REVIEW 4 major objections 4 minor 37 references
Semi-Implicit Neural Ordinary Differential Equations
T0 review · 4 major / 4 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read The paper claims that splitting a neural ODE into a nonlinear part handled explicitly and a fixed linear part handled implicitly, with a discrete-adjoint backward pass, allows stable training of stiff systems at time steps two orders of…
desk verdict A practical semi-implicit neural ODE trainer with real speedups on stiff partitioned systems, but the headline claim is narrower than it looks: every experiment hard-codes the linear part. 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 additive split of the vector field into a nonlinear $G(u)$ and a linear $J u$, plus an IMEX-RK time integrator that advances $G$ explicitly and $Ju$ implicitly. Each implicit stage reduces to the constant-coefficient linear system in Eq. (12), $(I-\Delta t\,\tilde a_{ii}J)U^{(i)}=u_n+\Delta t\sum_{j<i}a_{ij}G^{(j)}+\Delta t\sum_{j<i}\tilde a_{ij}JU^{(j)}$, which avoids nonlinear Newton iteration altogether; with $J$ constant, the factorization of $I-\Delta t\,\tilde a_{ii}J$ is reused across stages, steps, and mini-batches. The backward pass is the discrete adjoint of the same IMEX-RK map (Theorem 4.1), giving a reverse-accurate gradient whose memory cost depends on the network, not on the number of solver iterations or time steps.
What would settle it
Run the GRAND Cora benchmark with the linear operator learned instead of fixed to $-I$ in Eq. (19), updating it every optimizer step so factorizations must be recomputed each epoch; if per-epoch time and loss no longer beat explicit Adams and RK4 by a large margin, then the reusable constant linear system, not IMEX stability alone, is the load-bearing mechanism and the method's generality is limited to problems with a known fixed linear split.
Extended reading notes
Core claim
The paper's central claim is that a learnable ODE $\frac{du}{dt}=G(u)+Ju$ can be trained stably and efficiently when $G$ is integrated explicitly and $Ju$ implicitly, using IMEX-RK time stepping and a reverse-accurate discrete adjoint. The linearity of the implicit term turns every implicit stage into the linear system $(I-\Delta t\,\tilde a_{ii}J)U^{(i)}=\text{known}$, whose coefficient matrix does not change across stages, time steps, or (when $J$ is fixed) mini-batches, so off-the-shelf linear solvers and reused factorizations become available during training. On the GRAND graph benchmark and on Kuramoto--Sivashinsky and viscous Burgers learning problems, the paper reports stable training at step sizes roughly two orders of magnitude larger than explicit Runge--Kutta methods permit, with far fewer network evaluations per epoch and successful training in cases where fully implicit methods blow up or fail to converge.
Load-bearing premise
The whole speed and stability advantage rests on the model's dynamics being expressible as a nonlinear part plus a fixed linear part, because without a fixed linear part there is no cheap reusable linear equation to solve.
Editorial extensions
If this is right
- Stiff neural ODEs that previously needed step sizes near $10^{-3}$ can use step sizes near $1$ in the tested problems, cutting per-epoch network evaluations from tens of thousands to tens or hundreds.
- On the GRAND graph benchmark, SINODE trains stably where explicit RK4 and explicit Adams diverge and where fully implicit Crank--Nicolson and implicit Adams blow up on Cora.
- For the Kuramoto--Sivashinsky and viscous Burgers learning tasks, SINODE remains trainable at fine grid resolutions where explicit methods require step sizes around $10^{-7}$ and fully implicit methods suffer nonlinear-solver divergence.
- The discrete adjoint computes gradients consistent with the discrete forward pass, so the large-step training is not biased by the discretization mismatch of continuous adjoint methods.
- When $J$ is fixed, one LU factorization is shared across every stage, time step, and mini-batch within an epoch, so training cost scales with network evaluations rather than with the cost of solving the linear systems.
Reading between the lines
- Beyond the tested benchmarks, any stiff learning problem with a known linear operator, such as a diffusion, Laplacian, or spectral term, could use the same split, but the paper does not demonstrate a general recipe for finding such splits in arbitrary neural networks.
- Allowing $J$ to be learned while held fixed for several epochs at a time would likely preserve most of the factorization-reuse benefit and extend SINODE to problems without a prescribed linear part; this is an extension, not a paper claim.
- Because each fixed factorization is reused more often as the grid resolution or the number of time steps grows, the speed advantage over explicit training should widen on larger problem sizes than those tested.
- The failures reported for fully implicit methods suggest that the practical bottleneck in stiff neural ODE training is the cost and fragility of Newton iterations on ill-conditioned systems, and that replacing only the stiff part with a linear implicit solve removes that bottleneck; this diagnosis is inferred from the reported blow-ups rather than proved.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes SINODE, a semi-implicit approach for training neural ODEs whose right-hand side can be additively split into a nonlinear part G(u) and a linear part J u. The forward pass uses IMEX-RK methods, treating G explicitly and the linear part implicitly, which reduces each stage to a linear system when J is constant. The backward pass uses a discrete adjoint (Theorem 4.1) to obtain reverse-accurate gradients. Experiments on graph classification with GRAND, and on learning the Kuramoto–Sivashinsky and viscous Burgers equations, show that SINODE can use much larger time steps than explicit methods and remains stable where fully implicit methods fail, yielding substantial speedups in training time.
Significance. If the central claim holds, SINODE provides a practical way to train stiff neural ODEs by leveraging well-understood IMEX time integrators and off-the-shelf linear solvers, with the advantage of reverse-accurate discrete adjoints. The paper includes a concrete algorithm, a stated adjoint formula, and a reproducible code repository, and the experiments demonstrate clear stability and efficiency gains on problems with a known linear part. The main caveat is that the method's applicability currently hinges on the availability of a constant linear-nonlinear split, which is supplied externally in every experiment; the paper's headline claim about general challenging neural ODEs is therefore broader than what is demonstrated.
major comments (4)
- [Abstract and Sec. 7 (Limitations)] The abstract claims SINODE 'can train challenging neural ODEs where both explicit methods and fully implicit methods are intractable.' This claim is not qualified by the requirement, evident from Secs. 5.1–5.3, that a constant linear part J must be known a priori or supplied externally (e.g., Eq. (19) for GRAND, the fixed finite-difference convolution for KS, and the fixed finite-difference layer for Burgers). The paper does not propose a procedure for discovering a suitable split when the dynamics are not already separated, and if J were learned, the reusable-factorization argument in Sec. 4.1 would weaken to at most once per training iteration. The Limitations section does not list this as a limitation, which makes the scope of the headline claim easy to overread. The authors should either provide a method for constructing the split or explicitly restrict the central claims to problems with a known constant linear part.
- [Sec. 5.1 (GRAND experiments)] The GRAND experiments report NFEs per epoch (Table 1) and show testing accuracy versus training time (Fig. 1), but no final test accuracy table is provided, and no standard deviations across multiple seeds are reported for any dataset or method. Given that graph classification results are typically reported as mean±std over seeds, the absence of such numbers makes it difficult to assess whether the observed accuracy differences are significant. The authors should add a table of final test accuracies with error bars for all methods on Cora, CoauthorCS, and Photo.
- [Tables 1 and 2; Sec. 5.1-5.3] The NFE comparisons exclude the cost of linear solves, which are central to IMEX and fully implicit methods. While the paper also shows wall-clock training-time curves (Figs. 1, 2, 4, 6), these are not tabulated, and the reader cannot easily compare the efficiency of the methods in a quantitative way. The claim that NFE is 'usually a good metric' (Sec. 5.1) needs support: for implicit methods, the per-step linear solve cost can dominate the NN evaluation cost, especially at higher grid resolutions. The authors should either justify NFE as a fair metric in the presence of linear solves or report per-epoch wall-clock times in a table for all datasets and step sizes.
- [Appendix A (Proof of Theorem 4.1)] Theorem 4.1 is the backbone of the training algorithm, but the proof in Appendix A is only carried out for explicit RK (Eqs. (22)–(28)); the extension to IMEX-RK is asserted in a single sentence without a derivation. Since the IMEX adjoint involves two coefficient tableaux and mixed explicit/implicit stage treatments, a complete derivation (or a precise reference to one) is needed to make the reverse-accuracy claim fully verifiable. Additionally, the appendix is titled 'Proof of Theorem 5.1' while the theorem in the main text is numbered 4.1.
minor comments (4)
- [Sec. 4.2, after Eq. (15)] The text says the same strategy as Sec. 4.1 can be used to solve the transposed linear system, but for a non-symmetric J the transpose is generally not the same matrix as the forward system, so the LU factorization from the forward pass cannot be reused directly; this should be clarified.
- [Sec. 4.1, Eq. (13)] The Kronecker-product expression in Eq. (13) is not fully clear: the identity matrix I_m acts on the batch dimension, but the product with (I - Δt a_ij J) seems to mix state and batch indices. The matrix-free reshape interpretation in Eq. (14) is correct, but the notation in (13) should be cleaned up to avoid confusion.
- [Appendix A, Eq. (26)] The adjoint equations in the ERK proof appear to have a minor indexing issue in the last line of Eq. (26) (the factor b_i is omitted on the right-hand side of the second equation). While the final implementation in Eq. (30) looks correct, the proof's intermediate formulas should be corrected for consistency.
- [Sec. 5 and Appendix C] The exact time-step sizes used for each method and dataset are mentioned in the text but are not collected in one place. A table summarizing the step sizes for explicit, implicit, and IMEX methods across GRAND, KS, and Burgers would improve reproducibility.
Circularity Check
No significant circularity: the SINODE derivation is self-contained and the efficiency gains are empirically benchmarked against external IMEX-RK and integrator baselines.
full rationale
The central construction is not a renamed fit or a self-citation chain. Eq. (10) defines the additive split du/dt = G(u) + J u, and Eqs. (11)-(12) specialize the standard IMEX-RK stage equations to the case where H is linear, giving the linear system (I - Delta t a_ii J) U = ... by direct algebra. The discrete adjoint in Eqs. (15)-(17) is derived in Appendix A from the stage equations via a Lagrangian calculation, not imported from a self-citation. Stability properties of IMEX-RK are attributed to the external numerical-analysis literature (Ascher-Ruuth-Spiteri; Kennedy-Carpenter) and then validated by direct comparisons of NFEs, time-to-accuracy, and loss trajectories against explicit RK/Adams and fully implicit Crank-Nicolson/Adams on GRAND, KS, and Burgers tasks. No fitted parameter is renamed as a prediction; J is supplied by the physics (identity in GRAND; fixed finite-difference operators in KS and Burgers) and is not tuned to reproduce the target metric. Self-citations to the authors' PNODE framework and checkpointing papers support implementation details and do not carry the load of either the theoretical derivation or the empirical claims. The main caveat is scope: the strongest general claim about 'challenging neural ODEs' relies on a known constant linear part, and that requirement is not listed in the Limitations section. This is a scope limitation, not circularity, because Eq. (12) and the stability comparison remain independently checkable without assuming the conclusion.
Assumptions & free parameters
assumptions (3)
- domain assumption The right-hand side of the ODE can be written as G(u) + J u with J linear and constant.
- standard math IMEX-RK methods have the stability and accuracy properties stated in the cited numerical analysis literature.
- ad hoc to paper Theorem 4.1 (the discrete adjoint for IMEX-RK) is correct as stated.
Cite this review
Pith. "Pith review of Semi-Implicit Neural Ordinary Differential Equations." pith.science (2026). https://pith.science/paper/VCBDYQBD
@misc{pith2026241211301,
author = {Pith},
title = {Pith review of: Semi-Implicit Neural Ordinary Differential Equations},
year = {2026},
howpublished = {\url{https://pith.science/paper/VCBDYQBD}},
note = {Machine review of arXiv:2412.11301}
}
read the original abstract
Classical neural ODEs trained with explicit methods are intrinsically limited by stability, crippling their efficiency and robustness for stiff learning problems that are common in graph learning and scientific machine learning. We present a semi-implicit neural ODE approach that exploits the partitionable structure of the underlying dynamics. Our technique leads to an implicit neural network with significant computational advantages over existing approaches because of enhanced stability and efficient linear solves during time integration. We show that our approach outperforms existing approaches on a variety of applications including graph classification and learning complex dynamical systems. We also demonstrate that our approach can train challenging neural ODEs where both explicit methods and fully implicit methods are intractable.
Figures
Figures from the paper (9 more)
Reference graph
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Reviewed August 11, 2026 · model on record in the stance chip above.
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