REVIEW 3 major objections 5 minor 31 references
A first-order method for constrained nonconvex-nonconcave minimax optimization
T0 review · 3 major / 5 minor · reviewed 2026-08-04 · deepseek-v4-flash
Pith's one-line read Under a local Kurdyka-Lojasiewicz condition on a lifted inner problem, constrained nonconvex-nonconcave minimax problems can be solved by an inexact proximal gradient method with explicit first-order oracle complexity.
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 mechanism is the lifted minimax reformulation together with the local KL condition imposed on it. The paper replaces the constraint $c(y)\leq 0$ by surrogate constraints $\bar c(y,z)\leq 0$ built from a linearization of c at a reference point z plus a quadratic term, so that the lifted feasible region becomes an intersection of Euclidean balls. Lemma 3 shows the lifted problem is exactly equivalent to the original one. The local KL inequality (Assumption 1(iv)) then forces both a local growth bound on the distance from y to the inner maximizers and a generalized Holder modulus for the maximal function. The SCP subroutine (Algorithm 1) uses these surrogate constraints as convex balls,
What would settle it
Run Algorithm 2 on a deterministic family of instances in which the inner problem has known KL exponent $\theta$ and level-set shrink rate $\sigma$ at the boundary of the allowed range—for example, extend Example 1 so that $\sigma>0$—and compare measured iteration counts against the $\epsilon$ exponents in Theorem 6. Alternatively, construct a problem where Assumption 1(iv) holds with $x$-dependent KL constants and check whether the algorithm still converges at the stated rate; if it does, the uniformity assumption is not necessary.
Extended reading notes
Core claim
On the paper's own terms, the discovery is that the constrained inner problem's maximal function $F^*(x) = \max_{c(y)\leq 0} (f(x,y)-q(y))$ inherits a local generalized Holder smoothness from a local KL condition imposed on the lifted function $\bar F(x,y,z) = f(x,y) - q(y) - \delta_{\bar c(y,z)\leq 0} - \delta_Y(z)$, where $\bar c_j(y,z) = c_j(z) + \nabla c_j(z)^T (y-z) + \frac{L_{c_j}}{2} \|y-z\|^2$. Theorem 1 bounds $\nabla F^*$ differences by $L_{\nabla f}\|x-x'\|$ plus $\|x-x'\|^{(1-\theta)/\theta}$ on the region away from stationarity. Using this, Algorithm 2—an inexact proximal gradient method whose inner subproblems are solved by a sequential convex programming routine—outputs an $(\gamma \epsilon^\sigma/(4L_f), \epsilon)$-stationa.
Load-bearing premise
The entire complexity picture rests on Assumption 1(iv): the lifted inner objective must satisfy a local KL inequality with constants $C$ and $\theta$ that do not depend on the outer variable $x$, on level sets that shrink no faster than $\mathrm{dist}(0,\partial \bar\Psi(x))^\sigma$.
Editorial extensions
If this is right
- Penalty reformulations of fully constrained minimax problems min_{d(x)<=0} max_{c(y)<=0} [f(x,y)+p(x)-q(y)] become solvable by the same method, because the penalty subproblems have exactly the form (1).
- The local-KL assumption covers problems where global PL or global KL conditions fail; Example 1 demonstrates a concrete instance satisfying the local condition but not the global one.
- The oracle complexity of Algorithm 2 is explicit: only proximal evaluations of p and q plus gradients of f are needed, no higher-order or stochastic oracles.
- If the KL exponent is \theta=1/2 (the PL-like case), the worst-case iteration complexity degenerates to O(\epsilon^{-2\max\{1,\sigma\}} \log^2(1/\epsilon)), showing how the level-set shrink rate \sigma enters the final cost.
- Preliminary numerical experiments on exponentially constrained problems show large objective reductions, indicating the method behaves as predicted in practice.
Reading between the lines
- If the uniform-KL assumption fails because the constants C or \theta genuinely depend on x, the stated exponents are not justified; one would expect the effective complexity to degrade by whatever factor the constants grow near stationarity.
- A natural test is to generate semi-algebraic instances where the KL exponent \theta and shrink parameter \sigma are known exactly, then measure iteration counts and compare them with the predicted \epsilon exponents; this would separate tightness of the analysis from looseness of the hidden constants.
- A parameter-free variant of Algorithm 2—which the paper notes as worthwhile—could be built by estimating L_f, C, \theta, \gamma, and \sigma on the fly; if such estimates converge, the method would retain its complexity while dropping the input-parameter requirement.
- The lifted reformulation is the key conceptual export: it converts a nonconvex constraint into a family of convex ball constraints, so the same lifting might extend to other first-order schemes such as accelerated or stochastic variants.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies constrained nonconvex-nonconcave minimax problems of the form (1), where the inner maximization is subject to smooth constraints c(y) ≤ 0. A lifted reformulation (10) is introduced, and under a local Kurdyka–Lojasiewicz condition on the lifted inner problem (Assumption 1(iv)), the authors claim that the maximal function F* is locally generalized Hölder smooth (Theorem 1) and enjoys a local growth property (Theorem 2). They then propose a sequential convex programming method (Algorithm 1) for a constrained KL minimization problem, prove its finite termination and convergence rate (Theorems 4–5), and use it as an inner solver in an inexact proximal gradient method (Algorithm 2). The headline results, Theorems 6–7, state iteration and gradient-evaluation complexity bounds for computing an approximate stationary point of (1) under Assumptions 1–2. Numerical experiments on random instances illustrate the behavior of Algorithm 2.
Significance. If the main results are correct, this is the first complexity guarantee for constrained nonconvex-nonconcave minimax optimization with nonlinear inner constraints. The SCP part of the paper (Theorems 3–5 and Lemma 1) is a genuine contribution and appears, from the arguments given, to be sound; the uniform Slater point obtained from MFCQ in Theorem 3 is a useful device, and the explicit convergence-rate analysis for the SCP subroutine is a strength. The local KL framework is clearly motivated and the paper does not fit parameters to data; the KL constants are assumptions rather than fitted quantities. However, the central complexity claims currently rest on two unproved pillars: the local Hölder smoothness lemma (Lemma 7) and the transfer of the entire complexity analysis from the authors' prior work [17]. In addition, the uniform Lagrange-multiplier bound used in the complexity theorems requires a boundedness condition that is not stated in Assumptions 1–2. These issues must be repaired before the significance claim can be accepted.
major comments (3)
- [§4, Lemma 2 and Theorems 6–7] Lemma 2 requires the additional condition G_f := max_{X×Y} ‖∇_y f(x,y)‖ < ∞ (Eq. (40)), but Theorems 6 and 7 are stated under only Assumptions 1–2. This condition is not implied by Assumption 1: Assumption 1(i) bounds ∇_x f through L_f-Lipschitz continuity in x, but ∇_y f can grow with x when X = dom p is unbounded. The uniform bound A_f is then used in the definitions of ω_f, α_f, and K_{f,θ} in Theorem 7, and it affects the SCP termination constants used through Lemma 10 in the proof of Theorem 6. Without a uniform bound, the stated finite complexity does not follow. Please add G_f < ∞ (or a sufficient condition such as boundedness of X) to the assumptions, or prove that Algorithm 2 keeps iterates in a set where such a uniform bound holds.
- [§6.1, Lemma 7; §6.3, Lemmas 10–11 and Theorems 6–7] The proof of Lemma 7 is omitted with the explanation that it follows from Lemma 6 and arguments parallel to [17, Theorem 1]. Similarly, Lemmas 10 and 11 are stated as ‘parallel’ to [17, Lemmas 9 and 10], and the proofs of Theorems 6 and 7 are only sketches referring to [17, Theorems 4 and 5]. Lemma 7 is the basis for the local generalized Hölder smoothness of F* (Theorem 1), which in turn justifies the inexact proximal model (38) used throughout the analysis. Because the lifted problem introduces the extra variable z, the term δ_Y(z), and the constraint mapping ¯c, the transfer from the unconstrained setting in [17] is not immediate. A journal submission needs full proofs or a precise statement of which inequalities carry over, with all constants verified. As written, the central complexity claims are not verifiable from the manuscript.
- [§3, Algorithm 1, line 4; §4, Theorem 7] Theorem 7 is billed as a first-order oracle complexity bound, but each iteration of Algorithm 1 requires solving the convex subproblem (27), which is a constrained problem with an intersection-of-balls feasible set. The text suggests solving this subproblem with a customized interior-point method (Remark after Algorithm 1). The count bN_ϵ bounds the number of SCP outer iterations, not the number of gradient/prox evaluations needed to solve the subproblems to the required accuracy. Unless the authors provide a first-order algorithm for (27) with an explicit cost, or explicitly declare a different oracle model in which each convex subproblem solve is counted as a unit, the claimed first-order oracle complexity is not an end-to-end guarantee.
minor comments (5)
- [§1.1, Theorem 4] The notation ⌈·⌉_+ in the definition of ¯i is not defined. Please define the positive part explicitly.
- [§2, Example 1] The verification that the local KL condition holds with C=(3/2)^{1/2}, θ=1/2, γ=1/2, σ=0 is too terse. Please expand the computation of the level set bL_x and the subdifferential distance, as this example is the only concrete illustration of Assumption 1(iv).
- [§4, Theorem 6] The constants bC_1, …, bC_8 are defined with no explanation of their roles. A sentence or two motivating each group would greatly improve readability.
- [§5, numerical experiments] The choice of KL parameters (C=0.1, θ=0.5, γ=0.01, σ=0.1) is said to be empirical. Since the convergence theory depends on these parameters, the paper should state whether problem (42) is known to satisfy Assumption 1(iv) with these values, or clearly indicate that the experiments are exploratory.
- [References] Reference [17] is listed as ‘Accepted by SIAM Journal on Optimization’. If it is now published, please give the full journal reference and year.
Circularity Check
No significant circularity: the main results are genuine derivations from stated KL/MFCQ assumptions; the heavy use of the authors' prior work [17] is reliance on an accepted external result, not an identity or fitted prediction.
full rationale
The paper's derivation chain is not circular. Assumption 1(iv) posits a local KL inequality on the lifted function F-bar; Theorem 1, Corollary 1, Theorem 2 and Lemmas 5-7 then derive generalized Holder smoothness, a descent inequality, and a growth/error bound. These are implications, not restatements: the KL inequality controls the slope of F-bar and yields the asserted bounds through Lemma 4 and elementary calculus, and no parameter appearing in the conclusions is fitted to the quantity being predicted. The SCP convergence theory (Theorems 4-5) is proved from Assumptions 1-3 and Theorem 3, with explicit constants; the termination bound K_theta is a consequence of the descent inequality plus KL, not an input. The complexity claims in Theorems 6-7 are not obtained by fitting: they follow from the stated assumptions, Lemma 2, and Lemmas 10-11, which are stated as consequences of Theorems 1, 2 and 5. The proofs of Theorems 6-7 are sketched by analogy to [17, Theorems 4-5], but [17] is a prior, separately published/accepted work by the same authors, and the current paper introduces genuinely new ingredients for the constrained setting (the lifted problem, MFCQ-based uniform Slater point in Theorem 3, the SCP subroutine, and the multiplier bound in Lemma 2). Citing one's own earlier work as a proof template is not circular unless the cited claim is itself unverified or equivalent to the current claim; that is not the case here. The numerical section sets KL parameters C=0.1, theta=0.5, gamma=0.01, sigma=0.1 by hand, but these are fixed inputs for a demonstration and do not enter the theorem statements as fitted predictions. The one substantive mathematical concern is that Lemma 2 requires G_f = max_{X x Y} ||grad_y f|| < infinity, while this condition is not stated among Assumptions 1-2 and can fail for unbounded X; the skeptic's example illustrates a gap in the hypotheses under which Theorems 6-7 are asserted. That is a missing assumption / correctness risk, not a circularity: it does not make any theorem's conclusion identical to its input. Accordingly, no circular step is identified and the score is 0.
Assumptions & free parameters
free parameters (5)
- KL constant C
- KL exponent θ =
0.5
- level-set coefficient γ
- level-set shrink exponent σ
- algorithm parameters L, ρ, β, ε =
L=1, ρ=1.25, β=10, ε=10^{-2}
assumptions (6)
- standard math Standard variational analysis: limiting subdifferential calculus, normal cones, slope/limiting-slope properties, KL theory (Rockafellar-Wets, Drusvyatskiy et al.)
- domain assumption Assumption 1(i)-(iii): f is L_f-Lipschitz in x and L_∇f-smooth on X×Y; p,q simple closed convex with computable prox; Y compact; c_j Lcj-smooth and C² on Y.
- domain assumption Assumption 1(iv): local KL condition on the lifted function ¯F with level sets L(x) = {(y,z): 0<¯F*−¯F ≤ γ dist(0,∂¯Ψ(x))^σ}.
- domain assumption Assumption 2 (MFCQ) at every feasible y∈S.
- domain assumption Assumption 3: SCP objective g is L-smooth and its lifted ¯h satisfies a KL condition on the level set {¯h*<¯h≤¯h*+η}.
- ad hoc to paper [17, Theorems 1, 3-5] are correct and directly applicable to the lifted problems with identical constants.
invented entities (2)
-
Lifted variable z and lifted constraint ¯c(y,z)
-
Local KL level-set function L(x) with shrink exponent σ
Cite this review
Pith. "Pith review of A first-order method for constrained nonconvex-nonconcave minimax optimization." pith.science (2026). https://pith.science/paper/G2KQUG7N
@misc{pith2026251001168,
author = {Pith},
title = {Pith review of: A first-order method for constrained nonconvex-nonconcave minimax optimization},
year = {2026},
howpublished = {\url{https://pith.science/paper/G2KQUG7N}},
note = {Machine review of arXiv:2510.01168}
}
read the original abstract
We study a class of constrained nonconvex-nonconcave minimax optimization problems in which the inner maximization involves potentially complex constraints. Under the assumption that the inner problem of a novel lifted minimax reformulation satisfies a local Kurdyka-Lojasiewicz (KL) condition, we show that the maximal function of the original problem enjoys a local generalized H\"{o}lder smoothness property. We also propose a sequential convex programming (SCP) method for solving constrained optimization problems and establish its convergence rate under a local KL condition. Leveraging these results, we develop an inexact proximal gradient method for the original minimax problem, where the inexact gradient of the maximal function is computed via the SCP method applied to a locally KL-structured subproblem. Finally, we establish complexity guarantees for the proposed method in computing an approximate stationary point of the original minimax problem.
Figures
Reference graph
Works this paper leans on
- [17]
-
[1]
Arjovsky, S
M. Arjovsky, S. Chintala, and L. Bottou. Wasserstein generative adversarial networks. In International Conference on Machine Learning, pages 214–223, 2017
2017
-
[2]
Attouch, J
H. Attouch, J. Bolte, P. Redont, and A. Soubeyran. Proximal alternating minimization and projection methods for nonconvex problems: An approach based on the Kurdyka- Lojasiewicz inequality.Mathematics of Operations Research, 35(2):438–457, 2010
2010
-
[3]
Bertsimas, D
D. Bertsimas, D. B. Brown, and C. Caramanis. Theory and applications of robust optimization. SIAM review, 53(3):464–501, 2011
2011
-
[4]
Blanchet, J
J. Blanchet, J. Li, S. Lin, and X. Zhang. Distributionally robust optimization and robust statistics.Statistical Science, 40(3):351–377, 2025
2025
-
[5]
A. B¨ ohm. Solving nonconvex-nonconcave min-max problems exhibiting weak Minty solutions. arXiv preprint arXiv:2201.12247, 2022
arXiv 2022
-
[6]
Bolte, A
J. Bolte, A. Daniilidis, and A. Lewis. The Lojasiewicz inequality for nonsmooth subanalytic functions with applications to subgradient dynamical systems.SIAM Journal on Optimization, 17(4):1205–1223, 2007
2007
-
[7]
Bolte, A
J. Bolte, A. Daniilidis, A. Lewis, and M. Shiota. Clarke subgradients of stratifiable functions. SIAM Journal on Optimization, 18(2):556–572, 2007
2007
Show all 31 references
-
[8]
Cai and W
Y. Cai and W. Zheng. Accelerated single-call methods for constrained min-max optimization. arXiv preprint arXiv:2210.03096, 2022
2022 arXiv
-
[9]
B. Dai, A. Shaw, L. Li, L. Xiao, N. He, Z. Liu, J. Chen, and L. Song. SBEED: Convergent reinforcement learning with nonlinear function approximation. InInternational Conference on Machine Learning, pages 1125–1134, 2018. 25
2018
-
[10]
Drusvyatskiy, A
D. Drusvyatskiy, A. D. Ioffe, and A. S. Lewis. Curves of descent.SIAM Journal on Control and Optimization, 53(1):114–138, 2015
2015
-
[11]
Drusvyatskiy, A
D. Drusvyatskiy, A. D. Ioffe, and A. S. Lewis. Nonsmooth optimization using Taylor-like models: error bounds, convergence, and termination criteria.Mathematical Programming, 185:357–383, 2021
2021
-
[12]
Goodfellow, J
I. Goodfellow, J. Pouget-Abadie, M. Mirza, B. Xu, D. Warde-Farley, S. Ozair, A. Courville, and Y. Bengio. Generative adversarial networks.Communications of the ACM, 63(11):139–144, 2020
2020
-
[13]
F. Huang. Enhanced adaptive gradient algorithms for nonconvex-PL minimax optimization. arXiv preprint arXiv:2303.03984, 2023
2023 arXiv
-
[14]
J. Li, L. Zhu, and A. M.-C. So. Nonsmooth nonconvex–nonconcave minimax optimization: Primal–dual balancing and iteration complexity analysis.Mathematical Programming, pages 1–51, 2025
2025
-
[15]
M. Liu, H. Rafique, Q. Lin, and T. Yang. First-order convergence theory for weakly-convex- weakly-concave min-max problems.Journal of Machine Learning Research, 22(169):1–34, 2021
2021
-
[16]
Z. Lu. Sequential convex programming methods for a class of structured nonlinear programming. arXiv preprint arXiv:1210.3039, 2012
2012 arXiv
-
[18]
Madry, A
A. Madry, A. Makelov, L. Schmidt, D. Tsipras, and A. Vladu. Towards deep learning models resistant to adversarial attacks.arXiv preprint arXiv:1706.06083, 2017
2017 arXiv
-
[19]
Nesterov
Y. Nesterov. Universal gradient methods for convex optimization problems.Mathematical Programming, 152(1):381–404, 2015
2015
-
[20]
Nouiehed, M
M. Nouiehed, M. Sanjabi, T. Huang, J. D. Lee, and M. Razaviyayn. Solving a class of non-convex min-max games using iterative first order methods.Advances in Neural Information Processing Systems, 32, 2019
2019
-
[21]
Omidshafiei, J
S. Omidshafiei, J. Pazis, C. Amato, J. P. How, and J. Vian. Deep decentralized multi-task multi-agent reinforcement learning under partial observability. InInternational Conference on Machine Learning, pages 2681–2690, 2017
2017
-
[22]
Pethick, P
T. Pethick, P. Latafat, P. Patrinos, O. Fercoq, and V. Cevher. Escaping limit cycles: Global convergence for constrained nonconvex-nonconcave minimax problems.arXiv preprint arXiv:2302.09831, 2023
2023 arXiv
-
[23]
Rahimian and S
H. Rahimian and S. Mehrotra. Frameworks and results in distributionally robust optimization. Open Journal of Mathematical Optimization, 3:1–85, 2022
2022
-
[24]
R. T. Rockafellar.Convex analysis, volume 28. Princeton University Press, 1997
1997
-
[25]
R. T. Rockafellar and R. J.-B. Wets.Variational analysis, volume 317. Springer Science & Business Media, 2009
2009
-
[26]
Sinha, H
A. Sinha, H. Namkoong, R. Volpi, and J. Duchi. Certifying some distributional robustness with principled adversarial training.arXiv preprint arXiv:1710.10571, 2017. 26
2017 arXiv
-
[27]
Xu, Z.-Q
Z. Xu, Z.-Q. Wang, J.-L. Wang, and Y.-H. Dai. Zeroth-order alternating gradient descent ascent algorithms for a class of nonconvex-nonconcave minimax problems.Journal of Machine Learning Research, 24(313):1–25, 2023
2023
-
[28]
J. Yang, A. Orvieto, A. Lucchi, and N. He. Faster single-loop algorithms for minimax optimization without strong concavity. InInternational Conference on Artificial Intelligence and Statistics, pages 5485–5517, 2022
2022
-
[29]
P. Yu, T. K. Pong, and Z. Lu. Convergence rate analysis of a sequential convex programming method with line search for a class of constrained difference-of-convex optimization problems. SIAM Journal on Optimization, 31(3):2024–2054, 2021
2024
-
[30]
Zheng, A
T. Zheng, A. M.-C. So, and J. Li. Doubly smoothed optimistic gradients: A universal approach for smooth minimax problems.arXiv preprint arXiv:2506.07397, 2025
2025 arXiv
-
[31]
Zheng, L
T. Zheng, L. Zhu, A. M.-C. So, J. Blanchet, and J. Li. Universal gradient descent ascent method for nonconvex-nonconcave minimax optimization.Advances in Neural Information Processing Systems, 36:54075–54110, 2023. 27
2023
Reviewed August 4, 2026 · model on record in the stance chip above.
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