REVIEW 1 major objections 5 minor 38 references
From Logistic Regression to the Perceptron Algorithm: Exploring Gradient Descent with Large Step Sizes
T0 review · 1 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read In the infinite-step-size limit, logistic regression with gradient descent becomes the batch perceptron algorithm, and this limit explains why arbitrarily large steps still solve linearly separable classification problems.
desk verdict LR+GD with infinite step size is exactly the batch perceptron; new, clean proof, plus a solid suboptimality lower bound and a normalized variant. 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 rescaling θ_t/γ together with the sigmoid-to-threshold convergence: (1 + exp(γ y_i a_i^T (θ_t/γ)))^{-1} → 1 if y_i a_i^T θ̂_t < 0 and → 0 otherwise. Because each batch perceptron iterate is a nonnegative combination of samples, Assumption 3.1 ensures no sample sits exactly on the decision boundary, so the thresholds are well defined. This pointwise limit is what lets one transfer the classical perceptron analysis to LR+GD, and its failure mode—samples on the boundary—is exactly the technical assumption the paper needs.
What would settle it
Run LR+GD on a linearly separable dataset that violates Assumption 3.1 (e.g., a dataset where a batch perceptron iterate passes exactly through a sample) with a sequence of step sizes γ_k → ∞; if the rescaled iterates θ_t/γ_k still match the batch perceptron for all t, the assumption is unnecessary, while if they deviate, the theorem's stated scope fails. A second check: on the paper's lower-bound dataset with n samples, count iterations to perfect accuracy for LR+GD with very large γ and compare with Ω(n); a matching linear dependence supports the claim, and a sublinear count would refute it.
Extended reading notes
Core claim
The paper's central discovery is a limit identity: under a mild non-degeneracy condition, for every fixed t, θ_t/γ → θ̂_t as γ→∞, where θ_t is the t-th LR+GD iterate from θ0=0 and θ̂_t is the t-th iterate of the batch perceptron algorithm. The mechanism is that the logistic sigmoid factors (1+exp(y_i a_i^T θ_t))^{-1} become hard thresholds 1[y_i a_i^T θ̂_t < 0] in the limit, so each gradient step converges to the batch perceptron update θ̂_t + (1/n)∑_{i∈S_t} y_i a_i. Because the classical perceptron bound guarantees separation in at most nR²/µ² steps, this gives a direct explanation of large-step-size success and identifies the effective iteration complexity of LR+GD in that regime. The paper further shows that this rate is suboptimal in n, proves an Ω(n) lower bound on a specific dataset, and introduces Normalized LR+GD, whose step is rescaled by the inverse average sigmoid, to recover the n-free rate R²/µ² while keeping the connection to the perceptron.
Load-bearing premise
The whole limit story rests on the non-degeneracy assumption that no training sample ever lies exactly on a decision boundary of a batch perceptron iterate, and on the unproven uniformity of the pointwise limit θ_t/γ → θ̂_t, which is needed to conclude that LR+GD finds a separator within the perceptron's nR²/µ² iteration bound.
Editorial extensions
If this is right
- For any separable dataset satisfying Assumption 3.1, LR+GD with γ→∞ solves the classification problem in at most nR²/µ² iterations.
- Larger step sizes in LR+GD drive the logistic loss and gradient norm up while speeding up the search for a separator, so loss and gradient values are unreliable indicators of progress toward a solution.
- The dependence on n in the iteration bound is essential for LR+GD with γ→∞: there are separable datasets with constant margin and radius on which it needs Ω(n) iterations.
- Normalized LR+GD, which rescales the gradient step by the inverse average sigmoid, solves the problem in at most R²/µ² + 2 log(2n−1)/(γµ²) iterations, improving on LR+GD by a factor of n as γ→∞.
- The max-margin implicit bias of small-step LR+GD does not carry over to the γ→∞ limit, since the batch perceptron does not generally return a max-margin separator.
Reading between the lines
- Extension: the same thresholding mechanism should apply to any smooth margin loss whose gradient approaches a hard indicator at large arguments, so other losses may inherit perceptron-like dynamics at large step sizes.
- Extension: if loss and gradient values are unreliable on separable problems, existing benchmark comparisons of optimizers may be ranking methods by the wrong metric; reporting iterations-to-perfect-accuracy could reorder those rankings.
- Extension: a formal measure-theoretic statement that degenerate datasets (Assumption 3.1 failures) have probability zero under any continuous data distribution would strengthen the paper's 'almost all datasets' claim without changing the argument.
- Extension: on imbalanced data, where the paper observes normalized variants winning, an adaptive per-class normalization of the step might yield further iteration-count gains; this is directly testable with the paper's experimental setup.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper analyzes gradient descent on logistic loss (LR+GD) for linearly separable data and shows that, as the step size γ tends to infinity, the normalized iterates θ_t/γ converge pointwise to the iterates of a batch perceptron (Theorem 3.2). This gives a simple mechanistic explanation for the empirically observed ability of LR+GD with very large step sizes to solve classification, and it identifies the limiting iteration count as nR²/μ². The paper further shows that logistic loss and gradient norms can be misleading indicators of progress (Theorem 4.1), proves a lower bound of Ω(n) iterations for the batch perceptron on a worst-case dataset (Theorem 5.1), and proposes Normalized LR+GD, with an iteration bound of R²/μ² + O(log n/(γμ²)) (Theorem 6.1). Numerical experiments on CIFAR-10, FashionMNIST, EuroSAT, and MNIST support the claimed alignment between large-step LR+GD and the batch perceptron.
Significance. If correct, the paper provides a clean and novel reduction of large-step-size logistic regression to the perceptron, connecting edge-of-stability behavior to a classical algorithm. The main theorem has a short, self-contained proof; the proof of Theorem 6.1 is a transparent potential argument; and the classical perceptron bound is reproved in the appendix. The paper also makes a useful conceptual point about evaluating optimization methods for classification by iteration count rather than by function values. No fitted parameters enter the theoretical claims, and the experiments are described in enough detail to be reproduced. The result should be of interest to the machine-learning theory community.
major comments (1)
- [Section 5.1, proof of Theorem 5.1] The case analysis misidentifies which class is misclassified. After θ̂1 = (0.25, (n−2)/(2n)), the misclassified sample is the class +1 sample (0.5,−1), not the class −1 sample; the displayed update +(1/n)(0.5,−1) is y_i a_i for the class +1 sample. The later stopping conditions are likewise written for the wrong class. The recurrence for θ̂_t is correct, and the Ω(n) bound follows once the labels are corrected, but as written the proof is not internally consistent.
minor comments (5)
- [Section 3, after Theorems 3.2 and 3.3] The transfer from pointwise convergence to a statement about finite γ is not spelled out. Since the batch perceptron terminates at a finite τ, one only needs the pointwise limit at t=τ plus the fact that θ̂_τ has positive margin on every sample; no uniformity in t is required. Please add this one-sentence argument to remove any appearance of a gap.
- [Section 6, paragraph before Theorem 6.1] The claimed limit '∇f(θ_t) → 1/|S_t| ∑_{i∈S_t} y_i a_i' is incorrect. From (3), ∇f(θ_t) → −(1/n)∑_{i∈S_t} y_i a_i and β_t → |S_t|, so the quantity that converges to 1/|S_t|∑_{i∈S_t} y_i a_i is −β_t∇f(θ_t), not ∇f(θ_t). Please correct the sign and normalization in this motivating statement.
- [Theorem 4.1 statement] The assumption 'θ1 = 0' does not match the proof, where θ0=0 and θ1 is computed explicitly. Please change it to θ0=0.
- [Theorem 5.1 proof, final paragraph] The displayed stopping condition involving 0.5×0.25(...) + (−1)×(n−2k)/(2n) > 0 is the margin of the positive sample, not of the negative sample; please correct the attribution or the expression.
- [Assumption 3.1] Consider noting explicitly that for θ̂_t = (1/n)∑_{i=1}^n (0.5+k_i)y_i a_i, the condition y_j a_j^T θ̂_t ≠ 0 is exactly the non-intersection of the hyperplane with (0.5+k_1, ..., 0.5+k_n), which will help readers connect the assumption to the proof.
Circularity Check
No circularity found: the perceptron-limit reduction is derived from the explicit gradient formula, and the auxiliary convergence proofs are self-contained.
full rationale
The central claim (Theorem 3.2) is a first-principles limit computation. The paper writes the exact LR+GD update as theta_{t+1}/gamma = theta_t/gamma + (1/n) sum_i [1/(1+exp(gamma * y_i a_i^T (theta_t/gamma)))] y_i a_i, and since each sigmoid factor converges to 1 or 0 according to the sign of y_i a_i^T hat_theta_t, the update converges to the Batch Perceptron update. The limit is proved for each fixed t and does not presuppose the conclusion; Assumption 3.1 only excludes boundary ties, and the paper notes it holds for almost all datasets. The transfer to finite large gamma is not circular: for any dataset, the Batch Perceptron terminates at some finite tau <= nR^2/mu^2, and because the limit is pointwise at each of the finitely many steps 1,...,tau, a single finite gamma threshold suffices. The perceptron iteration bound (Theorem 3.3) is reproved from scratch in Appendix B using the margin assumption, not imported from the authors' own work. The Normalized LR+GD result (Theorem 6.1) is proven directly with a potential-function argument in Appendix C, and the lower bound for Batch Perceptron (Theorem 5.1) is an explicit worst-case construction. No fitted parameter is relabeled as a prediction, no load-bearing premise is justified only by self-citation, and no known result is merely renamed. Therefore the derivation chain is self-contained and no circular step is present.
Assumptions & free parameters
assumptions (2)
- domain assumption Assumption 1.1: the dataset is linearly separable with margin μ > 0.
- ad hoc to paper Assumption 3.1: no data point lies exactly on a batch perceptron decision boundary at any iterate, i.e., the hyperplane in the statement avoids all points (0.5+k_1,...,0.5+k_n).
Cite this review
Pith. "Pith review of From Logistic Regression to the Perceptron Algorithm: Exploring Gradient Descent with Large Step Sizes." pith.science (2026). https://pith.science/paper/NLTMMNO4
@misc{pith2026241208424,
author = {Pith},
title = {Pith review of: From Logistic Regression to the Perceptron Algorithm: Exploring Gradient Descent with Large Step Sizes},
year = {2026},
howpublished = {\url{https://pith.science/paper/NLTMMNO4}},
note = {Machine review of arXiv:2412.08424}
}
abstract
We focus on the classification problem with a separable dataset, one of the most important and classical problems from machine learning. The standard approach to this task is logistic regression with gradient descent (LR+GD). Recent studies have observed that LR+GD can find a solution with arbitrarily large step sizes, defying conventional optimization theory. Our work investigates this phenomenon and makes three interconnected key observations about LR+GD with large step sizes. First, we find a remarkably simple explanation of why LR+GD with large step sizes solves the classification problem: LR+GD reduces to a batch version of the celebrated perceptron algorithm when the step size $\gamma \to \infty.$ Second, we observe that larger step sizes lead LR+GD to higher logistic losses when it tends to the perceptron algorithm, but larger step sizes also lead to faster convergence to a solution for the classification problem, meaning that logistic loss is an unreliable metric of the proximity to a solution. Surprisingly, high loss values can actually indicate faster convergence. Third, since the convergence rate in terms of loss function values of LR+GD is unreliable, we examine the iteration complexity required by LR+GD with large step sizes to solve the classification problem and prove that this complexity is suboptimal. To address this, we propose a new method, Normalized LR+GD - based on the connection between LR+GD and the perceptron algorithm - with much better theoretical guarantees.
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Reviewed August 11, 2026 · model on record in the stance chip above.
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