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Convergence rates of regularized quasi-Newton methods without strong convexity

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Pith's one-line read This paper proves that regularized SR1 quasi-Newton methods enjoy explicit non-asymptotic superlinear convergence rates on nonconvex nonsmooth problems satisfying a Kurdyka–Łojasiewicz inequality, without strong convexity, line search, or…

desk verdict Smooth-case rates are a real advance; the nonsmooth composite claim is an oracle result that needs either a proof of Remark 4.5 or an honest caveat. read the letter →

arxiv 2506.00521 v6 pith:CMUHJFA6 submitted 2025-05-31 math.OC

classification math.OC MSC 90C5390C2690C2549J52
keywords quasi-NewtonmethodsSR1updateKurdyka–Łojasiewiczpropertynon-asymptoticconvergenceratescubicregularizationgradientnonconvexnonsmoothoptimizationsuperlinear
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The pith

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

The reading

Strong convexity is the usual engine behind fast quasi-Newton convergence; this paper shows the Kurdyka–Łojasiewicz (KL) property can play that role instead. It analyzes two regularized SR1 proximal quasi-Newton methods—a cubic-regularized one for general nonconvex nonsmooth composite objectives and a gradient-regularized one for convex objectives—and derives explicit non-asymptotic bounds on the subgradient norm after an initial number of iterations. The headline result is a superlinear rate of order $\left(\frac{C}{(k-k_0)^{1/2}}\right)^{(k-k_0)/2}$ when the KL desingularizing function is $\phi(t)=ct^{1/2}$. These are the first such rates for regularized proximal SR1 methods without strong convexity, and they require no line search, trust region, Dennis–Moré condition, or assumptions on the quasi-Newton metrics.

What carries the argument

The trace potential $V(G)=\operatorname{tr} G$ together with the SR1 update formula (5) is the central object. Each SR1 step decreases the trace by $\nu(A,G,u)=\frac{u^\top(G-A)^2u}{u^\top(G-A)u}$, which measures how much closer the metric $G$ gets to the average Hessian $J_k$; cubic or gradient regularization adds enough curvature to keep $J_k\preceq G_{k+1}\preceq\tilde G_{k+1}$ and to control the trace. The KL inequality then converts the guaranteed function decrease into control of $\|F'(x_k)\|$, and summing the trace decreases over iterations yields a geometric-mean contraction that becomes superlinear.

What would settle it

Take a KL function $F=g+f$ with $g=\|x\|_1$ and smooth $f$ satisfying Assumptions 1–2 with $\phi(t)=ct^{1/2}$, run the Cubic SR1 PQN update (6) using a standard proximal gradient solver with a fixed tolerance for the cubic subproblem, and check whether $\|F'(x_k)\|$ obeys the bound (10) for all $k\ge k_0$. A violation, or the need to solve the subproblem to machine precision to observe the bound, would show the theorem depends on exact subproblem minimization rather than on the structure analyzed.

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Extended reading notes

Core claim

The central claim is that the KL inequality with a desingularizing function $\phi$ replaces strong convexity as the driver of superlinear convergence. Under Assumptions 1 and 2 (Lipschitz smoothness plus KL) and boundedness of the generated sequence, both algorithms have $\|F'(x_k)\| o 0$, and for $k\ge k_0$ explicit rates hold; for example, when $\phi(t)=ct^{1/2}$, Cubic SR1 PQN satisfies $\|F'(x_{N+k_0})\| \le \left(\frac{3c^2}{4}\left(\frac{C^{\mathrm{CR}}_1}{N}+\frac{C^{\mathrm{CR}}_2}{N^{1/2}}\right)\right)^{N/2}\|F'(x_{k_0})\|$. The convex counterpart Grad SR1 PQN achieves an analogous bound with gradient regularization instead of cubic terms. The paper presents this as the first non-asymptotic explicit superlinear convergence result for regularized proximal SR1 methods on nonconvex nonsmooth KL objectives, and notes the rates are new even for smooth nonconvex problems.

Load-bearing premise

Each iteration must compute an exact global minimizer of a cubic-regularized proximal subproblem; for general nonsmooth $g$ no algorithm is supplied that can do this, so the non-asymptotic rates are not established for any implementable inexact version.

Editorial extensions

If this is right

  • For any nonconvex nonsmooth KL objective with desingularizer $\phi(t)=ct^{1/2}$, Cubic SR1 PQN attains a subgradient-norm rate of order $\left(\frac{C}{(k-k_0)^{1/2}}\right)^{(k-k_0)/2}$ for all $k\ge k_0$, which is superlinear.
  • The same type of guarantee holds for Grad SR1 PQN on convex KL objectives, with gradient regularization instead of cubic terms, at lower per-iteration cost.
  • No line search, trust region, Dennis–Moré condition, or strong convexity is needed for these rates; the only global mechanism is the restarting rule that resets the metric to $LI$ when its trace exceeds $n\bar\kappa$.
  • When $F$ satisfies a global Łojasiewicz inequality, the rates become global, holding from the first iteration rather than after an initial $k_0$.
  • The analysis covers nonsmooth additive composite problems $F=g+f$ with nonconvex $g$, so the result applies beyond smooth objectives.

Reading between the lines

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

  • The practical bottleneck is the cubic subproblem: the theorems assume an exact global minimizer, and the experiments solve only smooth problems; if inexact solvers are used, the rates may need a tolerance-dependent correction to remain valid.
  • The trace-restart mechanism looks transferable: any quasi-Newton update that preserves $J_k\preceq G_{k+1}\preceq\tilde G_{k+1}$ and decreases the trace could inherit the same superlinear argument under KL, so other metric updates may admit similar bounds.
  • For the $\theta=1/2$ case, the KL exponent coincides with gradient domination conditions, suggesting the result applies to overparameterized models where strong convexity fails but such domination often holds.
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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 / 5 minor

Summary. The manuscript studies non-asymptotic convergence rates for two regularized SR1 quasi-Newton methods: Cubic SR1 PQN (Algorithm 1) for possibly nonconvex nonsmooth composite objectives F = g + f satisfying the Kurdyka-Łojasiewicz property, and Grad SR1 PQN (Algorithm 2) for convex composite objectives. Under Assumptions 1-2 (Lipschitz-smooth f with Lipschitz Hessian, KL property of F), Theorems 4.2 and 4.9 establish global subsequential convergence and explicit rates for the subgradient norm, with last-iterate superlinear rates when the desingularizing function is φ(t)=ct^{1-θ} for θ≤1/2 and window-minimum sublinear rates for θ∈(1/2,1). Theorem 4.13 and Appendix A add global non-asymptotic rates under a gradient-domination (Łojasiewicz) inequality. The proof relies on a trace potential V(G)=tr G and rank-one SR1 update identities, following the framework of [54]. Section 6 reports experiments on smooth quadratic, logistic-regression, and image-deblurring problems.

Significance. If the main theorems are read as conditional mathematical statements, the trace-potential argument is a genuine and nontrivial extension of the strongly convex analysis in [54] to KL functions, and the explicit rates for smooth nonconvex objectives appear novel. The paper also deliberately avoids line search, trust regions, the Dennis-Moré condition, and strong convexity, which are standard assumptions in quasi-Newton theory. The numerical experiments confirm the expected superlinear behavior on smooth problems. However, the advertised nonsmooth composite results currently rely on an exact global-minimization oracle for the subproblem, and Remark 4.5, which claims an inexact relaxation, is unproved; the practical contribution is therefore weaker than the abstract suggests.

major comments (3)
  1. [Section 4.2.1, Eq. (6) and (15); Remark 4.5; Section 6] The nonsmooth composite rates in Theorems 4.2 and 4.9 apply to an oracle algorithm. Algorithm 1, Step 1a, Eq. (6), and Algorithm 2, Step 1, Eq. (15), require a global minimizer of a cubic-regularized composite subproblem. For general nonconvex g this subproblem is nonconvex, and even for nonsmooth convex g exact global minimization is not generally implementable. Remark 4.5 asserts that a stationary point satisfying Eq. (12) suffices, but no proof is given that Eq. (12) preserves the sufficient-decrease, sumability, and rate arguments of Section 5. Section 6 explicitly tests only smooth problems and states that nonsmooth subproblem solving is left to future work. To support the abstract's nonsmooth composite claim, the authors should either provide an implementable (possibly inexact) subproblem solver together with a proof that the rates survive, prove the relaxation in Remark 4.5, or restrict the theoretical claims to an exact-oracle setting and reword the abstract and introduction accordingly.
  2. [Abstract; Theorem 4.2, Eqs. (8), (9), (10), (11)] The abstract states that 'after a number of iterations k0, Cubic SR1 PQN exhibits non-asymptotic explicit super-linear convergence rates', but this overstates the theorem. For a general desingularizing function φ, Theorem 4.2, Eq. (8), gives a window-minimum rate with exponent N/(N+1), which is sublinear, not superlinear. Last-iterate superlinear rates are proven only for φ(t)=ct^{1-θ} with θ∈(0,1/2] (Eqs. (9) and (10)). For θ∈(1/2,1), Eq. (11) is again a window-minimum sublinear rate. The abstract and the introductory summary should qualify the superlinearity claim by the class of desingularizing functions.
  3. [Theorem 4.13, Eq. (21), and Proof of Theorem 4.13, Eq. (176)] The global rate in Theorem 4.13, Eq. (21), states ∥∇f(x_N)∥ ≤ (c^2 C_G D/(2N))^{N/2} ∥∇f(x_0)∥^{2/(N+1)}, but the proof in Eq. (176) derives the same bound with ∥∇f(x_0)∥ to the first power and no extra factor. The telescoping product in the proof does not produce the exponent 2/(N+1). The theorem statement and the proof therefore disagree, and the printed global-rate claim is not established as written. This should be corrected before the result can be used.
minor comments (5)
  1. [Proof of Theorem 4.2, Eq. (87)] The proof of Theorem 4.2 ends with an exponent 2/(N+1) on g_{k0}, while the theorem statement in Eq. (8) and the intervening derivation give 1/(N+1). This is likely a typographical error, but it should be aligned.
  2. [Remark 4.6 and Appendix A, Theorem A.2] The displayed rate in Remark 4.6, Eq. (14), appears as '≤ μ 6 (...)' and should read '≤ (6/μ) (...)' as in Theorem A.2. Please correct the notation.
  3. [Algorithm 2, Step 3, and Lemma 5.19, Eq. (109)] Lemma 5.19 states ∥F'(x_k)∥ ≤ λ_k^2/L_H for k ≥ k0, but Algorithm 2 defines λ_{k+1} = sqrt(L_H ∥F'(x_{k+1})∥) + L_H r_k, so the inequality is valid for indices shifted by one and not for k=0 with λ_0=0. The index convention should be stated explicitly.
  4. [Section 6.1.1, logistic regression] The text says the Lipschitz constant of the Hessian is L_H = 4 max_i ∥a_i∥, but the experiment sets L_H = 4 heuristically. If the theoretical rates are to be compared to the experiment, the authors should report whether the heuristic value is actually a valid upper bound for the chosen data set and parameters.
  5. [Theorem 4.2 and proof, constants] The constant C_CR1 in the theorem statement is (n+1)L + nκ̄ + 2nL_H R, while in the proof of Eq. (82) it is defined as (n+1)L + 2nκ̄ + 2L_H R. These should be reconciled, including the factor of n on κ̄.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the rates follow from stated KL/smoothness assumptions through self-contained trace-potential estimates.

Assumptions & free parameters 1 free parameters · 4 assumptions · 0 invented entities

The central claim rests on standard smoothness and KL assumptions plus a strong oracle assumption: the exact solution of a cubic-regularized proximal subproblem. No new physical or mathematical entities are introduced; the only hand-chosen algorithmic parameter is kappa bar.

free parameters (1)
  • kappa bar (restart threshold) = user-chosen, kappa bar >= L
    Restart threshold and trace bound in Algorithms 1 and 2; appears in all rate constants. The theorems hold for any kappa bar >= L, so it is a legitimate algorithmic parameter rather than a fitted constant.
assumptions (4)
  • domain assumption F = g + f is bounded below, g is proper lsc, f is twice differentiable with L-Lipschitz gradient and LH-Lipschitz Hessian, argmin F nonempty (Assumption 1).
    Standing assumptions for the algorithms' decrease lemmas and subgradient bounds.
  • domain assumption F is a KL function with desingularizing function phi (Assumption 2).
    Central condition replacing strong convexity; rates are expressed in terms of phi.
  • standard math Uniformized KL property over the cluster set (Lemma 3.1, from [10]).
    Allows passing from pointwise KL to a uniform neighborhood inequality, used to get non-asymptotic bounds.
  • ad hoc to paper Exact global minimization of the cubic-regularized subproblem in each iteration (Algorithm 1 Step 1a equation (6), Algorithm 2 Step 1 equation (15)).
    Required by the optimality conditions (40)/(92); no inexactness tolerance or solver is provided, and Section 6 limits experiments to smooth problems.

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Pith. "Pith review of Convergence rates of regularized quasi-Newton methods without strong convexity." pith.science (2026). https://pith.science/paper/CMUHJFA6

@misc{pith2026250600521,
  author       = {Pith},
  title        = {Pith review of: Convergence rates of regularized quasi-Newton methods without strong convexity},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/CMUHJFA6}},
  note         = {Machine review of arXiv:2506.00521}
}
abstract

In this paper, we study convergence rates of the cubic regularized proximal quasi-Newton method (\csr) for solving non-smooth additive composite problems that satisfy the so-called Kurdyka-\L ojasiewicz (K\L ) property with respect to some desingularization function $\phi$ rather than strong convexity. After a number of iterations $k_0$, Cubic SR1 PQN exhibits non-asymptotic explicit super-linear convergence rates for any $k\geq k_0$. In particular, when $\phi(t)=ct^{1/2}$, Cubic SR1 PQN has a convergence rate of order $\left(\frac{C}{(k-k_0)^{1/2}}\right)^{(k-k_0)/2}$, where $k$ is the number of iterations and $C>0$ is a constant. For the special case, i.e. functions which satisfy \L ojasiewicz inequality, the rate becomes global and non-asymptotic. This work presents, for the first time, non-asymptotic explicit convergence rates of regularized (proximal) SR1 quasi-Newton methods applied to non-convex non-smooth problems with K\L\ property. Actually, the rates are novel even in the smooth non-convex case. Notably, we achieve this without employing line search or trust region strategies, without assuming the Dennis-Mor\'e condition, without any assumptions on quasi-Newton metrics and without assuming strong convexity. Furthermore, for convex problems, we focus on a more tractable gradient regularized quasi-Newton method (Grad SR1 PQN) which can achieve results similar to those obtained with cubic regularization. We also demonstrate, for the first time, the non-asymptotic super-linear convergence rate of Grad SR1 PQN for solving convex problems with the help of the \L ojasiewicz inequality instead of strong convexity.

Figures

Figures reproduced from arXiv: 2506.00521 by the authors.

Figure 1
Figure 1. SR1 method significantly outperforms first-order methods in number of iterations and in terms of time. 6.1.1 Logistic regression with a convex regularization The second experiment is a logistic regression problem with regularization on the benchmark dataset “mushroom” from UCI Machine Learning Repository [53]: min x∈Rn f(x), with f(x) := 1 m Xm i=1 log(1 + exp(−bia ⊤ i x)) + µ p ∥x∥ 2 + ϵ , (178) where ai ∈ R n, bi … view at source ↗
Figure 2
Figure 2. Cubic- and Grad SR1 PQN significantly outperform first-order methods in number of [PITH_FULL_IMAGE:figures/full_fig_p027_2.png] view at source ↗
Figure 3
Figure 3. In the number of iterations and the norm of the gradients, Cubic Newton and Cubic [PITH_FULL_IMAGE:figures/full_fig_p028_3.png] view at source ↗

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