REVIEW 4 major objections 5 minor 32 references
Kolmogorov-Arnold Representation for Symplectic Learning: Advancing Hamiltonian Neural Networks
T0 review · 4 major / 5 minor · reviewed 2026-08-05 · deepseek-v4-flash
Pith's one-line read Univariate function sums let neural Hamiltonians hold energy longer
desk verdict KAN-HNN paper whose own tables contradict its energy-drift claim, and whose architecture comparison is confounded by unequal training setups. 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 Kolmogorov-Arnold Representation: any continuous multivariate function can be written as a sum of 2n+1 outer univariate functions applied to sums of n inner univariate functions. KAR-HNN uses this decomposition as the Hamiltonian, with univariate blocks for the inner and outer functions. The mechanism carries the argument by concentrating each variable's contribution into separate one-dimensional channels, which the paper says yields localized approximation and smoother gradient fields than a fully connected MLP; because the Hamiltonian is still one scalar function, the induced vector field J∇Hθ is automatically Hamiltonian and hence symplectic.
What would settle it
Run the same four benchmarks with the architecture as the only variable—same optimizer, same number of training steps, same parameter budget, same data—comparing MLP-HNN against KAR-HNN. If KAR-HNN's test loss and energy drift advantages disappear or reverse under matched settings, the claim that univariate localized approximation drives the improvement is falsified.
Extended reading notes
Core claim
The central claim is that replacing the MLP inside an HNN with a Kolmogorov-Arnold Representation improves both accuracy and long-term physical consistency. Concretely, KAR-HNN writes the Hamiltonian as Hθ(q,p) = Σ_r Φ_r(Σ_s φ_{r,s}(z_s)) and uses automatic differentiation to form ∂Hθ/∂p and −∂Hθ/∂q. Because every continuously differentiable Hamiltonian generates a symplectic flow, the model inherits energy conservation and phase-space volume preservation without extra constraints. The paper's experiments show this architecture achieving the lowest test MSE on all four benchmarks—roughly 28.6 versus 35.9 for spring-mass, 34.3 versus 38.5 for the pendulum, 0.281 versus 1.92 for the two-body p
Load-bearing premise
The central claim assumes the observed accuracy and stability gains come from replacing the MLP with univariate function sums, but the experiments change optimizer, training length, and network size at the same time, so that specific cause is not separately tested.
Editorial extensions
If this is right
- Learned dynamics are symplectic by construction: as long as the Hamiltonian is differentiable, trajectories preserve the phase-space 2-form, giving the same long-horizon energy behavior as classical Hamiltonian systems.
- The architecture needs only small univariate layers (2–15 nodes) and hundreds rather than thousands of training steps on these benchmarks, suggesting a lower parameter cost than MLP-based HNNs.
- Across the four benchmark systems, KAR-HNN reports the lowest test error in predicted derivatives, indicating better generalization to unseen trajectories.
- The results expose a trade-off: MLP-HNN often enforces near-perfect energy at the cost of poorer derivatives, while KAR-HNN sacrifices a little energy exactness for much better local predictive fidelity.
- Because the Kolmogorov-Arnold theorem covers continuous multivariate functions generally, the same construction extends in principle to higher-dimensional Hamiltonian systems than the four benchmarks shown.
Reading between the lines
- The reported comparisons change optimizer, iteration count, and layer size at the same time as the architecture, so the experiments do not isolate univariate locality as the cause of the gains; a matched ablation that varies only the architecture would settle that.
- If the architectural benefit holds under matched settings, KAR-HNN's small parameter count makes it a plausible backbone for Hamiltonian learning in high-dimensional or data-sparse settings, and its univariate channels could be inspected or symbolically regressed to recover interpretable energy terms.
- A natural extension is to weight the loss to interpolate between exact energy conservation and derivative accuracy, formalizing the trade-off the paper observes rather than treating it as a side effect.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript proposes KAR-HNN, a Hamiltonian Neural Network variant that replaces the usual MLP Hamiltonian with a KAN-style sum of univariate B-spline/function expansions. It argues that this localized, univariate representation better captures high-frequency and multi-scale dynamics, reduces energy drift, improves long-term stability, and preserves the symplectic form of Hamiltonian systems. The method is evaluated on four benchmark systems—spring–mass, simple pendulum, two-body, and three-body—against a baseline network and an MLP-based HNN, reporting train/test MSE and an energy-error metric.
Significance. If the empirical claims were established, the paper would provide a useful architectural improvement for Hamiltonian dynamics learning. The paper does present explicit Hamiltonian forms for the four benchmarks and reports mean-with-standard-deviation summaries, which is a helpful starting point for reproducibility. However, the central advertised advantage—reduced energy drift—is contradicted by the paper's own tables on two of the four benchmarks, and the comparisons are confounded by unequal training protocols. The theoretical section reproduces the standard fact that every differentiable scalar Hamiltonian generates a symplectic flow, so it does not isolate any KAR-HNN-specific benefit. As presented, the contribution is not supported.
major comments (4)
- [Abstract and §IV-C] The abstract and §IV-C claim that KAR-HNN reduces energy drift and improves long-term predictive stability relative to MLP-HNN. The paper's own Tables I and IV contradict this on two of the four benchmarks. In Table I (spring–mass), the MLP-HNN energy error is 0.376 ± 0.0798 while KAR-HNN is 1.63 ± 0.284, roughly 4× larger. In Table IV (three-body), HNN energy error is 2.97 ± 0.743 while KAR-HNN is 5.80 ± 1.54, roughly 2× larger. The claim as stated is therefore false on half of the reported benchmarks. The authors must either substantially weaken the claim or provide additional experiments that establish the advertised energy-drift reduction.
- [§V-A–D] The architecture comparison is confounded. KAR-HNN is trained with LBFGS for 200–4000 steps, uses 2–15 hidden nodes, and batch size 50, while the baseline and MLP-HNN use Adam for 2000–10000 steps, 200 hidden units, and different batch sizes. Any observed difference in test MSE or energy error could be due to optimizer, training budget, model capacity, or data handling rather than the univariate KAN-style representation. To support the central claim that KAR-HNN's architecture is responsible for superior accuracy, the authors need matched experiments or ablations in which only the architecture is varied and all training hyperparameters are held fixed.
- [§IV-C] The symplectic-preservation argument is tautological and does not distinguish KAR-HNN from MLP-HNN. The proof that X_H = J∇H generates a symplectic flow uses only the fact that H is a continuously differentiable scalar function; it holds identically for any MLP-based Hamiltonian as well. Thus the statement 'preserving the symplectic form' is not a KAR-HNN-specific property. The paper should either provide an architectural argument—for example, an approximation-theoretic or gradient-smoothness bound that depends on the univariate expansion—or explicitly acknowledge that both architectures share this property and focus the claimed advantage on approximation quality.
- [§IV-A] The use of the Kolmogorov–Arnold Representation is presented as a constructive advantage, but the paper provides no proof or analysis that the univariate decomposition yields better approximation of Hamiltonian dynamics, especially in high dimensions. The KAR theorem is existential, and the implemented B-spline KAN architecture is a heuristic approximation. All benchmarks are low-dimensional (2–6 dimensional phase spaces), so the claim of high-dimensional advantage is not tested. Without an approximation-error analysis or a high-dimensional experiment, the theoretical motivation remains unsupported.
minor comments (5)
- [Figure 1 caption, §III] The caption contains a self-referential artifact: 'I would like to check whether it is the good name of the caption of the graph.' This is clearly an editing leftover and should be removed before any formal submission.
- [§V-E] There is a typo at the end of the summary paragraph: 'priorities..' should be a single period.
- [§III] The text says 'Finally, Section II situates our approach...' but the preceding text is in Section III; this cross-reference appears incorrect.
- [References] Several references are incomplete or marked as forthcoming/in press without volume or page details (e.g., [12], [16], [17]). This makes verification difficult and should be corrected.
- [General] No code or data release is mentioned. Given the small scale of the experiments, releasing the data-generation and training code would greatly aid reproducibility.
Circularity Check
The empirical core is not circular, but the symplectic-preservation 'theoretical insight' is a generic Hamiltonian-flow property identical for MLP-HNN, so the KAR-HNN-specific claim reduces by construction.
-
self definitional
[Section IV-C, 'Symplectic Preservation']
"Any continuously differentiable scalar function H(z) generates a symplectic flow because ιXH (ω0) = dH implies LXH ω0 = 0. In the Kolmogorov–Arnold representation-based Hamiltonian Neural Network (KAR-HNN), the Hamiltonian Hθ(z) is composed of univariate building blocks but remains a single, continuous scalar function: ... Substituting Hθ into the Hamiltonian equations ensures ˙z = J ∇z Hθ(z), thereby making the induced flow symplectic."
The 'preservation of symplectic structure' is derived solely from Hθ being a continuously differentiable scalar function and the identity ˙z=J∇Hθ. It holds identically for the MLP-HNN baseline, whose Hθ is also a differentiable scalar. The Kolmogorov–Arnold univariate decomposition plays no role in the proof. Thus the claimed KAR-HNN-specific guarantee is equivalent, by construction, to the standard definition of a Hamiltonian flow; it cannot explain the paper's advertised reduction in energy drift or improved stability relative to MLP-HNN.
full rationale
The empirical part of the paper is self-contained: KAR-HNN and baselines are trained with a standard MSE loss on (q,p, qdot,pdot), and the reported train/test losses and energy errors are evaluated, not fitted parameters renamed as predictions. No load-bearing self-citation chain appears; the self-citations in the references are background and not used to justify the architecture. The central empirical comparison is, however, confounded by unequal training protocols (LBFGS vs Adam, 200–4000 vs 2000–10000 steps, 2–15 vs 200 hidden units), and the abstract's 'reducing energy drift' is contradicted by Tables I and IV where MLP-HNN has lower energy error than KAR-HNN. These are correctness risks, not circularity. The one genuine circularity-adjacent step is the theoretical 'symplectic preservation' contribution: it is a known property of every differentiable Hamiltonian flow, relabeled as a KAR-HNN advantage, so the architecture-specific claim reduces by construction. There is also an editing artifact in the Fig. 1 caption ('I would like to check whether it is the good name...'), which indicates lack of polish but has no scientific bearing. Overall score 4: one theoretical claim reduces by construction, while the empirical core retains independent content.
Assumptions & free parameters
free parameters (6)
- KAN univariate layer sizes (nodes per layer) =
2 (spring), 10/10 (two-body), 15/10 (three-body)
- B-spline order k =
5 (spring), 3 (pendulum, two-body, three-body)
- B-spline grid size =
2 (spring, three-body), 3 (two-body)
- Training steps and optimizer =
LBFGS 200 steps (spring), 4000 (two-body), 200 (three-body) vs Adam 2000-10000 for baselines
- Noise variance sigma^2 =
0.1, 0.05, 0.2
- Dataset sizes =
25/25 trajectories (spring, pendulum), 1000 (two-body, 80/20 split), 5000 (three-body)
assumptions (4)
- standard math Kolmogorov-Arnold Representation theorem (Kolmogorov 1957, Arnold 1957)
- standard math Any continuously differentiable scalar function H(z) on R^{2d} defines a Hamiltonian vector field J grad H whose flow preserves the standard symplectic form
- domain assumption B-spline basis functions can represent the univariate components
- domain assumption The benchmark systems are exactly Hamiltonian and noise is zero-mean Gaussian
Cite this review
Pith. "Pith review of Kolmogorov-Arnold Representation for Symplectic Learning: Advancing Hamiltonian Neural Networks." pith.science (2026). https://pith.science/paper/WMYYPHYB
@misc{pith2026250819410,
author = {Pith},
title = {Pith review of: Kolmogorov-Arnold Representation for Symplectic Learning: Advancing Hamiltonian Neural Networks},
year = {2026},
howpublished = {\url{https://pith.science/paper/WMYYPHYB}},
note = {Machine review of arXiv:2508.19410}
}
read the original abstract
We propose a Kolmogorov-Arnold Representation-based Hamiltonian Neural Network (KAR-HNN) that replaces the Multilayer Perceptrons (MLPs) with univariate transformations. While Hamiltonian Neural Networks (HNNs) ensure energy conservation by learning Hamiltonian functions directly from data, existing implementations, often relying on MLPs, cause hypersensitivity to the hyperparameters while exploring complex energy landscapes. Our approach exploits the localized function approximations to better capture high-frequency and multi-scale dynamics, reducing energy drift and improving long-term predictive stability. The networks preserve the symplectic form of Hamiltonian systems, and thus maintain interpretability and physical consistency. After assessing KAR-HNN on four benchmark problems including spring-mass, simple pendulum, two- and three-body problem, we foresee its effectiveness for accurate and stable modeling of realistic physical processes often at high dimensions and with few known parameters.
Figures
Figures from the paper (2 more)
Reference graph
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Reviewed August 5, 2026 · model on record in the stance chip above.
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