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REVIEW 2 major objections 5 minor 59 references

Effect of Activation Functions on the Training of Overparametrized Neural Nets

T0 review · 2 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

Pith's one-line read The paper proves that activation functions with a derivative kink guarantee large minimum eigenvalues of the training Gram matrix, while smooth activations can force it exponentially small on low-dimensional data, and depth restores the…

desk verdict You should know two things before reading: this is the first paper I know that gives quantitative eigenvalue bounds for activations with a kink and explicit negative results for smooth ones, and I believe the headline ReLU bound is not proven as stated. read the letter →

arxiv 1908.05660 v4 pith:BGJ72746 submitted 2019-08-16 cs.LG stat.ML

classification cs.LGstat.ML MSC 68Q3268T07
keywords G-matrixminimumeigenvalueactivationfunctionoverparametrizationsmoothvsnon-smoothactivationsReLUtanhHermiteexpansion
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper asks what makes one activation function train faster than another in overparametrized two-layer networks, where gradient descent provably reaches zero training error. Its answer is that a single quantity, the minimum eigenvalue of the Gradient Gram matrix at initialization, separates activations into two classes. Activations with a derivative kink have this eigenvalue polynomially large under mild separation of the data, which predicts fast convergence. Smooth activations have it zero or exponentially small when the data occupies a low-dimensional subspace, which predicts slow training, while enough depth or data smoothing restores the large-eigenvalue regime.

What carries the argument

The central object is the Gradient Gram matrix at initialization, with entries $g_{ij} = \frac{1}{m}\sum_{k=1}^{m} a_k^2 \varphi'(w_k^{\mathsf{T}} x_i)\varphi'(w_k^{\mathsf{T}} x_j)\langle x_i, x_j\rangle$, whose eigenvalue decomposition governs the loss dynamics. The proofs split a Gaussian weight vector into a large part $w'$ and a small part $w''$ to isolate the kink, then use Taylor expansion of $\sum_i \zeta_i \varphi'(w^{\mathsf{T}} x_i)$ to convert a jump in the $r$-th derivative into a lower bound on the norm over an $\epsilon$-net. For smooth activations the counterpoint is polynomial approximation: $\tanh'$ is approximated by Chebyshev or Hermite polynomials whose decaying coefficients give the exponentially small upper bound on $\lambda_{\min}(G(0))$, while the lower bounds for smoothed data go through Khatri-Rao powers of the data matrix and anticoncentration of polynomial maps.

What would settle it

Compute $\lambda_{\min}(G(0))$ at initialization for a two-layer tanh network on $n$ points whose span has dimension $d' = O(\log^{0.75} n)$: the theorem predicts $\lambda_{\min}(G(0)) \le \exp(-\Omega(n^{1/(2d')}))$, so a measurement showing it is instead polynomially large would refute the upper-bound claim.

Watch

Extended reading notes

Core claim

The central claim is that the smoothness of the activation determines the conditioning of the G-matrix at initialization. For activations whose $r$-th derivative has a jump discontinuity, $\lambda_{\min}(G(0))$ is large: at least $\Omega(\delta^3 n^{-8}(\log n)^{-1})$ for the $J_2$ class and $\Omega((\delta/\log n)^{1.5})$ for ReLU. For smooth activations such as tanh, if the span of the $n$ data points has dimension $d' = O(\log^{0.75} n)$, then $\lambda_{\min}(G(0)) \le \exp(-\Omega(n^{1/(2d')}))$, and for polynomial activations many eigenvalues are exactly zero when $d' = O(n^{1/p})$. Thus the same architecture trains fast or slow depending on whether the activation has a kink and on the intrinsic dimension of the data, and the paper shows these bounds are tight in a precise sense.

Load-bearing premise

The training-speed conclusions hold only in the small-perturbation (lazy) regime, where weights move little enough that the G-matrix stays close to its value at initialization; the paper guarantees this only when the learning rate satisfies $\eta \le O(n^{-2}\lambda_{\min}(G(0)))$.

Editorial extensions

If this is right

  • For activations in the $J_r$ classes (e.g. ReLU, SELU, ELU), a sufficiently wide two-layer network trained by gradient descent reaches near-zero training error in polynomially many steps, with the step count controlled by $\lambda_{\min}(G(0))$.
  • For smooth activations on low-dimensional data, the G-matrix has many small eigenvalues, so loss decays slowly even in the ideal perturbative training regime.
  • The low-dimensional limitation of smooth activations disappears with depth: for tanh with depth $L = \Theta(\log(1/\delta))$, $\lambda_{\min}(G(0)) \ge e^{-O(\sqrt{\log n})}$, which is much larger than any inverse polynomial in $n$.
  • For smoothed data whose span has dimension at least $n^{\gamma}$, tanh regains a polynomial lower bound on $\lambda_{\min}(G(0))$, so the earlier slow-training result is not robust to small data perturbation.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • One could use $\lambda_{\min}(G(0))$ as a cheap diagnostic for predicting whether a given activation will train slowly on a particular dataset, prior to actually running gradient descent.
  • The results suggest a design principle for new activations: introduce a derivative kink of some order if the target data is low-dimensional, or rely on depth or data smoothing to rescue smooth activations.
  • Because the training-speed conclusions hold only under learning rates bounded by $O(n^{-2}\lambda_{\min}(G(0)))$, an immediate extension is to test whether the gap between kinked and smooth activations persists at practical learning rates; the eigenvalue bounds alone do not decide this.
  • The Hermite-coefficient perspective links activation smoothness to training speed, implying that activations with slowly decaying Hermite coefficients (like the step function underlying ReLU) should train fast, which could guide automated activation search.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 5 minor

Summary. The paper studies training of highly overparametrized two-layer neural networks in the DZPS (Du et al.) random-initialization setting, focusing on the minimum eigenvalue at initialization of the Gradient Gram matrix G(0), which controls the convergence rate of gradient descent in the lazy/perturbative regime. The main results are: for activations with a kink (class Jr, including ReLU, SELU, ELU), all eigenvalues of G(0) are polynomially large under mild data separation; for smooth activations (tanh, swish, polynomial activations), the minimum eigenvalue can be zero or exponentially small when the data span has low dimension; positive results are given for smoothed high-dimensional data and for sufficiently deep networks. The paper contains detailed appendix proofs, applications to training bounds, and experiments on synthetic data and a CIFAR-10 subset.

Significance. If the results hold as stated, the paper would provide the first quantitative eigenvalue bounds distinguishing smooth from nonsmooth activations in the NTK/Gram-matrix analysis of overparametrized networks. The proofs are built from standard tools (Hermite and Chebyshev approximation, Carbery-Wright anticoncentration, Gershgorin's theorem) and no constants are fitted to data; the predicted smooth-vs-kink dichotomy is falsifiable and is qualitatively supported by the included experiments. However, one of the headline quantitative claims, the ReLU lower bound of Theorem 4.2, appears to rest on an incorrect translation of Assumption 2, and the training-speed conclusions are explicitly valid only in the small-learning-rate lazy regime acknowledged by the authors.

major comments (2)
  1. [Appendix L, proof of Theorem L.2; Theorem 4.2] The proof of Theorem L.2 states that Assumption 2 implies |x_i^T x_j| ≤ 1−δ, but Assumption 2 gives |x_i^T x_j| ≤ sqrt(1−δ^2) = 1−Θ(δ^2) for unit vectors. Consequently, the threshold r0 should be Θ(log n / δ^2) rather than Θ(log n / δ), and the r^{−3/2} decay of the squared Hermite coefficients of ReLU′ yields λmin(G∞) ≥ Ω(δ^3 / log^{3/2} n), not Ω((δ/log n)^{1.5}). The neuron requirement m = Ω(n^4 δ^{−3} log^4 n) also needs to be re-derived with the corrected eigenvalue. Therefore Theorem 4.2, advertised as the best known ReLU bound, is not proved as stated; the kink-versus-smooth dichotomy may survive with weaker exponents, but the quantitative claim needs revision.
  2. [Section 4.2, Theorem 4.6] The abstract and introduction phrase the results as "leading to slow training" for smooth activations on low-dimensional data, but the formal transfer from eigenvalue bounds to training speed is proved only in the small-learning-rate perturbative regime: Theorem 4.6 requires η ≤ O(n^{−2} λmin(G(0))) and m ≥ Ω(n^5 κ^{−1} λmin(G(0))^{−4} ε^{−2}), and the text explicitly says that analyzing higher learning rates remains open. The claim should be qualified accordingly; as it stands, the eigenvalue bounds alone do not establish slow training outside the lazy regime.
minor comments (5)
  1. [Theorem 4.2] The displayed bound is malformed: it reads "Ω((δ1.5 log−1.5n)" and should be Ω((δ/log n)^{1.5}) (or the corrected exponent after the scaling issue described in the major comment).
  2. [Section 6, Theorem 6.4] The informal Hermite-approximation statement for tanh′ is attributed to "Theorem G.2", but Theorem G.2 in the appendix is the corresponding statement for swish; the correct reference appears to be Corollary F.7.2.
  3. [Section 2, notation] The notation "‖u‖2 := ‖u‖2^2 := ∑ u_i^2" defines the norm as the square of the norm; please use distinct symbols, e.g., ‖u‖_2 for the Euclidean norm and ‖u‖_2^2 for its square.
  4. [Section 7, Figure 2(b)] The caption says "Semilog plot of sum of squares of top k singular values" while the text says "cumulative sums of eigenvalues"; please make the caption consistent with the quantity plotted.
  5. [Theorem 4.4] The expression exp(−Ω(n^{1/2d′})) is ambiguous; it should be written as exp(−Ω(n^{1/(2d′)})) to make clear that the exponent is 1/(2d′).

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the eigenvalue bounds are derived from explicit spectral/approximation arguments, with no fitted input or self-citation chain carrying the central claims.

full rationale

The paper's central results are quantitative bounds on λmin(G(0)) for kinked and smooth activations. These are proved from the definition of the G-matrix (Eq. 2), Hermite/Chebyshev approximation theorems (Hille 1940; Boyd 1984), standard linear-algebra tools (Weyl, Gershgorin, Carbery-Wright), and concentration inequalities. No parameter is fitted to data and then relabeled as a prediction: the constants (δ, n, m) appear in theorem statements and are not tuned to match the experiments. The training-speed interpretation (Theorem 4.6) explicitly invokes the prior perturbative/lazy-training framework of Du et al. and Arora et al., and the paper openly acknowledges that higher learning rates remain outside the analysis; this is an honest reliance on published external frameworks, not a circular import of the target result. The only self-citation, Anderson et al. (2014) with coauthor Navin Goyal, supplies an anticoncentration argument for smoothed-analysis lower bounds; the mathematical content is the external Carbery-Wright inequality, and the citation is not load-bearing for the core kink-versus-smooth dichotomy. The skeptic's concern about the ReLU bound (using |x_i^T x_j| ≤ 1−δ instead of sqrt(1−δ^2) from Assumption 2) is a potential correctness/scaling error in a proof step, not a definitional equivalence or a fitted-input-as-prediction; even if true, it would weaken an exponent without making the derivation circular. Accordingly, the derivation chain is self-contained with respect to circularity.

Assumptions & free parameters 0 free parameters · 6 assumptions · 0 invented entities

The paper introduces no new free parameters fitted to data; the constants in the Jr conditions are normalized to 1 for simplicity. The main load-bearing external premises are the δ-separation data assumption and the lazy-training/NTK regime imported from prior work, which connects eigenvalue bounds to actual training speed.

assumptions (6)
  • domain assumption Data satisfies Assumption 1 and 2: unit norm and δ-separation
    All positive eigenvalue lower bounds scale polynomially in δ; without separation the bounds degrade or vanish. Stated in Section 2.
  • domain assumption The NTK/lazy training regime applies: G(t) stays close to G(0) and training speed is governed by λmin(G(0))
    Imported from Du et al. (2019a,b) and Arora et al. (2019c); connects eigenvalue bounds to convergence. The paper's Theorem 4.6 proves slow convergence only under a small learning rate constraint, and the paper notes higher learning rates remain open.
  • domain assumption Smoothed data assumption (Assumption 3): data points are Gaussian-perturbed within their span
    Used only for the positive results for smooth activations (Theorem 4.7); rules out non-robust low-dimensional spans.
  • standard math Lemma I.1 (from Oymak & Soltanolkotabi 2019) relating G∞ to Khatri-Rao powers and Hermite coefficients
    Cited lemma used to lower bound λmin via Hermite coefficient decay; not proven in the paper.
  • standard math Hille's theorem on Hermite series decay and Boyd's asymptotic for Hermite coefficients of tanh'
    Used to bound polynomial approximation error; classical analytic results.
  • domain assumption Activation function belongs to class Jr (derivative jump discontinuity of order r)
    Defines the scope of the positive results. The constants in the Jr conditions are normalized to 1 for simplicity, which is a modeling choice, not a fitted parameter.

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Pith. "Pith review of Effect of Activation Functions on the Training of Overparametrized Neural Nets." pith.science (2026). https://pith.science/paper/BGJ72746

@misc{pith2026190805660,
  author       = {Pith},
  title        = {Pith review of: Effect of Activation Functions on the Training of Overparametrized Neural Nets},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/BGJ72746}},
  note         = {Machine review of arXiv:1908.05660}
}
read the original abstract

It is well-known that overparametrized neural networks trained using gradient-based methods quickly achieve small training error with appropriate hyperparameter settings. Recent papers have proved this statement theoretically for highly overparametrized networks under reasonable assumptions. These results either assume that the activation function is ReLU or they crucially depend on the minimum eigenvalue of a certain Gram matrix depending on the data, random initialization and the activation function. In the later case, existing works only prove that this minimum eigenvalue is non-zero and do not provide quantitative bounds. On the empirical side, a contemporary line of investigations has proposed a number of alternative activation functions which tend to perform better than ReLU at least in some settings but no clear understanding has emerged. This state of affairs underscores the importance of theoretically understanding the impact of activation functions on training. In the present paper, we provide theoretical results about the effect of activation function on the training of highly overparametrized 2-layer neural networks. A crucial property that governs the performance of an activation is whether or not it is smooth. For non-smooth activations such as ReLU, SELU and ELU, all eigenvalues of the associated Gram matrix are large under minimal assumptions on the data. For smooth activations such as tanh, swish and polynomials, the situation is more complex. If the subspace spanned by the data has small dimension then the minimum eigenvalue of the Gram matrix can be small leading to slow training. But if the dimension is large and the data satisfies another mild condition, then the eigenvalues are large. If we allow deep networks, then the small data dimension is not a limitation provided that the depth is sufficient. We discuss a number of extensions and applications of these results.

Figures

Figures reproduced from arXiv: 1908.05660 by the authors.

Figure 1
Figure 1. Experiments on synthetic dataset (From left to right) (a)Rate of convergence of 2-layer [PITH_FULL_IMAGE:figures/full_fig_p010_1.png] view at source ↗
Figure 2
Figure 2. Experiments on a random subset of 104 images from CIFAR10 dataset: (a) L 2 -distances between all pairs of preprocessed images (b) Semilog plot of sum of squares of top k singular values of data matrix (c) Eigenvalue distribution of G-matrix at initialization (d) Convergence speed of 2 layer networks using different activation functions. 8 CONCLUSION In this paper we characterized the effect of activation function o… view at source ↗

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