REVIEW 3 major objections 4 minor 40 references
Best Subset Selection: Optimal Pursuit for Feature Selection and Elimination
T0 review · 3 major / 4 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read This paper proposes two objective-based criteria—one for adding features, one for removing them—that are provably optimal for the current support-set update, and shows that substituting them for classical criteria in greedy algorithms…
desk verdict Solid greedy-selection upgrade with credible experiments, but the CoSaOP convergence claim rides on an assumption that smuggles in the true support. 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 the support-restricted least-squares value $f(S)=\min_{\operatorname{supp}(\beta)=S}\|y-X\beta\|_2^2$, whose one-step behavior the new criteria optimize exactly. The mechanical engine is a pair of matrix-inverse update lemmas: the forward inverse (Lemma 3.2) writes $(X_S^{\top}X_S)^{-1}$ for $S=S_{k-1}\cup\{j\}$ as a rank-one update of the already-available inverse $C_{k-1}$, converting the exact selection subproblem into the explicit criterion (8); the backward inverse (Lemma 3.10) performs the matching downdate for $S=S_{k-1}\setminus\{j\}$, yielding criterion (10). These lemmas are what keep the new criteria at the same computational order as the classical ones: the required inverse or Cholesky factor was already computed when the coefficients on $S_{k-1}$ were fit, so the criteria add no new factorization cost.
What would settle it
Take a small design matrix and a sparse vector, enumerate $f(S\cup\{j\})$ for every $j\notin S$ and $f(S\setminus\{j\})$ for every $j\in S$ by fitting least squares on each candidate support, and check that the indices selected by criteria (8) and (10) attain the minima; the first violation would refute Theorems 4.1 and 4.2. For the convergence-preservation claim, construct a design matrix with restricted-isometry constant 0.1 where the true support intersects the disagreement set in the CoSaOP proof and run CoSaOP; a residual sequence that fails the claimed linear bound would refute Theorem 4.9 as stated.
Extended reading notes
Core claim
Stated on the paper's own terms, the discovery is that the classical selection and elimination rules of greedy subset selection are only first steps of block coordinate descent on the support-restricted least-squares objective $f(S)=\min_{\beta:\,\operatorname{supp}(\beta)=S}\|y-X\beta\|_2^2$. The classical entry rule (3) maximizes the immediate drop in $\|y-X\beta\|^2$ from adding $j$ with all other coefficients frozen, and the exit rule (4) minimizes the immediate rise from deleting $j$ with other coefficients frozen; the subsequent coefficient refit is invisible to both rules. The paper formulates the exact subproblems (P1) and (Q1), in which coefficients are refit after the support change, solves them in closed form, and obtains criterion (8) for entry and criterion (10) for exit. Theorem 4.1 then states that the index $j^*$ from (8) satisfies $f(S\cup\{j^*\})\le f(S\cup\{j\})$ for every candidate $j$ outside $S$, and Theorem 4.2 states the symmetric statement for (10) and deletion. Replacing criteria throughout existing algorithms yields OP, CoSaOP, and OP-(A)BESS; Theorem 4.9 gives CoSaOP a CoSaMP-style linear convergence rate under RIP, and Theorems 4.10 and 4.11 quantify advantages over the classical rules when features are correlated.
Load-bearing premise
The load-bearing premise is an extra disjointness condition in the CoSaOP proof—the true support must avoid every index where the elimination rule's output and the best K-sparse approximation to the current fit disagree—and the paper does not show this follows from its stated restricted-isometry bound.
Editorial extensions
If this is right
- Theorems 4.1 and 4.2 make the one-step choices optimal: at a given support $S$, the selected entry or exit index produces the smallest possible support-restricted least-squares objective among all candidates.
- Meta-substitution turns the paper's criteria into a general recipe: any greedy subset-selection algorithm that uses correlation-based entry or T-statistic exit can be upgraded, and the paper demonstrates this for OMP, CoSaMP, and (A)BESS.
- Because both new criteria reuse the inverse already computed for the current support, OP, CoSaOP, and OP-(A)BESS stay in the same computational-complexity class as their classical counterparts.
- Under restricted-isometry assumptions, CoSaOP inherits a CoSaMP-style linear convergence bound, with the constants the paper computes in Theorem 4.9.
- In the correlated-feature regime where RIP fails, criterion (8) has a lower bound that grows as the correlation between two true features approaches one, while the classical criterion's upper bound shrinks to zero; criterion (10) can identify pseudo-correlated spurious features for removal.
Reading between the lines
- Inference: the entry criterion (8) is effectively correlation with the residual after the current support's contribution is projected out, so the same logic should transfer to matching-pursuit variants beyond the three tested families, such as LARS-style or kernel-based greedy fits; this is a direct extension the paper does not run.
- Inference: the elimination criterion (10) is a support-agnostic pruning rule, so it could be used as a generic post-processing or inner-loop step in any local-search subset-selection solver, not only in greedy pipelines; the authors' 'meta-substitution' language suggests this, but the experiments only cover three families.
- Inference: the Optimal Gradient Pursuit appendix hints that the exact-subproblem principle extends to non-quadratic objectives; a testable formulation would define entry and exit gains relative to gradient updates with exact line search and compare against current greedy methods on logistic or quantile losses.
- Inference: the claimed same-order cost depends on the current-support inverse being available; in ultra-high-dimensional settings where even forming the projection in criterion (8) is prohibitive, the paper's own Optimal Gradient Pursuit version is the practical route, but its one-step optimality is not proved to the same standard as Theorem 4.1.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript proposes two new criteria for best-subset selection: a selection criterion (8) and an elimination criterion (10), obtained by solving one-entry and one-exit least-squares subproblems exactly via forward and backward matrix-inversion formulas. The paper proves that these criteria give per-step optimal decisions (Theorems 4.1 and 4.2), claims that substituting them into OMP, CoSaMP, and (A)BESS preserves the original algorithms' theoretical guarantees while achieving substantial empirical gains, and reports experiments on compressed sensing, sparse regression, column subset selection, and line-spectrum estimation.
Significance. The algebraic derivations of (8) and (10) are clean, the per-step optimality theorems are correct, and the correlated-feature counterexample in Theorem 4.11 provides a concrete demonstration of a real failure mode of Wald-T-based pruning. The experimental evaluation is extensive, with multiple datasets, metrics, and runtime comparisons, and the authors provide code and detailed appendices. The contribution would be substantial if the guarantee-preservation claim were established. However, as detailed below, the preservation claim currently rests on an extra assumption about the new elimination rule, so the main theoretical selling point is not yet proven.
major comments (3)
- [§4, Lemma 4.7 / Appendix I, Assumption 4] The convergence proof for CoSaOP is not established because Lemma 4.7 relies on Assumption 4 in Appendix I: supp(β*)⊆(S_k △ supp(ã_K))^c. This assumption is not derived from the RIP condition δ≤0.1 used in Lemma 4.4, nor from any earlier result. In the sparse-recovery setting S_k, supp(ã_K), and supp(β*) all have cardinality K, so in the generic case where a true index lies in S_k, the assumption forces S_k = supp(ã_K) and S_k = supp(β*); that is, it assumes that the new elimination criterion (10) has already recovered the true support and that pruning does not remove any true index. If a true feature with a large coefficient were dropped, then ∥β*−ã_{S_k}∥ could be proportional to that large coefficient even when a≈β*, so the bound ∥β*−ã_{S_k}∥ ≤ (2+δ)∥β*−a∥ of Lemma 4.7 can fail by an arbitrarily large factor. Consequently, Theorem 4.9's linear convergence rate for CoSaOP is not proven, and the Abstract's claim that enhanced algorithms 'preserve the theoretical properties of the original algorithms' is unsupported for CoSaOP, which is the paper's worked example of preservation.
- [Abstract / Conclusion / §5.1, Appendix O] The claim that the enhanced algorithms achieve gains 'without increasing computational cost' overstates what is shown. Theorem 4.3 establishes only that the computational complexity is of the same order of magnitude, and Appendix O explicitly reports that the new criteria result in 'slightly higher computational time'. The abstract and conclusion should be reworded to claim 'same asymptotic order of complexity' rather than 'no increase in computational cost', so that the main text, the abstract, and the runtime data in Figure 5 are mutually consistent.
- [§4, Theorem 4.11(2) / Appendix M] The proof of Theorem 4.11(2) ends with the sentence that criterion (10) 'will always identify Xp by Theorem 4.2'. This is stronger than what is proved. Theorem 4.2 states only that criterion (10) minimizes f(S\{j}) over j in the current S; it does not by itself guarantee that the minimizer is the pseudo-correlated feature in every configuration satisfying the stated correlation condition. The specific 3×3 counterexample is sufficient to demonstrate the 'could' claim of the theorem, but the 'always' phrasing should be removed or supported by a separate argument.
minor comments (4)
- [§4, Theorems 4.1 and 4.2] Theorems 4.1 and 4.2 are immediate restatements of the derivations of (8) and (10); they should be explicitly described as per-step optimality statements rather than standalone convergence or recovery guarantees, to avoid overstating their role.
- [Appendix F / Appendix G / Appendix H / Appendix I] The appendix headings refer to 'Theorem 4.4', 'Theorem 4.5', etc., while the main text labels these results as Lemmas 4.4–4.8; the numbering should be harmonized.
- [Appendix O, Figure 5] The caption for Figure 5 should state explicitly whether the reported times are total wall-clock time over 500 runs or per-run averages, and the discussion of CoSaMP's larger runtime should be moved closer to the figure for clarity.
- [§5.1.1, Appendix P] There is a typo 'signiaficant' in Section 5.1.1 and 'pseudo-correlated' is misspelled in Theorem 4.11(2); these should be corrected in the revision.
Circularity Check
No substantive circularity: The optimality theorems 4.1 and 4.2 restate the construction of criteria (8) and (10), but the empirical meta-gains and the convergence analysis are independent of any fitted or self-cited input.
-
self definitional
[Section 4, Theorems 4.1 and 4.2; criteria constructed in Section 3.2, Eqs. (8) and (10)]
"Theorem 4.1. For index j ∗ selected by criterion(8), f(S∪ {j∗})≤f(S∪ {j}),∀j∈S c. Theorem 4.2. For index j ∗ selected by criterion(10), f(S\{j∗})≤f(S\{j}),∀j∈S."
Criterion (8) is derived by solving the exact one-step optimization (P1)/(P2): Lemma 3.1 rewrites (P2) as maximizing y^T X_S(X_S^T X_S)^{-1} X_S^T y over S=S_{k-1}∪{j}, i.e. minimizing f(S∪{j}); Theorem 3.3 identifies (8) as the argmax. Criterion (10) is likewise the solution of (Q1)/(Q2) in Theorem 3.11. Theorems 4.1 and 4.2 thus restate the defining optimization used to construct the criteria. The algebraic closed forms are genuine derivations, but the optimality assertions add no independent evidence beyond the construction; this is a minor definitional identity, not a fitted-input prediction.
full rationale
The paper's central empirical claim—that substituting criteria (8) and (10) into OMP, CoSaMP, and (A)BESS yields meta-gains across compressed sensing, sparse regression, column subset selection, and complex-signal tasks—is supported by external experiments against standard baselines and does not reduce to the criteria's definitions. No parameters are fitted and later renamed as predictions, and no self-citation chain is load-bearing: the ABESS and CoSaMP citations are external prior work, and the similarity to SMP (Tohidi et al. 2025) is disclosed in Remark 3.8. The only construction-relative step is the optimality claim in Theorems 4.1 and 4.2, which restates the exact subproblems used to define the criteria; because the paper explicitly frames these as solved subproblems, it is a definitional restatement rather than a hidden circularity. The main weakness is not circularity but proof completeness: Theorem 4.9's linear rate for CoSaOP relies on Assumption 4 in Appendix I, supp(β*)⊆(S_k △ supp(ã_K))^c, which is not shown to follow from the stated RIP condition δ≤0.1. That is a correctness gap in the preservation-of-guarantees claim, not a circular reduction, since the assumption is not equivalent to the theorem's inputs. Overall circularity is low.
Assumptions & free parameters
assumptions (4)
- domain assumption Design matrix X satisfies the Restricted Isometry Property with δ_K ≤ δ_{K+1} ≤ δ_{2K} ≤ δ_{4K} ≤ 0.1, and columns are normalized to unit norm.
- ad hoc to paper Assumption 4 in Appendix I: supp(β*)⊆(S_k △ supp(ã_K))^c, where S_k is the support chosen by criterion (10) and ã_K is the best K-sparse approximation of the least-squares estimate on the merged support.
- standard math For every subset S considered, X_S has full column rank so (X_S^T X_S) is invertible.
- domain assumption In the synthetic compressed sensing experiments, the true support is unique and recoverable; experiments measure exact support recovery.
Cite this review
Pith. "Pith review of Best Subset Selection: Optimal Pursuit for Feature Selection and Elimination." pith.science (2026). https://pith.science/paper/RBWSZ6SF
@misc{pith2026250116815,
author = {Pith},
title = {Pith review of: Best Subset Selection: Optimal Pursuit for Feature Selection and Elimination},
year = {2026},
howpublished = {\url{https://pith.science/paper/RBWSZ6SF}},
note = {Machine review of arXiv:2501.16815}
}
read the original abstract
This paper introduces two novel criteria: one for feature selection and another for feature elimination in the context of best subset selection, which is a benchmark problem in statistics and machine learning. From the perspective of optimization, we revisit the classical selection and elimination criteria in traditional best subset selection algorithms, revealing that these classical criteria capture only partial variations of the objective function after the entry or exit of features. By formulating and solving optimization subproblems for feature entry and exit exactly, new selection and elimination criteria are proposed, proved as the optimal decisions for the current entry-and-exit process compared to classical criteria. Replacing the classical selection and elimination criteria with the proposed ones generates a series of enhanced best subset selection algorithms. These generated algorithms not only preserve the theoretical properties of the original algorithms but also achieve significant meta-gains without increasing computational cost across various scenarios and evaluation metrics on multiple tasks such as compressed sensing and sparse regression.
Figures
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Reference graph
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Reviewed August 10, 2026 · model on record in the stance chip above.
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