REVIEW 2 major objections 5 minor 37 references
Low-rank approximation in the Frobenius norm by column and row subset selection
T0 review · 2 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read A deterministic algorithm finds cross approximations within a factor k+1 of the best rank-k error.
desk verdict A genuinely new deterministic polynomial-time cross approximation algorithm with a (k+1)^2 guarantee—worth serious refereeing once the bulge-chasing step is actually described and proved. 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 central object is the conditional expectation of the cross-approximation error after t index pairs have been fixed. Lemma 4 shows this equals (k - t + 1)^2 times the ratio of characteristic polynomial coefficients c_{m-k+t-1}(BB^T) / c_{m-k+t}(BB^T) of the Schur-complement remainder B. The algorithm minimizes these ratios, and the paper evaluates them by forming a thin SVD B = U Sigma V^T, reducing each candidate update to a rank-one modification Sigma - x y^T of a diagonal matrix, and computing the singular values of that modification via O($m^{2}$) bulge-chasing and bidiagonalization steps. This replaces the cancellation-prone characteristic-polynomial updates of Deshpande and Rademacher with direct singular value computations, maintaining the same favorable complexity while improving numerical robustness.
What would settle it
For a random diagonal matrix Sigma with known singular values and random vectors x and y, compute the singular values of Sigma - x y^T using the bulge-chasing procedure as described in Section 3.2.2 and compare them against a direct high-precision SVD; a mismatch beyond roundoff would invalidate the O($m^{2}$) update that the polynomial-time claim depends on.
Extended reading notes
Core claim
The paper's central claim is Theorem 5: Algorithm 3 returns row and column index sets I and J such that the cross approximation A(:,J)A(I,J)^{-1}A(I,:) satisfies the Frobenius-norm error bound ||A - A(:,J)A(I,J)^{-1}A(I,:)||$_F^{2}$ <= (k+1)^2 (sigma_{k+1}^2 + ... + $sigma_m^{2}$). This matches the best rank-k approximation error up to a factor k+1 in norm, and the paper asserts it is the first deterministic polynomial-time algorithm attaining a polynomial-in-k bound for cross approximation. The argument sequentially selects index pairs by minimizing conditional expectations of the approximation error under a volume-sampling distribution, and it computes the needed ratios from singular values of rank-one modifications of the current remainder matrix. Along with this, the paper contributes a numerically robust and faster variant of the Deshpande-Rademacher column subset selection algorithm, an early-stopping heuristic, and an extension to low multilinear rank tensor approximation.
Load-bearing premise
The algorithm's claimed O(k $m^{3}$ n) running time and its deterministic guarantees rest on an unproven assertion that the bulge-chasing procedure computes the singular values of each rank-one modified diagonal matrix in O($m^{2}$) operations, since the paper says 'We refrain from giving a detailed description' and provides only a figure.
Editorial extensions
If this is right
- If Theorem 5 is correct, cross approximation joins column subset selection as a low-rank tool with a certified polynomial error bound, closing a gap the paper identifies in the literature.
- The algorithm's sequential index-pair selection can be stopped early once a candidate pair satisfies the sufficient condition (21), yielding large practical speedups in the experiments.
- The same derandomization strategy applied to the Zamarashkin-Osinsky existence result suggests that other volume-sampling existence bounds could be converted into deterministic algorithms.
- The tensor extension (Algorithm 4) gives a deterministic construction of a Tucker approximation from fibers of the tensor, with error within a factor sqrt(k1+k2+k3+3) of the best multilinear-rank approximation.
- The early-stopping variant appears numerically more stable than the full search, as the experiments show that the full search can produce nearly singular intersection matrices A(I,J) for larger k.
Reading between the lines
- The bulge-chasing step that the paper states without a full description is the most fragile link in the polynomial-time claim; if that step were replaced by a fully specified and verified O(m^2) SVD update, the deterministic complexity result would rest on firmer ground.
- The conditional-expectation ratios depend on the volume-sampling distribution, so analogous derandomized algorithms could likely be designed for other distributions, such as those favoring max-volume submatrices, potentially yielding deterministic cross approximations with maximum-norm bounds.
- The early-stopping heuristic introduces a data-dependent number of candidate pairs, so a rigorous average-case or expected complexity analysis might reveal that the practical running time is far below the worst-case O(k m^3 n).
- The observed near-optimal performance on slowly decaying singular values suggests that the guaranteed (k+1) factor is loose for typical matrices, and a sharper analysis of the conditional-expectation chain might yield tighter constants.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper develops deterministic algorithms for column subset selection and cross approximation in the Frobenius norm. For column subset selection it replaces the characteristic-polynomial updates of Deshpande and Rademacher with singular-value updates, claims the same O(k m^2 n) complexity with better numerical robustness, and adds an early-stopping heuristic. For cross approximation, the main contribution is Algorithm 3, which derandomizes the existence result of Zamarashkin and Osinsky and is claimed to return row/column index sets I, J such that the cross approximation error is bounded by (k+1)^2 times the tail sum of squared singular values, in deterministic time O(k m^3 n). The paper also gives a CUR approximation bound from two column subset selections, a tensor extension with a quasi-optimal multilinear-rank bound, and numerical experiments on matrices and tensors.
Significance. If the algorithmic claims are fully established, the cross approximation result is significant: it would be the first deterministic polynomial-time algorithm with a polynomial-in-k error bound for cross approximation in the Frobenius norm, a gap the paper correctly identifies in the literature. The singular-value-based reformulation of the column subset selection algorithm is a genuinely useful practical improvement over characteristic-polynomial updates, and the experiments support the robustness and speed claims. The paper is also careful in its claims: it explicitly flags the absence of a numerical stability analysis, the small/medium-scale scope, and the heuristic nature of early stopping. The tensor extension is straightforward but gives a clean, explicit bound. However, the main cross approximation algorithm is not fully specified: its correctness and complexity rest on an unproven bulge-chasing step described only by a figure.
major comments (2)
- [§3.2.2, step 2 and Figure 5] The bulge-chasing procedure that transforms D1 into the banded matrix D2 is load-bearing for both correctness and complexity, but it is not actually specified. The text says 'We refrain from giving a detailed description of the algorithm and refer to Figure 5'; no pseudocode, invariant, or count of Givens rotations is supplied. The ratios in equation (18) are computed from the singular values of this D2, and the claimed O(m^2) cost per pair (and therefore the total O(k m^3 n) complexity) depends on this step. As written, a reader cannot verify either the correctness of the computed singular values or the stated operation count. This is a major gap: supply a complete description of the bulge-chasing reduction, or a precise reference with a theorem proving that D1 can be reduced to upper banded form in O(m^2) Givens rotations, and state the fill-in pattern explicitly.
- [Algorithm 3, lines 6–14 and equation (19)] The pseudocode loops over all pairs (i,j) and defines y = (1/B(i,j)) Sigma U(i,:)^T, but it does not exclude pairs with B(i,j)=0. When B(i,j)=0, the rank-one update (19) is undefined, while Lemma 4 conditions on a pair with positive probability, which is precisely the condition that the pivot submatrix has nonzero volume and hence B(i,j) is nonzero. A correct implementation must skip zero pivots and restrict the search to pairs with B(i,j) != 0; this restriction is not stated in the algorithm or in Theorem 5. This is a well-definedness gap in the algorithm as written, though it is locally fixable.
minor comments (5)
- [§2.2, p. 6] The sentence 'The cost of computing this bidiagonal matrix is O(m^2), which is identical with the cost of computing its singular values [20, Section 8.6]' is slightly ambiguous; it would be clearer to say that both the reduction and the subsequent bidiagonal SVD cost O(m^2) operations.
- [Figure 5] The caption should define the symbols × and ⊗ and state the matrix dimensions; in the current form, the figure is difficult to use for verifying the fill-in pattern or the number of rotations.
- [§3.2.2] There is a typo: 'implemended' should be 'implemented'.
- [References] References [31] and [32] are identical (both are Sorensen and Embree, 'A DEIM induced CUR factorization'); one of the two entries should be removed or replaced with a distinct citation.
- [§2.4 and §3.2.2] The early-stopping variant is presented as a heuristic with empirical support, but the paper could note explicitly that the same induction as in Theorem 5 applies to any pair satisfying (21), which would make the theoretical guarantee for the early-stopping variant immediate.
Circularity Check
No circularity: Algorithm 3's bound derives from external existence result [38] and standard derandomization.
full rationale
After walking the derivation chain, I find no step in which a claimed prediction or derived result reduces by construction to an input, fitted parameter, or prior claim by the same authors. The existence bound (14) is imported from Zamarashkin and Osinsky [38], an external published result, and the derandomization framework from Deshpande and Rademacher [14]; the paper's contribution is a new deterministic algorithm and its analysis. The only self-citation, [12] in Section 3.2.3, is an aside about symmetric cross approximation and does not support Theorem 5 or Algorithm 3. The load-bearing numerical ingredient, the O(m^2) bulge-chasing step in Section 3.2.2, is asserted with a figure rather than a complete algorithm, and Algorithm 3 does not state the nonzero-pivot restriction needed in Lemma 4; these are correctness and completeness gaps, not circularity, because the paper does not assume the truth of its target bound when proving it. No fitted values are used as predictions. Accordingly the circularity score is 0.
Assumptions & free parameters
assumptions (5)
- domain assumption The distribution (13) satisfies E[||A - A(:,Y) A(X,Y)^{-1} A(X,:)||_F^2] <= (k+1)^2 (sigma_{k+1}^2 + ... + sigma_m^2) (eq. (14)).
- domain assumption The error of a cross approximation equals a sum of squared volumes, and volume sums relate to characteristic polynomial coefficients via Cauchy-Binet (Lemma 4 uses [38, Lemma 1] and [26, Theorem 7]).
- domain assumption The Summation Algorithm [29] computes characteristic polynomial coefficients from squared singular values in forward-stable O(m^2) operations.
- domain assumption The bulge-chasing reductions in Section 3.2.2 transform the bidiagonal-plus-rank-one matrix D1 into bidiagonal form D3 in O(m^2) Givens rotations.
- domain assumption A is an m x n matrix with m <= n and rank at least k.
Cite this review
Pith. "Pith review of Low-rank approximation in the Frobenius norm by column and row subset selection." pith.science (2026). https://pith.science/paper/WCSMVHMQ
@misc{pith2026190806059,
author = {Pith},
title = {Pith review of: Low-rank approximation in the Frobenius norm by column and row subset selection},
year = {2026},
howpublished = {\url{https://pith.science/paper/WCSMVHMQ}},
note = {Machine review of arXiv:1908.06059}
}
abstract
A CUR approximation of a matrix $A$ is a particular type of low-rank approximation $A \approx C U R$, where $C$ and $R$ consist of columns and rows of $A$, respectively. One way to obtain such an approximation is to apply column subset selection to $A$ and $A^T$. In this work, we describe a numerically robust and much faster variant of the column subset selection algorithm proposed by Deshpande and Rademacher, which guarantees an error close to the best approximation error in the Frobenius norm. For cross approximation, in which $U$ is required to be the inverse of a submatrix of $A$ described by the intersection of $C$ and $R$, we obtain a new algorithm with an error bound that stays within a factor $k + 1$ of the best rank-$k$ approximation error in the Frobenius norm. To the best of our knowledge, this is the first deterministic polynomial-time algorithm for which this factor is bounded by a polynomial in $k$. Our derivation and analysis of the algorithm is based on derandomizing a recent existence result by Zamarashkin and Osinsky. To illustrate the versatility of our new column subset selection algorithm, an extension to low multilinear rank approximations of tensors is provided as well.
Figures
Figures from the paper (6 more)
Reference graph
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