REVIEW 1 major objections 4 minor 1 cited by
Information-Theoretic Lower Bounds for Compressive Sensing with Generative Models
T0 review · 1 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read For compressive sensing with Lipschitz generative models, $m = \Omega(k \log L)$ measurements are information-theoretically unavoidable.
desk verdict Solid first lower bounds for generative-model CS, but Theorem 7's proof has a real amplitude error that needs fixing. 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 a specially designed generative model that reduces the general problem to group-sparse recovery. For the Lipschitz case, the model divides the output into $k$ blocks and maps each scalar input $z_i$ through a double-triangular waveform into one block, so that exactly one entry per block is nonzero; its Lipschitz constant is $L = \frac{2 n x_{\max}}{k r}$. The argument then applies a minimax lower bound for estimating $k$-group-sparse signals from Gaussian measurements, obtained via a distance-based Fano inequality. The proof chooses the amplitude $x_{\max}$ and the output dimension $n$ so that the hard signal set fits inside the model range while the Lipschitz constraint is respected, forcing $m = \Omega\big(k \log \frac{n}{k}\big)$, which translates into the stated $k \log L$ bound. For ReLU networks, the same group-sparse signals are generated using recursive sawtooth and trapezoidal constructions, and standard ReLU implementation results control the width and depth, producing the stated depth/width dependence.
What would settle it
Instantiate the paper's double-triangular generative model with the prescribed parameters and compute the minimax risk over its range for a number of measurements $m$ below the claimed bound; if any estimator achieves average squared error $O(\alpha)$ on that constructed group-sparse signal set, then Lemma 2 would be false and the lower bound would collapse.
Extended reading notes
Core claim
The central discovery is that the sample-complexity scaling established in [14] for compressive sensing with generative models is not an artifact of any particular algorithm: it is an information-theoretic requirement. For bounded $L$-Lipschitz generative models with $k$-dimensional inputs, the paper constructs a generative model whose range consists of $k$-group-sparse signals, built from blockwise double-triangular scalar mappings. A minimax lower bound for group-sparse recovery then implies that any algorithm achieving average squared error $O(\alpha)$ in the idealized no-representation-error, no-optimization-error, i.i.d. Gaussian-noise setting needs $m = \Omega\big(k \log \frac{L r \sqrt{k}}{\sqrt{\alpha}}\big)$ measurements for rectangular domains and $m = \Omega\big(k \log \frac{L r}{\sqrt{\alpha}}\big)$ for spherical domains. For ReLU networks, a recursive construction generates $(k k_0)$-group-sparse signals and yields a necessary measurement count of $\Omega\big(k k_0 \log \frac{n}{k k_0}\big)$, which specialises to $\Omega\big(k d \frac{\log w}{\log n}\big)$ in general and $\Omega(k d \log w)$ in some regimes, matching the $O(k d \log w)$ upper bound up to a log factor or exactly. The lower bounds hold for every fixed measurement matrix with normalized Frobenius norm, so the bottleneck is intrinsic to the model class itself.
Load-bearing premise
The lower bounds hold only in the idealized setting where the signal lies exactly in the model range, optimization error is zero, and the noise is i.i.d. Gaussian, and the Lipschitz results additionally require the Lipschitz constant $L$ to be above a stated threshold relative to $\alpha$, $k$, and $r$; below that threshold the problem can become trivial and the bounds do not apply.
Editorial extensions
If this is right
- The known upper bounds for Lipschitz generative models are tight up to constants in the idealized setting, so any further reduction in measurements must come from extra assumptions beyond Lipschitzness and bounded inputs.
- For ReLU-network generative models, the $O(k d \log w)$ upper bound is optimal or near-optimal; in particular, the depth-2 high-width construction gives a regime with matching scaling up to mild logarithmic terms.
- The lower bounds hold for any algorithm and any fixed measurement matrix with normalized Frobenius norm, meaning the sample-complexity bottleneck is a property of the model class, not of specific recovery procedures.
- Because the proof uses a hard prior distribution over signals, the same lower bound applies even to average-case recovery guarantees over that prior, strengthening the impossibility result.
- If one wants fewer measurements, one must impose structure beyond the Lipschitz or ReLU architectural assumptions, such as distributional or weight-based assumptions, since the model classes considered here alone do not lower the information-theoretic cost.
Reading between the lines
- I read this result as a warning that worst-case Lipschitz generative priors have fundamentally limited sample efficiency; the empirical gains seen with generative priors may therefore rely on real signal distributions being far better than the worst case.
- A natural testable extension would be to check whether natural image priors avoid the adversarial geometry of the constructed group-sparse model by attempting recovery with fewer than $k \log L$ measurements on realistic datasets.
- The paper leaves open whether the extra $\log n$ factor in the general ReLU lower bound can be removed by sharper constructions; closing that gap would settle the exact constant-order sample complexity for deep generative models.
- The same group-sparse reduction technique could plausibly be applied to other generative-model inverse problems, such as phase retrieval or non-Gaussian measurement models, to see whether the same scaling laws survive.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper establishes algorithm-independent lower bounds on the number of measurements for compressive sensing with generative models. It constructs an explicit L-Lipschitz generative model whose range contains k-group-sparse signals, and combines this with a minimax lower bound for group-sparse recovery (Lemma 2) to show that, in a no-representation-error, no-optimization-error Gaussian-noise setting, any estimator with expected squared error O(alpha) needs m = Omega(k log L) measurements for both rectangular and spherical input domains (Theorems 3 and 4). These match the upper bounds of Bora et al. up to constants. For ReLU-network generative models, the paper constructs a more complex (k k0)-group-sparse signal generator and claims lower bounds that depend on depth and width (Theorem 7), showing the O(kd log w) upper bound is optimal or near-optimal in various regimes. The proofs are based on Fano-type inequalities; the main auxiliary result, Lemma 2, is proved in Appendix B, with a self-contained derivation of the mutual information bound and the covariance of the group-sparse prior.
Significance. If correct, the paper fills an important gap in the theory of compressive sensing with deep generative priors by showing that the sample-complexity scaling laws of Bora et al. cannot be improved without further assumptions. The Lipschitz lower bound construction is explicit, elementary, and directly matches the covering-number-based upper bound. The group-sparse minimax bound in Lemma 2 is carefully proved in an appendix, and the reciprocity between the constructed generative model and the group-sparse hard instance is transparent. These contributions are likely to become a standard reference for lower bounds in this area.
major comments (1)
- [Section IV-B (Theorem 7) and Appendix B (Lemma 2)] In the proof of Theorem 7, Lemma 2 is applied with K = k k0 groups, sigma^2 = alpha/m, and ||A||_F^2 = C_A n. Lemma 2 requires the hard-signal amplitude to satisfy xi^2 = n sigma^2 log(n/K)/(4||A||_F^2) = alpha log(n/K)/(4 C_A m). With m = m* chosen as the largest integer below K log(n/K)/(64 C1 C_A), this yields xi^2 = Theta(alpha/(k k0)), not Theta(alpha/k). The theorem instead states xi = sqrt(C2 alpha/k), which is a factor sqrt(k0) larger. For the stated xi, the mutual information bound I(V;y) <= xi^2/(2 sigma^2) * (K/n)||A||_F^2 becomes Theta(k0) times the term (K/3) log(n/K) in the denominator of the Fano lower bound in (32)-(35), so the bound is vacuous for large k0. Thus the proof as written does not establish the first claim of Theorem 7 for its stated parameter xi. The theorem should either be corrected to xi = Theta(sqrt(alpha/(k k0))) with a suitable constant, or the proof must exhibit a hard subset of Range(G) at the smaller amplitude; the recursive pulse construction of Section IV-B does not obviously provide such a subset, since the pulses are defined with fixed amplitude xi on their respective intervals. This is a load-bearing gap in the main ReLU lower bound.
minor comments (4)
- [Abstract] In the abstract, the phrase 'ReLU networks with high depth and/or high depth' should be 'with high depth and/or high width'; this appears to be a typographical error.
- [Proof of Theorem 3] After setting C' = 1/sqrt(128 C1), the text says (21) reduces to m >= 1/(128 C' C_A) k log(n/k); substituting C' gives 1/(128 C1 C_A) k log(n/k), so the printed denominator appears to have a typographical error, replacing C1 with C'.
- [Theorem 6] The statement 'depth O(log n/(kr))' is ambiguous; it should read O(log(n/(kr))) to clearly indicate the logarithm of the ratio.
- [Proof of Theorem 7, part (b)] The claim that the width is a constant multiple of the number of outputs, w = O(n), would benefit from a brief justification that each of the n parallel outputs can be realized with O(1) width and the specified depth using the sawtooth construction.
Circularity Check
No circularity: the lower bounds are derived from explicit group-sparse hard-instance constructions and Fano/minimax arguments, not from the target upper bounds.
full rationale
The central derivation chain is self-contained. Lemma 2 proves a minimax lower bound for k-group-sparse recovery by placing a uniform prior on group-sparse ±1 patterns and applying Fano's inequality; the only self-citation is [34], used in Appendix B for the Gaussian mutual-information bound I(V;y) ≤ ξ²/(2σ²) E[||AV||²]. That bound is an auxiliary, parameter-free inequality for Gaussian channels with stated assumptions; it does not assert or assume any of the paper's target lower bounds, and the surrounding proof independently establishes Cov[V]=(k/n)I_n. Theorems 3 and 4 convert the resulting k log(n/k) necessity into k log(Lr√k/√α) and k log(Lr/√α) by explicitly constructing a generative model and choosing the free output dimension n and amplitude xmax via Lemma 1; this is a valid existential construction rather than an importation of the upper-bound scaling. Theorem 7 likewise applies Lemma 2 with K=kk0 groups and builds a ReLU-implementable recursive group-sparse generator, with depth/width trade-offs obtained from standard piecewise-linear ReLU representation results. The comparison with Corollaries 1 and 2 is for interpretation only and is not used as input to the lower-bound proofs. The only apparent circularity candidate, the self-citation [34], is non-load-bearing and independently verifiable, so it does not raise the score. Separately, the skeptical arithmetic check of Theorem 7's stated ξ=√(C2 α/k) against Lemma 2 is a proof-consistency/correctness concern, not a circularity, because the Fano argument does not assume the upper-bound conclusion; no prediction in the paper reduces by construction to its inputs.
Assumptions & free parameters
free parameters (4)
- Output dimension n (hard-instance construction) =
C' L r k^{3/2} / sqrt(alpha)
- Signal magnitude xi (hard prior) =
sqrt(alpha log(n/k)/(4 C_A m)) in Lemma 2; sqrt(C2 alpha/k) in Theorem 7
- x_max (output magnitude bound) =
sqrt(alpha)/(2 C' sqrt(k))
- ReLU block parameter k0 (and n0) =
arbitrary subject to n0 >= C0 k0 and the chosen d-w tradeoff
assumptions (5)
- standard math Fano's inequality and the distance-based minimax lower bound (Lemma 4 from [24])
- domain assumption Gaussian noise model eta ~ N(0, (alpha/m) I) and chi-square concentration
- domain assumption Signals lie exactly in Range(G), with no representation error and no optimization error
- domain assumption Lipschitz lower-bound condition L >= Omega((1/r) sqrt(alpha/k)) for rectangular and L >= Omega(sqrt(alpha)/r) for spherical domains
- standard math ReLU representation facts from [30]-[32]: piecewise-linear functions with N pieces are implementable by depth-2 width-O(N) ReLU networks, and sawtooth functions need depth O(log R)
Cite this review
Pith. "Pith review of Information-Theoretic Lower Bounds for Compressive Sensing with Generative Models." pith.science (2026). https://pith.science/paper/BRSCL7IL
@misc{pith2026190810744,
author = {Pith},
title = {Pith review of: Information-Theoretic Lower Bounds for Compressive Sensing with Generative Models},
year = {2026},
howpublished = {\url{https://pith.science/paper/BRSCL7IL}},
note = {Machine review of arXiv:1908.10744}
}
abstract
It has recently been shown that for compressive sensing, significantly fewer measurements may be required if the sparsity assumption is replaced by the assumption the unknown vector lies near the range of a suitably-chosen generative model. In particular, in (Bora {\em et al.}, 2017) it was shown roughly $O(k\log L)$ random Gaussian measurements suffice for accurate recovery when the generative model is an $L$-Lipschitz function with bounded $k$-dimensional inputs, and $O(kd \log w)$ measurements suffice when the generative model is a $k$-input ReLU network with depth $d$ and width $w$. In this paper, we establish corresponding algorithm-independent lower bounds on the sample complexity using tools from minimax statistical analysis. In accordance with the above upper bounds, our results are summarized as follows: (i) We construct an $L$-Lipschitz generative model capable of generating group-sparse signals, and show that the resulting necessary number of measurements is $\Omega(k \log L)$; (ii) Using similar ideas, we construct ReLU networks with high depth and/or high depth for which the necessary number of measurements scales as $\Omega\big( kd \frac{\log w}{\log n}\big)$ (with output dimension $n$), and in some cases $\Omega(kd \log w)$. As a result, we establish that the scaling laws derived in (Bora {\em et al.}, 2017) are optimal or near-optimal in the absence of further assumptions.
Figures
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