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REVIEW 3 major objections 6 minor 78 references

First-ish Order Methods: Hessian-aware Scalings of Gradient Descent

T0 review · 3 major / 6 minor · reviewed 2026-08-09 · deepseek-v4-flash

Pith's one-line read Hessian-aware gradient scaling gives gradient descent a unit step guarantee.

desk verdict Solid, checkable theory for Hessian-aware gradient scalings, undermined in the empirical section by selection bias and an assumption that likely fails on the paper's own benchmarks — still deserves peer review. read the letter →

arxiv 2502.03701 v3 pith:TTQPPPWG submitted 2025-02-06 math.OC cs.LG

classification math.OCcs.LG MSC 90C5390C2690C30
keywords Hessian-awarescalingscaledgradientdescentunitstepsizeguaranteesecond-orderconditionnonconvexoptimizationsufficient-decreaselinesearchdirectionalHessiansmoothnessinexact
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

Gradient descent's main practical headache is choosing a learning rate: too small crawls, too large diverges, and backtracking line search costs extra evaluations and needs a sensible starting guess. This paper argues that a single piece of local curvature information, one Hessian-vector product along the gradient, can remove that headache. Its adaptive scaling picks a step direction of the form $p=-s g$, with $s$ chosen from three curvature regimes, so the direction always satisfies the second-order descent condition $\langle g,p\rangle + \langle p,Hp\rangle \le 0$. Under a directional smoothness assumption on the Hessian, the paper proves that the standard sufficient-decrease line-search condition accepts the unit step $\alpha=1$ once the gradient is small, and near nondegenerate local minima the unit step is accepted every iteration with linear convergence. If right, this gives gradient descent a second-order-like natural step while keeping first-order per-iteration cost, and it extends to inexact Hessians.

What carries the argument

The object that carries the argument is Algorithm 1's three-regime scaling selection: it converts one directional curvature measurement $\langle g,Hg\rangle$ into a scalar $s$ that is small when curvature is strong, capped at $1/\sigma$ in the limited-positive regime, and free in the negative-curvature regime. Proposition 2.2 upper-bounds all these scalings, and Proposition 2.3 turns those bounds into the second-order descent condition. The analytic machinery is the cubic Taylor upper bound $|f(x+p)-f(x)-\langle g,p\rangle - \frac12\langle p,Hp\rangle| \le (L_2/6)\|p\|^3$, which follows from the paper's Hessian directional smoothness assumption, combined with the sufficient-decrease line-search condition; together they produce explicit thresholds on $\|g\|$ under which $\alpha=1$ is guaranteed.

What would settle it

Run the exact line-search test from Theorem 3.3 on $f(x)=\frac12 x^T x + \epsilon\sum_i x_i^3$ with $\epsilon$ chosen so the Hessian is directionally smooth with known $L_2=6\epsilon$, sampling $x$ with $\|g\|$ below the threshold (8); any rejection of $\alpha=1$ contradicts the theorem.

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Extended reading notes

Core claim

The central claim is that a gradient direction scaled by curvature, $p=-s g$, inherits the step-size wisdom of second-order methods without the cost of solving a full second-order system. Algorithm 1 checks the curvature $\langle g,Hg\rangle$ along the gradient: if it is strongly positive it sets $s$ to one of $s_{\mathrm{CG}}=\|g\|^2/\langle g,Hg\rangle$, $s_{\mathrm{MR}}=\langle g,Hg\rangle/\|Hg\|^2$, or $s_{\mathrm{GM}}=\|g\|/\|Hg\|$; if curvature is limited positive it caps $s\le 1/\sigma$; if curvature is negative it uses a scaling in a user-specified range and exploits the negative curvature. Proposition 2.3 shows all three cases keep $\langle g,p\rangle + \langle p,Hp\rangle \le 0$. Theorem 3.3 then shows that under Hessian directional smoothness, the sufficient-decrease condition holds at $\alpha=1$ whenever $\|g\|$ is below the explicit threshold (8), even for nonconvex functions. Theorem 3.7 upgrades this to a local linear rate with unit steps accepted at every iteration near a minimum satisfying the second-order sufficient conditions, and Theorem 3.9 shows the MR scaling gives linear contraction of the gradient norm. The same guarantees survive inexact Hessian estimates under a directional error bound.

Load-bearing premise

The proof's load-bearing premise is that the Hessian changes slowly along the negative-gradient direction at every point and every future step; if that directional smoothness constant does not exist along the optimization trajectory, the unit-step guarantee and the global convergence rate are unproven.

Editorial extensions

If this is right

  • Near a nondegenerate local minimum, the method runs with the unit step accepted at every iteration, so backtracking is bypassed and the function value converges linearly.
  • Globally, the method reaches a gradient norm below $\varepsilon_g$ in $O(\varepsilon_g^{-2})$ iterations under smoothness assumptions weaker than global Lipschitz gradient continuity.
  • Inexact Hessian information, such as a subsampled Hessian, preserves the unit-step guarantee and the global complexity bound provided the error along the gradient direction is bounded.
  • With the MR scaling, the gradient norm decreases monotonically and linearly, so the method implicitly minimizes the gradient norm as a secondary objective.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Editorial inference: because the curvature check needs only one Hessian-vector product, the scaling is a plausible drop-in for stochastic minibatch optimizers; the paper's proofs are deterministic, so a stochastic variant with subsampled Hessians is a testable extension rather than a stated result.
  • Editorial inference: the worst-case threshold (8) is much smaller than the region where unit steps are observed in the experiments, so a sharper analysis that averages curvature along the trajectory rather than bounding it globally would likely close that gap.
  • Editorial inference: the MR scaling's automatic gradient-norm contraction suggests a parameter-free implicit bias toward flat regions; comparing the generalization of MR-scaled runs with explicit gradient-norm penalties on identical architectures would test this directly.
  • Editorial inference: the paper notes that large scalings near $1/\mu$ remove the condition-number dependence from the local rate; a decreasing-$\sigma$ schedule that keeps the method in the strong-positive-curvature regime might reproduce that benefit more often, but the paper does not analyze such a schedule.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

3 major / 6 minor

Summary. This paper proposes Hessian-aware scalar scalings of the gradient direction, p = -s g, where s is selected in three regimes (SPC, LPC, NC) according to the directional curvature <g,Hg>. The main theoretical results are: Proposition 2.3, which shows that the resulting direction satisfies the second-order descent condition <g,p> + <p,Hp> <= 0; Theorem 3.3, which gives a sufficient condition under Assumption 3.1 for the Armijo condition to hold with unit step size; Proposition 3.6, which establishes an O(epsilon_g^{-2}) global complexity bound under Assumptions 3.1 and 3.4; Theorem 3.7, which shows local linear convergence near a second-order sufficient minimum with a unit step; and Theorem 3.9, which proves linear convergence in the gradient norm for the MR scaling. Numerical experiments on logistic regression, an MLP, and a ResNet18 compare the method against fixed-step, accelerated, and line-search baselines, reporting oracle-call counts and observing frequent acceptance of alpha=1.

Significance. If the stated results hold, the paper makes a useful conceptual contribution: it shows that a one-dimensional curvature-aware scaling of the gradient can inherit some of the unit-step behavior usually associated with second-order methods, at the cost of one Hessian-vector product per iteration. The analysis is self-contained and the algebra in Propositions 2.2, 2.3, and 2.5 is checkable. The authors are careful to phrase the best-case improved rate in Section 3.2 as conditional rather than as a theorem, and the inexact-Hessian extension is a genuine addition. The numerical study uses oracle-call counts, covers convex and nonconvex problems, and the authors report code availability. The main reservation is that the central unit-step and global-convergence theorems rest on a global directional Hessian smoothness assumption that is not verified for the benchmarks and can fail for smooth nonconvex objectives; the empirical behavior is therefore not fully explained by the theory. In addition, the ResNet experiment uses rho=0, outside the assumptions of all convergence results.

major comments (3)
  1. [Section 3.1, Assumption 3.1 and Eq. (11)] Assumption 3.1 is load-bearing for the central claims: it is used to derive the cubic bound (11), which is the only bridge from the second-order descent condition (3) to the unit-step Armijo guarantee in Theorem 3.3, and it is also used in Proposition 3.6 and Theorem 3.7. The assumption is global (all x in R^d and all t >= 0) and is not verified on any of the Section 4 problems. For smooth nonconvex objectives with unbounded higher-order growth, e.g., f(x) = (a^T x)^4, the Hessian along a negative-gradient ray is unbounded and no finite L2 exists, so (11) and the results built on it do not apply. Remark 3.2 only says the assumption is difficult to verify; the paper should either replace it by a local or checkable condition (for example, with an L2 that depends on the current iterate) or provide empirical estimates of the directional Hessian variation for the tested benchmarks. Without this, the frequent acceptance of alpha=1 in Figures 2-4 is not explained by Theorem 3.3.
  2. [Appendix A.1 and Theorem 3.3] The recommended hyperparameter setting is incompatible with the only unit-step theorem. Appendix A.1 recommends sigma small, "or even zero", and for strongly convex problems sigma = 0. But Theorem 3.3 and its proof use the bound s <= 1/sigma and the first term 6 sigma^2(1/2 - rho)/L2 in (8); for sigma = 0 the theorem has no content in the non-negative curvature case, and the proof divides by sigma. If sigma is merely small, 6 sigma^2(1/2 - rho)/L2 is tiny, making the provable unit-step region minuscule. The limitations paragraph acknowledges this, but the claim in the abstract that the method "ensures a local unit step size guarantee" should be qualified by the parameter regime in which (8) is actually satisfied.
  3. [Appendix D.5 (ResNet18 experiment)] For the ResNet18 benchmark, the paper sets rho = 0 in the Armijo condition (7) (Appendix D.5, "Parameter Settings"), while every convergence result in Section 3 and Appendix B assumes rho in (0,1/2). With rho = 0, the descent constants cSPC, cLPC, cNC in Proposition B.1 vanish, so the O(epsilon_g^{-2}) complexity bound and the local linear rate in Theorem 3.7 have no content for this experiment. The main text and figure caption should either report a run with rho > 0 or state explicitly that the ResNet experiment is outside the theoretical framework.
minor comments (6)
  1. [Section 2.2] "a Lg-Lipschitz gradient" should read "an Lg-Lipschitz gradient".
  2. [Section 3.2] "This reflected in the practical performance" should be "This is reflected in the practical performance".
  3. [Appendix B.3] "the factor based on L1 and L2 could are smaller" should be "the factor based on L1 and L2 could be smaller".
  4. [Appendix B.4] The heading "Amijo condition" is a typo for "Armijo condition".
  5. [Appendix D.1] "Following the typical convection" should be "Following the typical convention".
  6. [References] Reference [73] lists the publisher as "PMLP"; this should be "PMLR".

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the Hessian-aware scalings are derived from explicit quadratic-model minimizations and the convergence theorems follow from stated assumptions without fitted inputs.

full rationale

The main derivation chain is self-contained. The scalings s_CG, s_MR, and s_GM are obtained by explicitly minimizing local quadratic models along p = -s g: s_CG = ||g||^2/<g,Hg> solves the Newton quadratic restricted to the gradient direction, s_MR = <g,Hg>/||Hg||^2 solves min ||Hp+g||^2, and s_GM is their geometric mean. Proposition 2.3 verifies the second-order descent condition <g,p>+<p,Hp> <= 0 directly from these definitions, so the key direction property is constructed rather than imported. Theorem 3.3 derives the unit-step Armijo guarantee from Assumption 3.1 through the cubic bound (11); the gradient threshold (8) is obtained algebraically from the scaling upper bounds s <= 1/sigma and s <= s_NC_max, not from any fitted parameter or from the convergence conclusion. Global convergence (Proposition 3.6 and Proposition B.1) follows from per-iteration descent estimates in Lemma B.2 and scaling lower bounds in Lemma B.4 under Assumptions 3.1 and 3.4; it is a conditional complexity bound with explicitly stated constants. Local results (Theorems 3.7 and 3.9) use second-order sufficient conditions and independent contraction arguments. Section 4 is empirical validation and is not used as an input to any theorem. The self-citations ([42], [43], [63], [66]) are used for motivation, for the Newton-MR/Newton-CG connection, and for the contextual remark that moral smoothness implies Assumption 3.4; none of these citations is load-bearing for the paper's proofs, which use Assumption 3.4 as a stated assumption. No uniqueness theorem from the authors is invoked, and no external result is renamed as a new finding. The unverifiability of Assumption 3.1 flagged in Remark 3.2 is a correctness and robustness concern, not a circularity concern.

Assumptions & free parameters 4 free parameters · 6 assumptions · 0 invented entities

The theorems depend on weakened smoothness assumptions (3.1, 3.4, B.9, B.12) rather than on fitting. The method's hyperparameters (sigma, s ranges) are user-chosen and appear in the constants of the convergence bounds. No new entities are introduced.

free parameters (4)
  • sigma = 0 for convex experiments, 1e-6 for nonconvex
    SPC tolerance; chosen by hand. Affects the classification of curvature cases and the upper bound 1/sigma on step scalings. Not fitted to data.
  • s_LPC_min, s_LPC_max = 1/sigma or 1
    Range for limited positive curvature scalings; set to fixed values in experiments. User-chosen.
  • s_NC_min, s_NC_max = 1
    Range for negative curvature scalings; set to 1 in experiments. User-chosen.
  • rho, theta = rho=1e-4, theta=0.5
    Line search parameters used in experiments; standard values, not fitted.
assumptions (6)
  • domain assumption f is twice continuously differentiable and bounded below
    Stated in the introduction; needed for Hessian-vector products and Taylor expansions.
  • domain assumption Assumption 3.1: Hessian directional smoothness, ||H(x - t g(x)) - H(x)|| <= t L2 ||g(x)|| for all x, t >= 0
    Used in Theorem 3.3 and Lemma B.2 to control the cubic remainder in the Taylor expansion; the paper notes it is difficult to verify directly.
  • domain assumption Assumption 3.4: if <g,Hg> > 0 then ||Hg|| <= L1 ||g||
    Used in Lemma B.4 to lower-bound SPC scalings, required for the global convergence proposition.
  • domain assumption Assumption B.9: existence of mu0, nu0 with |<g,Hg>| >= mu0||g||^2 and cos^2(angle(g,Hg)) >= nu0 along MR iterates
    Used only for Theorem 3.9's gradient-norm convergence; the authors show it is implied near a second-order sufficient minimum.
  • domain assumption Assumption B.12: |<g,(H - tilde H)g>| <= Delta_H ||g||^2 for the inexact Hessian
    Used in Appendix B.7 to extend unit step and convergence results to inexact Hessians.
  • standard math Taylor expansion with integral remainder and cubic upper bound (11)
    Standard calculus result used in all proofs.

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Pith. "Pith review of First-ish Order Methods: Hessian-aware Scalings of Gradient Descent." pith.science (2026). https://pith.science/paper/TTQPPPWG

@misc{pith2026250203701,
  author       = {Pith},
  title        = {Pith review of: First-ish Order Methods: Hessian-aware Scalings of Gradient Descent},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/TTQPPPWG}},
  note         = {Machine review of arXiv:2502.03701}
}
read the original abstract

Gradient descent is the primary workhorse for optimizing large-scale problems in machine learning. However, its performance is highly sensitive to the choice of the learning rate. A key limitation of gradient descent is its lack of natural scaling, which often necessitates expensive line searches or heuristic tuning to determine an appropriate step size. In this paper, we address this limitation by incorporating Hessian information to scale the gradient direction. By accounting for the curvature of the function along the gradient, our adaptive, Hessian-aware scaling method ensures a local unit step size guarantee, even in nonconvex settings. Near a local minimum that satisfies the second-order sufficient conditions, our approach achieves linear convergence with a unit step size. We show that our method converges globally under a significantly weaker version of the standard Lipschitz gradient smoothness assumption. Even when Hessian information is inexact, the local unit step size guarantee and global convergence properties remain valid under mild conditions. Finally, we validate our theoretical results empirically on a range of convex and nonconvex machine learning tasks, showcasing the effectiveness of the approach.

Figures

Figures reproduced from arXiv: 2502.03701 by the authors.

Figure 1
Figure 1. Logistic regression on the Mushrooms dataset [1]. We compare our scaling approach to standard GD with backtracking line search, using different initialization of the backtracking proce￾dure (α 0 ), and assess performance based on oracle calls (Appendix D.1). The av￾erage number of backtracking steps per iteration is reported in brackets. With an arbitrary initialization, x0, the only hyperparameter is the step size,… view at source ↗
Figure 2
Figure 2. Multi-class logistic regression on CIFAR10. (a) Objective value. (b) Scaling utilized by the [PITH_FULL_IMAGE:figures/full_fig_p008_2.png] view at source ↗
Figure 3
Figure 3. MLP on the FashionMNIST. (a) Objective value (b) Scaling utilized by the CGMR method [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗
Figures from the paper (16 more)
Figure 4
Figure 4. Figure 4: ResNet18 on Imagenette. (a) Objective value. (b) Scaling utilized by the MRCG method. [PITH_FULL_IMAGE:figures/full_fig_p009_4.png]
Figure 5
Figure 5. Figure 5: Multi-class logistic regression on CIFAR10 (Figure 2 with PoNo line search included). (a) [PITH_FULL_IMAGE:figures/full_fig_p036_5.png]
Figure 6
Figure 6. Figure 6: Comparison between scaling method performance for multiclass logistic regression on [PITH_FULL_IMAGE:figures/full_fig_p036_6.png]
Figure 7
Figure 7. Figure 7: Performance of the CG scaling on the multi-class logistic regression problem on CIFAR10. [PITH_FULL_IMAGE:figures/full_fig_p037_7.png]
Figure 8
Figure 8. Figure 8: Performance of the MR scaling on the multi-class logistic regression problem on CIFAR10. [PITH_FULL_IMAGE:figures/full_fig_p037_8.png]
Figure 9
Figure 9. Figure 9: Wall-clock time for multi-class logistic regression on the CIFAR10 dataset. [PITH_FULL_IMAGE:figures/full_fig_p037_9.png]
Figure 10
Figure 10. Figure 10: MLP on the FashionMNIST (Figure 3 with PoNo line search included). (a) Objective [PITH_FULL_IMAGE:figures/full_fig_p038_10.png]
Figure 11
Figure 11. Figure 11: Comparison between scaling methods for MLP on FashionMNIST dataset. [PITH_FULL_IMAGE:figures/full_fig_p038_11.png]
Figure 12
Figure 12. Figure 12: Performance of the CG scaling for the MLP on the FashionMNIST dataset. (a) The [PITH_FULL_IMAGE:figures/full_fig_p039_12.png]
Figure 13
Figure 13. Figure 13: Performance of the MR scaling for the MLP on the FashionMNIST dataset. (a) The [PITH_FULL_IMAGE:figures/full_fig_p039_13.png]
Figure 14
Figure 14. Figure 14: Wall-clock time results for MLP on the FashionMNIST dataset [ [PITH_FULL_IMAGE:figures/full_fig_p039_14.png]
Figure 15
Figure 15. Figure 15: ResNet18 on Imagenette (Figure 4 with PoNo line search included.) (a) Objective value. [PITH_FULL_IMAGE:figures/full_fig_p040_15.png]
Figure 16
Figure 16. Figure 16: Comparison of scaling method performance on for the ResNet18 model on the Imagenette [PITH_FULL_IMAGE:figures/full_fig_p041_16.png]
Figure 17
Figure 17. Figure 17: Performance of the CG scaling for the ResNet18 model on Imagenette dataset. (a) The [PITH_FULL_IMAGE:figures/full_fig_p041_17.png]
Figure 18
Figure 18. Figure 18: Performance of the MR scaling for the ResNet18 model on Imagenette dataset. (a) The [PITH_FULL_IMAGE:figures/full_fig_p041_18.png]
Figure 19
Figure 19. Figure 19: Wall-clock time results for the ResNet18 model on Imagenette dataset. [PITH_FULL_IMAGE:figures/full_fig_p042_19.png]

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    Therefore, when (33) is satisfied, we have f (x + α˜p) − f (x) ≤ −αρ ∥˜p∥ ∥g∥ ≤ −ρ − 3∆H 2L2 + s 9∆2 H 4L2 2 + 6(1 − ρ) ∥˜p∥ L2sNCmax ! ∥g∥ < −ρ − 3∆H 2L2 + s 9∆2 H 4L2 2 + 6(1 − ρ)sNC minεg L2sNCmax ! εg ≤ −ρ   6(1 − ρ)sNC min/(L2sNC max) 3∆H 2L2 + r 9∆2 H 4L2 2 + 6(1−ρ)s...

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    ∥g∥, which follows from (34), and ∥g∥ > εg in the fourth line, while in the fifth line we apply Lemma B.16. In the final line we define ˜cSPC ≜ ρ min    12σ2(1/2 − ρ)/ ˜L2 1 3∆H + q 9∆2 H + 24σ2(1/2 − ρ)L2/ ˜L2 1 , σ ˜L2 1    . The proof now proceeds similarly to the det...

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    large p” case for the exact Hessian, a better dependence on εg is obtained when ˜p is large and ∆H is controlled. In fact, when θ = 0 we have ∆H = 0 and (39) matches the “large p

    Additionally, applying (38) to the lower bound on ∥˜p∥ we obtain ∥˜p∥ ≥6σ(1/2 − ρ) − 3∆H L2 ≥ 6σ(1 − θ)(1/2 − ρ) L2 . By combining the line search condition (33), α ≥ α⋆ 2, (32), the SPC curvature test and the lower bound on ∥˜p∥ we have f (x + α˜p) − f (x) ≤ αρ ⟨˜p, g⟩ ≤ −α⋆ ...

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    This modification has previously been considered in [ 70]

    layers with LayerNorm [4]. This modification has previously been considered in [ 70]. We utilize the “160px” version of the Imagenette dataset [33], which is available from PyTorch [58]. The Imagenette is a 10 class, subset of the full Imagenet dataset [21]. We process the dat...

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Reviewed August 9, 2026 · model on record in the stance chip above.