REVIEW 4 major objections 4 minor 1 cited by
A Quasilinear Algorithm for Computing Higher-Order Derivatives of Deep Feed-Forward Neural Networks
T0 review · 4 major / 4 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read The paper claims that n-TangentProp computes exact high-order input derivatives of dense feed-forward networks in quasilinear time, reducing the exponential cost it attributes to autodifferentiation and making derivative-heavy PINN…
desk verdict A correct but well-known recurrence is marketed as a novel algorithm against a strawman baseline; the PINN engineering results are real, but the complexity claims do not survive contact with Taylor-mode AD. 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 Faà di Bruno's formula, the chain rule generalized to $n$-th derivatives of a composition. The paper inserts it into the TangentProp forward pass: instead of differentiating the network by building and traversing a computational graph, each activation layer directly produces all derivatives $(\xi^{(m)})$ of its input, and the weight layers combine them linearly. The sum over integer partitions of $n$ with Bell-polynomial coefficients $C_p$ is what couples lower-order derivatives into the $n$-th one, and the derivatives must be computed in increasing order because $\xi^{(n)}$ depends on all $\xi^{(k)}$ with $k<n$. The partition function $p(n)$ therefore sets the combinatorial size of each activation step, and its asymptotic behavior is exactly what turns the recurrence into a quasilinear algorithm.
What would settle it
Run a Taylor-mode automatic differentiation implementation that propagates truncated Taylor series layer by layer on the same dense feed-forward benchmark and compare its runtime for $d^n/dx^n f(x)$ with n-TangentProp across $n$ and $M$; a polynomial-in-$(M,n)$ runtime comparable to or faster than n-TangentProp's would falsify the paper's claim of an exponential-to-quasilinear improvement over autodifferentiation.
Extended reading notes
Core claim
The paper's central claim, stated on its own terms, is that applying Faà di Bruno's formula layer by layer converts the forward pass into a complete derivative computation. For each layer, the activation's $n$-th derivative is obtained by summing over all integer partitions of $n$: $(\xi^{(n)})^{\ell-1}_j = \sum_p C_p \sigma^{(|p|)}(a^\ell_j) \prod_{m=1}^n (\xi^{(m)})^\ell_j{}^{p_m}$, then the linear weights propagate it by $(\gamma^{(n)})^\ell_i = \sum_j w^\ell_{ij}(\xi^{(n)})^{\ell-1}_j$. Since the number of partition terms is the partition number $p(n)$, and the classical bound $p(n)=O(e^{\sqrt{n}}/n)$ holds, the total cost for a network with $M$ parameters is $O(n p(n) M)=O(e^{\sqrt{n}}M)$. The authors emphasize that the method is exact, computes only the input derivatives actually needed, and that autodifferentiation instead implicitly pays an $O(e^{\sqrt{n}}/n\,M^n)$ cost because it builds all partial derivatives.
Load-bearing premise
The speedup claim rests on the assumption that autodifferentiation must compute and store all partial derivatives with respect to every input and parameter, making $O(M^n)$ time and memory unavoidable; if a polynomial-time method such as Taylor-mode automatic differentiation already computes the same input derivatives, the exponential baseline being beaten is not the true state of the art.
Editorial extensions
If this is right
- Physics-informed training with Sobolev losses can use higher derivative orders, such as $m=4$ or more, at practical cost instead of stopping at one or two extra derivatives.
- The third and fourth self-similar smooth Burgers profiles become computable on a single GPU; the paper reports the third profile in under an hour and projects the autodifferentiation route to exceed 25 hours.
- Memory use drops from exponential, $O(M^n)$, to linear, $O(nM)$, so more derivatives fit on the same hardware than autodifferentiation can store.
- Because the method is exact, it can replace autodifferentiation for derivative computation inside a PINN training loop without changing the loss definition or the solution accuracy.
- For optimizers such as L-BFGS, which evaluate many forward passes during line search, the cheap forward pass compounds into large end-to-end training speedups.
Reading between the lines
- Inference: the asymptotic comparison is against reverse-mode computational-graph autodifferentiation as implemented in common deep learning libraries; Taylor-mode automatic differentiation, which propagates truncated Taylor series layer by layer, also computes input derivatives without materializing all $O(M^n)$ partials, so a direct benchmark against Taylor-mode AD would likely shrink the headlin
- Inference: the recurrence is stated for a scalar input and parameter-free activations, but the same Faà di Bruno summation should extend to mixed partial derivatives and multi-dimensional inputs, which would broaden the method to general PDE residuals.
- Inference: the practical crossover point depends on implementation constants; the reported advantage already appears around three derivatives for a small PINN architecture, and a compiled or fully vectorized implementation would likely make the method win at lower derivative orders and larger batch sizes.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes n-TangentProp, an extension of the TangentProp formalism that computes the n-th derivative of a dense feed-forward neural network with respect to its input in a single forward pass. The derivation applies Faà di Bruno's formula layer-by-layer, leading to the recurrence in Eq. (5b) and Algorithm 1, with a claimed runtime of O(e^{sqrt(n)} M) and memory O(nM). The authors contrast this with an alleged O(e^{sqrt(n)}/n M^n) cost for autodifferentiation, and they present empirical scaling experiments and an end-to-end PINN application to unstable self-similar Burgers profiles, with code available on GitHub.
Significance. If the quasilinear claim were correct, the algorithm would be a significant contribution to physics-informed neural networks, where high-order input derivatives are a bottleneck. The Faà di Bruno-based recurrence is exact and self-contained, and the released code is a concrete engineering artifact. The empirical forward-pass timings for moderate n are consistent with the paper's own complexity formula when compared against PyTorch reverse-mode autodifferentiation. However, the central theoretical significance is undermined by the fact that Taylor-mode forward automatic differentiation already computes input-only derivatives in polynomial time per layer; against that state of the art, the proposed algorithm is asymptotically slower, not faster. The paper's contribution is therefore best understood as an empirical acceleration over a specific implementation (PyTorch reverse-mode), not as a quasilinear alternative to automatic differentiation in general.
major comments (4)
- [Abstract; Section III-A] The claimed exponential baseline for autodifferentiation is not the relevant baseline for input-only derivatives. Taylor-mode forward automatic differentiation (univariate jets) propagates a truncated Taylor series through each elementary operation and computes d^n f/dx^n in O(n^2 M) time for a network with M parameters, without enumerating all partial derivatives. The paper's assertion that repeated autodifferentiation necessarily costs O(M^n) because there are O(M^n) n-th order partial derivatives conflates the number of possible derivatives with the cost of computing one specific input derivative. Against this true polynomial baseline, the proposed O(e^{O(sqrt(n))} M) runtime is asymptotically slower, so the central 'quasilinear instead of exponential' claim is unsupported and likely false. The authors should either compare against Taylor-mode AD explicitly or substantially reframe the contribution as an engineering acceleration over a particular reverse-mode implementation.
- [Section III-B, Eq. (5b)] The displayed recurrence is not index-consistent. The left-hand side is (xi^(n))^{ell-1}_j, while the right-hand side evaluates sigma and xi at layer ell (sigma^{(|p|)}(a^ell_j) and (xi^(m))^ell_j). Following Eq. (3b), the correct recurrence should use the layer-(ell-1) quantities, namely sigma^{(|p|)}(a^{ell-1}_j) and the derivatives of a^{ell-1}_j, not those of a^ell_j. As printed, the equations do not directly imply the update steps in Algorithm 1; the indexing convention must be corrected or explicitly defined before the derivation can be verified.
- [Section III-B, Hardy-Ramanujan bound] The Hardy-Ramanujan asymptotic is misstated. The correct classical bound is p(n) = O(exp(pi sqrt(2n/3))/n), which gives n p(n) = O(exp(2.565 sqrt(n))), not O(exp(sqrt(n))). Consequently the refined runtime O(np(n)M) is O(exp(2.565 sqrt(n)) M), and the term 'quasilinear' is inaccurate for a superpolynomial (though subexponential) function. Even with the corrected constant, the runtime is asymptotically worse than any polynomial in n, which further undermines the comparison with Taylor-mode AD.
- [Section IV-B and IV-C] The empirical evaluation compares the proposed method only against PyTorch reverse-mode autodifferentiation. Since the theoretical baseline is invalid, these experiments do not establish an improvement over the state of the art in higher-order AD. The observed speedups for n >= 3 may still support a narrower engineering claim, but the paper should clearly state that the comparison is against a specific implementation and should not imply that the algorithm outperforms all existing exact higher-order differentiation methods.
minor comments (4)
- [Section I, contributions] The phrase 'for it's implementation' should be 'for its implementation'.
- [Throughout] There are several typographical issues, including 'classifictation' in Section III-B and the broken formatting of 'Fa `a di Bruno' in the same section.
- [Section IV-C1 and Appendix A] The main text states that the fourth profile was 'able to compute' and reports timings, while Appendix A says the method 'failed to adequately converge' to the fourth profile. This contradiction should be resolved and clearly reported.
- [Algorithm 1] The variable names y, a, and the use of 'BELL' are only loosely described in the text; a short explanation of the data structures and of how the Bell coefficients are precomputed would improve reproducibility.
Circularity Check
No significant circularity: n-TangentProp is a direct Faà di Bruno forward-pass derivation; the exponential-autodiff baseline is a correctness concern, not a circular step.
full rationale
The derivation chain is self-contained. Section III-B starts from Faà di Bruno's formula (Eq. 4) and applies it to the layer activations, producing the recurrence (5a)–(5b); the claimed runtime O(n p(n) M) follows by counting the partition sum and the number of layers. No fitted parameter, target derivative value, or external benchmark is used to construct the formula, so the central claim is not circular. The paper's only self-citation ([7], Chickering et al.) appears as related work on self-similar Burgers perturbations and is not load-bearing. The genuinely weak step is Section III-A's assertion that autodifferentiation costs O(M^n) because there are O(M^n) n-th order partial derivatives; this conflates the number of possible partials with the cost of computing one input derivative and is contradicted by Taylor-mode forward AD, which computes input derivatives in polynomial time. That is a correctness/novelty risk, not circularity: the n-TangentProp recurrence does not assume the exponential baseline. The appendix's admitted limitations (not reproducing [40], not converging the fourth profile) are honest reports rather than circular reasoning. The Hardy–Ramanujan bound is also misstated as O(e^{√ n}/n) without the constant π√(2/3), but this is a mathematical error, not a circular step. Accordingly, no circular step can be exhibited and the score is 0.
Assumptions & free parameters
assumptions (4)
- standard math Faà di Bruno's formula applies to the composition of the network's smooth activation and linear layers
- domain assumption Automatic differentiation for the n-th input derivative has runtime O(M^n) and memory O(M^n)
- standard math Hardy-Ramanujan asymptotic p(n) = O(e^{sqrt(n)}/n)
- domain assumption The partition-sum product in Eq. (5b) costs O(|p|) and the Bell tables are precomputed
Cite this review
Pith. "Pith review of A Quasilinear Algorithm for Computing Higher-Order Derivatives of Deep Feed-Forward Neural Networks." pith.science (2026). https://pith.science/paper/BJLQXN6P
@misc{pith2026241209752,
author = {Pith},
title = {Pith review of: A Quasilinear Algorithm for Computing Higher-Order Derivatives of Deep Feed-Forward Neural Networks},
year = {2026},
howpublished = {\url{https://pith.science/paper/BJLQXN6P}},
note = {Machine review of arXiv:2412.09752}
}
abstract
The use of neural networks for solving differential equations is practically difficult due to the exponentially increasing runtime of autodifferentiation when computing high-order derivatives. We propose $n$-TangentProp, the natural extension of the TangentProp formalism \cite{simard1991tangent} to arbitrarily many derivatives. $n$-TangentProp computes the exact derivative $d^n/dx^n f(x)$ in quasilinear, instead of exponential time, for a densely connected, feed-forward neural network $f$ with a smooth, parameter-free activation function. We validate our algorithm empirically across a range of depths, widths, and number of derivatives. We demonstrate that our method is particularly beneficial in the context of physics-informed neural networks where \ntp allows for significantly faster training times than previous methods and has favorable scaling with respect to both model size and loss-function complexity as measured by the number of required derivatives. The code for this paper can be found at https://github.com/kyrochi/n\_tangentprop.
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