REVIEW 2 major objections 4 minor 1 cited by
Phase retrieval of complex and vector-valued functions
T0 review · 2 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read Phase retrieval reduces to a perpendicular-split test for complex and vector-valued functions.
desk verdict The vector-valued characterization is sound in finite dimensions, but Assumption 3.1 fails for natural infinite-dimensional measurements, so the infinite-dimensional claims overreach; the quaternion and graph results are genuinely worth keeping. 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 mechanism is the perpendicular-split identity. If a rival function $g$ has the same magnitude data as $f$ and is not a unitary image of $f$, then $u=(f+g)/2$ and $v=(f-g)/2$ produce exactly such a split: the measurement orthogonality comes from the equality of magnitudes, and the nonzero symmetric cross-term comes from the difference between the inner products of measurements of $g$ and of $f$. Conversely, any perpendicular split yields a rival $g=u-v$ with identical magnitudes and different pairwise measurement inner products. Assumption 3.1 is the bridge that lets magnitude data control those inner products: it states that equal pairwise inner products for all measurement pairs imply the two functions differ by a unitary, a condition that is automatic in finite dimensions but is an extra hypothesis in infinite dimensions.
What would settle it
Construct an infinite-dimensional real Hilbert space $H$, a unitary-invariant space $S$ of $H$-valued functions on a two-point domain, and a unitary-invariant measurement family $\Phi$ whose measurement map is injective but whose image does not determine all pairwise inner products—for instance, choose the two point evaluations so that the span of one function's measurements has an orthogonal complement of different dimension from the span of a competitor's measurements. If for such $f$ the pairwise-inner-product condition (3.8) holds for some $g$ that is not a unitary image of $f$, and no perpendicular split $u,v$ exists, then the 'if' direction of Theorem 3.2 fails and Assumption 3.1 is pinpointed as the obstruction.
Extended reading notes
Core claim
On the paper's own terms, the central claim is Theorem 3.2: in a unitary-invariant space $S$ of $H$-valued functions with unitary-invariant measurements $\Phi$ satisfying Assumption 3.1, a function $f$ is phase retrieval—determined up to a unitary operator on $H$ by the magnitudes $\|\varphi(f)\|$—if and only if there are no $u,v\in S$ with $f=u+v$, $\langle\varphi(u),\varphi(v)\rangle=0$ for every $\varphi\in\Phi$, and $\langle\varphi_0(u),\varphi_1(v)\rangle+\langle\varphi_1(u),\varphi_0(v)\rangle\neq 0$ for some $\varphi_0,\varphi_1\in\Phi$. The same perpendicular-split condition, with the trivial ambiguity adjusted, characterizes complex conjugate phase retrieval of complex functions and quaternion conjugate phase retrieval of quaternion-valued functions. Direct corollaries are that a real scalar space is phase retrieval exactly when the measurement set has the complement property, that phase retrieval passes to projections onto subspaces, and that a vector field on a complete graph is determined up to an orthogonal matrix by its absolute and neighboring relative magnitudes, with a connected-simplex-graph condition covering non-complete graphs. Affine phase retrieval of vector-valued functions is characterized by injectivity of a family of linear maps built from the reference vectors.
Load-bearing premise
For the vector-valued theorems, the argument assumes that any two functions whose measurements have the same pairwise dot products must be related by a single global rotation; this is automatic in finite dimensions but is a strictly stronger, unproved condition in infinite dimensions.
Editorial extensions
If this is right
- In finite-dimensional Hilbert spaces, the vector-valued characterization is an 'if and only if' once the measurement map is injective, and in the real scalar case it reproduces the complement property as the exact condition for phase retrieval.
- Phase retrieval is hereditary: if $f$ is phase retrieval in $S$, then its projection onto any subspace $\tilde H$ is phase retrieval in the projected space, so uniqueness on a subspace is forced by uniqueness on the whole space.
- A velocity field on a complete graph is determined up to an orthogonal matrix by absolute speeds at vertices and relative speeds between every pair of vertices; for a general graph the same holds when the associated $d$-simplex graph is connected and covers all vertices.
- With at least two reference vectors per measurement, affine phase retrieval of a vector-valued space is equivalent to injectivity of the measurement map whenever the reference differences span the Hilbert space.
- For the shift-invariant space generated by the hat function, the paper identifies precisely which coefficient sequences are complex conjugate phase retrieval: all coefficients in the support interval must be nonzero and at most one adjacent pair may have a nonzero imaginary cross term.
Reading between the lines
- If Assumption 3.1 could be established for natural infinite-dimensional measurement systems—sampling at points or frames—the perpendicular-split test would become a practical certificate for uniqueness in those systems; the paper proves the assumption in finite dimensions and leaves the infinite-dimensional case open.
- The perpendicular-split condition is algebraic: in the finite-dimensional range-space case it is equivalent to excluding a real matrix of rank at most two with zero diagonal traces, which suggests a low-rank matrix test for conjugate phase retrieval that could be checked by semidefinite relaxation.
- The graph reconstruction theorem implies a distributed reconstruction algorithm: each $(d+1)$-clique fixes a local orthogonal frame, and connectivity of the $d$-simplex graph is exactly the condition for gluing these local frames into one global orthogonal transformation; where the graph disconnects, one should expect gauge ambiguities between components.
- In affine phase retrieval, the paper's necessary-and-sufficient condition suggests that when the reference differences fail to span the space, the number of reference vectors required will grow with the codimension of their span—a quantitative question the paper does not address.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper develops phase-retrieval characterizations for complex-valued, quaternion-valued, and vector-valued functions from the magnitudes of linear measurements. Section 2 characterizes complex conjugate phase retrieval in a conjugate-invariant linear space by the non-existence of a split f=u+v satisfying the orthogonality condition (2.8) and the non-vanishing cross-correlation condition (2.9), with applications to matrix range spaces, shift-invariant spaces, and quaternion-valued functions. Section 3 introduces unitary-invariant spaces of vector-valued functions and, under Assumption 3.1, characterizes phase retrieval of a single function and of the whole space by the absence of a perpendicular split (Theorems 3.2 and 3.3), together with a projection theorem and a graph-vector-field reconstruction theorem. Section 4 treats affine phase retrieval through injectivity of the maps T_u. The paper states that all proofs are collected in Section 5.
Significance. If the results hold as stated, the paper provides a useful unifying framework: the scalar characterizations recover the classical complement-property equivalence, the quaternion characterization is new, and the graph-vector-field theorem gives a concrete connectivity condition for reconstruction up to an orthogonal matrix. The proofs that are actually supplied are generally clean, and the isomorphism argument in Theorem 2.10 is elegant. However, the infinite-dimensional vector-valued results rest on Assumption 3.1, for which the paper proves no infinite-dimensional instance and for which a natural point-evaluation family fails; moreover, Theorem 4.1 is missing from the proof section despite the paper's declared policy. The finite-dimensional and scalar parts appear sound, but the advertised infinite-dimensional vector-valued claims are not established.
major comments (2)
- [Section 3.1 (Assumption 3.1; Theorems 3.2–3.3)] The central vector-valued characterization is conditional on Assumption 3.1, but the paper only proves this assumption in the finite-dimensional case (Remark 3.4) and supplies no infinite-dimensional example satisfying it. In infinite dimensions the assumption is strictly stronger than injectivity of T_Φ, and the natural point-evaluation family on the unitary-invariant space S=ℓ²(N;ℓ²(N)) violates it: let f(n)=2^{-n}e_{n+1} and g(n)=2^{-n}e_{n+2}. Then ⟨g(n),g(m)⟩=4^{-n}δ_{nm}=⟨f(n),f(m)⟩ for all n,m, so (3.8) holds for Φ={δ_n}, but any U with g=Uf would satisfy Ue_{n+1}=e_{n+2} for all n≥1; such a U is not onto H, contradicting unitarity. Thus the abstract's promise of an infinite-dimensional vector-valued characterization is not established; the theorems hold only under a hypothesis whose verified instances are finite-dimensional. The authors should either prove Assumption 3.1 for a nontrivial infinite-dimensional measurement family or explicitly restrict the vector-valued theorems to the finite-dimensional case.
- [Section 4 (Theorem 4.1)] Theorem 4.1 is stated as a characterization of affine phase retrieval of a linear space, yet Section 5—which the paper says collects all proofs—does not prove it. The paragraph preceding the theorem only derives the identity ⟨φ(f−g),φ(f+g)+2b_{φ,i}⟩=0 from equal affine magnitudes; it does not establish the asserted equivalence, in particular the necessity direction that injectivity of every T_u follows from affine phase retrieval. Please provide a complete proof of both directions, or explicitly include the missing argument in Section 5.
minor comments (4)
- [Section 5.6 (proof of Proposition 2.8)] In the displayed expansion after (5.37), the final term '2µ_iµ_j a_1 a_1^T' should presumably be '2µ_iµ_j a_2 a_2^T'.
- [Section 2.3] The notation Q_8 for the quaternions is nonstandard and is easily confused with the quaternion group of order 8; consider using H or a different symbol.
- [Figure 1 caption] The caption is hard to parse, especially the clause beginning 'where we assume that the hyperplane containing two vectors ...'; please rephrase it for clarity.
- [Section 1 and Section 5] The statement 'All proofs are collected in Section 5' is inaccurate also for Theorem 3.3, whose proof is not listed there, although it is a direct consequence of Theorem 3.2.
Circularity Check
No circular derivation: the central vector-valued characterization is a direct equivalence under an explicit assumption, and the only self-citation is a non-load-bearing prior lemma.
full rationale
I find no circular step. The main claims are derived from definitions and explicit hypotheses rather than from fitted quantities or self-referential renaming. Theorem 3.2 is proved in Section 5.1 by a genuine equivalence: given u, v satisfying (3.11)-(3.13), the function g = u - v has the same magnitude measurements but a different pairwise-geometry, contradicting phase retrieval; conversely, given g in M_{f,Phi} that is not a unitary image, Assumption 3.1 supplies a pairwise inner-product mismatch, and setting u = (f+g)/2, v = (f-g)/2 produces exactly the forbidden split. Assumption 3.1 is explicitly stated, not derived from the conclusion, and the finite-dimensional sufficiency in Remark 3.4 is a proof, not a definitional shortcut. The scalar corollaries are consequences, with the known complement-property equivalence credited to [4,8,9,12]. The only self-citation used as a proof input is [17, Theorem 3.2] (Lemma 5.2) in the hat-function application Proposition 2.9; this is a parameter-free real-scalar lemma with stated assumptions and is not the target vector-valued result, so it is external support rather than load-bearing circularity. The fragility of Assumption 3.1 in infinite dimensions, where injectivity of T_Phi is necessary but not sufficient and a point-evaluation counterexample can violate it, is a limitation of applicability rather than an equivalence-by-construction: the characterization is conditional but not circular.
Assumptions & free parameters
assumptions (7)
- domain assumption Unitary invariant space S and unitary invariant measurement family Phi (Section 3, equations (3.1)-(3.3)).
- domain assumption Assumption 3.1: equal pairwise measurement Gram matrices imply unitary equivalence.
- domain assumption The linear space C is complex conjugate invariant (Section 2.1).
- domain assumption The linear space W is quaternion conjugate invariant (Section 2.3, equation (2.17)).
- domain assumption Lemma 5.2 from [17] characterizing phase retrieval in the real hat-function shift-invariant space.
- standard math Complement property equivalence for scalar frames (Corollary 3.6).
- standard math Gram-Schmidt orthonormalization and unitary operators on Hilbert space.
invented entities (1)
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d-simplex graph G_f associated to a vector field f
Cite this review
Pith. "Pith review of Phase retrieval of complex and vector-valued functions." pith.science (2026). https://pith.science/paper/GIJAPTQT
@misc{pith2026190902078,
author = {Pith},
title = {Pith review of: Phase retrieval of complex and vector-valued functions},
year = {2026},
howpublished = {\url{https://pith.science/paper/GIJAPTQT}},
note = {Machine review of arXiv:1909.02078}
}
abstract
The phase retrieval problem in the classical setting is to reconstruct real/complex functions from the magnitudes of their Fourier/frame measurements. In this paper, we consider a new phase retrieval paradigm in the complex/quaternion/vector-valued setting, and we provide several characterizations to determine complex/quaternion/vector-valued functions $f$ in a linear space $S$ of (in)finite dimensions, up to a trivial ambiguity, from the magnitudes $\|\phi(f)\|$ of their linear measurements $\phi(f), \phi\in \Phi$. Our characterization in the scalar setting implies the well-known equivalence between the complement property for linear measurements $\Phi$ and the phase retrieval of linear space $S$. In this paper, we also discuss the affine phase retrieval of vector-valued functions in a linear space and the reconstruction of vector fields on a graph, up to an orthogonal matrix, from their absolute magnitudes at vertices and relative magnitudes between neighboring vertices.
Figures
Forward citations
Cited by 1 Pith paper
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Random phaseless sampling for causal signals in shift-invariant spaces: a zero distribution perspective
If a signal generator satisfies a new zero-set condition, causal signals in complex-generated shift-invariant spaces can be recovered from three random magnitude samples per unit interval, with probability one.
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