REVIEW 3 major objections 3 minor 1 cited by
Persistent (Co)Homology in Matrix Multiplication Time
T0 review · 3 major / 3 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read Persistent homology representatives can be computed in matrix multiplication time, matching the cost of the persistence diagram itself.
desk verdict Solid exhaustive-reduction algorithm, but the lazy-reduction row algorithm looks broken as written and is not adequately proven. 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 identity is $R=DV$, where $D$ is the filtration-ordered boundary matrix, $V$ is a full-rank upper-triangular matrix of column operations, and $R$ is column-reduced; the columns of $V$ and $R$ provide the cycle and cocycle representatives. The second mechanism is the Schur complement: when a submatrix of pivot rows is lower triangular (hence invertible), the update $R[\bar{L},C] \leftarrow R[\bar{L},C] - R[\bar{L},B]\cdot R[L,B]^{-1}\cdot R[L,C]$ applies all column reductions from block $B$ to block $C$ in one matrix product. The row algorithm replaces inversion with column permutations that keep every column with a lower pivot to the left of the current pivot, allowing one batched row-zeroing update per pivot. These batch operations are what bring the complexity down from cubic time to matrix multiplication time.
What would settle it
Implement Algorithm 5 over a fixed field and compare its output against Algorithm 4 (the incremental lazy reduction) on all simplicial complexes with up to six vertices and every total-order extension of their face posets; any filtration where the pivot positions (the low values) of the two algorithms differ, or where the recovered cycle representatives differ, would refute Theorem 5.3. A smaller targeted check is to construct a boundary matrix where the row algorithm's column permutation places a column with a lower pivot to the right of the current pivot and verify that the batched row zeroing in Line 5 still matches the lazy reduction.
Extended reading notes
Core claim
On its own terms, the paper establishes that there is no asymptotic price for the standard representatives in ordinary persistent (co)homology. For the exhaustive reduction, Algorithm 3 uses batched Schur-complement updates that reduce blocks of columns at once after permuting pivot rows to the bottom, yielding $R=DV$ with the same column-reduced form as a full look-ahead reduction. For the lazy reduction, Algorithm 5 works bottom-up on rows, using column permutations to align leftmost eligible pivots and then zeroing whole rows in one batch; Lemma 5.1 and Theorem 5.3 assert this reproduces exactly the standard lazy reduction's $R$, $V$, and hence its representatives. Because $V$ is upper triangular and $U = 2I - \Lambda$ in the lazy case, representatives, pairings, and the persistence diagram all come out of the same $O(n^\omega)$ computation.
Load-bearing premise
The fast row algorithm's correctness rests on the invariant that after every column permutation, every column whose lowest nonzero entry lies below the current row has been moved to the left of the current pivot position; if that invariant ever failed, zeroing the whole row to the right would silently skip reductions that the lazy algorithm would still perform.
Editorial extensions
If this is right
- Persistence diagrams and the cycle representatives produced by either the lazy or exhaustive reduction become simultaneously available in $O(n^\omega)$ time, so applications that currently run a second, slower reduction to obtain representatives pay no asymptotic extra cost.
- All the algorithms transfer to persistent cohomology by applying the same reduction to the anti-transpose of the boundary matrix, giving cocycle representatives with the same time bound.
- Combined with the fast zigzag construction for reducing zigzag persistence to ordinary persistence, these algorithms provide another matrix-multiplication-time route to zigzag persistence, extending the reach of the $R=DV$ formulation beyond ordinary filtrations.
- Because the row algorithm returns the same representatives as the standard lazy persistence algorithm, it can serve as a drop-in replacement for standard pipelines that need the usual basis, not just the barcode.
- The exhaustive reduction's representatives are the lexicographically optimal cycles used in point-cloud triangulation, so those optimal representatives now come with the same asymptotic guarantee as the barcode.
Reading between the lines
- With both target reductions at the matrix-multiplication bound, the practical bottleneck shifts from asymptotic complexity to constant factors and the cost of field operations; implementations could plausibly switch between block matrix products and standard column reduction depending on block size, a trade-off the paper does not explore.
- The invariant behind the row algorithm suggests a general recipe: batched reductions are safe exactly when the permutation removes all later columns with lower pivots to the left. Adapting that recipe to other reduction orders, such as output-sensitive or parallel schemes, could yield additional algorithms with the same complexity bound, though the paper makes no such claim.
- If the paper's stated open question about transforming exhaustive representatives into lazy ones (and vice versa) without rerunning the reduction is resolved, a single matrix-multiplication-time reduction would serve every application that needs either basis.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents two algorithms for computing the R = DV decomposition in persistent (co)homology in matrix multiplication time, with the goal of recovering cycle representatives in the same asymptotic time as computing the persistence diagram. Algorithm 3 is a fast "exhaustive" reduction based on Schur complements, and Algorithm 5 is a fast "lazy" reduction based on bottom-up batched row updates. The authors claim that the latter reproduces exactly the lazy reduction of the standard persistence algorithm, including its cycle representatives, in O(n^ω) time (O(n^2 log n) when ω = 2).
Significance. If both algorithms were correct, the paper would close a notable gap: persistence diagrams were already known to be computable in matrix multiplication time, but extracting the specific cycle representatives used in applications was not known to have the same bound. The column algorithm in Section 4 is an elegant and apparently correct specialization of the zigzag algorithm of Milosavljevic et al., and the running-time analysis for it is convincing. However, the row algorithm in Section 5 is the load-bearing contribution for the lazy reduction, and it has a concrete correctness problem that is not addressed by the proof of Theorem 5.3. Since the central claim of the paper depends on this algorithm, the manuscript needs substantial revision before the claimed result can be accepted.
major comments (3)
- [§5.1, Algorithm 5, line 11] The batched row update in line 11 applies the full row (P·Λ·P)[B,·], including the diagonal identity entries that Λ inherits from its initialization as I_{m+n}. These identity entries are self/pivot coefficients, not reduction coefficients, and using them adds the pivot columns themselves to the rows in Cr. This corrupts the lazy reduction. Concretely, over F_2 with n = 4 and D having columns c1 = c2 = c4 = 0 and c3 = (1,1,0,0), augmented with I_4, running Algorithm 5 literally makes column 7 (the third identity column) equal to (1,0,1,0) after the row-2 update, whereas Algorithm 4 and standard lazy reduction leave R[1,7] = 0. Thus Algorithm 5, as written, does not compute the lazy reduction.
- [Theorem 5.3 and Remark 5.2] The proof of Theorem 5.3 asserts that the batched operations reproduce Algorithm 4, but it never explains why the identity entries on the diagonal of P·Λ·P should not be applied during the row updates. Remark 5.2 only guarantees that columns whose pivots lie below the current row have been moved to the left of the current pivot; it does not guarantee that those columns are not modified. In the example above, the corrupted column lies to the left of the row-1 pivot and is therefore never zeroed by the base-case row clearing, so the invariant cannot catch the error. A correct proof must either exclude the diagonal entries from the row update or show they cancel through some additional mechanism.
- [Theorem 5.4 and §2.2] Because Theorem 5.4 and the claimed recovery of lazy cycle representatives from U and V depend on Algorithm 5 actually performing the lazy reduction, the failure described above invalidates the advertised representative-recovery result for the lazy reduction. The paper's central claim in the introduction that Algorithm 5 computes the lazy reduction is therefore not established by the current manuscript.
minor comments (3)
- [Theorem 5.4 proof] The proof contains a duplicated sentence about a column that 'has not been used to reduce any other column'; the repetition should be removed.
- [Section 6] The sentence about using the column algorithm to compute the lazy basis and the row algorithm for the exhaustive basis is easy to misread as reversing the roles of the two algorithms; consider rephrasing for clarity.
- [Lemma 5.1] The proof of Lemma 5.1 is a bare citation to [3, Theorem 3.1]. Since the terminology 'lazy reduction' may not match the cited theorem's conventions, a short explanation of the correspondence would improve readability.
Circularity Check
No significant circularity: the algorithms are explicit reductions, and author-overlapping citations are independent published theorems.
full rationale
The paper's derivation chain is algorithmic rather than definitional: it constructs R and V through explicit batched column/row reductions, and the matrix-multiplication bounds follow from standard lemmas (Schur complement, triangular inversion, Lemma 3.1, Theorem 4.2). The central reduction steps do not presuppose the target decomposition. The only load-bearing citations to prior work with overlapping authors are [1] and [3, Theorem 3.1]. These are published, parameter-free mathematical results with stated assumptions that do not include the fast batched algorithms proved here; under the independence rule, such citations are genuine evidence and do not raise the circularity score. Algorithm 5's equivalence to the lazy reduction is argued by matching the recorded coefficients and recursion order in Theorem 5.3, and the underlying row-algorithm/lazy equivalence is an external theorem, not a renaming. There are no fitted parameters, no predictions of closely related quantities, and no empirical pattern renamed as organization. Any implementation-level concern about the batched row update's treatment of identity entries (which has been raised externally) would be a correctness or bug issue, not a circularity of definition, and is therefore not scored here.
Assumptions & free parameters
assumptions (5)
- standard math Matrix multiplication of n x n matrices over a fixed field F can be done in O(n^omega) time for some omega >= 2.
- standard math Inverting a non-singular triangular matrix can be done in matrix multiplication time.
- domain assumption The filtration can be assumed to be a total order with one simplex added at a time.
- standard math The Pairing Uniqueness Lemma of Cohen-Steiner et al. holds, so pivot pairs are independent of the reduction algorithm.
- standard math Algorithm 4 (the incremental row algorithm) produces the same decomposition as the standard lazy reduction.
Cite this review
Pith. "Pith review of Persistent (Co)Homology in Matrix Multiplication Time." pith.science (2026). https://pith.science/paper/AJT6AC3V
@misc{pith2026241202591,
author = {Pith},
title = {Pith review of: Persistent (Co)Homology in Matrix Multiplication Time},
year = {2026},
howpublished = {\url{https://pith.science/paper/AJT6AC3V}},
note = {Machine review of arXiv:2412.02591}
}
read the original abstract
Most algorithms for computing persistent homology do so by tracking cycles that represent homology classes. There are many choices of such cycles, and specific choices have found different uses in applications. Although it is known that persistence diagrams can be computed in matrix multiplication time [8] for the more general case of zigzag persistent homology, it is not clear how to extract cycle representatives, especially if specific representatives are desired. In this paper, we provide the same matrix multiplication bound for computing representatives for the two choices common in applications in the case of ordinary persistent (co)homology. We first provide a fast version of the reduction algorithm, which is simpler than the algorithm in [8], but returns a different set of representatives than the standard algorithm [6] We then give a fast version of a different variant called the row algorithm [4], which returns the same representatives as the standard algorithm.
Figures
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Forward citations
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Reference graph
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