REVIEW 4 major objections 3 minor 43 references
Is the neural tangent kernel of PINNs deep learning general partial differential equations always convergent ?
T0 review · 4 major / 3 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read The paper proves that the PINN neural tangent kernel converges to a deterministic limit only when the output scaling $1/N^s$ has $s$ large enough—$s\ge1$ for non-homogeneous operators and $s\ge s_1$ for homogeneous ones—so the standard…
desk verdict A plausible and experimentally supported scaling rule for PINN initialization, but the main theorem rests on an unproven law-of-large-numbers step; worth refereeing with a clear demand for a rigorous proof. 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 central object is the PINN kernel of (15), assembled from gradients of the operator $F[q]$ and the boundary trace $q_x^m$ with respect to all network parameters, with the network output scaled by $1/N^s$. The argument decomposes $\partial F/\partial\theta$ as $\sum_i F_i[q,q_x,\dots]\,\partial q_{ix}/\partial\theta$ and writes each kernel entry as a sum over the $N$ neurons; the homogeneity definition (33) assigns each monomial a degree $\sum_l t_{i_l}$, and the proof balances powers of $N$—the factor $N^{1-2s}$ coming from the sum against the degree-dependent growth of the $F_i$—so that the law of large numbers yields a deterministic limit. The threshold $s_1$ in (39) is exactly the exponent at which that balance holds.
What would settle it
Recompute the KdV threshold from formula (39): the monomial $q q_x$ has homogeneity degree $2$, so the formula gives $s_1 = (2\cdot2+1)/(2\cdot2+2) = 5/6$, whereas the paper states $3/4$. Simulating the initialized NTK of the KdV equation at a sequence of widths with scaling exponents $s=0.75$ and $s=0.83$ and checking whether $\|K_{N+k}-K_N\|_2$ decays in probability will show which threshold the dynamics actually follow.
Extended reading notes
Core claim
Theorem 2.2 states that for the two-layer network (13) with output scaling $1/N^s$, the PINN kernel (15) converges in probability to a deterministic limiting kernel only above certain scaling thresholds. In case A—when $F_0[q,q_x,\dots,q_{nx}]F_0[\hat q,\hat q_x,\dots,\hat q_{nx}]\ne 0$, or when some monomial in the expansion $\partial F/\partial\theta = \sum_i F_i \,\partial q_{ix}/\partial\theta$ is non-homogeneous—the threshold is $s\ge 1$. In case B—when that product vanishes and every monomial is homogeneous—the threshold is $s\ge s_1$, where $s_1 = \frac{2\sum_l t_{i'_l}+1}{2\sum_l t_{i'_l}+2}$ and $\sum_l t_{i'_l}$ is the maximum homogeneity degree of the operator, with a secondary exponent $s_2$ for the cross-block $K_{fb}$. The standard choice $s=1/2$ therefore does not universally deliver convergence; the paper identifies the sine-Gordon equation, with the non-homogeneous term $\sin(q)$, as a case-A example and the KdV equation, with the quadratic term $q q_x$, as a case-B example, reporting thresholds $s_1=3/4$, $s_2=2/3$ for KdV. Theorem 2.3 supplements this by proving that, under uniform boundedness of parameters, operator derivatives, and integrated residuals, the NTK stays asymptotically constant during training whenever $s>1/4$.
Load-bearing premise
The load-bearing premise is that the sums over neurons making up the kernel entries obey a law of large numbers toward deterministic limits, even though the summed terms contain the operator coefficients $F_i$, which are nonlinear functions of the whole random network output; no concentration bound is given for those dependent, nonlinear terms.
Editorial extensions
If this is right
- Under the paper's result, NTK-based convergence analyses of PINNs that silently assume $s=1/2$ do not transfer to general nonlinear PDEs; a practitioner must either raise the output scaling to $s\ge 1$ or first verify that the operator is homogeneous and compute its maximum degree.
- For homogeneous operators such as KdV, there is a window $s_1 \le s < 1$ in which the initialized kernel converges even though the classical scaling does not, so kernel convergence at initialization and kernel frozenness during training are governed by different exponents ($s_1$ versus $1/4$).
- The homogeneity degree of the differential operator becomes a computable design parameter: operators with higher-degree nonlinear terms demand larger scaling exponents, so the same architecture may need different $s$ for different PDEs.
- The theorem gives a sufficient condition for the kernel gradient-flow description (5) to be well posed, which is a prerequisite for using NTK-based diagnostics to explain or fix PINN training failures.
Reading between the lines
- A rigorous proof of the sharp thresholds would require concentration bounds: the law-of-large-numbers step in the proof averages terms that are nonlinear, dependent functions of all $N$ random parameters, so the thresholds $s_1,s_2$ are best treated as scaling predictions until such bounds are supplied.
- Formula (39) and the paper's stated KdV threshold $3/4$ are in tension if the quadratic monomial $q q_x$ is assigned homogeneity degree $2$, since the formula then gives $s_1=5/6$; a direct check of which value the KdV kernel follows would settle the discrepancy.
- The same balancing argument suggests a practical design rule beyond the paper's examples: compute the maximum homogeneity degree of the PDE's nonlinear terms and set $s$ accordingly, which would also apply to other homogeneous equations such as Burgers' equation.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the neural tangent kernel (NTK) of physics-informed neural networks (PINNs) for general nonlinear PDEs. It introduces an output-layer scaling 1/N^s and claims that the initialized NTK converges in probability to a deterministic kernel only above thresholds that depend on the homogeneity of the differential operator (Theorem 2.2): s >= 1 for non-homogeneous operators (case A) and s >= s1 for homogeneous operators (case B), with s1 defined in Eq. (39). It further claims that the NTK stays constant during training for any s > 1/4 (Theorem 2.3). The claims are tested on the sine-Gordon and KdV equations by plotting ||K_{N+k} - K_N|| for widths up to 50000 at s = 0.5 and s = 1. The paper reports divergence at s = 0.5 and convergence at s = 1, and interprets this as confirmation of the theory.
Significance. If the quantitative thresholds were correct, the paper would provide useful design guidance for PINN initialization and identify a genuine limitation of standard NTK scaling for nonlinear PDEs. The basic qualitative phenomenon is plausible and the numerical experiments, while limited to two equations and a proxy quantity, show a clear difference between s = 0.5 and s = 1. The paper also has a useful negative message: the standard NTK convergence argument does not automatically extend to nonlinear PDEs. However, the theoretical derivation of the thresholds is the load-bearing part of the manuscript, and it contains substantial gaps: the law-of-large-numbers step is not justified for dependent nonlinear terms, the scaling analysis appears to ignore the N^{1/2} fluctuations of the hidden-layer sum, and the norm estimates used in the proof of Theorem 2.3 are not valid for the claimed range of s. I therefore cannot regard the main quantitative claims as established.
major comments (4)
- [Appendix B, Eqs. (31)-(40)] The proof of Theorem 2.2 invokes the law of large numbers for sums whose summands are not independent. In the expression for K_ff, the coefficients F_i[q,q_x,...] are nonlinear functions of q(x,theta), which is itself a sum over all N hidden units, so the summands indexed by k are dependent. No variance or concentration bound is supplied for these terms. This is not a cosmetic issue: for s = 1/2 and F[q] = sin(q), q converges to a nondegenerate Gaussian process, and cos(q(x))cos(q(hat x)) has positive variance, so a deterministic limiting kernel cannot exist. For s > 1/2 a Slutsky-type argument might be possible, and for the homogeneous case a delta-method argument controlling the normalized fluctuations of F_i would be needed. Since neither is provided, the thresholds s1 and s2 in Theorem 2.2 are not proved as stated.
- [Appendix B, Eqs. (34), (39)-(40)] The scaling analysis appears to neglect the N^{1/2} fluctuation of the sum defining q. For mean-zero iid summands, q(x,theta) = N^{-s} sum_k W^{(1)}_k sigma(...) is of order O_P(N^{1/2-s}), not O_P(N^{-s}). For a homogeneous coefficient F_i of degree d = sum_l t_{il}, this gives F_i = O_P(N^{d(1/2-s)}), and the corresponding K_ff contribution has size N^{1+d-2s(d+1)}. Balancing this yields s = 1/2, not the formula s1 = (2d+1)/(2d+2) in Eq. (39). Consequently, the claimed KdV thresholds s1 = 3/4 and s2 = 2/3 do not follow from the displayed scaling; for the homogeneous terms in KdV the contribution would already vanish for every s > 1/2. The experiments only test s = 0.5 and s = 1.0, which cannot distinguish the paper's thresholds from the simpler condition s > 1/2.
- [Appendix C, proof of Theorem 2.3] The proof of Theorem 2.3 uses Frobenius-norm estimates that are not sufficient for the claimed range of s. Lemma C.1 gives a uniform bound O(N^{-s}) on each entry of the Jacobian, but the Jacobian has 3N+1 columns, so ||J(t)||_F = O(N^{1/2-s}), which diverges for s < 1/2. The step following Eq. (72), which treats ||J(t)||_F as bounded for all s > 1/4, is therefore unjustified. Similarly, the bound ||J(t)-J(0)||_F = O(N^{1/2-2s}) leads to a product in Eq. (72) that vanishes under these estimates only for s > 1/3, and a spectral-norm or concentration argument would be needed to reach s > 1/4. As written, Theorem 2.3 is not established for s in (1/4, 1/2].
- [Sec. 3.2 and Fig. 5] The experimental validation of Theorem 2.3 is internally inconsistent: the text states that the convergence coefficient s is set to 0.3, while the caption of Fig. 5 says s = 0.2. Since 0.2 is below the claimed threshold 1/4, the figure as captioned would not validate the theorem. The authors should correct this mismatch and, more importantly, provide experiments at several values of s in the claimed critical ranges (for example s = 0.55, 0.6, 0.7 for the KdV equation) to distinguish between their thresholds and the simpler s > 1/2 condition.
minor comments (3)
- [Theorem 2.2 statement] The statement of Theorem 2.2 says the kernel converges in probability to a deterministic limiting kernel K*, but K* is never defined in the theorem; it appears only implicitly in the proof. The statement should be made precise.
- [Sec. 1, Fig. 1 and Sec. 3] The paper uses ||K_{N+k} - K_N|| as a proxy for ||K_N - K*||. Convergence of the former is necessary but not sufficient for convergence to a fixed limit, and this caveat should be stated more carefully in the interpretation of the experiments.
- [Throughout] There are several typos and minor inconsistencies, including 'Theorem 3' in Sec. 3.2 instead of Theorem 2.3, and the LaTeX corruption in Eq. (59). The notation in Remark 1 is also difficult to parse because the case distinction between 'F0 ... != 0' and 'every monomial is homogeneous' is not presented cleanly.
Circularity Check
No significant circularity: the s-thresholds derive from operator homogeneity (Eqs. 33-39), the experiments validate rather than fit the theory, and self-citations are contextual; the unproven law-of-large-numbers step is a proof gap, not a circular reduction.
full rationale
The derivation chain is self-contained. The central thresholds of Theorem 2.2 are computed analytically from the differential operator's homogeneity: Eq. (33) defines homogeneity of the coefficients F_i, and the critical exponents s1 (Eq. (39)) and s2 (Eq. (47)) are algebraic functions of the maximal homogeneity degree only. For the KdV operator (22), the degree-1 coefficients F0 = 6q_x and F1 = 6q give s1 = 3/4, matching the paper's value; the value is consistent with Eq. (39) as written, which uses the degree of the coefficient F_i rather than of the monomial q q_x. Nothing in the sine-Gordon or KdV experiments feeds back into this computation, so the predictions (divergence at s = 1/2, convergence at s >= 1 in case A and s >= 3/4 in case B) are not fitted inputs renamed as predictions. The 1/N^s scaling is transparently introduced at Eq. (13) as a modification of the standard initialization, not smuggled via citation, and no uniqueness theorem is imported from prior work. Self-citations (refs [28] and [30]) are application examples in the introduction and do not carry the proof chain. The genuinely fragile loci are proof gaps rather than circular reductions: Appendix B (Eqs. (31)-(40)) invokes "the law of large numbers" for sums whose summands include F_i[q] dq/dtheta_k dq_hat/dtheta_k, where F_i depends on all N hidden units through q, and no variance or concentration bound for these dependent, nonlinear terms is supplied; and Theorem 2.3's assumption (iii), Eq. (20), bounds the time-integral of residuals as an explicit premise rather than a derived fact. The Figure 5 caption (s = 0.2) also disagrees with the Section 3.2 text (s = 0.3) for the KdV training experiment, which weakens that validation but is not circular. Under the analytical rules, an omitted or unjustified proof step is a correctness risk, not circularity, because the theorem's claim does not reduce by construction to its own inputs. No circular step is therefore identified; the score of 2 merely records minor non-load-bearing self-citations.
Assumptions & free parameters
free parameters (2)
- s (convergence coefficient) =
Not fitted; chosen by the user (0.5, 1, 0.25, 0.3, etc.)
- alpha, beta (loss weights) =
unspecified
assumptions (5)
- ad hoc to paper The law of large numbers applies to the kernel sums even when the operator coefficients F_i are nonlinear functions of the network output q.
- domain assumption Network parameters remain uniformly bounded during training (Assumption (i) of Theorem 2.3).
- domain assumption All derivatives of the PDE operator up to order n are uniformly bounded along the training trajectory (Assumption (ii) of Theorem 2.3).
- domain assumption The activation function is smooth and all its derivatives are bounded (Assumption (iv) of Theorem 2.3).
- ad hoc to paper The output-layer bias b^(1) is deterministic (implicitly zero) so that the limiting kernel is deterministic.
Cite this review
Pith. "Pith review of Is the neural tangent kernel of PINNs deep learning general partial differential equations always convergent ?." pith.science (2026). https://pith.science/paper/L5GLRHTI
@misc{pith2026241206158,
author = {Pith},
title = {Pith review of: Is the neural tangent kernel of PINNs deep learning general partial differential equations always convergent ?},
year = {2026},
howpublished = {\url{https://pith.science/paper/L5GLRHTI}},
note = {Machine review of arXiv:2412.06158}
}
read the original abstract
In this paper, we study the neural tangent kernel (NTK) for general partial differential equations (PDEs) based on physics-informed neural networks (PINNs). As we all know, the training of an artificial neural network can be converted to the evolution of NTK. We analyze the initialization of NTK and the convergence conditions of NTK during training for general PDEs. The theoretical results show that the homogeneity of differential operators plays a crucial role for the convergence of NTK. Moreover, based on the PINNs, we validate the convergence conditions of NTK using the initial value problems of the sine-Gordon equation and the initial-boundary value problem of the KdV equation.
Figures
Figures from the paper (2 more)
Reference graph
Works this paper leans on
- [1]
-
[2]
I. Goodfellow , Y . Bengio, A. Courville, Deep Learning (M IT Press, 2016)
work page 2016
-
[3]
M. Dissanayake, N. Phan-Thien, Neural-network-based a pproximations for solving partial differential equations , Commun. Numer. Methods Eng. 10 (1994) 195-201
work page 1994
-
[4]
I. E. Lagaris, A. Likas, D. I. Fotiadis, Artificial neural networks for solving ordinary and partial differential equ ations, IEEE Trans. Neural Netw . 9 (1998) 987-1000. 16
work page 1998
-
[5]
Cybenko, Approximation by superpositions of a sigmoi dal function, Math
G. Cybenko, Approximation by superpositions of a sigmoi dal function, Math. Control Signal Systems, 2 (1989) 3
work page 1989
-
[6]
K. Hornik, Approximation Capabilities of Muitilayer Fe edforward Networks, Neural Networks, 4 (1991) 251-257
work page 1991
-
[7]
J. Sirignano, K. Spiliopoulos, DGM: a deep learning algo rithm for solving partial differential equations, J. Compu t. Phys. 375 (2018) 1339-1364
work page 2018
- [8]
Show all 43 references
-
[9]
W. E, B. Yu, The Deep Ritz Method: A Deep Learning-Based Nu merical Algorithm for Solving Variational Problems, Com- mun. Math. Stat. 6 (2018) 1-12
2018
-
[10]
Z. Long, Y . Lu, X. Ma, B. Dong, PDE-Net: Learning PDEs fro m Data, PMLR 80 (2018) 3208-3216
2018
-
[11]
Z. Long, Y . Lu, B. Dong, PDE-Net 2.0: Learning PDEs from d ata with a numeric-symbolic hybrid deep network, J. Comput. Phys. 399 (2019) 108925
2019
-
[12]
Bar-Sinaia, S
Y . Bar-Sinaia, S. Hoyerb, J. Hickeyb, and M. P . Brennera , Learning data-driven discretizations for partial differ ential equa- tions, PNAS, 116 (2019) 15344-15349
2019
-
[13]
Y . Wang, Z. Shen, Z. Long and B. Dong, Learning to Discretize: Solving 1D Scalar Conservation Laws via Deep Reinforcement Learning, (2019) arXiv:1905.11079
2019 arXiv
-
[14]
L. Lu, P . Jin, and G. E. Karniadakis, DeepONet: Learning nonlinear operators for identifying differential equatio ns based on the universal approximation theorem of operators, (2019) a rXiv:1910.03193
2019 arXiv
-
[15]
Z. Li, N. Kovachki, K. Azizzadenesheli, B. Liu, K. Bhatt acharya, A. Stuart, and A. Anandkumar, Fourier Neural Opera tor for parametric partial differential equations, (2021) arXiv: 2010.08895v2
2021 arXiv
-
[16]
K. Wu, D. Xiu, Data-driven deep learning of partial diff erential equations in modal space, J. Comput. Phys. 408 (2020) 109307
2020
-
[17]
Y . Khoo, J. Lu and L. Ying, Solving parametric PDE proble ms with artificial neural networks, Euro. J. Appl. Math. 32 (2 021) 421-435
-
[18]
G. M. Rotskoff, E. Vanden-Eijnden, Learning with rare d ata: using active importance sampling to optimize objectiv es domi- nated by rare events. PMLR 145 (2022) 757-780
2022
-
[19]
D. Pfau, J. S. Spencer, A. G. Matthews, W. M. C. Foulkes, A b initio solution of the many electron Schr ¨ odinger equation with deep neural networks. Phys. Rev . Res. 2 (2020) 033429
2020
-
[20]
Goswamia, M
S. Goswamia, M. Yinb, Y . Yu, G. E. Karniadakis, A physics -informed variational DeepONet for predicting crack path i n quasi-brittle materials, Comput. Methods Appl. Mech. Engr g. 391 (2022) 114587
2022
-
[21]
S. Lin, Y . Chen, A two-stage physics-informed neural ne twork method based on conserved quantities and application s in localized wave solutions, J. Comput. Phys. 457 (2022) 11105 3
2022
-
[22]
S. Lin, Y . Chen, Physics-informed neural network metho ds based on Miura transformations and discovery of new local ized wave solutions, Physica D 445 (2023) 133629
2023
-
[23]
J. Li, Y . Chen, A deep learning method for solving third- order nonlinear evolution equations, Commun. Theor. Phys. 72 (2020) 115003
2020
-
[24]
J. Li, J. Chen, B. Li, Gradient-optimized physics-info rmed neural networks (GOPINNs): a deep learning method for s olving the complex modified KdV equation, Nonlinear Dyn. 107 (2022) 781-792
2022
-
[25]
Z. Miao, Y . Chen, VC-PINN: Variable Coefficient Physica l Information Neural Network For Forward And Inverse PDE Problems with Variable Coefficient, arXiv preprint arXiv:2 305.07479
-
[26]
J. Pu, Y . Chen, Complex dynamics on the one-dimensional quantum droplets via time piecewise PINNs, Physica D 454 (2023) 133851
2023
-
[27]
L. Wang, Z. Yan, Data-driven peakon and periodic peakon solutions and parameter discovery of some nonlinear disper sive equations via deep learning, Physica D 428 (2021) 133037
2021
-
[28]
Z. Zhou, Z. Yan, Deep learning neural networks for the th ird-order nonlinear Schr?dinger equation: bright soliton s, breathers, and rogue waves, Commun. Theor. Phys. 73 (2021) 1 05006
2021
-
[29]
J. Song, Z. Yan, Deep learning soliton dynamics and comp lex potentials recognition for 1D and 2D PT-symmetric satur able nonlinear Schr ¨ odinger equations, Physica D 448 (2023) 133729
2023
-
[30]
Zhong, S
M. Zhong, S. Gong, S.-F. Tian, Z. Yan, Data-driven rogue waves and parameters discovery in nearly integrable PT-sym metric Gross-Pitaevskii equations via PINNs deep learning, Physi ca D 439 (2022) 133430
2022
-
[31]
Shuning Lin a, Yong Chen, A two-stage physics-informed neural network method based on conserved quantities and app li- cations in localized wave solutions, J. Comput. Phys. (2022 )
2022
-
[32]
S., Li, Z., Salakhutdinov , R., Wang, R
Arora, S., Du, S. S., Li, Z., Salakhutdinov , R., Wang, R. , and Yu, D. (2019c). Harnessing the power of infinitely wide d eep nets on small-data tasks. arXiv preprint arXiv:1910.01663
2019 arXiv
-
[33]
Y . Shin, J. Darbon, G. E. Karniadakis, On the convergenc e of physics informed neural networks for linear second-ord er elliptic and parabolic type PDEs, Commun. Comput. Phys. 28 ( 2020) 2042-2074
2020
-
[34]
T. Luo, H. Yang, Two-layer neural networks for partial d ifferential equations: optimization and generalization t heory . (2020) arXiv:2006.15733
2020 arXiv
-
[35]
Jacot, F
A. Jacot, F. Gabriel, C. Hongler, Neural tangent kernel : convergence and generalization in neural networks. Adv . Neural Inf. Process. Syst. 31 (2018) 8571-8580
2018
-
[36]
Martens, A
J. Martens, A. Ballard, G. Desjardins, G. Swirszcz, V . D alibard, J. SohlDickstein, and S. S. Schoenholz, Rapid trai ning of deep neural networks without skip connections or normali zation layers using deep kernel shaping. arXiv preprint arXiv:2110.01765, (2021). 17
2021 arXiv
-
[37]
Dyer and G
E. Dyer and G. Gur-Ari, Asymptotics of wide networks fro m feynman diagrams. In International Conference on Learnin g Representations. (2020)
2020
-
[38]
Meanti, L
G. Meanti, L. Carratino, L. Rosasco, and A. Rudi, Kernel methods through the roof: handling billions of points effici ently . arXiv preprint arXiv:2006.10350.(2020)
2020 arXiv
-
[39]
Arora, S
S. Arora, S. S. Du, W. Hu, Z. Li, R. R. Salakhutdinov , and R . Wang, On exact computation with an infinitely wide neural ne t. In Advances in Neural Information Processing Systems, (201 9b) pages 8141-8150
-
[40]
S.Wang, X. Yu, P . Perdikaris, When and why PINNs fail to t rain: a neural tangent kernel perspective. J. Comput. Phys. 449 (2022) 110768
2022
-
[41]
Tancik, P
M. Tancik, P . P . Srinivasan, B. Mildenhall, S. Fridovic h-Keil, N. Raghavan, U. Singhal, R. Ramamoorthi, J. T. Barro n, and R. Ng, Fourier features let networks learn high frequenc y functions in low dimensional domains. arXiv preprint arXiv:2006.10739.(2020)
2020 arXiv
-
[42]
S. Wang, H. Wang, P . Perdikaris, On the eigenvector bias of Fourier feature networks: From regression to solving mul ti-scale PDEs with physics-informed neural networks, Comput. Metho ds Appl. Mech. Engrg. 384 (2021) 113938
2021
-
[43]
M. J. Ablowitz, P . A. Clarkson, Solitons, Nonlinear Evo lution Equations and Inverse Scattering (Cambridge Univer sity Press, Cambridge, 1991). 18
1991
Reviewed August 11, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.