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REVIEW 3 major objections 5 minor 86 references

The paper claims that the continuous-time trajectory of an optimizer can be learned from a short window of its own history, then integrated forward as a cheap surrogate that skips expensive objective and gradient evaluations while reaching

Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →

T0 review · deepseek-v4-flash

2026-08-02 23:29 UTC pith:MACPO3D4

load-bearing objection LGF is a genuinely new SINDy-on-optimizer-dynamics idea with two convincing well-posed demos, but the headline accelerations partly rest on underdetermined fits and unmeasured overhead. the 3 major comments →

arxiv 2602.13513 v2 pith:MACPO3D4 submitted 2026-02-13 math.OC cs.CEcs.LGcs.NAmath.DSmath.NA

Learning Gradient Flow: Using Equation Discovery to Accelerate Engineering Optimization

classification math.OC cs.CEcs.LGcs.NAmath.DSmath.NA MSC 90C3090C5337N3034A55
keywords equation discoverygradient flowSINDysurrogate modelstopology optimizationfull wave inversionDeep Ritz methodADAM optimization
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

Optimization is normally a loop: compute the objective and gradient, take a step, repeat. This paper proposes that the loop itself is a dynamical system that can be learned from data. The authors test a short stretch of true gradient steps, fit a low-order polynomial ODE (via sparse-regression equation discovery) to the observed trajectory, and then integrate that ODE forward for many steps, deferring the next true gradient evaluation. Across five engineering and scientific-machine-learning problems they report the same final solutions with 33% to 250% fewer gradient evaluations. The wider point is that expensive physics or backpropagation need not be run at every step if the optimizer's own motion is predictable.

Core claim

The load-bearing claim is that a SINDy-style polynomial ODE (Eqs. 5-6), fit to a short history of optimizer states (K iterations), can act as a surrogate for the true gradient flow and be integrated for M−K additional iterations without meaningful loss of solution quality. The LGF optimizer alternates between these two phases: a data-generating window with true gradient steps, then a surrogate rollout; retraining on a schedule keeps the model fresh. For gradient descent and Newton, the surrogate models the full time derivative of the state; for ADAM, it models only the expensive gradient term, keeping the moment dynamics explicit. Demonstrations include a 13,824-variable topology optimizatio

What carries the argument

The central mechanism is the Learned Gradient Flow (LGF) loop. From a K-step history of the optimization variables, the algorithm estimates time derivatives by finite differences, forms a library of polynomial terms (up to order P), and solves for a sparse coefficient matrix via sequentially thresholded least squares. The identified ODE ȧ = Ξ^T φ(a) is then integrated with an adaptive solver from iteration K to M, skipping true gradient evaluations. For high-dimensional problems, a truncated SVD projects the state to a rank-r latent space before modeling; for ADAM, the moment equations are left analytical and only the unknown gradient term is learned.

Load-bearing premise

A K-iteration window of optimizer states is representative enough of future dynamics that an autonomous, smooth polynomial ODE can extrapolate reliably over M−K steps; the paper's own high-acceleration experiments show this assumption breaks when the window is too short or the horizon too long.

What would settle it

On the Deep Ritz problem with M=70 and K=35, the surrogate rollout drives the total potential energy upward while active (Figure 12); a general falsifier is to measure, for any problem, whether the surrogate-only segment monotonically decreases the true objective. If it does not, the learned ODE has failed to capture the dynamics, and the acceleration claim for that setting is refuted.

Watch this falsifier. Get emailed when new claim-graph text bears on it.

If this is right

  • Gradient-evaluation cost drops by a factor of M/K per retraining window, with the surrogate integration essentially free by comparison.
  • The same recipe applies to gradient descent, Newton, and ADAM, so it covers PDE-constrained design optimization and neural-network training alike.
  • A rank-2 surrogate capturing a 13,824-variable topology optimization suggests many engineering optimization trajectories are far lower-dimensional than their design spaces.
  • Overly aggressive acceleration (e.g., M=70 vs K=35) makes the surrogate actively increase the loss, establishing a reliability ceiling tied to how far the model extrapolates.
  • Because the surrogate never touches the true loss, the method naturally extends to any iterative optimizer whose update direction can be learned from trajectory data.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If low-rank, smooth optimization dynamics are the norm rather than the exception, this style of surrogate could transfer to high-dimensional shape optimization and multi-parameter inverse problems beyond the five examples shown.
  • The failure modes suggest an adaptive schedule: monitor a cheap mismatch between the surrogate's predicted state and rare true-gradient checkpoints, and retrain only when the mismatch grows — a testable alternative to the fixed M schedule.
  • For mini-batched or stochastic objectives, the paper conjectures that weak-form or regularized variants of the equation-discovery step would improve extrapolation; this can be measured directly by comparing surrogate rollout error with and without those variants.
  • An implicit consequence is that the surrogate is learning the geometry of one trajectory; when the optimization enters a new regime (e.g., constraint switching, active-set changes), the learned autonomous ODE should be expected to fail — matching the paper's high-acceleration cases.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

3 major / 5 minor

Summary. The paper introduces Learned Gradient Flow (LGF), a framework that applies SINDy-style equation discovery to the continuous-time dynamics of optimization algorithms. After K iterations of a base optimizer (gradient descent, Newton, or ADAM), LGF fits a polynomial ODE (Eqs. 5-6) to the recorded state trajectory and integrates this surrogate from iteration K to M, thereby skipping expensive objective/gradient evaluations. For ADAM, the unknown gradient is modeled while the moment dynamics are kept partially known (Eqs. 12-13); for high-dimensional problems, the state is first projected to an r-dimensional latent space (Sec. 3.5). The method is demonstrated on five engineering/scientific ML problems and reports 33-250% reductions in gradient evaluations (Table 6), while the paper also documents cases where too large a retrain interval M causes the surrogate to degrade.

Significance. The core idea is original and timely: using online equation discovery as a surrogate for optimizer dynamics is a natural but, to my knowledge, underexplored connection. The paper is honest about limitations and includes useful negative results for large M. The latent-space result for topology optimization (Sec. 4.2) is especially striking and suggests that the method can exploit low-dimensional structure. However, the manuscript currently provides no identifiability or validation analysis for the SINDy solves in the high-dimensional examples, and the headline accelerations are measured in skipped gradient evaluations rather than wall-clock time. These issues do not invalidate the concept but make the central claim stronger than the evidence supports.

major comments (3)
  1. [Sec. 4.4 (also 4.3, 4.5), Eq. (6)] In the FWI example, n=346, P=2 gives p=binom(348,2)=60,378, while K=20 recorded states yield at most 18 independent rows for \dot{A}. Eq. (6) is therefore severely underdetermined, and with alpha=0 and max_iter=5 the returned Xi is an artifact of the solver's regularization path, not of the optimization dynamics. The same issue occurs in Sec. 4.3 (n=225, P=1, p=226, K=15) and Sec. 4.5 (n=825, P=1, p=826, K=35). Since these three examples constitute a large fraction of the reported accelerations, the claim that LGF reliably accelerates optimization is not yet supported. Please add identifiability checks, validation on held-out trajectory segments, or reformulate these cases in a latent space where K>p, and report the resulting Xi sensitivity.
  2. [Alg. 1, lines 6-10; Secs. 3.3-3.4, 4.3-4.5] The method's success hinges on the surrogate extrapolating for M-K steps, as the paper itself states ('hinges on how well the data-generating phase represents the training as a whole'). The negative results in Secs. 4.3 (M=30), 4.4 (M=40), and 4.5 (M=70) show that the surrogate can actively increase the loss when M is too large. No criterion for choosing M, or for detecting surrogate failure during the surrogate phase, is provided; Sec. 3.6 describes K and M as matters of trial and error. Because the reported accelerations (Table 6) all use favorable M values, the method needs at least an empirical sensitivity analysis or an online validation/retraining trigger before the central claim can be accepted as robust.
  3. [Table 6; Sec. 4] Acceleration is defined solely as 100*(M/K-1), i.e., the reduction in the number of true gradient evaluations. No wall-clock timings are reported for the overhead of library construction, SVD, STLSQ, or the dopri5 integration. In examples where the objective/gradient is not overwhelmingly expensive (e.g., Deep Ritz with a small network and 125,000 integration points), this overhead could materially reduce or eliminate the speedup. Please report run times (or at least state explicitly that the claim is limited to gradient-evaluation savings), and include the cost of the per-cycle SVD in Sec. 4.2.
minor comments (5)
  1. [Eq. (12)] The m and v equations use a_k where a(t) is needed in the continuous-time formulation; this is a notational inconsistency that should be corrected.
  2. [Sec. 4.2] The text says 'aim for 100% cost savings' with K=20, M=40; the gradient-evaluation savings is 50% (M/K=2), while the acceleration is 100%. Please rephrase to avoid confusion.
  3. [Sec. 4.1] '3x savings' is ambiguous; M/K=3 means a factor-3 reduction in gradient evaluations, not necessarily a '3x savings' in runtime. Please use consistent terminology.
  4. [Sec. 4.4] The statement 'within the STLSQ solver, we set alpha=0 and max_iter=5 due to the high cost' is concerning because max_iter=5 may terminate before thresholding converges. Please justify this choice or report its effect.
  5. [General] The manuscript does not state whether the PyTorch optimizer or experiment scripts will be released, which would aid reproducibility.

Circularity Check

0 steps flagged

No circularity: LGF is an online surrogate-forecasting method whose errors are empirically observable; self-citations are incidental.

full rationale

The paper's derivation chain is not circular. The LGF surrogate is a SINDy polynomial ODE fit to the first K recorded optimizer states (Algorithm 1, lines 2-10), and it is then integrated for the subsequent M-K steps. The training input (states 1..K) and prediction target (states K+1..M) are distinct, and the fit does not encode the future states by construction. The paper's own high-acceleration experiments (Secs. 4.3-4.5) show the surrogate can fail and drive up the loss, which confirms the prediction is not forced by the fitting procedure. The reported 'acceleration' 100*(M/K-1) is an accounting identity for the chosen hyperparameters, not a fitted parameter renamed as a prediction; the claim that the method is useful rests on separate empirical comparisons of solution quality. Self-citations (e.g., [66], [67], [68], [25], [26], [34], [76]) appear in background, dimensionality-reduction alternatives, or side remarks on interpretability, and none carries the central claim; there is no imported uniqueness theorem or author-justified ansatz. Appendix A is a consistency check showing that a posited continuous-time ADAM model discretizes back to the ADAM update, not a derivation that folds the target result into the input. The severe underdetermination in the FWI example (n=346, P=2, K=20, p~6e4) is a numerical identifiability and robustness concern, not circularity: the fitted coefficients are underdetermined, not trivially equal to the data or to the prediction. The paper is self-contained in its empirical demonstrations, and no load-bearing step reduces to its own inputs.

Axiom & Free-Parameter Ledger

6 free parameters · 5 axioms · 0 invented entities

No new physical entities are introduced. The load-bearing choices are hyperparameters K/M/P/r and the autonomous-smooth-polynomial extrapolation assumption. Several SINDy solves are underdetermined, so implicit regularization is doing unseen work that is not validated.

free parameters (6)
  • K (history size) = 10, 20, 15, 20, 35 per example
    Number of state samples used to build the SINDy surrogate; chosen by trial and error per problem, explicitly noted in Sec. 3.6.
  • M (retrain interval) = 30, 40/70, 20/30, 30/40, 50/70 per example
    Total steps per surrogate cycle; controls acceleration; hand-tuned, with larger values shown to degrade results.
  • Polynomial order P = 1 or 2 (P=1 in four examples; P=2 in FWI)
    Library complexity; chosen by hand, with no model-selection criterion.
  • Truncation rank r = 2 in topology optimization only
    Dimension of latent space for reduced dynamics; chosen informally because dynamics appeared low-rank.
  • STLSQ hyperparameters (alpha, threshold, max_iter) = alpha=1e-6, threshold=1e-8, max_iter=20 in most; FWI: alpha=0, max_iter=5
    Regularization and thresholding settings for coefficient recovery; hand-set per example; paper claims insensitivity in tested cases but not in general.
  • Penalty beta in topology objective = not reported
    Volume-constraint penalty in Eq. (25) changes the objective and is a tuning parameter; its value is not given.
axioms (5)
  • domain assumption Optimization variables evolve according to an autonomous polynomial ODE of total order P.
    Eqs. (5)-(6) define da/dt = Xi^T phi(a) with a polynomial library. This is an assumption, not derived from the objective; for gradient descent it is only exact in the eta->0 limit (Eqs. 3-4).
  • domain assumption A K-iteration history is representative of the M-K future iterations.
    Core extrapolation premise, stated in Sec. 3.3-3.4: surrogate performance hinges on how well the data-generating phase represents training as a whole.
  • domain assumption Finite-difference estimates of the derivative are accurate enough for SINDy recovery.
    Algorithm 1 line 7 and Sec. 3.1; in the mini-batch Deep Ritz example, gradients are noisy, yet no denoising or derivative-robust variant is used.
  • ad hoc to paper STLSQ solves with more unknowns than samples (K < p) still recover useful dynamics.
    Sec. 4.3 (K=15, p=226), 4.4 (K=20, p~6e4), and 4.5 (K=35, p=826) solve underdetermined linear systems; no identifiability or cross-validation is provided.
  • domain assumption Continuous-time ADAM ODE (Eq. 12) integrated with an adaptive Runge-Kutta method remains close to discrete ADAM.
    Appendix A shows a semi-implicit discretization recovers ADAM, but Algorithm 1 integrates with dopri5; equivalence over finite horizons is assumed, not proved.

pith-pipeline@v1.3.0-alltime-deepseek · 25277 in / 14964 out tokens · 141105 ms · 2026-08-02T23:29:58.211427+00:00 · methodology

0 comments
read the original abstract

In this work, we investigate the use of data-driven equation discovery for dynamical systems to model and forecast continuous-time dynamics of unconstrained optimization problems. To avoid expensive evaluations of the objective function and its gradient, we leverage trajectory data on the optimization variables to learn the continuous-time dynamics associated with gradient descent, Newton's method, and ADAM optimization. The discovered gradient flows are then solved as a surrogate for the original optimization problem. To this end, we introduce the Learned Gradient Flow (LGF) optimizer, which is equipped to build surrogate models of variable polynomial order in full- or reduced-dimensional spaces at user-defined intervals in the optimization process. We demonstrate the efficacy of this approach on several standard problems from engineering mechanics and scientific machine learning, including two inverse problems, structural topology optimization, and two forward solves with different discretizations. Our results suggest that the learned gradient flows can significantly expedite convergence by capturing critical features of the optimization trajectory while avoiding expensive evaluations of the objective and its gradient.

Figures

Figures reproduced from arXiv: 2602.13513 by Alireza Doostan, Conor Rowan, Grant Norman, Kurt Maute.

Figure 1
Figure 1. Figure 1: Scheduled retraining of gradient flow. We alternate between collecting histories for [PITH_FULL_IMAGE:figures/full_fig_p013_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Convergence of the two optimizers to the minimum of the data loss defining the inverse problem. The [PITH_FULL_IMAGE:figures/full_fig_p018_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Design domain, boundary conditions, and loading for the compliance minimization design problem. We take [PITH_FULL_IMAGE:figures/full_fig_p021_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Results of the density-based topology optimization with [PITH_FULL_IMAGE:figures/full_fig_p022_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Results of the density-based topology optimization with [PITH_FULL_IMAGE:figures/full_fig_p023_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Comparing the surrogate for the full Newton dynamics to the true dynamics. Using the LGF optimizer [PITH_FULL_IMAGE:figures/full_fig_p026_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Comparing the surrogate model against the true Newton dynamics with a [PITH_FULL_IMAGE:figures/full_fig_p027_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Problem setup for the full wave inversion problem. Three of the four surfaces have homogeneous Dirichlet [PITH_FULL_IMAGE:figures/full_fig_p028_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Comparing standard ADAM optimization to the continuous time ADAM with gradient surrogate. Note [PITH_FULL_IMAGE:figures/full_fig_p031_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Comparing standard ADAM optimization to the continuous-time ADAM with gradient surrogate and [PITH_FULL_IMAGE:figures/full_fig_p032_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Comparing standard ADAM optimization with the LGF version of ADAM. We save [PITH_FULL_IMAGE:figures/full_fig_p034_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: When the acceleration is increased to 100%, the surrogate model does not accurately reflect the loss landscape. When switched on, it consistently drives the total potential energy up, providing no benefit to the optimization process. The normalized difference in the displacement fields obtained by minimizing the stochastic objective with the two optimizers is R Ω ∥ˆuADAM(x) − ˆuLGF(x)∥ 2dΩ R Ω ∥ˆuADAM(x)∥… view at source ↗
Figure 13
Figure 13. Figure 13: Comparing the two optimizers with a Monte Carlo approximation of the total potential energy. The loss [PITH_FULL_IMAGE:figures/full_fig_p035_13.png] view at source ↗

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