REVIEW 5 major objections 4 minor 18 references
Training NTK to Generalize with KARE
T0 review · 5 major / 4 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read Directly training the kernel of a deep network, using an estimator of test error, can produce a predictor that matches or beats the network's own output.
desk verdict Explicit NTK training with KARE is a new and plausible method with consistent empirical wins, but the missing theory for KARE as an optimized objective and some protocol issues in the benchmarks mean the central claim is not yet 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 machinery has three pieces. The data-dependent NTK $K(x,\tilde x;\theta_T)=\nabla_\theta f(x;\theta_T)^\top\nabla_\theta f(\tilde x;\theta_T)$ is the kernel induced by a trained finite-width network. KARE, the Kernel Alignment Risk Estimator, is a data-only approximation of expected squared error for kernel ridge regression, written in terms of the kernel matrix and ridge parameter: $\mathrm{KARE}(y,K(X,X),z)=\frac{1}{n} y^\top(\frac{1}{n}K(X,X)+zI)^{-2}y\,/\,\bigl(\frac{1}{n}\mathrm{tr}\bigl((\frac{1}{n}K(X,X)+zI)^{-1}\bigr)\bigr)^2$. The paper uses KARE as the objective for gradient steps on kernel parameters, producing the NTK-KARE predictor. The gradient-boosting decomposition of Proposition 1, $f(x;\theta_t)\approx f(x;\theta_T)+K(x,X;\theta_T)U_t$, is what ties the after-kernel's predictive power to the trained network, and it motivates dropping the DNN output entirely.
What would settle it
Track the KARE objective and fresh-sample test error jointly during kernel training on a fixed dataset; if there is a step where KARE decreases but held-out error increases substantially, then minimizing KARE is not minimizing generalization and the claimed mechanism fails.
Extended reading notes
Core claim
The paper's central claim is that the after-training neural tangent kernel $K(x,\tilde x;\theta_T)=\nabla_\theta f(x;\theta_T)^\top\nabla_\theta f(\tilde x;\theta_T)$ is the object that carries generalization in deep learning, and that it can be trained directly. Proposition 1 shows that once the NTK stabilizes, late-time gradient descent on the DNN is equivalent to kernel ridge regression on the residuals, so the network output is redundant once the after-kernel is known. The paper therefore proposes NTK-KARE, which trains the kernel parameters by minimizing KARE, a consistent estimator of the expected risk of kernel ridge regression, rather than by minimizing empirical risk of the network. Empirically, NTK-KARE matches or outperforms the trained DNN and the after-training NTK across synthetic data, MNIST, Higgs, and the UCI benchmark, and the authors argue this is a form of over-parameterized feature learning.
Load-bearing premise
The method requires that KARE remains a faithful estimate of test error while the kernel parameters are being optimized, not only for a fixed kernel; the paper does not prove this for finite-width NTKs.
Editorial extensions
If this is right
- The trained DNN's output function can be discarded: prediction can be carried by the after-training NTK or the KARE-trained version of it.
- The after-training NTK is improvable by direct optimization, so implicit kernel learning during empirical risk minimization is not the only route to good generalization.
- KARE-based kernel training applies beyond NTK to any heavily parameterized kernel, including deep kernel learning models.
- Kernel methods with many learnable parameters are feature learners, not just fixed-feature regressors.
- In settings where the after-NTK already matches the DNN, explicit kernel training can convert that tie into a consistent advantage.
Reading between the lines
- If the result extends to larger architectures and harder benchmarks, part of what is credited to end-to-end representation learning may be reproducible by direct kernel optimization, which would make generalization easier to audit.
- A useful stress test is to record true test error alongside KARE during kernel training: if the two ever move apart sharply, the objective is not doing what the paper assumes.
- KARE could be applied to kernel selection over random feature or deep feature maps, where parameter count is high but gradient flow is cheaper.
- One open scaling question the paper leaves implicit: whether NTK-KARE's advantage grows or shrinks with width and depth beyond the tested ranges (up to depth 5 and width 512 on UCI).
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes NTK-KARE, a method that explicitly trains the parameters of a neural tangent kernel by minimizing the Kernel Alignment Risk Estimator (KARE) rather than by minimizing empirical risk. The authors interpret the after-training NTK as a gradient booster (Proposition 1), then present experiments on simulated low-rank regression, MNIST, Higgs, and the UCI classification benchmark, claiming that NTK-KARE consistently matches or outperforms both the trained DNN and the after-NTK, and is competitive with Recursive Feature Machines on UCI.
Significance. If the central claim holds, the paper would provide evidence that explicit kernel learning can outperform end-to-end DNN training, and that the after-training NTK is the main event in network training. The paper is clearly written, includes a public code repository, and reports experiments across several settings. However, the validity of KARE as a training objective for a label-dependent, finite-width NTK is not established, and the UCI comparison is weakened by test-set hyperparameter selection and by mismatched loss functions. These issues affect the two most important empirical claims.
major comments (5)
- [Section 3.3, Eq. (7); Section 5] KARE's risk-estimation property is established for a fixed kernel matrix that does not depend on the labels y used to evaluate the estimator. In NTK-KARE, the kernel parameters are updated by minimizing KARE on the same y, so K(X,X;theta) becomes label-dependent; the leave-one-out / GCV-style argument no longer applies. Minimizing KARE may then reduce the KARE objective artificially without reducing true test error, especially when P is comparable to n. The paper acknowledges this gap in Section 5. I recommend adding a diagnostic experiment that tracks KARE and test risk over KARE-training iterations, and a variant in which KARE is computed on a label-split or on a hold-out subset, to show that KARE-based optimization tracks generalization rather than merely overfitting the objective.
- [Appendix A.2; Table 1] For the UCI experiments, the text states that for NTK and NTK-KARE the authors "report the highest out-of-sample accuracy obtained over the grid of hyperparameters." Selecting hyperparameters by test-set accuracy makes the reported averages, P90/P95, PMA, and Friedman ranks systematically optimistic, and it is unfair relative to baseline models whose hyperparameters were presumably selected by a validation procedure. The UCI comparison should be repeated using a validation-based selection rule, or the full grid results should be reported so that the selection protocol is transparent.
- [Section 4.3; Appendix A.2] The kernel methods and the DNN baseline are trained with different loss functions on classification tasks: NTK-KARE and after-NTK use MSE on {-1,1} (or one-hot) targets, while the DNN baseline uses cross-entropy. This confounds the comparison, since the models are optimizing different objectives and the reported accuracy gains could reflect the choice of training loss rather than the kernel-learning mechanism. Please train the DNN baseline on the same {-1,1} MSE objective, or report results for both losses.
- [Section 3.2, Proposition 1] The proof of Proposition 1 bounds the difference between the true dynamics and the frozen-kernel dynamics by epsilon * eta/n * (t-T) * sup ||ell_hat||. This is not O(epsilon) uniformly in t: for t arbitrarily large after stabilization, the error grows linearly in t-T. The proposition should be restated with a bounded horizon (t-T fixed) or with a more careful dependence on t; as written, the claimed O(epsilon) approximation is not justified for late-time gradient boosting.
- [Figures 1 and 2; Section 4] The experiments are repeated k=10 times, but the figures report only averages, with no error bars, standard deviations, or confidence intervals. Since the central claim is that NTK-KARE "consistently" outperforms DNNs and after-NTK, the reported averages alone do not support that claim; the authors should provide variance information or a paired-test analysis.
minor comments (4)
- [Section 5] The text attributes KARE to "Jacot et al. (2018)"; the correct reference is Jacot et al. (2020).
- [Section 3.2, proof of Proposition 1] In the displayed equation for the time derivative of the empirical loss, the kernel should be K(X,X;theta_t), not K(x,X;theta_t); as written, the expression mixes a test-point kernel with the training loss.
- [Section 3.3] The statement that NTK-KARE "is unaware of the parametric model f(x;theta)" is misleading: the kernel is constructed from the gradients of f, so the architecture and parameterization of f determine the search space. Please clarify.
- [Section 3.3, Eq. (8)] The ridge parameter in the final predictor is denoted lambda, while the KARE objective uses z_KARE; the relationship between these two regularization parameters should be stated explicitly, since the main text only defines lambda as a scaled trace.
Circularity Check
No significant circularity: KARE is used as an externally introduced risk estimator, and the central NTK-KARE comparison is not defined in terms of its own outputs.
full rationale
The paper's derivation chain is self-contained with respect to its central claim. The NTK-KARE estimator in Eq. (8) is obtained by gradient descent on KARE in Eq. (7), where KARE is explicitly taken from Jacot et al. (2020) as an external estimator of expected risk. The paper does not redefine the target in terms of the output, nor does it fit a parameter that is then renamed as a prediction. Proposition 1 derives the after-NTK as a gradient booster from a first-order Taylor expansion and the NTK definition, so Eq. (4) follows from Eqs. (1)-(2) rather than being assumed. Eq. (6) is presented as an empirical finding of prior work, not as a consequence of KARE. The only self-citation, Kelly et al. (2024), appears in Section 4.2 alongside Simon et al. (2023) to support the claim that ridge tuning matters for large-DNN kernels; it is not load-bearing for the main NTK-KARE result and is not used to define KARE or the predictor. Section 5's admission that rigorous theoretical guarantees for KARE-based training are left to future work is a limitation about the validity of KARE as an optimization objective, not a circular step: KARE remains an independently introduced quantity, and the experiments are benchmarked against out-of-sample performance rather than against KARE itself. No equation or fitted hyperparameter is renamed as a prediction, and no uniqueness claim is imported from the authors' own prior work.
Assumptions & free parameters
free parameters (4)
- zKARE =
0.1
- eta_KARE =
1, 10, or 100 depending on network depth
- lambda_tilde ridge multiplier =
grid 10^-5 to 10^5 for simulations; 10^-3 to 10^3 for MNIST and Higgs
- network depth and width =
depth in {1,2,4}, width in {32,64,128}
assumptions (4)
- domain assumption The NTK stabilizes at late training: ||K(x,X;theta_t)-K(x,X;theta_T)|| <= epsilon for all t >= T.
- domain assumption KARE(y,K(X,X),z) is a consistent approximation of the expected risk of the kernel ridge predictor.
- domain assumption For a suitably adjusted ridge, the residual kernel predictor approximates a plain kernel predictor: f(x;theta_T) + K(x,X)(zI+n^-1K)^-1(y-f) is approximately n^-1 K(x,X)(z~I+n^-1K)^-1 y.
- standard math The DNN output f is differentiable, the loss derivative is continuous, sublevel sets of the loss are bounded, and the loss is bounded below.
Cite this review
Pith. "Pith review of Training NTK to Generalize with KARE." pith.science (2026). https://pith.science/paper/QHMP4W2Q
@misc{pith2026250511347,
author = {Pith},
title = {Pith review of: Training NTK to Generalize with KARE},
year = {2026},
howpublished = {\url{https://pith.science/paper/QHMP4W2Q}},
note = {Machine review of arXiv:2505.11347}
}
read the original abstract
The performance of the data-dependent neural tangent kernel (NTK; Jacot et al. (2018)) associated with a trained deep neural network (DNN) often matches or exceeds that of the full network. This implies that DNN training via gradient descent implicitly performs kernel learning by optimizing the NTK. In this paper, we propose instead to optimize the NTK explicitly. Rather than minimizing empirical risk, we train the NTK to minimize its generalization error using the recently developed Kernel Alignment Risk Estimator (KARE; Jacot et al. (2020)). Our simulations and real data experiments show that NTKs trained with KARE consistently match or significantly outperform the original DNN and the DNN- induced NTK (the after-kernel). These results suggest that explicitly trained kernels can outperform traditional end-to-end DNN optimization in certain settings, challenging the conventional dominance of DNNs. We argue that explicit training of NTK is a form of over-parametrized feature learning.
Figures
Reference graph
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Reviewed August 15, 2026 · model on record in the stance chip above.
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