REVIEW 3 major objections 5 minor 16 references
Nonlinearly Preconditioned Gradient Methods under Generalized Smoothness
T0 review · 3 major / 5 minor · reviewed 2026-08-08 · deepseek-v4-flash
Pith's one-line read A single generalized smoothness condition unifies the convergence analysis of gradient clipping, Adam, Adagrad, and (L0,L1)-smooth methods.
desk verdict Solid two-constant smoothness framework with correct conditional convergence rates, but the injectivity proof for nonsmooth reference functions has a gap that should be fixed. 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 pair (reference function $\phi$, dual $\phi^*$) through the forward operator $T_{\gamma,\lambda}(x)=x-\gamma\nabla\phi^*(\lambda\nabla f(x))$, together with the $(L,\bar L)$-anisotropic smoothness inequality. The argument's engine is that convexity and evenness of $\phi$ make the descent gap $\phi((1-\alpha)v)-\phi(v)$ bounded by $(|1-\alpha|-1)\phi(v)$, which gives the factor $\beta=1-|1-\alpha|$ in the rates. A second engine is the envelope identity $f=\inf_y \bar L(L^{-1}\star\phi)(\cdot-y)+\xi(y)$, which turns the algorithm into a nonlinear proximal point method and yields the convex anisotropic rate.
What would settle it
Choose a twice differentiable function such as a variant of $\frac14\|x\|^4$ with an added non-quartic term and a reference function from Table 1, then scan a fine grid of pairs $(x,\bar x)$ for a violation of inequality (4) with the constants claimed by the paper's sufficient conditions; finding any pair that violates (4) while satisfying the second-order condition and forward-operator coercivity would disprove Proposition 2.9. Conversely, running iteration (2) on an $(L_0,L_1)$-smooth function and checking that the stationarity measure decays by the predicted $O(1/K)$ bound would confirm the mechanism.
Extended reading notes
Core claim
The central discovery is that replacing the quadratic upper bound of Lipschitz smoothness with a reference-function upper bound $f(x)\le f(\bar x)+\bar L[(L^{-1}\star\phi)(x-\bar y)-(L^{-1}\star\phi)(\bar x-\bar y)]$, where $\bar y=\bar x-L^{-1}\nabla\phi^*(\bar L^{-1}\nabla f(\bar x))$, yields a strictly larger class of smooth functions still amenable to first-order methods. The iteration that minimizes this bound is gradient descent with a nonlinear, coordinate-wise or radial step determined by the conjugate of $\phi$. On this class the paper proves the nonconvex stationarity bound $\min_{0\le k\le K}\phi(\nabla\phi^*(\bar L^{-1}\nabla f(x^k)))\le L(f(x^0)-f^*)/(\bar L \beta(K+1))$, and convex $O(1/K)$ suboptimality-gap rates for isotropic and $2$-subhomogeneous anisotropic reference functions.
Load-bearing premise
The whole convergence theory stands on the assumption that at every point the objective $f$ satisfies the anisotropic descent inequality (4) with the particular point $\bar y$ produced by the algorithm; if that inequality is violated for even one iterate, the descent step used in every rate proof breaks down.
Editorial extensions
If this is right
- Stepsizes up to $\gamma=2/L$ are allowed in the nonconvex case, with rate depending on $\beta=1-|1-\alpha|$ (Theorem 3.2).
- Under isotropic reference functions, iterates are Fej\'er monotone, gradient norms decrease monotonically, and the function-value gap decays as $O(1/K)$ (Theorem 3.6).
- For convex objectives with $2$-subhomogeneous anisotropic reference functions, the gap decays as $O(1/K)$ without an initial-gradient-dependent constant (Theorem 3.7).
- Every $(L_0,L_1)$-smooth $C^2$ function satisfies the generalized smoothness property for a specific kernel, giving new guarantees and characterizations for that class (Corollary 2.11).
- Gradient clipping, Adam and Adagrad without memory are recovered as special cases, so their convergence is covered by the same proof.
Reading between the lines
- The kernel view suggests a way to design new preconditioners by choosing $\phi$, with the second-order condition in Theorem 2.4 as a checkable design criterion; one could generate a family of sigmoid-step methods beyond the listed examples.
- The framework's weakest point is the need to verify the anisotropic descent inequality itself; the second-order sufficient condition relies on a coercivity/injectivity assumption, and a useful extension would be to characterize the class of functions satisfying (4) for non-smooth kernels such as the clipping kernel without the extra monotonicity assumption.
- The convex results might extend to composite objectives with an additive nonsmooth term, which the paper explicitly leaves as future work; a proof alongside the anisotropic proximal gradient would likely carry the same rates.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces (L,\bar L)-anisotropic smoothness, a two-constant, possibly non-full-domain generalization of the anisotropic descent property of Laude and Patrinos, and studies the nonlinearly preconditioned gradient iteration x^{k+1}=x^k-\gamma\nabla\phi^*(\bar L^{-1}\nabla f(x^k)). The main theoretical results are a nonconvex stationarity rate (Theorem 3.2), convex O(1/K) suboptimality rates for isotropic and 2-subhomogeneous anisotropic reference functions (Theorems 3.6 and 3.7), and a second-order sufficient condition route (Definition 2.4, Propositions 2.6 and 2.9) intended to certify the new smoothness condition for classes of functions including clipping, Adagrad-like, Adam-like, and (L_0,L_1)-smooth objectives. The paper also contains numerical experiments on a quartic problem, phase retrieval, and a small neural network, with public code.
Significance. If the breadth claims are fully established, this is a valuable unifying framework: it places gradient clipping, signed/adaptive preconditioners, and recent (L_0,L_1)-smooth methods under one majorization-minimization umbrella and gives explicit convergence rates that are not conditional on a Euclidean Lipschitz gradient. The core convergence argument is refreshingly simple and appears correct: Theorem 3.2 derives the stationarity rate directly from the defining descent inequality, and the convex rates in Theorems 3.6 and 3.7 are nontrivial extensions of existing results. The paper ships reproducible code and states its constants explicitly, which I regard as strengths. The main caveat is that the convergence theory is conditional on Definition 2.1, whose breadth is supposed to be certified by the second-order sufficiency theory; as detailed below, that certification has a gap for nonsmooth reference functions, and the worked example in Appendix D.1 contains a false inequality. These issues do not invalidate the conditional convergence theorems, but they do affect the advertised scope of the framework.
major comments (3)
- [Appendix B.4, Proposition 2.6] The proof that T_{L^{-1},\bar L^{-1}} is a local homeomorphism is incomplete. From the chain-rule inclusion, each element A of the Clarke Jacobian satisfies that A(x-\bar x) lies in the convex hull of matrices of the form M_H = I - L^{-1}\bar L^{-1} H \nabla^2 f(\bar x) applied to (x-\bar x). Condition (5) only shows that each individual M_H has eigenvalues in (0,\infty) (when the product has real eigenvalues). It does not imply that every matrix in the convex hull is nonsingular: for example, the invertible matrices [[1,2],[0,1]] and [[1,0],[2,1]] both have all eigenvalues equal to 1, yet their 1/2-convex combination is the singular matrix [[1,1],[1,1]]. Thus the application of Clarke's inverse function theorem as stated is not justified. Since Proposition 2.9 requires injectivity of T, the sufficient route to (L,\bar L)-anisotropic smoothness is left unproved exactly for nonsmooth reference functions such as the clipping kernel in Example 1.7 and Section D.2, where the generalized Jacobian of \nabla\phi^* is multi-valued and H\nabla^2 f is generally nonsymmetric. Proposition D.2 does not cover this case because it assumes H\nabla^2 f is symmetric. The convergence theorems in Section 3 are conditional on Definition 2.1 and are not affected; what needs repair is the sufficiency/breadth claim. A fix could impose a condition on the entire convex hull or prove strong monotonicity of the forward operator under additional structure.
- [Appendix D.1, Example D.3] The proof of the constants in Table 2 asserts that (p-1)a_i(\bar L^{-1}\|x\|^{p-1}) \ge b_i(\bar L^{-1}\|x\|^{p-1}) for all reference functions considered. For \phi_3, one has a_3(t)=1/(1+t)^2 and b_3(t)=1/(1+t), so the inequality fails as soon as t>p-2. Consequently the eigenvalue formula (37) is not maximized by (p-1)a_i in that regime, and the subsequent bound is not justified as written. The final value of L_{\rm norm} appears to remain valid for p=4 (the maximum of the second branch at the switch point t=2 is smaller than the maximum of the first branch at t=1/2), but the argument should be replaced by a genuine case analysis rather than the stated universal inequality.
- [Definition 2.1 and Proposition 2.9] The main circularity risk of the framework deserves an explicit statement: the defining inequality (4) is almost literally the descent condition used in the proof of Theorem 3.2, so the convergence results are conditional on a condition that is not algorithm-independent. The paper does provide an independent sufficiency theory, which is the right way to address this concern, but because that theory currently hinges on the unproved injectivity step in Proposition 2.6, the reader cannot yet conclude that the advertised classes (clipping, Adam, Adagrad, (L_0,L_1)-smooth) satisfy Definition 2.1 in the nonsmooth cases. I would ask the authors to either repair Proposition 2.6 or explicitly state the injectivity condition as an additional hypothesis in Proposition 2.9 for the nonsmooth-reference-function applications.
minor comments (5)
- [Throughout, numbering] The text repeatedly cites numbered display items as 'Theorem' even when they are Assumptions, Definitions, Examples, or Propositions (for instance, 'Theorem 1.1' for Assumption 1.1, 'Theorem 1.7' for Example 1.7, and 'Theorem 2.2' for Proposition 2.2). This is confusing and should be normalized to the correct labels.
- [Section 1.5] The sentence 'Theorem 1.1 is considered valid throughout the paper' should read 'Assumption 1.1 is assumed throughout', since the item is an assumption, not a theorem.
- [Appendix C.4, proof of Theorem 3.6] In the display around equation (24), the quotient h^{*\prime}(\|\bar L^{-1}\nabla f(x^k)\|)/\|\bar L^{-1}\nabla f(x^k)\| is undefined when \nabla f(x^k)=0. The stationary case should be handled separately, or the argument should be stated for the active regime \nabla f(x^k)\ne 0.
- [Table 2 / Section 4.1] The experiment in Section 4.1 says that L is computed 'according to the rules established in section D', but the precise values of L used for each \bar L are not listed in the text or caption. Since Table 2 gives formulas, adding the numerical constants would improve reproducibility; the public code presumably contains them, but a short sentence would help the reader.
- [Example D.4] The bounds leading to the L_{\rm logistic} entries are stated as a chain of inequalities without indicating which bound corresponds to which \phi_i. A short annotation would make the case analysis much easier to follow.
Circularity Check
No significant circularity: the convergence rates are conditional theorems under the stated anisotropic smoothness assumption, which is independently supported by second-order and (L0,L1) sufficiency results.
full rationale
The paper's main convergence results, Theorems 3.2, 3.6, and 3.7, are valid conditional statements: they assume (L,bar-L)-anisotropic smoothness (Definition 2.1) and derive rates by summing the descent inequality (4). This is a standard majorization-minimization structure, not a circular reduction, because the smoothness condition is stated as an a priori property of f and is not fitted to the target rate. The paper provides independent routes for verifying the condition: Proposition 2.3 shows Lipschitz-smooth functions satisfy it, Corollary 2.11 connects it to (L0,L1)-smoothness, and Section D verifies it for concrete examples using second-order conditions. The self-citations to Laude and Patrinos, Laude et al., and Laude are used for context, for the definitional ancestry of the framework, and for a technical proximal-equivalence lemma in the proof of Theorem 3.7; none of these import an unverified uniqueness claim, and the cited lemma does not assume the target convergence rate. The gap in Proposition 2.6 noted by the skeptic is a soundness concern about one sufficient route to injectivity, not a circularity: even if that route fails, Theorem 3.2's proof follows directly from the assumed inequality, and alternative sufficiency conditions such as Proposition D.2 remain available. The paper is self-contained in its main derivation chain and does not rename a known result as a prediction or fit a parameter and call it a prediction.
Assumptions & free parameters
free parameters (1)
- Smoothness constants L and \bar L and stepsizes \gamma, \lambda =
not fitted to data; chosen per problem, e.g. L > L_norm in Appendix D
assumptions (6)
- domain assumption Reference function \phi is proper, lsc, strongly convex, even, with \phi(0)=0 (Assumption 1.1).
- domain assumption \phi \in C^2(int dom \phi) and is essentially smooth (Assumption 1.2).
- domain assumption f is C^1, (L,\bar L)-anisotropically smooth relative to \phi, and bounded below (Assumption 3.1).
- domain assumption arg min f is nonempty and f is convex in the convex rate theorems (Assumption 3.4).
- domain assumption In Theorem 3.7, dom \phi = R^n and \phi is 2-subhomogeneous, i.e. \phi(\theta x) \le \theta^2 \phi(x) for \theta \in [0,1].
- standard math Standard tools from convex analysis and Clarke generalized calculus (Rockafellar and Wets, Clarke, Dontchev and Rockafellar).
Cite this review
Pith. "Pith review of Nonlinearly Preconditioned Gradient Methods under Generalized Smoothness." pith.science (2026). https://pith.science/paper/4JWMPV33
@misc{pith2026250208532,
author = {Pith},
title = {Pith review of: Nonlinearly Preconditioned Gradient Methods under Generalized Smoothness},
year = {2026},
howpublished = {\url{https://pith.science/paper/4JWMPV33}},
note = {Machine review of arXiv:2502.08532}
}
abstract
We analyze nonlinearly preconditioned gradient methods for solving smooth minimization problems. We introduce a generalized smoothness property, based on the notion of abstract convexity, that is broader than Lipschitz smoothness and provide sufficient first- and second-order conditions. Notably, our framework encapsulates algorithms associated with the gradient clipping method and brings out novel insights for the class of $(L_0,L_1)$-smooth functions that has received widespread interest recently, thus allowing us to extend beyond already established methods. We investigate the convergence of the proposed method in both the convex and nonconvex setting.
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Reference graph
Works this paper leans on
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[1]
Thus we can further bound (30): AK ξ(xK)−ξ(x ⋆) ≤ D0 K−1X k=0 a2 k+1 Ak+1
from (27). Thus we can further bound (30): AK ξ(xK)−ξ(x ⋆) ≤ D0 K−1X k=0 a2 k+1 Ak+1 . We chooseA k =k 2 and by using the fact that PK k=1 a2 k Ak ≤4K(Doikov & Nesterov, 2020, Equation (35)): AK ξ(xK)−ξ(x ⋆) ≤4D 0K. Dividing byA K we obtain: ξ(xK)−ξ(x ⋆)≤ 4D0 K . Noting thatf(x K)≤ξ(x K), from the envelope representation off, andξ(x ⋆) =f(x ⋆)we obtain th...
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Proof.The gradient and the hessian offare given respectively by ∇f(x) =− α 1 + exp(α⊤x) ,∇ 2f(x) = αα⊤ exp(−α⊤x)(1 + exp(α⊤x))2 .(38) 22 Nonlinearly Preconditioned Gradient Methods In this case the second-order condition becomes ai( ¯L−1∇f(x)) ∥α∥2 exp(−α⊤x)(1 + exp(α⊤x))2 < L¯L.(39) The results from Table 2 forϕ 1,ϕ 2 andϕ 3 then follow respectively from...
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Sincehis even, we have from (Bauschke et al., 2017b, Example 13.8) thath ∗ = (h◦ | · |)∗ =h ∗ ◦ | · |, which means thath∗ is also even. Therefore, through (Bauschke et al., 2017b, Proposition 11.7) we get thath ∗ is increasing onR +. Now, note that the functiong=h− µ 2 | · |2 is proper, lsc and convex whereµis the strong convexity parameter ofh. Moreover,...
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Then, from (Rockafellar & Wets, 1998, Theorem 11.23) we have thath ∗(y) = inf x σ[−1,1](x) + 1 2 (y−x) 2, whereσ [−1,1] is the support function of[−1,1]and in light of (Rockafellar & Wets, 1998, Exercise 11.27),h ∗′(y) = Π [−1,1](y), where Π[−1,1](y) = min(1,max(−1, y))is the projection on the closed convex set[−1,1]. Using Theorem 1.3 we thus obtain ∇ϕ∗(...
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In light of Theorem 2.2,f ∗ −L −1( ¯L ⋆ ϕ∗)is a convex function. By definition∇f(x)∈dom∂f ∗ ⊆domf ∗ for allx∈R n and as such we can consider the convex subgradient inequality forf ∗ −L −1( ¯L ⋆ ϕ∗)between points∇f(x)and∇f(x ⋆) and obtain: f ∗(∇f(x))−L −1 ¯Lϕ∗( ¯L−1∇f(x))≥f ∗(∇f(x ⋆)) +⟨x ⋆,∇f(x)⟩,(21) where we have used the fact that∇f(x ⋆) = 0,∇ϕ ∗(0) = ...
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Reviewed August 8, 2026 · model on record in the stance chip above.
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