REVIEW 3 major objections 5 minor 17 references
Totally Positive Matrices and the Highest-Order Coefficients of the Characteristic Polynomial
T0 review · 3 major / 5 minor · reviewed 2026-08-01 · deepseek-v4-flash
Pith's one-line read The paper claims that the three highest-order characteristic-polynomial coefficients (a_{n-1}, a_{n-2}, a_{n-3}), which are signed sums of all 1x1, 2x2, and 3x3 principal minors, already separate totally positive from non-totally positive m
desk verdict Clear empirical finding for the sampled families, but the TP/non-TP claim outruns the data because the negative class is too easy; worth refereeing with hard-negative tests. 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 engine of the argument is the projection π(A)=(a_{n-1},a_{n-2},a_{n-3}) combined with the identity a_{n-k}=(-1)^k E_k(A), where E_k is the sum of all k×k principal minors. This links the three chosen scalars directly to the minor conditions that define total positivity. On top of that projection, the paper fits Mahalanobis ellipsoids — sets {x:(x−μ)^T Σ^{-1}(x−μ) ≤ τ} — to each family's point cloud, using the mean and covariance of the coefficients to give the cloud a concrete geometric shape. The three TP families exploited are products of oscillatory bidiagonal pairs (each pair is an oscillatory matrix, and n pairs guarantee total positivity), Vandermonde matrices with sorted positive
What would settle it
Check all minors exactly for any positive-entry matrix that the three-coefficient classifier labels TP: a single non-positive minor would be a concrete counterexample to the claimed separation. Symmetrically, sample totally positive matrices outside the three generative families (for instance Pascal matrices or general oscillatory powers) and test whether their (a_{n-1},a_{n-2},a_{n-3}) projections land inside the fitted ellipsoids; a TP matrix landing far outside would show the signatures are family-specific rather than intrinsic to total positivity.
Extended reading notes
Core claim
The paper's central claim is that the map π(A)=(a_{n-1},a_{n-2},a_{n-3}) — the three highest-order characteristic-polynomial coefficients — is a concise spectral fingerprint of total positivity. Up to sign, these coefficients are the elementary symmetric sums E_1, E_2, E_3 of the principal minors of orders 1, 2, and 3. Using feedforward networks trained on only these three inputs, the authors obtain test accuracies of 0.9998 (n=5), 0.9998 (n=10), and 0.9999 (n=30) on the binary TP/non-TP problem, while a linear SVM plateaus around 0.84 and an RBF-kernel SVM reaches about 0.99, showing the boundary is nonlinear but geometrically simple. In the same coordinate space, the TP samples from each o
Load-bearing premise
The results stand or fall with the assumption that the randomly sampled positive-entry non-TP matrices and the three generative TP families (with the n=30 bidiagonal guarantee resting on extensive sampling rather than exhaustive minor checks) are representative of the full classes of totally positive and non-totally positive matrices.
Editorial extensions
If this is right
- Across n=5, 10, and 30, a classifier restricted to three coefficients reaches 0.9998–0.9999 test accuracy, so a three-number screen can flag total positivity with few errors for these structured families, bypassing the roughly 5×10^7 minors needed at n=30.
- The failure of linear separation (0.805–0.877) and success of quadratic/RBF separation (~0.99) implies the TP/non-TP boundary in coefficient space is nonlinear but low-complexity.
- The pairwise family overlap of the 95% Mahalanobis ellipsoids drops to 0% already at n=10 and stays 0% through n=100, while centroid Mahalanobis separation grows to tens of standard deviations for two of the three pairs.
- Conjecture 1: with n large enough, the Mahalanobis ellipsoids enclosing any fixed fraction of each family's samples are pairwise disjoint, with separation bounded below in spread units.
Reading between the lines
- If the separation is a general phenomenon, total positivity testing for large structured matrices could be reduced to evaluating three symmetric functions and checking one quadratic inequality, a dramatic cost cut relative to minor enumeration.
- The paper's mapping suggests a testable extension: other structured TP classes (Toeplitz, Hankel, Pascal, or TP matrices built from other bidiagonal distributions) should also form concentrated ellipsoidal clusters; if they do not, the signatures are artifacts of the three generative recipes.
- The observed quadratic boundary hints that the projection of the TP cone may have a semi-algebraic description of low degree; proving or refuting that would connect the numerics to algebraic geometry.
- Because E_k = tr C_k(A) with C_k(A) the k-th compound matrix, the three-coefficient projection is equivalently a three-trace fingerprint of the first three compound matrices, suggesting other trace invariants (power sums, compound traces) as alternative or complementary fingerprints.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper investigates whether the three highest-order characteristic-polynomial coefficients (a_{n-1}, a_{n-2}, a_{n-3}) can separate totally positive (TP) matrices from non-totally-positive matrices. TP examples are generated from three structured families—products of positive bidiagonal matrices, Vandermonde matrices, and Cauchy matrices—while non-TP examples are random strictly-positive-entry matrices that fail total positivity. The authors train neural-network classifiers and SVMs on the three coefficients and report test accuracies of 0.9998–0.9999 for dimensions n=5, 10, and 30 (Section 5). They fit Mahalanobis ellipsoids to the TP clouds, observe that the TP/non-TP boundary is nonlinear (Section 6), and find family-specific ellipsoidal signatures whose pairwise overlap vanishes for n≥10 (Section 7.3). Based on this evidence, they formulate Conjecture 1 on the asymptotic pairwise disjointness of the three family ellipsoids.
Significance. The paper proposes a concise spectral fingerprint for structured families of totally positive matrices and a concrete, falsifiable conjecture about their geometric separation. The empirical protocol is largely transparent: dataset construction is described in detail, and the authors include a convergence analysis of the ellipsoid parameters (Section 7.2) and give some exact-arithmetic verification for small n. The central TP-versus-non-TP claim, however, is only as strong as the sampling of the 'non-TP' class. Because that class consists only of generic positive-entry matrices that fail total positivity, the reported near-perfect accuracy may reflect distribution separation rather than a property of total positivity itself. The family-separation conjecture is better supported since it is explicitly scoped to the three generative models, but its evaluation contains an in-sample component. The paper is potentially interesting and the conjecture is a genuine contribution, but the headline claim needs additional hard-negative testing before it can be accepted at face value.
major comments (3)
- [Section 2 (Non-TP sampling)] The non-TP class is constructed only as random matrices with strictly positive entries that fail total positivity. This omits near-boundary totally nonnegative matrices, such as products of k=n−1 bidiagonal pairs, which the paper itself notes are TN but not TP (Section 2.1). With only generic random matrices as negatives, the classifier may separate structured TP families from unstructured random matrices rather than detecting the total-positivity condition. The abstract's claim about 'separating totally positive from non-totally positive matrices' is therefore overbroad. Please add hard-negative families (e.g., products of n−1 bidiagonal pairs, matrices with a single small negative minor) and report per-family accuracy; without this, the 0.9998–0.9999 test accuracies do not establish the stated claim.
- [Section 2.1 (TP guarantee for n=30)] The assertion that products of k=n bidiagonal pairs are totally positive is verified exhaustively only for n≤8; for n=10 and n=30 the support is 'extensive sampling' rather than exact minor checks. Since the paper's headline accuracy includes n=30, a mislabeled training point could in principle alter the reported numbers. Please provide a proof of the structural guarantee (or a reference to the oscillatory-matrix result) and state how many n=30 samples were checked in exact arithmetic. This is load-bearing for the n=30 classification results.
- [Section 7.3 and Table 3 (In-sample ellipsoid overlap)] The overlap percentages in Table 3 are computed as the worst-case fraction of points of one family inside the other family's 95% Mahalanobis ellipsoid, using the same 5000 matrices per family used to fit those ellipsoids. This is an in-sample evaluation: it describes the fitted object rather than demonstrating predictive separation. To support Conjecture 1, evaluate the overlap on held-out samples or via cross-validation. The convergence analysis in Section 7.2 addresses parameter stability but does not remove this in-sample bias.
minor comments (5)
- [Section 7.1] Typo: 'charactheristic' should be 'characteristic'.
- [Section 7.3] The verification of ellipsoid disjointness via a 50^3 grid over a bounding box is not a rigorous intersection test. A computational-geometry or semidefinite-programming based check would be more convincing, though not necessary for the conjecture's plausibility.
- [Section 3.2] The statement that test accuracies were 'identical ... achieving a perfect accuracy of 1.0' would be more informative with standard errors or results over multiple seeds, given the stochastic training procedure.
- [Data availability] The datasets and code are 'available from the corresponding author upon reasonable request.' For reproducibility, a public repository would be preferable, especially since the paper's conclusions are empirical.
- [Section 6.2] The text says the RBF kernel reaches accuracies 'near 0.99,' but Table 1 lists 0.9945, 0.9962, and 0.9885. Consider reporting these values explicitly to avoid ambiguity.
Circularity Check
No load-bearing circularity: held-out classification is genuine; the only circular phrasing is the descriptive TP-enclosing ellipsoid, which is a fit restated.
-
self definitional
[Section 7.1 (Mahalanobis Distance and Ellipsoids), Figure 9]
"As shown in Figure 9, the ellipsoids capture all TP matrices, while most non-TP matrices lie outside the boundaries."
Immediately before, the ellipsoid is fitted to the TP samples: 'Given a set of such vectors corresponding to TP samples, their mean and covariance matrix μ∈R^3, Σ∈R^{3×3} are calculated' and τ is the radius of the fitted Mahalanobis ellipsoid. Therefore 'capture all TP matrices' is true by construction when τ is the maximum sample Mahalanobis distance (or a quantile chosen on the same sample); it restates the fit rather than testing it. The non-TP exclusion is empirical, and the paper's central classification result uses an independent test split, so this tautology is descriptive and not load-bearing.
full rationale
The paper's headline claim that (a_{n-1}, a_{n-2}, a_{n-3}) separate TP from non-TP matrices is supported by a genuine held-out evaluation: Section 5 reports FFNs trained on the three coefficients and tested on a 10^4-matrix test set, with accuracies 0.9998–0.9999. That accuracy is not a fitted parameter renamed as a prediction. The Mahalanobis-ellipsoid geometry in Section 7 is explicitly a fitted description of the TP point clouds; the statement that the fitted ellipsoid 'captures all TP matrices' is tautological, but the paper does not use it as a predictive result, and Conjecture 1 is explicitly conjectural. The main validity concerns are not circularity: the non-TP class is sampled only as generic positive matrices (Section 2) and omits near-boundary totally nonnegative matrices (k=n−1 bidiagonal products), and the n=30 TP guarantee rests on 'extensive sampling' rather than exhaustive minor checks; these affect representativeness and label confidence, not the logic of the derivation. The self-citation [8] appears only in a general list of related work and is not load-bearing. No central result reduces by construction to its own inputs.
Assumptions & free parameters
free parameters (4)
- bidiagonal entry distribution
- Vandermonde/Cauchy node cutoff ε =
unspecified (U(ε,1))
- number of bidiagonal pairs k =
k = n
- Mahalanobis radius τ =
122.426 / 251.705 / 138.201 for n=30 (Table 2)
assumptions (4)
- domain assumption Products of at least n−1 oscillatory matrices are totally positive, and each L_i U_i pair is oscillatory.
- domain assumption The generative distributions in Section 2 are representative of TP and non-TP matrices.
- standard math Standard total-positivity facts: all minors of TP matrices are positive; a_{n-k}=(-1)^k E_k; Vandermonde and Cauchy matrices with positive ordered parameters are TP.
- domain assumption Floating-point computation of characteristic-polynomial coefficients is accurate enough at n=30 for the reported separation.
Cite this review
Pith. "Pith review of Totally Positive Matrices and the Highest-Order Coefficients of the Characteristic Polynomial." pith.science (2026). https://pith.science/paper/N3GPUANL
@misc{pith2026260718148,
author = {Pith},
title = {Pith review of: Totally Positive Matrices and the Highest-Order Coefficients of the Characteristic Polynomial},
year = {2026},
howpublished = {\url{https://pith.science/paper/N3GPUANL}},
note = {Machine review of arXiv:2607.18148}
}
read the original abstract
We investigate the extent to which totally positive matrices can be distinguished through the highest-order coefficients of their characteristic polynomials. To identify the most informative coefficients, we also employed neural-network classifiers together with feature-attribution methods. Using datasets built from several structured totally positive families, including products of positive bidiagonal matrices, Vandermonde matrices, and Cauchy matrices, we find that the coefficients (a_{n-1}, a_{n-2}, a_{n-3}) already contain strong discriminatory information for separating totally positive from non-totally positive matrices in dimensions 5, 10, and 30. The resulting separation is markedly nonlinear and admits a natural geometric description in the corresponding three-dimensional coefficient space by means of Mahalanobis ellipsoids. These ellipsoids enclose the totally positive samples while excluding most non-totally positive ones. Moreover, different structured totally positive families exhibit distinct ellipsoidal signatures, and the separation between these signatures increases with the dimension. These observations lead us to formulate a conjecture on the geometric separation of structured totally positive families in the space determined by the three highest-order characteristic coefficients.
Figures
Figures from the paper (9 more)
Reference graph
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Reviewed August 1, 2026 · model on record in the stance chip above.
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