REVIEW 2 major objections 4 minor 1 cited by
Quasi-optimal hierarchically semi-separable matrix approximation
T0 review · 2 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read This paper proves that a randomized matrix-vector algorithm can approximate any N×N matrix by a hierarchically semi-separable rank-k matrix with expected squared error within O(log(N/k)) of optimal, using O(k log(N/k)) matrix-vector…
desk verdict First provable quasi-optimal HSS approximation from matvecs, with a clean meta-theorem and a glaring-but-fixable typo: the factor p in Theorem 4.2 is spurious and should be removed. 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 argument is carried by a greedy level-by-level construction of the HSS telescoping factorization: at each level, the algorithm fits a sequentially semi-separable (SSS) matrix to the current compressed matrix, then compresses the residual. A lemma shows the best HSS error cannot increase under the projection used to pass to the next level, so the $L$ independent SSS fits accumulate only an $O(L)$ factor. In the matvec setting, block nullification simulates Gaussian sketches of block rows and columns from full-matrix matvecs with $A$ and $A^T$, and fresh random sketches per level keep the errors independent. A projection-cost-preserving sketch bound (Theorem 4.2) converts those sketches into near-optimal subspaces, and a robust diagonal-block estimator (Theorem 4.4) recovers the diagonal blocks without needing exact HSS structure.
What would settle it
Compute, for a tall matrix $B$ with a fixed number of rows and many columns $p$, the ratio $\mathbb{E}[\|B - UU^T B\|_F^2]/\|B - [B]_k\|_F^2$ where $U$ comes from a Gaussian sketch with $q$ columns, and compare it with the bound $1 + 2e q p/((q-k)^2 - 1)$. If the ratio grows with $p$, the proof of Theorem 4.2 is invalid as stated and the main theorem's level-by-level argument collapses; if it does not, the missing uniform bound is available.
Extended reading notes
Core claim
The central discovery is that quasi-optimal HSS approximation does not require exact recovery of each block's singular subspaces, nor reuse of sketches across levels. Theorem 4.1 states that for $s \geq 3k+2$, Algorithm 4.1 uses $O(sL)$ matvecs and $O(N k s L + N s^2)$ runtime to return $B \in \mathrm{HSS}(L,k)$ with $\mathbb{E}[\|A-B\|_F^2] \leq O(L) \min_{C \in \mathrm{HSS}(L,k)} \|A-C\|_F^2$; choosing $s=5k$ gives $O(k \log(N/k))$ matvecs and an $O(\log(N/k))$ quasi-optimality factor. The proof instantiates a general greedy meta-algorithm (Algorithm 3.1) by arguing that solving an SSS approximation at each level, using independent projection-cost-preserving sketches and block nullification, accumulates only one $O(1)$ error factor per level. The paper also proves the first relative-error guarantee for explicit-access HSS approximation (a $2 \log_2(N/k)$ factor) and explicit error bounds for projection-cost-preserving sketches that are of independent interest. It further shows the greedy method cannot be tuned to $(1+\varepsilon)$-optimality: a constructed matrix forces a factor arbitrarily close to 2.
Load-bearing premise
The main theorem depends on the projection-cost-preserving sketch bound in Theorem 4.2 being essentially independent of the width $p$ of a block row; if the factor $p$ in the stated bound is genuine, each level's basis degrades with the matrix dimension and the $O(L)$ quasi-optimality conclusion does not follow.
Editorial extensions
If this is right
- HSS matrices can now be compressed from black-box matvec queries with a provable guarantee: $O(k \log(N/k))$ queries suffice for an $O(\log(N/k))$ approximation factor.
- The same level-by-level greedy scheme with truncated SVDs on block rows and columns gives the first polynomial-time relative-error bound for explicit-access HSS approximation, at $O(N^2 k)$ cost.
- Markov's inequality converts the expected-error bound into a high-probability guarantee: with probability $1-\delta$, the returned HSS matrix satisfies the desired relative-error bound with $\Gamma = O(\log(N/k)/\delta)$.
- Because the diagonal-block estimator is tolerant of matrices that are only approximately HSS, the method applies to integral operators, inverse covariance matrices, and other nearly-HSS matrices, not just exactly HSS ones.
- The fresh-sketch variant is less query-efficient than sketch reuse in practice, so the paper leaves open whether the logarithmic query factor can be removed while keeping a strong guarantee.
Reading between the lines
- Inference: If the $p$-dependence in Theorem 4.2 is an artifact of the proof rather than a genuine phenomenon, the same machinery likely yields a $p$-independent constant and the $O(\log(N/k))$ factor in the main theorem might be improvable to a constant; checking that bound numerically on matrices with many columns would settle this.
- Inference: The greedy level-by-level decomposition transfers naturally to other hierarchical formats with shared bases, such as H2 matrices; the paper's observation that shared-bases classes lack polynomial optimal approximation algorithms suggests fresh-sketch analyses could produce the first quasi-optimal guarantees there too.
- Inference: The lower-bound example in Theorem 3.7 indicates the greedy strategy itself is limited to a factor of 2, so any hope of a $(1+\varepsilon)$-optimal HSS approximation would require a different algorithmic principle, not a sharper analysis of the greedy recursion.
- Inference: Experiments showing sketch reuse beats fresh sketches at fixed query budget suggest the true query complexity of quasi-optimal HSS approximation may be $O(k)$ rather than $O(k \log(N/k))$; a refined analysis of reuse dependencies would be the natural test.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents a randomized algorithm for computing a quasi-optimal hierarchically semi-separable (HSS) approximation to an N x N matrix A using only matrix-vector products with A and A^T. The main result, Theorem 4.1, claims that with O(k log(N/k)) matvecs and O(N k^2 log(N/k)) additional runtime, the algorithm returns an HSS(L,k) approximation B with E[||A-B||_F^2] at most O(log(N/k)) times the optimal HSS approximation error. The analysis proceeds through a greedy meta-algorithm (Algorithm 3.1), a theorem bounding its performance via level-wise SSS approximation (Theorem 3.1), a near-optimal SSS approximation result (Theorem 3.3), a monotonicity lemma for HSS optima (Lemma 3.4), an explicit projection-cost-preserving sketch bound (Theorem 4.2), a diagonal-block estimator (Theorem 4.4), and a construction showing that the explicit greedy algorithm cannot beat a factor arbitrarily close to 2 (Theorem 3.7). Numerical experiments compare the fresh-sketch variant with the sketch-reuse algorithm of Levitt and Martinsson.
Significance. If the corrections below are made, this is a substantial contribution: it appears to give the first polynomial-time quasi-optimality guarantee for HSS approximation in the explicit-access model and in the matrix-vector query model. The proof architecture is clean: the greedy level-by-level view, Lemma 3.4 connecting each level's optimum to the global HSS optimum, and the induction in Theorem 3.1 together provide a genuine O(L) bound. The explicit PCPS constants in Theorem 4.2 are of independent interest, the lower bound for greedy in Theorem 3.7 is a useful counterpoint to the upper bounds, and the experiments are reproducible with released code. The main issues are two formal errors in central theorem statements, both of which appear to be typos that the proofs themselves indicate how to fix.
major comments (2)
- [Theorem 3.1, Eq. (3.1)] The displayed bound is written as (Gamma_r + Gamma_c)(1 + Gamma_d)^L, but the proof does not establish an exponential factor. Combining Eq. (3.10) and Eq. (3.11) gives (Gamma_r + Gamma_c)(1 + Gamma_d) L, since the first term contributes the factor (1 + Gamma_d)(Gamma_r + Gamma_c) and the inductive second term contributes (1 + Gamma_d)(Gamma_r + Gamma_c)(L - 1). With the exponential form as written, the choice s = 5k in Theorem 4.1 gives Gamma_d roughly 2/3 and the claimed Gamma = O(L) would be false for fixed constants. Eq. (3.1) should be corrected to the linear-in-L bound, and all subsequent citations of this factor should be updated accordingly.
- [Theorem 4.2 and Theorem 4.1, Eqs. (4.6)-(4.7); Theorem C.4] The factor p in the numerator of the PCPS bound is not supported by the proof. The two probabilistic estimates E||Sigma_2 Omega_2||_F^2 = q||Sigma_2||_F^2 and E||Omega_1^dagger||_2^2 <= e^2 q/((q-k)^2 - 1) contain no p, so the inequality should be dimension-free in the column count p. As stated, the p is load-bearing: in Theorem 4.1 the matrix B is a block row with p = 2^{ell+1} k - 2k, so a genuine p factor would make Gamma_r and Gamma_c grow with N and would invalidate the claimed O(L) quasi-optimality factor. The p should be removed from Theorem 4.2, from the Gamma_r/Gamma_c formulas in Theorem 4.1, from Eqs. (4.6)-(4.7), and from the analogous formula in Theorem C.4.
minor comments (4)
- [Theorem 3.1, assumption statement] The conditioning in the three expectation assumptions is written as conditional on A_i^{(ell+1)}, which is a block entry rather than a sigma-algebra; it should be conditional on A^{(ell+1)} (or on the algorithmic history up to that level). The proof's use of the law of total expectation appears consistent with the intended conditioning, but the notation should be clarified.
- [Proof of Theorem 3.7] The displayed ratio after the construction is garbled; the reasoning uses the ratio (2^{2L+1} - 2^{L+2})/(2^{2L} + delta 2^{L+1}), which tends to 2 as L grows. Please correct the displayed simplification so the limiting factor 2 - epsilon is transparent.
- [Appendix D] The closed-set characterization is written as HSS(L,k) = {B : f(A) = 0}; it should be f(B) = 0.
- [Algorithm C.1, lines 5 and 7] The indices in the basis computations appear to be swapped: the row basis U_i should be computed from Y_{R_i} P_{R_i} and the column basis V_j from Z_{C_j} Q_{C_j}, rather than from the blocks named YRj PCj and ZCj QRi in the current text.
Circularity Check
No circularity: the quasi-optimality guarantee follows from in-paper explicit bounds, and the self-citations to prior PCPS work are not load-bearing.
full rationale
The derivation chain is self-contained. Theorem 4.1 instantiates Theorem 3.1 with the basis-quality estimate of Theorem 4.2 and the diagonal estimate of Theorem 4.4, both proved in the paper from standard Gaussian matrix facts (Eckart-Young truncation, pseudo-inverse moment bounds); neither assumes the HSS quasi-optimality it helps establish. Theorem 3.1 is proved by induction using Theorem 3.3 and Lemma 3.4, with the existence of an optimal HSS approximation supplied by Appendix D, so no step defines the target error as an input or fits a parameter and then reports it as a prediction. The cited PCPS framework [26,58] is prior work by two of the authors, but the explicit bound actually used (Theorem 4.2) is derived in this paper, so the self-citation is motivational rather than load-bearing. The lower bound in Theorem 3.7 is an explicit construction. The only substantive issue found is a mathematical typo, not a circularity: Theorem 4.2's displayed constant contains a factor p that its own proof does not produce, since the proof uses a k by q matrix Omega_1 and the identity E ||Sigma_2 Omega_2||_F^2 = q ||Sigma_2||_F^2, which gives a q/sqrt((q-k)^2-1) dependence rather than q p. If the p were genuine, Theorem 4.1's constants would depend on block width, but that is a correctness or typo concern, not a reduction of the result to its inputs.
Assumptions & free parameters
assumptions (3)
- standard math Standard properties of Gaussian random matrices and Eckart-Young theorem
- domain assumption HSS matrices admit a telescoping factorization (Definition 2.3) and an optimal HSS approximation exists (Appendix D)
- ad hoc to paper The matrix is square of size N = 2^{L+1}k with a perfect binary partition and leaf blocks of size 2k
Cite this review
Pith. "Pith review of Quasi-optimal hierarchically semi-separable matrix approximation." pith.science (2026). https://pith.science/paper/RW4HAINF
@misc{pith2026250516937,
author = {Pith},
title = {Pith review of: Quasi-optimal hierarchically semi-separable matrix approximation},
year = {2026},
howpublished = {\url{https://pith.science/paper/RW4HAINF}},
note = {Machine review of arXiv:2505.16937}
}
abstract
We present a randomized algorithm for producing a quasi-optimal hierarchically semi-separable (HSS) approximation to an $N\times N$ matrix $A$ using only matrix-vector products with $A$ and $A^T$. We prove that, using $O(k \log(N/k))$ matrix-vector products and ${O}(N k^2 \log(N/k))$ additional runtime, the algorithm returns an HSS matrix $B$ with rank-$k$ blocks whose expected Frobenius norm error $\mathbb{E}[\|A - B\|_F^2]$ is at most $O(\log(N/k))$ times worse than the best possible approximation error by an HSS rank-$k$ matrix. In fact, the algorithm we analyze in a simple modification of an empirically effective method proposed by [Levitt & Martinsson, SISC 2024]. As a stepping stone towards our main result, we prove two results that are of independent interest: a similar guarantee for a variant of the algorithm which accesses $A$'s entries directly, and explicit error bounds for near-optimal subspace approximation using projection-cost-preserving sketches. To the best of our knowledge, our analysis constitutes the first polynomial-time quasi-optimality result for HSS matrix approximation, both in the explicit access model and the matrix-vector product query model.
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