericity, so the dimension thresholds may not apply to it.
Editorial extensions
If this is right
- In the high-noise regime, signals that satisfy the dimension conditions are recoverable from the second moment with $n=\omega(\sigma^4)$ samples, improving on the $\omega(\sigma^6)$ cost of third-moment methods.
- Cryo-EM structure determination becomes a second-moment problem whenever the radial discretization satisfies $R\ge 2L+1$ and the molecule lies in a sufficiently low-dimensional semialgebraic prior: generic uniqueness up to sign follows from the dimension count $K\approx L^2(R+2L/3)$.
- Classical phase-retrieval software can be lifted to any compact-group setting by swapping the magnitude-matching projection for a Procrustes projection, so sparsity, support, and generative-model priors plug in through their usual projection operators.
- Phase retrieval's known dimension thresholds ($N\ge 2M$ generic, $N\ge 4M$ for all signals) reappear as the special case of one-dimensional irreducible representations, giving a uniform explanation across applications.
- If the bi-Lipschitz conjecture holds, the recovery map from Gram matrices to $x$ has a noise-robustness constant, so small perturbations of the empirical second moment translate to linearly controlled recovery error under linear priors.
Reading between the lines
- The $GL(V)$-generic assumption is the fragile bridge to practice: for a fixed natural prior such as exact sparsity with known support, or a specific trained generative model, the paper provides no proof of genericity. A direct test would be to apply the second-moment map to random linear embeddings of that prior and check numerically whether any two distinct signals share a Gram tuple at $K>M$.
- The dimension thresholds are probably not improvable without extra structure: at $K=M$ the intersection of an $M$-dimensional prior with generic orbits of dimension $k(H)$ should generically have isolated self-intersections, so uniqueness up to sign is the best one can expect—this is our inference, not a claim of the paper.
- If the bi-Lipschitz conjecture is proven, it would convert the existing uniqueness statements into finite-sample guarantees by standard concentration of the empirical second moment: the number of samples needed to reach a target error would scale as $\sigma^4$ times a condition-number factor, which is exactly the regime the paper motivates.
- The same Procrustes-projection framework should extend to third-moment recovery, where the missing objects are not unitary matrices but elements of larger representation-theoretic tensor products; that would give algorithms for signals whose second moment is not injective, a direction the authors flag but do not develop.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces a generalized phase retrieval problem over compact groups, in which the goal is to recover a signal x in a finite-dimensional representation V of a compact group G from the second moment of the multi-reference alignment model. The second moment is shown, following earlier work of the authors, to determine the tuple of Gram matrices X_ℓ^* X_ℓ up to the action of an ambiguity group H = ∏ U(N_ℓ). The paper then states a transversality theorem for semialgebraic priors (Theorem III.1) and derives uniqueness-up-to-sign guarantees under a dimension inequality K > M, with specializations to classical phase retrieval and to a cryo-EM model. It also describes projection-based algorithms borrowed from [14], presents numerical experiments on a linear-prior example, and proposes a bi-Lipschitz stability conjecture (Conjecture V.1). The exposition is clear, but the main theoretical results are surveyed rather than proved in this manuscript, and one of the stated corollaries contains an internal inconsistency that affects its conclusion.
Significance. If the stated results are taken at face value, the paper offers a useful unifying algebraic framework for phase retrieval, MRA, and cryo-EM, and the effective-dimension criterion K = dim V − k(H) provides a clean heuristic for when semialgebraic priors remove the ambiguity group. The transversality-based approach is elegant and the specialization to cryo-EM is timely. However, the novelty of the present manuscript is mostly expository: Theorem II.1 is attributed to [12], Theorem III.1 to [10], and the algorithms to [14]. The paper's original contributions are the unified presentation, the numerical experiments, and the bi-Lipschitz conjecture. The experiments are reproducible in principle but lack error bars and code, and the conjecture is supported only by a narrow linear-prior example. The corrected effective dimension for cryo-EM changes the quantitative uniqueness threshold, so the paper needs revision before its claims can be relied upon.
major comments (2)
- [§III-B, Corollary III.6] The effective-dimension formula in Corollary III.6 contains a sign error that is internal to the paper's own definitions. Equation (III.1) defines K = dim V − k(H). For the cryo-EM representation V = ⊕_{ℓ=0}^L V_ℓ^{⊕R}, we have dim V = R(L+1)^2 ≈ R L^2. With R ≥ 2L+1, the generic H-orbit under H = ∏_{ℓ=0}^L O(2ℓ+1) has dimension k(H) = Σ_{ℓ=0}^L ℓ(2ℓ+1) ≈ 2L^3/3, because each O(2ℓ+1) acts freely on a full-rank (2ℓ+1)×R matrix. Hence K ≈ L^2(R − 2L/3), not L^2(R + 2L/3) as printed. The printed expression overstates K by about 4L^3/3, so the condition K > M is claimed for priors of dimension up to roughly twice the actual generic threshold. This is a load-bearing error in the cryo-EM uniqueness statement and must be corrected, together with any downstream discussion of the threshold.
- [Theorem III.1 and Corollary III.4] The uniqueness statements are conditional on a GL(V)-genericity hypothesis that is not carried through to the applications. Theorem III.1 states the transversality result for a GL(V)-generic translate A(M) of a semialgebraic set M, but Corollary III.4 is phrased as if any semialgebraic set of dimension M satisfying K > M is sufficient. For the motivating priors—exact sparsity with known support, or a fixed deep generative model—there is no proof that the set is GL(V)-generic, and a generic linear translate is not the same as the original prior. The paper should either prove the genericity condition for concrete instances of sparsity and generative-model priors, or explicitly state in Corollaries III.4–III.6 that the guarantee applies only after a generic linear translate of the prior. Without this qualification, the practical claim that a fixed prior ensures uniqueness is not established.
minor comments (5)
- [Theorem III.1] The proof of Theorem III.1 is only sketched, and the text refers to [10] for the detailed formulation. Since the paper's abstract describes itself as a survey, this is acceptable, but the introduction should state explicitly that the main theorems are surveyed from prior work, and the present manuscript's original contribution should be clearly delineated.
- [Numerical experiments, §IV] The numerical experiments report median errors over 10,000 runs but provide no error bars, no confidence intervals, and no code or detailed parameter settings beyond the matrix size and noise level. Adding these would substantially strengthen the empirical support for Conjecture V.1.
- [Corollary II.2 and §II-B] The notation is inconsistent between the complex and real settings: Corollary II.2 uses H = ∏ U(N_ℓ), while the cryo-EM discussion uses H = ∏ O(2ℓ+1). Please clarify that the unitary group is used for complex representations and the orthogonal group for real representations.
- [Conjecture V.1] The target space in Conjecture V.1 is written as R^N = ∏_{i=1}^L R^{N_ℓ×R_ℓ}, which conflates the total dimension with the product of matrix spaces. The intended meaning is a direct sum of matrix spaces, so the notation should be cleaned up to avoid dimension confusion.
- [Corollary III.6] The text says that a precise statement of Corollary III.6 is provided in [10]. After correcting the sign error, it would be helpful to include the precise statement in the main text as well, so the corollary is self-contained.
Circularity Check
No circular derivation: the paper transparently cites prior work by the same authors for the main theorems, and the new conjecture and numerics are independent contributions.
full rationale
The paper is a survey/exposition that explicitly attributes the core theorems to prior work: Theorem II.1 is credited to [12], Theorem III.1 to [10], and the projection algorithm to [14]. These citations are not disguised as new derivations; the authors state the assumptions (e.g., GL(V)-generic semialgebraic priors, K = dim V - k(H)) and give proof sketches, and the cited results do not assume the target conclusions. No fitted parameter is renamed as a prediction, and no quantity is defined in terms of the claimed output. The numerical experiments are new tests of an alternating-projection algorithm and are not used to prove the uniqueness theorems, so no 'prediction' reduces to a fit. The heavy self-citation reflects the authors building on their own prior work, but it does not constitute circularity because the cited results are independent prior theorems with stated assumptions. The only notable issue found is a likely sign error in Corollary III.6's estimate of K (the paper states K ≈ L^2(R + 2L/3), but using Eq. (III.1) and the cryo-EM decomposition dim V = R(L+1)^2 and k(H) ≈ 2L^3/3 yields K ≈ L^2(R - 2L/3)); this is an arithmetic/consistency error, not a circular step, and should be treated as a correctness risk rather than circularity.
Assumptions & free parameters
assumptions (7)
- standard math Schur's lemma and the decomposition of finite-dimensional representations of compact groups into irreducibles (II.1).
- domain assumption The MRA observation model with uniform Haar-distributed group action and Gaussian noise (I.2, I.3).
- domain assumption The cryo-EM approximation uses radial discretization and band-limiting to make V finite-dimensional (Section II.B).
- ad hoc to paper The semialgebraic set M is GL(V)-generic (Theorem III.1).
- standard math The Fiber Lemma from [10, Lemma 6.1] used to prove Theorem III.1.
- domain assumption For the conjecture, the second moment is injective up to sign on M (Conjecture V.1 preamble).
- domain assumption The cryo-EM band-limit and radial discretization with R samples and bandlimit L, plus the condition R ≥ 2L+1 (Section II.B and Corollary III.6).
Cite this review
Pith. "Pith review of The generalized phase retrieval problem over compact groups." pith.science (2026). https://pith.science/paper/KQYT2HIK
@misc{pith2026250103549,
author = {Pith},
title = {Pith review of: The generalized phase retrieval problem over compact groups},
year = {2026},
howpublished = {\url{https://pith.science/paper/KQYT2HIK}},
note = {Machine review of arXiv:2501.03549}
}
read the original abstract
The classical phase retrieval problem involves estimating a signal from its Fourier magnitudes (power spectrum) by leveraging prior information about the desired signal. This paper extends the problem to compact groups, addressing the recovery of a set of matrices from their Gram matrices. In this broader context, the missing phases in Fourier space are replaced by missing unitary or orthogonal matrices arising from the action of a compact group on a finite-dimensional vector space. This generalization is driven by applications in multi-reference alignment and single-particle cryo-electron microscopy, a pivotal technology in structural biology. We define the generalized phase retrieval problem over compact groups and explore its underlying algebraic structure. We survey recent results on the uniqueness of solutions, focusing on the significant class of semialgebraic priors. Furthermore, we present a family of algorithms inspired by classical phase retrieval techniques. Finally, we propose a conjecture on the stability of the problem based on bi-Lipschitz analysis, supported by numerical experiments.
Figures
Reference graph
Works this paper leans on
-
[12]
Tamir Bendory and Dan Edidin. The sample complexity of sparse multireference alignment and single-particle cryo-electron microscopy. SIAM Journal on Mathematics of Data Science , 6(2):254–282, 2024
work page 2024
-
[10]
Tamir Bendory, Nadav Dym, Dan Edidin, and Arun Suresh. A transversality theorem for semi-algebraic sets with application to signal recovery from the second moment and cryo-EM. arXiv preprint arXiv:2405.04354, 2024
arXiv 2024
-
[14]
Tamir Bendory, Yuehaw Khoo, Joe Kileel, Oscar Mickelin, and Amit Singer. Autocorrelation analysis for cryo-EM with sparsity constraints: improved sample complexity and projection-based algorithms. Proceed- ings of the National Academy of Sciences , 120(18):e2216507120, 2023
work page 2023
-
[1]
Estimation in the group action channel
Emmanuel Abbe, Joao M Pereira, and Amit Singer. Estimation in the group action channel. In 2018 IEEE International Symposium on Information Theory (ISIT) , pages 561–565. IEEE, 2018
work page 2018
-
[2]
On Lipschitz analysis and Lipschitz synthesis for the phase retrieval problem
Radu Balan and Dongmian Zou. On Lipschitz analysis and Lipschitz synthesis for the phase retrieval problem. Linear Algebra and its Applications, 496:152–181, 2016
work page 2016
-
[3]
Estimation under group actions: recovering orbits from invariants
Afonso S Bandeira, Ben Blum-Smith, Joe Kileel, Jonathan Niles-Weed, Amelia Perry, and Alexander S Wein. Estimation under group actions: recovering orbits from invariants. Applied and Computational Harmonic Analysis, 66:236–319, 2023
work page 2023
-
[4]
Mul- tireference alignment using semidefinite programming
Afonso S Bandeira, Moses Charikar, Amit Singer, and Andy Zhu. Mul- tireference alignment using semidefinite programming. In Proceedings of the 5th conference on Innovations in theoretical computer science , pages 459–470, 2014
work page 2014
-
[5]
Non-unique games over compact groups and orientation estimation in cryo-EM
Afonso S Bandeira, Yutong Chen, Roy R Lederman, and Amit Singer. Non-unique games over compact groups and orientation estimation in cryo-EM. Inverse Problems, 36(6):064002, 2020
work page 2020
Show all 31 references
-
[6]
Geometry of the Phase Retrieval Problem: Graveyard of Algorithms
Alexander H Barnett, Charles L Epstein, Leslie Greengard, and Jeremy Magland. Geometry of the Phase Retrieval Problem: Graveyard of Algorithms. Cambridge University Press, 2022
2022
-
[7]
Single-particle cryo-electron microscopy: Mathematical theory, computational chal- lenges, and opportunities
Tamir Bendory, Alberto Bartesaghi, and Amit Singer. Single-particle cryo-electron microscopy: Mathematical theory, computational chal- lenges, and opportunities. IEEE signal processing magazine , 37(2):58– 76, 2020
2020
-
[8]
Fourier phase retrieval: Uniqueness and algorithms
Tamir Bendory, Robert Beinert, and Yonina C Eldar. Fourier phase retrieval: Uniqueness and algorithms. In Compressed Sensing and its Applications: Second International MATHEON Conference 2015 , pages 55–91. Springer, 2017
2015
-
[9]
Bispectrum inversion with application to multireference align- ment
Tamir Bendory, Nicolas Boumal, Chao Ma, Zhizhen Zhao, and Amit Singer. Bispectrum inversion with application to multireference align- ment. IEEE Transactions on signal processing , 66(4):1037–1050, 2017
2017
-
[11]
Algebraic theory of phase retrieval
Tamir Bendory and Dan Edidin. Algebraic theory of phase retrieval. Not. AMS , 69(9):1487–1495, 2022
2022
-
[13]
Non-convex phase retrieval from STFT measurements
Tamir Bendory, Yonina C Eldar, and Nicolas Boumal. Non-convex phase retrieval from STFT measurements. IEEE Transactions on Information Theory, 64(1):467–484, 2017
2017
-
[15]
Bi-Lipschitz quotient embedding for Euclidean group actions on data
Harm Derksen. Bi-Lipschitz quotient embedding for Euclidean group actions on data. arXiv preprint arXiv:2409.06829 , 2024
2024 arXiv
-
[16]
Sparse and redundant representations: from theory to applications in signal and image processing, 2010
Michael Elad. Sparse and redundant representations: from theory to applications in signal and image processing, 2010
2010
-
[17]
Phase retrieval by iterated projections
Veit Elser. Phase retrieval by iterated projections. JOSA A, 20(1):40–55, 2003
2003
-
[18]
Benchmark problems for phase retrieval
Veit Elser, Ti-Yen Lan, and Tamir Bendory. Benchmark problems for phase retrieval. SIAM Journal on Imaging Sciences , 11(4):2429–2455, 2018
2018
-
[19]
Searching with iterated maps
Veit Elser, I Rankenburg, and P Thibault. Searching with iterated maps. Proceedings of the National Academy of Sciences , 104(2):418– 423, 2007
2007
-
[20]
The numerics of phase retrieval
Albert Fannjiang and Thomas Strohmer. The numerics of phase retrieval. Acta Numerica , 29:125–228, 2020
2020
-
[21]
Phase retrieval algorithms: a comparison
James R Fienup. Phase retrieval algorithms: a comparison. Applied optics, 21(15):2758–2769, 1982
1982
-
[22]
Phase retrieval: uniqueness and stability
Philipp Grohs, Sarah Koppensteiner, and Martin Rathmair. Phase retrieval: uniqueness and stability. SIAM Review, 62(2):301–350, 2020
2020
-
[23]
Phase retrieval from local measurements: Improved robustness via eigenvector-based angular synchronization
Mark A Iwen, Brian Preskitt, Rayan Saab, and Aditya Viswanathan. Phase retrieval from local measurements: Improved robustness via eigenvector-based angular synchronization. Applied and Computational Harmonic Analysis , 48(1):415–444, 2020
2020
-
[24]
STFT phase retrieval: Uniqueness guarantees and recovery algorithms
Kishore Jaganathan, Yonina C Eldar, and Babak Hassibi. STFT phase retrieval: Uniqueness guarantees and recovery algorithms. IEEE Journal of selected topics in signal processing , 10(4):770–781, 2016
2016
-
[25]
An accelerated expectation- maximization algorithm for multi-reference alignment
Noam Janco and Tamir Bendory. An accelerated expectation- maximization algorithm for multi-reference alignment. IEEE Transac- tions on Signal Processing , 70:3237–3248, 2022
2022
-
[26]
The reconstruction of structure from electron micrographs of randomly oriented particles
Zvi Kam. The reconstruction of structure from electron micrographs of randomly oriented particles. Journal of Theoretical Biology , 82(1):15– 39, 1980
1980
-
[27]
Relaxed averaged alternating reflections for diffraction imaging
D Russell Luke. Relaxed averaged alternating reflections for diffraction imaging. Inverse problems, 21(1):37, 2004
2004
-
[28]
The sample complexity of multireference alignment
Amelia Perry, Jonathan Weed, Afonso S Bandeira, Philippe Rigollet, and Amit Singer. The sample complexity of multireference alignment. SIAM Journal on Mathematics of Data Science , 1(3):497–517, 2019
2019
-
[29]
Phase retrieval with ap- plication to optical imaging: a contemporary overview
Yoav Shechtman, Yonina C Eldar, Oren Cohen, Henry Nicholas Chap- man, Jianwei Miao, and Mordechai Segev. Phase retrieval with ap- plication to optical imaging: a contemporary overview. IEEE signal processing magazine, 32(3):87–109, 2015
2015
-
[30]
Computational methods for single- particle electron cryomicroscopy
Amit Singer and Fred J Sigworth. Computational methods for single- particle electron cryomicroscopy. Annual review of biomedical data science, 3(1):163–190, 2020
2020
-
[31]
Advances in cryo-ET data processing: meeting the demands of visual proteomics
Abigail JI Watson and Alberto Bartesaghi. Advances in cryo-ET data processing: meeting the demands of visual proteomics. Current Opinion in Structural Biology , 87:102861, 2024
2024
Reviewed August 10, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.