Accelerated Zeroth-Order and First-Order Momentum Methods from Mini to Minimax Optimization

Feihu HuangShangqian GaoJian PeiHeng Huang

article2022JMLR73 citations

Develops momentum-based accelerated zeroth-order and first-order algorithms for nonconvex minimization and minimax problems that achieve state-of-the-art query and gradient complexities without relying on large batch sizes.

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Modern machine learning applications increasingly rely on complex optimization, ranging from training models against adversarial attacks to solving multi-agent decision problems. However, calculating exact mathematical gradients is often impractical or impossible, especially in black-box systems where practitioners can only observe model outputs rather than internal calculations. While gradient-free and minimax optimization techniques exist, standard approaches suffer from high computational noise and typically require impractically large data batches at every iteration, leading to excessive evaluation costs and slow processing.

The article aims to resolve these computational bottlenecks by introducing a family of accelerated momentum-based algorithms for both single-objective minimization and two-player minimax optimization. Specifically, the authors evaluate whether combining momentum-based noise reduction with uniform smoothing techniques can achieve provably faster convergence and lower query costs in both gradient-free and gradient-accessible settings, with and without variable constraints.

To demonstrate this, the authors develop theoretical proofs establishing query and gradient complexity bounds for both constrained and unconstrained problems. They also validate their theoretical guarantees through practical computational experiments, including generating black-box adversarial attacks against deep neural networks across standard image benchmarks (MNIST, FashionMNIST, CIFAR-10, and SVHN) and simulating data poisoning attacks on logistic regression models.

The findings show that the proposed gradient-free minimization method reduces function query complexity while operating efficiently with a single data sample per step, eliminating the need for large batch updates required by existing baseline methods. For black-box minimax problems, the accelerated method matches or improves upon existing convergence rates without large batches. For transparent minimax problems where gradients can be calculated directly, the proposed first-order algorithm achieves state-of-the-art computational efficiency. In empirical testing, these algorithms consistently required substantially fewer function evaluations to successfully compromise target models compared to previous techniques.

These results demonstrate that organizations conducting security audits, adversarial robustness testing, and complex optimization can significantly cut computing time and infrastructure expenses. Removing the requirement for massive batch sizes enables real-world deployment in resource-constrained environments where querying a black-box model is computationally expensive or rate-limited.

Technical leaders and engineering teams should consider adopting these momentum-based variance-reduction methods when auditing machine learning models for adversarial vulnerabilities or solving saddle-point games. Practitioners should evaluate these techniques in pilot benchmarking workflows to optimize hyperparameter selection across their specific domain tasks before wide-scale deployment.

Readers should note that the theoretical guarantees rely on component smoothness and strong concavity in the minimax sub-problems, which may not hold uniformly across every non-standard architecture. Nevertheless, given the consistent performance across both mathematical proofs and empirical benchmarks, confidence in the algorithms' computational advantages remains high.

arXiv: 2008.08170
Cover for Accelerated Zeroth-Order and First-Order Momentum Methods from Mini to Minimax Optimization

Abstract

In the paper, we propose a class of accelerated zeroth-order and first-order momentum methods for both nonconvex mini-optimization and minimax-optimization. Specifically, we propose a new accelerated zeroth-order momentum (Acc-ZOM) method for black-box mini-optimization where only function values can be obtained. Moreover, we prove that our Acc-ZOM method achieves a lower query complexity of Õ(d3/4ϵ−3) for finding an ϵ-stationary point, which improves the best known result by a factor of O(d1/4) where d denotes the variable dimension. In particular, our Acc-ZOM does not need large batches required in the existing zeroth-order stochastic algorithms. Meanwhile, we propose an accelerated zeroth-order momentum descent ascent (Acc-ZOMDA) method for black-box minimax optimization, where only function values can be obtained. Our Acc-ZOMDA obtains a low query complexity of Õ((d1 + d2)3/4κ4.5y ϵ−3) without requiring large batches for finding an ϵ-stationary point, where d1 and d2 denote variable dimensions and κy is condition number. Moreover, we propose an accelerated first-order momentum descent ascent (Acc-MDA) method for minimax optimization, whose explicit gradients are accessible. Our Acc-MDA achieves a low gradient complexity of Õ(κ4.5y ϵ−3) without requiring large batches for finding an ϵ-stationary point. In particular, our Acc-MDA can obtain a lower gradient complexity of Õ(κ2.5y ϵ−3) with a batch size O(κ4y), which improves the best known result by a factor of O(κ1/2y ). Extensive experimental results on black-box adversarial attack to deep neural networks and poisoning attack to logistic regression demonstrate efficiency of our algorithms.

Table of Contents

  • 1. Introduction
  • 2. Related Works
  • 2.1 Zeroth-Order Mini-Optimization
  • 2.2 Zeroth-Order Minimax Optimization
  • 2.3 First-Order Minimax Optimization
  • 3. Preliminaries
  • 3.1 Notations
  • 3.2 Preliminaries for Mini-Optimization
  • 3.3 Preliminaries for Minimax-Optimization
  • 4. Accelerated Zeroth-Order Momentum Method for Mini-Optimization
  • 5. Accelerated Zeroth-Order Momentum Descent Ascent Method for Minimax Optimization
  • 6. Accelerated First-Order Momentum Descent Ascent Method for Minimax Optimization
  • 7. Convergence Analysis
  • 7.1 Convergence Analysis of the Acc-ZOM Algorithm
  • 7.1.1 Convergence Analysis of the Acc-ZOM Algorithm for Constrained Mini-Optimization
  • 7.1.2 Convergence Analysis of Acc-ZOM Algorithm for Unconstrained Mini-Optimization
  • 7.2 Convergence Analysis of the Acc-ZOMDA Algorithm
  • 7.2.1 Convergence Analysis of the Acc-ZOMDA Algorithm for Constrained Minimax Optimization
  • 7.2.2 Convergence Analysis of the Acc-ZOMDA Algorithm for Unconstrained Minimax Optimization
  • 7.3 Convergence Analysis of the Acc-MDA Algorithm
  • 7.3.1 Convergence Analysis of the Acc-MDA Algorithm for Constrained Minimax Optimization
  • 7.3.2 Convergence Analysis of Acc-MDA Algorithm for Unconstrained Minimax Optimization
  • 8. Numerical Experiments
  • 8.1 Black-Box Adversarial Attack to DNNs
  • 8.2 Poisoning Attack to Logistic Regression
  • 9. Conclusions
  • Acknowledgments
  • Appendix A. Detailed Convergence Analysis
  • A.1 Convergence Analysis of Acc-ZOM Algorithm for Constrained Mini-Optimization
  • A.2 Convergence Analysis of Acc-ZOM Algorithm for Unconstrained Mini-Optimization
  • A.3 Convergence Analysis of the Acc-ZOMDA Algorithm for Constrained Minimax Optimization
  • A.4 Convergence Analysis of Acc-ZOMDA Algorithm for Unconstrained Minimax Optimization
  • A.5 Convergence Analysis of Acc-MDA Algorithm for Constrained Minimax Optimization
  • A.6 Convergence Analysis of Acc-MDA Algorithm for Unconstrained Minimax Optimization
  • Appendix B. Comparison of Assumptions Used in Zeroth-Order Methods
  • Appendix C. Query Complexity of ZO-Min-Max Method in (Liu et al., 2019b)
  • References

Knowls

  1. Knowl 1 — Accelerated Zeroth-Order Momentum Algorithm for Mini-Optimization

    algorithm

    Accelerated Zeroth-Order Momentum (Acc-ZOM) is a stochastic gradient-free optimization algorithm for solving constrained or unconstrained nonconvex stochastic mini-optimization problems: min⁡x∈Xf(x)=Eξ∼D[f(x;ξ)]\min_{x \in \mathcal{X}} f(x) = \mathbb{E}_{\xi \sim \mathcal{D}}[f(x; \xi)] where X⊆Rd\mathcal{X} \subseteq \mathbb{R}^d is a closed convex set and only stochastic function evaluations f(x;ξ)f(x; \xi) are accessible. The method applies a momentum-based variance-reduction estimator inspired by STORM and extends it to constrained domains via projected momentum steps.

    At step tt, the algorithm updates an adaptive learning rate ηt=k(m+t)1/3\eta_t = \frac{k}{(m+t)^{1/3}} and momentum parameter αt+1=cηt2\alpha_{t+1} = c\eta_t^2. In the unconstrained setting (X=Rd\mathcal{X} = \mathbb{R}^d), the update is xt+1=xt−γηtvtx_{t+1} = x_t - \gamma \eta_t v_t. In the constrained setting (X⊂Rd\mathcal{X} \subset \mathbb{R}^d), it computes an auxiliary projected point x~t+1=PX(xt−γvt)=arg⁡min⁡x∈X⟨vt,x−xt⟩+12γ∥x−xt∥2\tilde{x}_{t+1} = \mathcal{P}_{\mathcal{X}}(x_t - \gamma v_t) = \arg\min_{x \in \mathcal{X}} \langle v_t, x - x_t \rangle + \frac{1}{2\gamma}\|x - x_t\|^2 and sets xt+1=xt+ηt(x~t+1−xt)x_{t+1} = x_t + \eta_t(\tilde{x}_{t+1} - x_t). The variance-reduced zeroth-order gradient estimate vt+1v_{t+1} is updated recursively using a random vector u∈Rdu \in \mathbb{R}^d drawn from the uniform distribution over the unit sphere Sd−1\mathbb{S}^{d-1}: vt+1=∇^f(xt+1;ξt+1)+(1−αt+1)(vt−∇^f(xt;ξt+1))v_{t+1} = \hat{\nabla} f(x_{t+1}; \xi_{t+1}) + (1 - \alpha_{t+1})\big(v_t - \hat{\nabla} f(x_t; \xi_{t+1})\big) where ∇^f(x;ξ)=f(x+μu;ξ)−f(x;ξ)μ/du\hat{\nabla} f(x; \xi) = \frac{f(x + \mu u; \xi) - f(x; \xi)}{\mu / d} u is the uniform smoothing gradient estimator with smoothing parameter μ>0\mu > 0.

    Input: Total iterations TT, parameters {γ,k,m,c\gamma, k, m, c}, smoothing radius μ\mu, initial point x1∈Xx_1 \in \mathcal{X}
    Sample ξ1∼D\xi_1 \sim \mathcal{D} and u∼Uniform(Sd−1)u \sim \text{Uniform}(\mathbb{S}^{d-1})
    Compute v1=f(x1+μu;ξ1)−f(x1;ξ1)μ/duv_1 = \frac{f(x_1 + \mu u; \xi_1) - f(x_1; \xi_1)}{\mu / d} u
    for t=1,2,…,Tt = 1, 2, \dots, T do
        ηt=k(m+t)1/3\eta_t = \frac{k}{(m+t)^{1/3}}
        if X=Rd\mathcal{X} = \mathbb{R}^d then
            xt+1=xt−γηtvtx_{t+1} = x_t - \gamma \eta_t v_t
        else
            x~t+1=PX(xt−γvt)\tilde{x}_{t+1} = \mathcal{P}_{\mathcal{X}}(x_t - \gamma v_t)
            xt+1=xt+ηt(x~t+1−xt)x_{t+1} = x_t + \eta_t(\tilde{x}_{t+1} - x_t)
        end if
        αt+1=cηt2\alpha_{t+1} = c \eta_t^2
        Sample ξt+1∼D\xi_{t+1} \sim \mathcal{D} and u∼Uniform(Sd−1)u \sim \text{Uniform}(\mathbb{S}^{d-1})
        ∇^f(xt+1;ξt+1)=f(xt+1+μu;ξt+1)−f(xt+1;ξt+1)μ/du\hat{\nabla} f(x_{t+1}; \xi_{t+1}) = \frac{f(x_{t+1} + \mu u; \xi_{t+1}) - f(x_{t+1}; \xi_{t+1})}{\mu / d} u
        ∇^f(xt;ξt+1)=f(xt+μu;ξt+1)−f(xt;ξt+1)μ/du\hat{\nabla} f(x_t; \xi_{t+1}) = \frac{f(x_t + \mu u; \xi_{t+1}) - f(x_t; \xi_{t+1})}{\mu / d} u
        vt+1=∇^f(xt+1;ξt+1)+(1−αt+1)(vt−∇^f(xt;ξt+1))v_{t+1} = \hat{\nabla} f(x_{t+1}; \xi_{t+1}) + (1 - \alpha_{t+1})(v_t - \hat{\nabla} f(x_t; \xi_{t+1}))
    end for
    Output: xζx_\zeta chosen uniformly at random from {xt}t=1T\{x_t\}_{t=1}^T (theoretical), or xTx_T (practical)
  2. Knowl 2 — Convergence and Query Complexity of Acc-ZOM for Mini-Optimization

    theoretical result

    Let f(x)=Eξ∼D[f(x;ξ)]f(x) = \mathbb{E}_{\xi \sim \mathcal{D}}[f(x; \xi)] defined on a closed convex set X⊆Rd\mathcal{X} \subseteq \mathbb{R}^d satisfy:

    1. Bounded zeroth-order gradient variance: Eu,ξ[∥∇^f(x;ξ)−∇fμ(x)∥2]≤σ2\mathbb{E}_{u, \xi}[\|\hat{\nabla} f(x; \xi) - \nabla f_\mu(x)\|^2] \le \sigma^2 for all x∈Xx \in \mathcal{X}, where fμ(x)=Eu∼UB[f(x+μu)]f_\mu(x) = \mathbb{E}_{u \sim \mathcal{U}_B}[f(x + \mu u)] is the uniform ball-smoothed function.
    2. Component LL-smoothness: ∥∇f(x;ξ)−∇f(x′;ξ)∥≤L∥x−x′∥\|\nabla f(x; \xi) - \nabla f(x'; \xi)\| \le L \|x - x'\| for all x,x′∈Xx, x' \in \mathcal{X}.
    3. Lower boundedness: f∗=inf⁡x∈Xf(x)>−∞f^* = \inf_{x \in \mathcal{X}} f(x) > -\infty.

    Set parameters ηt=k(m+t)1/3\eta_t = \frac{k}{(m+t)^{1/3}}, αt+1=cηt2\alpha_{t+1} = c\eta_t^2, c≥23k3+54c \ge \frac{2}{3k^3} + \frac{5}{4}, k>0k > 0, m≥max⁡(2,k3,(ck)3)m \ge \max(2, k^3, (ck)^3), step size 0<γ≤min⁡(m1/32Lk,126dL)0 < \gamma \le \min\left(\frac{m^{1/3}}{2Lk}, \frac{1}{2\sqrt{6d}L}\right), and smoothing parameter 0<μ≤1d(m+T)2/30 < \mu \le \frac{1}{d(m+T)^{2/3}}.

    1. Constrained Setting (X⊂Rd\mathcal{X} \subset \mathbb{R}^d): 1T∑t=1TE[∥GX(xt,∇f(xt),γ)∥]≤1T∑t=1TE[Gt]≤2Mm1/6T1/2+2MT1/3+L2(m+T)2/3\frac{1}{T}\sum_{t=1}^T \mathbb{E}[\|G_{\mathcal{X}}(x_t, \nabla f(x_t), \gamma)\|] \le \frac{1}{T}\sum_{t=1}^T \mathbb{E}[G_t] \le \frac{\sqrt{2M}m^{1/6}}{T^{1/2}} + \frac{\sqrt{2M}}{T^{1/3}} + \frac{L}{2(m+T)^{2/3}} where Gt=1γ∥x~t+1−xt∥+∥∇f(xt)−vt∥G_t = \frac{1}{\gamma}\|\tilde{x}_{t+1} - x_t\| + \|\nabla f(x_t) - v_t\|, GX(xt,∇f(xt),γ)=1γ(xt−PX(xt−γ∇f(xt)))G_{\mathcal{X}}(x_t, \nabla f(x_t), \gamma) = \frac{1}{\gamma}(x_t - \mathcal{P}_{\mathcal{X}}(x_t - \gamma \nabla f(x_t))), and M=fμ(x1)−f∗kγ+m1/3σ2k2+9L24k2+2k2c2σ2ln⁡(m+T)=O~(d).M = \frac{f_\mu(x_1) - f^*}{k\gamma} + \frac{m^{1/3}\sigma^2}{k^2} + \frac{9L^2}{4k^2} + 2k^2 c^2 \sigma^2 \ln(m+T) = \tilde{\mathcal{O}}(\sqrt{d}).

    2. Unconstrained Setting (X=Rd\mathcal{X} = \mathbb{R}^d): 1T∑t=1TE[∥∇f(xt)∥]≤2Mm1/6T1/2+2MT1/3+L2(m+T)2/3.\frac{1}{T}\sum_{t=1}^T \mathbb{E}[\|\nabla f(x_t)\|] \le \frac{\sqrt{2M}m^{1/6}}{T^{1/2}} + \frac{\sqrt{2M}}{T^{1/3}} + \frac{L}{2(m+T)^{2/3}}.

    Because γ=O(d−1/2)\gamma = \mathcal{O}(d^{-1/2}) and M=O~(d1/2)M = \tilde{\mathcal{O}}(d^{1/2}), the algorithm achieves an O~(d1/4T−1/3)\tilde{\mathcal{O}}(d^{1/4} T^{-1/3}) convergence rate. For an ϵ\epsilon-stationary point (E[Gζ]≤ϵ\mathbb{E}[G_\zeta] \le \epsilon or E[∥∇f(xζ)∥]≤ϵ\mathbb{E}[\|\nabla f(x_\zeta)\|] \le \epsilon), Acc-ZOM requires T=O~(d3/4ϵ−3)T = \tilde{\mathcal{O}}(d^{3/4}\epsilon^{-3}) iterations. Since each iteration queries exactly 4 function evaluations (batch size O(1)\mathcal{O}(1)), the total query complexity is O~(d3/4ϵ−3)\tilde{\mathcal{O}}(d^{3/4}\epsilon^{-3}), improving upon coordinate-smoothing SPIDER zeroth-order methods by a factor of O(d1/4)\mathcal{O}(d^{1/4}).

  3. Knowl 3 — Accelerated Zeroth-Order Momentum Descent Ascent Algorithm

    algorithm

    Accelerated Zeroth-Order Momentum Descent Ascent (Acc-ZOMDA) is a single-loop stochastic zeroth-order method for solving black-box nonconvex-strongly-concave minimax optimization problems: min⁡x∈Xmax⁡y∈Yf(x,y)=Eξ∼D′[f(x,y;ξ)]\min_{x \in \mathcal{X}} \max_{y \in \mathcal{Y}} f(x, y) = \mathbb{E}_{\xi \sim \mathcal{D}'}[f(x, y; \xi)] where X⊆Rd1\mathcal{X} \subseteq \mathbb{R}^{d_1} and Y⊆Rd2\mathcal{Y} \subseteq \mathbb{R}^{d_2} are compact convex sets, and explicit gradients are inaccessible.

    Acc-ZOMDA generates stochastic partial gradient approximations using mini-batches Bt+1={ξt+1i}i=1b\mathcal{B}_{t+1} = \{\xi_{t+1}^i\}_{i=1}^b and uniform sphere random vectors u^i∈Sd1−1\hat{u}_i \in \mathbb{S}^{d_1-1}, u~i∈Sd2−1\tilde{u}_i \in \mathbb{S}^{d_2-1} via: ∇^xf(x,y;B)=1b∑i=1bf(x+μ1u^i,y;ξi)−f(x,y;ξi)μ1/d1u^i\hat{\nabla}_x f(x, y; \mathcal{B}) = \frac{1}{b}\sum_{i=1}^b \frac{f(x + \mu_1 \hat{u}_i, y; \xi^i) - f(x, y; \xi^i)}{\mu_1 / d_1} \hat{u}_i ∇^yf(x,y;B)=1b∑i=1bf(x,y+μ2u~i;ξi)−f(x,y;ξi)μ2/d2u~i\hat{\nabla}_y f(x, y; \mathcal{B}) = \frac{1}{b}\sum_{i=1}^b \frac{f(x, y + \mu_2 \tilde{u}_i; \xi^i) - f(x, y; \xi^i)}{\mu_2 / d_2} \tilde{u}_i

    Input: Iterations TT, parameters {γ,λ,k,m,c1,c2,b\gamma, \lambda, k, m, c_1, c_2, b}, smoothing radii μ1,μ2\mu_1, \mu_2, initial x1∈X,y1∈Yx_1 \in \mathcal{X}, y_1 \in \mathcal{Y}
    Draw mini-batch B1={ξ1i}i=1b\mathcal{B}_1 = \{\xi_1^i\}_{i=1}^b and vectors {u^i∈Sd1−1}i=1b\{\hat{u}_i \in \mathbb{S}^{d_1-1}\}_{i=1}^b, {u~i∈Sd2−1}i=1b\{\tilde{u}_i \in \mathbb{S}^{d_2-1}\}_{i=1}^b
    Compute v1=∇^xf(x1,y1;B1)v_1 = \hat{\nabla}_x f(x_1, y_1; \mathcal{B}_1) and w1=∇^yf(x1,y1;B1)w_1 = \hat{\nabla}_y f(x_1, y_1; \mathcal{B}_1)
    for t=1,2,…,Tt = 1, 2, \dots, T do
        ηt=k(m+t)1/3\eta_t = \frac{k}{(m+t)^{1/3}}
        if X=Rd1\mathcal{X} = \mathbb{R}^{d_1} then
            xt+1=xt−γηtvtx_{t+1} = x_t - \gamma \eta_t v_t
        else
            x~t+1=PX(xt−γvt)\tilde{x}_{t+1} = \mathcal{P}_{\mathcal{X}}(x_t - \gamma v_t)
            xt+1=xt+ηt(x~t+1−xt)x_{t+1} = x_t + \eta_t(\tilde{x}_{t+1} - x_t)
        end if
        y~t+1=PY(yt+λwt)\tilde{y}_{t+1} = \mathcal{P}_{\mathcal{Y}}(y_t + \lambda w_t)
        yt+1=yt+ηt(y~t+1−yt)y_{t+1} = y_t + \eta_t(\tilde{y}_{t+1} - y_t)
        αt+1=c1ηt2\alpha_{t+1} = c_1 \eta_t^2
        βt+1=c2ηt2\beta_{t+1} = c_2 \eta_t^2
        Draw mini-batch Bt+1={ξt+1i}i=1b\mathcal{B}_{t+1} = \{\xi_{t+1}^i\}_{i=1}^b and vectors {u^i∈Sd1−1}i=1b\{\hat{u}_i \in \mathbb{S}^{d_1-1}\}_{i=1}^b, {u~i∈Sd2−1}i=1b\{\tilde{u}_i \in \mathbb{S}^{d_2-1}\}_{i=1}^b
        vt+1=∇^xf(xt+1,yt+1;Bt+1)+(1−αt+1)(vt−∇^xf(xt,yt;Bt+1))v_{t+1} = \hat{\nabla}_x f(x_{t+1}, y_{t+1}; \mathcal{B}_{t+1}) + (1 - \alpha_{t+1})(v_t - \hat{\nabla}_x f(x_t, y_t; \mathcal{B}_{t+1}))
        wt+1=∇^yf(xt+1,yt+1;Bt+1)+(1−βt+1)(wt−∇^yf(xt,yt;Bt+1))w_{t+1} = \hat{\nabla}_y f(x_{t+1}, y_{t+1}; \mathcal{B}_{t+1}) + (1 - \beta_{t+1})(w_t - \hat{\nabla}_y f(x_t, y_t; \mathcal{B}_{t+1}))
    end for
    Output: (xζ,yζ)(x_\zeta, y_\zeta) sampled uniformly from {xt,yt}t=1T\{x_t, y_t\}_{t=1}^T (theoretical), or (xT,yT)(x_T, y_T) (practical)
  4. Knowl 4 — Convergence and Query Complexity of Acc-ZOMDA

    theoretical result

    Let f(x,y)=Eξ∼D′[f(x,y;ξ)]f(x, y) = \mathbb{E}_{\xi \sim \mathcal{D}'}[f(x, y; \xi)] defined on compact convex sets X⊆Rd1\mathcal{X} \subseteq \mathbb{R}^{d_1} and Y⊆Rd2\mathcal{Y} \subseteq \mathbb{R}^{d_2} satisfy:

    1. LfL_f-Lipschitz gradient: ∥∇f(x,y;ξ)−∇f(x′,y′;ξ)∥≤Lf∥(x,y)−(x′,y′)∥\|\nabla f(x, y; \xi) - \nabla f(x', y'; \xi)\| \le L_f \|(x, y) - (x', y')\| for all x,x′∈Xx, x' \in \mathcal{X} and y,y′∈Yy, y' \in \mathcal{Y}.
    2. τ\tau-strong concavity in yy: ∥∇yf(x,y)−∇yf(x,y′)∥≥τ∥y−y′∥\|\nabla_y f(x, y) - \nabla_y f(x, y')\| \ge \tau \|y - y'\|, with condition number κy=Lf/τ\kappa_y = L_f / \tau.
    3. Bounded zeroth-order partial gradient variance: E[∥∇^xf(x,y;ξ)−∇xfμ1(x,y)∥2]≤δ12\mathbb{E}[\|\hat{\nabla}_x f(x, y; \xi) - \nabla_x f_{\mu_1}(x, y)\|^2] \le \delta_1^2 and E[∥∇^yf(x,y;ξ)−∇yfμ2(x,y)∥2]≤δ12\mathbb{E}[\|\hat{\nabla}_y f(x, y; \xi) - \nabla_y f_{\mu_2}(x, y)\|^2] \le \delta_1^2.
    4. Lower bounded primal function: F∗=inf⁡x∈XF(x)>−∞F^* = \inf_{x \in \mathcal{X}} F(x) > -\infty, where F(x)=max⁡y∈Yf(x,y)F(x) = \max_{y \in \mathcal{Y}} f(x, y).

    Let d~=d1+d2\tilde{d} = d_1 + d_2, Lg=Lf+Lf2/τL_g = L_f + L_f^2 / \tau, and choose parameters:

    • ηt=k(m+t)1/3\eta_t = \frac{k}{(m+t)^{1/3}}, c1≥23k3+9τ24c_1 \ge \frac{2}{3k^3} + \frac{9\tau^2}{4}, c2≥23k3+625d~Lf23bc_2 \ge \frac{2}{3k^3} + \frac{625\tilde{d}L_f^2}{3b}, k>0k > 0, 1≤b≤d~1 \le b \le \tilde{d}
    • m≥max⁡(2,k3,(c1k)3,(c2k)3)m \ge \max(2, k^3, (c_1 k)^3, (c_2 k)^3), 0<λ≤min⁡(16Lf,75τ24)0 < \lambda \le \min\left(\frac{1}{6L_f}, \frac{75\tau}{24}\right)
    • 0<γ≤min⁡(λτ2Lf6b/d~36λ2+625κy2,m1/32Lgk)0 < \gamma \le \min\left(\frac{\lambda \tau}{2L_f}\sqrt{\frac{6b / \tilde{d}}{36\lambda^2 + 625\kappa_y^2}}, \frac{m^{1/3}}{2L_g k}\right)
    • 0<μ1≤1d1(m+T)2/30 < \mu_1 \le \frac{1}{d_1(m+T)^{2/3}} and 0<μ2≤1d~1/2d2(m+T)2/30 < \mu_2 \le \frac{1}{\tilde{d}^{1/2} d_2(m+T)^{2/3}}

    Convergence Rate: 1T∑t=1TE[∥GX(xt,∇F(xt),γ)∥]≤1T∑t=1TE[Ht]≤23M′m1/6T1/2+23M′T1/3+Lf2(m+T)2/3\frac{1}{T}\sum_{t=1}^T \mathbb{E}[\|G_{\mathcal{X}}(x_t, \nabla F(x_t), \gamma)\|] \le \frac{1}{T}\sum_{t=1}^T \mathbb{E}[H_t] \le \frac{2\sqrt{3M'}m^{1/6}}{T^{1/2}} + \frac{2\sqrt{3M'}}{T^{1/3}} + \frac{L_f}{2(m+T)^{2/3}} where Ht=1γ∥x~t+1−xt∥+∥∇xf(xt,yt)−vt∥+Lf∥yt−y∗(xt)∥H_t = \frac{1}{\gamma}\|\tilde{x}_{t+1} - x_t\| + \|\nabla_x f(x_t, y_t) - v_t\| + L_f \|y_t - y^*(x_t)\|, Δ1=∥y1−y∗(x1)∥2\Delta_1 = \|y_1 - y^*(x_1)\|^2, and M′=Fμ1(x1)−F∗γk+25d~Lf2Δ1kλτb+2m1/3δ2bτ2k2+36τ2Lf2+625Lf48bτ2(m+T)−2/3+9Lf24bτ2k2+2(c12+c22)δ2k2bτ2ln⁡(m+T).M' = \frac{F_{\mu_1}(x_1) - F^*}{\gamma k} + \frac{25\tilde{d}L_f^2 \Delta_1}{k\lambda \tau b} + \frac{2m^{1/3}\delta^2}{b\tau^2 k^2} + \frac{36\tau^2 L_f^2 + 625L_f^4}{8b\tau^2}(m+T)^{-2/3} + \frac{9L_f^2}{4b\tau^2 k^2} + \frac{2(c_1^2 + c_2^2)\delta^2 k^2}{b\tau^2}\ln(m+T).

    Query Complexity: For batch size b=1b = 1, M′=O~(d~κy3+d~2κy2)M' = \tilde{\mathcal{O}}(\sqrt{\tilde{d}\kappa_y^3 + \tilde{d}^2 \kappa_y^2}). When κy≥d~3/2\kappa_y \ge \tilde{d}^{3/2}, the convergence rate is O~(κy3/2d~1/4T1/3)\tilde{\mathcal{O}}\left(\frac{\kappa_y^{3/2}\tilde{d}^{1/4}}{T^{1/3}}\right), resulting in a total query complexity of: 8T=O~((d1+d2)3/4κy4.5ϵ−3)8T = \tilde{\mathcal{O}}\big((d_1 + d_2)^{3/4} \kappa_y^{4.5} \epsilon^{-3}\big) for finding an ϵ\epsilon-stationary point without requiring large batches.

  5. Knowl 5 — Accelerated First-Order Momentum Descent Ascent Algorithm

    algorithm

    Accelerated First-Order Momentum Descent Ascent (Acc-MDA) is a single-loop stochastic first-order algorithm for solving transparent nonconvex-strongly-concave minimax optimization problems: min⁡x∈Xmax⁡y∈Yf(x,y)=Eξ∼D′[f(x,y;ξ)]\min_{x \in \mathcal{X}} \max_{y \in \mathcal{Y}} f(x, y) = \mathbb{E}_{\xi \sim \mathcal{D}'}[f(x, y; \xi)] where stochastic partial gradients ∇xf(x,y;ξ)\nabla_x f(x, y; \xi) and ∇yf(x,y;ξ)\nabla_y f(x, y; \xi) are explicitly accessible.

    Acc-MDA tracks both partial gradients using STORM-style momentum estimators vtv_t and wtw_t and applies projected gradient updates with step size ηt=k(m+t)1/3\eta_t = \frac{k}{(m+t)^{1/3}}.

    Input: Iterations TT, parameters {γ,λ,k,m,c1,c2\gamma, \lambda, k, m, c_1, c_2}, batch size bb, initial x1∈X,y1∈Yx_1 \in \mathcal{X}, y_1 \in \mathcal{Y}
    Draw mini-batch B1={ξ1i}i=1b\mathcal{B}_1 = \{\xi_1^i\}_{i=1}^b
    Compute v1=1b∑i=1b∇xf(x1,y1;ξ1i)v_1 = \frac{1}{b}\sum_{i=1}^b \nabla_x f(x_1, y_1; \xi_1^i) and w1=1b∑i=1b∇yf(x1,y1;ξ1i)w_1 = \frac{1}{b}\sum_{i=1}^b \nabla_y f(x_1, y_1; \xi_1^i)
    for t=1,2,…,Tt = 1, 2, \dots, T do
        ηt=k(m+t)1/3\eta_t = \frac{k}{(m+t)^{1/3}}
        if X=Rd1\mathcal{X} = \mathbb{R}^{d_1} then
            xt+1=xt−γηtvtx_{t+1} = x_t - \gamma \eta_t v_t
        else
            x~t+1=PX(xt−γvt)\tilde{x}_{t+1} = \mathcal{P}_{\mathcal{X}}(x_t - \gamma v_t)
            xt+1=xt+ηt(x~t+1−xt)x_{t+1} = x_t + \eta_t(\tilde{x}_{t+1} - x_t)
        end if
        y~t+1=PY(yt+λwt)\tilde{y}_{t+1} = \mathcal{P}_{\mathcal{Y}}(y_t + \lambda w_t)
        yt+1=yt+ηt(y~t+1−yt)y_{t+1} = y_t + \eta_t(\tilde{y}_{t+1} - y_t)
        αt+1=c1ηt2\alpha_{t+1} = c_1 \eta_t^2
        βt+1=c2ηt2\beta_{t+1} = c_2 \eta_t^2
        Draw mini-batch Bt+1={ξt+1i}i=1b\mathcal{B}_{t+1} = \{\xi_{t+1}^i\}_{i=1}^b
        vt+1=1b∑i=1b∇xf(xt+1,yt+1;ξt+1i)+(1−αt+1)(vt−1b∑i=1b∇xf(xt,yt;ξt+1i))v_{t+1} = \frac{1}{b}\sum_{i=1}^b \nabla_x f(x_{t+1}, y_{t+1}; \xi_{t+1}^i) + (1 - \alpha_{t+1})\left(v_t - \frac{1}{b}\sum_{i=1}^b \nabla_x f(x_t, y_t; \xi_{t+1}^i)\right)
        wt+1=1b∑i=1b∇yf(xt+1,yt+1;ξt+1i)+(1−βt+1)(wt−1b∑i=1b∇yf(xt,yt;ξt+1i))w_{t+1} = \frac{1}{b}\sum_{i=1}^b \nabla_y f(x_{t+1}, y_{t+1}; \xi_{t+1}^i) + (1 - \beta_{t+1})\left(w_t - \frac{1}{b}\sum_{i=1}^b \nabla_y f(x_t, y_t; \xi_{t+1}^i)\right)
    end for
    Output: (xζ,yζ)(x_\zeta, y_\zeta) chosen uniformly from {xt,yt}t=1T\{x_t, y_t\}_{t=1}^T (theoretical), or (xT,yT)(x_T, y_T) (practical)
  6. Knowl 6 — Convergence and Gradient Complexity of Acc-MDA

    theoretical result

    Let f(x,y)=Eξ∼D′[f(x,y;ξ)]f(x, y) = \mathbb{E}_{\xi \sim \mathcal{D}'}[f(x, y; \xi)] have LfL_f-Lipschitz gradient, τ\tau-strong concavity in yy (condition number κy=Lf/τ\kappa_y = L_f / \tau), bounded partial gradient variance E[∥∇xf(x,y;ξ)−∇xf(x,y)∥2]≤δ22\mathbb{E}[\|\nabla_x f(x, y; \xi) - \nabla_x f(x, y)\|^2] \le \delta_2^2 and E[∥∇yf(x,y;ξ)−∇yf(x,y)∥2]≤δ22\mathbb{E}[\|\nabla_y f(x, y; \xi) - \nabla_y f(x, y)\|^2] \le \delta_2^2, and lower-bounded envelope F∗=inf⁡x∈Xmax⁡y∈Yf(x,y)>−∞F^* = \inf_{x \in \mathcal{X}} \max_{y \in \mathcal{Y}} f(x, y) > -\infty.

    Hyperparameters are chosen as ηt=k(m+t)1/3\eta_t = \frac{k}{(m+t)^{1/3}}, c1≥23k3+9τ24c_1 \ge \frac{2}{3k^3} + \frac{9\tau^2}{4}, c2≥23k3+75Lf22c_2 \ge \frac{2}{3k^3} + \frac{75L_f^2}{2}, m≥max⁡(2,k3,(c1k)3,(c2k)3)m \ge \max(2, k^3, (c_1 k)^3, (c_2 k)^3), 0<λ≤min⁡(16Lf,27bτ16)0 < \lambda \le \min\left(\frac{1}{6L_f}, \frac{27b\tau}{16}\right), and 0<γ≤min⁡(λτ2Lf2b8λ2+75κy2b,m1/32Lgk)0 < \gamma \le \min\left(\frac{\lambda \tau}{2L_f}\sqrt{\frac{2b}{8\lambda^2 + 75\kappa_y^2 b}}, \frac{m^{1/3}}{2L_g k}\right).

    Convergence Bound: 1T∑t=1TE[∥GX(xt,∇F(xt),γ)∥]≤1T∑t=1TE[Ht]≤23M′′m1/6T1/2+23M′′T1/3\frac{1}{T}\sum_{t=1}^T \mathbb{E}[\|G_{\mathcal{X}}(x_t, \nabla F(x_t), \gamma)\|] \le \frac{1}{T}\sum_{t=1}^T \mathbb{E}[H_t] \le \frac{2\sqrt{3M''}m^{1/6}}{T^{1/2}} + \frac{2\sqrt{3M''}}{T^{1/3}} where Δ1=∥y1−y∗(x1)∥2\Delta_1 = \|y_1 - y^*(x_1)\|^2 and M′′=F(x1)−F∗γk+9Lf2Δ1kλτ+2m1/3δ2bτ2k2+2(c12+c22)δ2k2bτ2ln⁡(m+T).M'' = \frac{F(x_1) - F^*}{\gamma k} + \frac{9L_f^2 \Delta_1}{k\lambda \tau} + \frac{2m^{1/3}\delta^2}{b\tau^2 k^2} + \frac{2(c_1^2 + c_2^2)\delta^2 k^2}{b\tau^2}\ln(m+T).

    Gradient Complexities:

    • Batch size b=1b = 1: M′′=O(κy3)M'' = \mathcal{O}(\kappa_y^3), yielding a gradient complexity of 4T=O~(κy4.5ϵ−3)4T = \tilde{\mathcal{O}}(\kappa_y^{4.5}\epsilon^{-3}) for finding an ϵ\epsilon-stationary point without large batches.
    • Batch size b=O(κyν)b = \mathcal{O}(\kappa_y^\nu) with ν>0\nu > 0 and κyν≤881Lfτ\kappa_y^\nu \le \frac{8}{81L_f \tau}: Acc-MDA achieves a gradient complexity of: 4bT=O~(κy4.5−ν/2ϵ−3).4bT = \tilde{\mathcal{O}}\big(\kappa_y^{4.5 - \nu/2}\epsilon^{-3}\big).
      • For b=O(κy3)b = \mathcal{O}(\kappa_y^3) (ν=3\nu = 3), the complexity is O~(κy3ϵ−3)\tilde{\mathcal{O}}(\kappa_y^3 \epsilon^{-3}).
      • For b=O(κy4)b = \mathcal{O}(\kappa_y^4) (ν=4\nu = 4), the complexity is O~(κy2.5ϵ−3)\tilde{\mathcal{O}}(\kappa_y^{2.5} \epsilon^{-3}), improving the best known rate by a factor of O(κy1/2)\mathcal{O}(\kappa_y^{1/2}).
  7. Knowl 7 — Stationarity Metrics for Constrained Stochastic Momentum Optimization

    definition

    For stochastic momentum algorithms over closed convex constraint sets, convergence is characterized using metrics tailored to momentum estimators vtv_t and projected auxiliary iterates x~t+1=PX(xt−γvt)=arg⁡min⁡x∈X⟨vt,x−xt⟩+12γ∥x−xt∥2\tilde{x}_{t+1} = \mathcal{P}_{\mathcal{X}}(x_t - \gamma v_t) = \arg\min_{x \in \mathcal{X}} \langle v_t, x - x_t \rangle + \frac{1}{2\gamma}\|x - x_t\|^2.

    1. Mini-Optimization Stationarity Metric GtG_t: For min⁡x∈Xf(x)\min_{x \in \mathcal{X}} f(x) with X⊂Rd\mathcal{X} \subset \mathbb{R}^d: Gt=1γ∥x~t+1−xt∥+∥∇f(xt)−vt∥.G_t = \frac{1}{\gamma}\|\tilde{x}_{t+1} - x_t\| + \|\nabla f(x_t) - v_t\|. GtG_t upper-bounds the gradient mapping metric ∥GX(xt,∇f(xt),γ)∥=1γ∥xt−PX(xt−γ∇f(xt))∥\|G_{\mathcal{X}}(x_t, \nabla f(x_t), \gamma)\| = \frac{1}{\gamma}\|x_t - \mathcal{P}_{\mathcal{X}}(x_t - \gamma \nabla f(x_t))\|, since: ∥GX(xt,∇f(xt),γ)∥≤∥∇f(xt)−vt∥+1γ∥x~t+1−xt∥=Gt.\|G_{\mathcal{X}}(x_t, \nabla f(x_t), \gamma)\| \le \|\nabla f(x_t) - v_t\| + \frac{1}{\gamma}\|\tilde{x}_{t+1} - x_t\| = G_t.

    2. Minimax Optimization Stationarity Metric HtH_t: For min⁡x∈Xmax⁡y∈Yf(x,y)\min_{x \in \mathcal{X}} \max_{y \in \mathcal{Y}} f(x, y) with primal envelope F(x)=max⁡y∈Yf(x,y)F(x) = \max_{y \in \mathcal{Y}} f(x, y) and unique maximizer y∗(x)=arg⁡max⁡y∈Yf(x,y)y^*(x) = \arg\max_{y \in \mathcal{Y}} f(x, y): Ht=1γ∥x~t+1−xt∥+∥∇xf(xt,yt)−vt∥+Lf∥yt−y∗(xt)∥H_t = \frac{1}{\gamma}\|\tilde{x}_{t+1} - x_t\| + \|\nabla_x f(x_t, y_t) - v_t\| + L_f \|y_t - y^*(x_t)\| where LfL_f is the Lipschitz constant of ∇f(x,y)\nabla f(x, y). HtH_t upper-bounds the primal gradient mapping ∥GX(xt,∇F(xt),γ)∥≤Ht\|G_{\mathcal{X}}(x_t, \nabla F(x_t), \gamma)\| \le H_t.

  8. Knowl 8 — Uniform Smoothing Gradient Estimator Formulation and Bounds

    model/method

    The Uniform Smoothing Gradient Estimator (UniGE) constructs a gradient estimate from randomized function evaluations over the unit Euclidean sphere Sd−1={u∈Rd:∥u∥=1}\mathbb{S}^{d-1} = \{u \in \mathbb{R}^d : \|u\| = 1\}.

    For a scalar function f(x;ξ):Rd→Rf(x; \xi): \mathbb{R}^d \to \mathbb{R} with smoothing parameter μ>0\mu > 0: ∇^f(x;ξ)=f(x+μu;ξ)−f(x;ξ)μ/du,u∼Uniform(Sd−1).\hat{\nabla} f(x; \xi) = \frac{f(x + \mu u; \xi) - f(x; \xi)}{\mu / d} u, \quad u \sim \text{Uniform}(\mathbb{S}^{d-1}).

    Properties:

    1. Let fμ(x;ξ)=Eu∼UB[f(x+μu;ξ)]f_\mu(x; \xi) = \mathbb{E}_{u \sim \mathcal{U}_B}[f(x + \mu u; \xi)] where UB\mathcal{U}_B is the uniform distribution on the dd-dimensional unit Euclidean ball B={u∈Rd:∥u∥≤1}\mathcal{B} = \{u \in \mathbb{R}^d : \|u\| \le 1\}. Then Eu,ξ[∇^f(x;ξ)]=∇fμ(x)=Eξ[∇fμ(x;ξ)]\mathbb{E}_{u, \xi}[\hat{\nabla} f(x; \xi)] = \nabla f_\mu(x) = \mathbb{E}_\xi[\nabla f_\mu(x; \xi)].
    2. If f(x)f(x) has LL-Lipschitz continuous gradient, then fμ(x)f_\mu(x) also has LL-Lipschitz continuous gradient, and: ∣fμ(x)−f(x)∣≤μ2L2,∥∇fμ(x)−∇f(x)∥≤μLd2,∀x∈Rd.|f_\mu(x) - f(x)| \le \frac{\mu^2 L}{2}, \quad \|\nabla f_\mu(x) - \nabla f(x)\| \le \frac{\mu L d}{2}, \quad \forall x \in \mathbb{R}^d.
    3. The estimation difference satisfies: Eu[∥∇^f(x;ξ)−∇^f(x′;ξ)∥2]≤3dL2∥x−x′∥2+3L2d2μ22,∀x,x′∈Rd.\mathbb{E}_u\big[\|\hat{\nabla} f(x; \xi) - \hat{\nabla} f(x'; \xi)\|^2\big] \le 3dL^2 \|x - x'\|^2 + \frac{3L^2 d^2 \mu^2}{2}, \quad \forall x, x' \in \mathbb{R}^d.

    In minimax optimization f(x,y;ξ)f(x, y; \xi), separate estimators with smoothing parameters μ1,μ2\mu_1, \mu_2 are used for partial derivatives: ∇^xf(x,y;B)=1b∑i=1bf(x+μ1u^i,y;ξi)−f(x,y;ξi)μ1/d1u^i,u^i∼Uniform(Sd1−1)\hat{\nabla}_x f(x, y; \mathcal{B}) = \frac{1}{b}\sum_{i=1}^b \frac{f(x + \mu_1 \hat{u}_i, y; \xi_i) - f(x, y; \xi_i)}{\mu_1 / d_1}\hat{u}_i, \quad \hat{u}_i \sim \text{Uniform}(\mathbb{S}^{d_1-1}) ∇^yf(x,y;B)=1b∑i=1bf(x,y+μ2u~i;ξi)−f(x,y;ξi)μ2/d2u~i,u~i∼Uniform(Sd2−1).\hat{\nabla}_y f(x, y; \mathcal{B}) = \frac{1}{b}\sum_{i=1}^b \frac{f(x, y + \mu_2 \tilde{u}_i; \xi_i) - f(x, y; \xi_i)}{\mu_2 / d_2}\tilde{u}_i, \quad \tilde{u}_i \sim \text{Uniform}(\mathbb{S}^{d_2-1}).

  9. Knowl 9 — Complexity Comparison of Nonconvex Zeroth-Order and First-Order Methods

    data/table

    The theoretical query and gradient complexities for finding an ϵ\epsilon-stationary point in stochastic mini-optimization min⁡x∈Xf(x)\min_{x \in \mathcal{X}} f(x) and minimax optimization min⁡x∈Xmax⁡y∈Yf(x,y)\min_{x \in \mathcal{X}} \max_{y \in \mathcal{Y}} f(x, y) (with τ\tau-strongly concave yy, condition number κy=Lf/τ\kappa_y = L_f / \tau, and dimensions d,d1,d2d, d_1, d_2 where d~=d1+d2\tilde{d} = d_1 + d_2) are compared below.

    Algorithm Problem Estimator Batch Size Complexity
    ZO-SGD Mini Gaussian O(1)\mathcal{O}(1) O(dϵ−4)\mathcal{O}(d \epsilon^{-4})
    ZO-AdaMM Mini Uniform O(ϵ−2)\mathcal{O}(\epsilon^{-2}) O(d2ϵ−4)\mathcal{O}(d^2 \epsilon^{-4})
    ZO-SVRG Mini Coordinate O(ϵ−2)\mathcal{O}(\epsilon^{-2}) O(dϵ−10/3)\mathcal{O}(d \epsilon^{-10/3})
    ZO-SPIDER-Coord Mini Coordinate O(ϵ−2)\mathcal{O}(\epsilon^{-2}) O(dϵ−3)\mathcal{O}(d \epsilon^{-3})
    SPIDER-SZO Mini Coordinate O(ϵ−2)\mathcal{O}(\epsilon^{-2}) O(dϵ−3)\mathcal{O}(d \epsilon^{-3})
    Acc-ZOM Mini Uniform O(1)\mathcal{O}(1) O~(d3/4ϵ−3)\tilde{\mathcal{O}}(d^{3/4} \epsilon^{-3})
    ZO-Min-Max Minimax Uniform O((d1+d2)κy2ϵ−2)\mathcal{O}((d_1+d_2)\kappa_y^2 \epsilon^{-2}) O((d1+d2)κy6ϵ−6)\mathcal{O}((d_1+d_2)\kappa_y^6 \epsilon^{-6})
    ZO-SGDA Minimax Gaussian O((d1+d2)ϵ−2)\mathcal{O}((d_1+d_2)\epsilon^{-2}) O((d1+d2)κy5ϵ−4)\mathcal{O}((d_1+d_2)\kappa_y^5 \epsilon^{-4})
    ZO-SGDMSA Minimax Gaussian O((d1+d2)ϵ−2)\mathcal{O}((d_1+d_2)\epsilon^{-2}) O~((d1+d2)κy2ϵ−4)\tilde{\mathcal{O}}((d_1+d_2)\kappa_y^2 \epsilon^{-4})
    ZO-SREDA-Boost Minimax Coordinate O(max⁡(κyϵ−1,d~)κyϵ−1)\mathcal{O}(\max(\kappa_y \epsilon^{-1}, \tilde{d})\kappa_y \epsilon^{-1}) O((d1+d2)κy3ϵ−3)\mathcal{O}((d_1+d_2)\kappa_y^3 \epsilon^{-3})
    Acc-ZOMDA Minimax Uniform O(1)\mathcal{O}(1) O~((d1+d2)3/4κy4.5ϵ−3)\tilde{\mathcal{O}}((d_1+d_2)^{3/4}\kappa_y^{4.5}\epsilon^{-3})
    PGSVRG Minimax Explicit O(ϵ−2)\mathcal{O}(\epsilon^{-2}) O(κy3ϵ−4)\mathcal{O}(\kappa_y^3 \epsilon^{-4})
    SGDA Minimax Explicit O(κyϵ−2)\mathcal{O}(\kappa_y \epsilon^{-2}) O(κy3ϵ−4)\mathcal{O}(\kappa_y^3 \epsilon^{-4})
    SREDA Minimax Explicit O(κy2ϵ−2)\mathcal{O}(\kappa_y^2 \epsilon^{-2}) O(κy3ϵ−3)\mathcal{O}(\kappa_y^3 \epsilon^{-3})
    SREDA-Boost Minimax Explicit O(κy2ϵ−2)\mathcal{O}(\kappa_y^2 \epsilon^{-2}) O(κy3ϵ−3)\mathcal{O}(\kappa_y^3 \epsilon^{-3})
    Acc-MDA Minimax Explicit O(1)\mathcal{O}(1) O~(κy4.5ϵ−3)\tilde{\mathcal{O}}(\kappa_y^{4.5}\epsilon^{-3})
    Acc-MDA Minimax Explicit O(κy3)\mathcal{O}(\kappa_y^3) O~(κy3ϵ−3)\tilde{\mathcal{O}}(\kappa_y^3 \epsilon^{-3})
    Acc-MDA Minimax Explicit O(κy4)\mathcal{O}(\kappa_y^4) O~(κy2.5ϵ−3)\tilde{\mathcal{O}}(\kappa_y^{2.5}\epsilon^{-3})

    The comparison shows that Acc-ZOM reduces the dimensional dependence for mini-optimization from O(d)\mathcal{O}(d) to O(d3/4)\mathcal{O}(d^{3/4}) with O(1)\mathcal{O}(1) batch size. For minimax optimization, Acc-ZOMDA is the only zeroth-order method achieving O(1)\mathcal{O}(1) batch size with sublinear query complexity O~(d~3/4κy4.5ϵ−3)\tilde{\mathcal{O}}(\tilde{d}^{3/4}\kappa_y^{4.5}\epsilon^{-3}). In the first-order setting, Acc-MDA achieves O~(κy2.5ϵ−3)\tilde{\mathcal{O}}(\kappa_y^{2.5}\epsilon^{-3}) with batch size O(κy4)\mathcal{O}(\kappa_y^4), improving upon the best existing bound by a factor of O(κy1/2)\mathcal{O}(\kappa_y^{1/2}).

  10. Knowl 10 — Empirical Performance on Black-Box Adversarial Attacks to DNNs

    empirical result

    Acc-ZOM was evaluated on designing universal black-box adversarial perturbations against pre-trained deep neural networks across four image classification benchmarks:

    Setup:

    • Datasets: MNIST (99.4% test accuracy), FashionMNIST (91.8%), CIFAR-10 (93.2%), and SVHN (80.8%).
    • Optimization Task: Untargeted perturbation x∈Rdx \in \mathbb{R}^d constrained to ∥x∥∞≤ε\|x\|_\infty \le \varepsilon over n=40n = 40 images: min⁡∥x∥∞≤ε1n∑i=1n(fbi(x+ai)−max⁡j≠bifj(x+ai)+ln⁡(1+exp⁡(max⁡j≠bifj(x+ai)−fbi(x+ai))))\min_{\|x\|_\infty \le \varepsilon} \frac{1}{n}\sum_{i=1}^n \left( f_{b_i}(x + a_i) - \max_{j \ne b_i} f_j(x + a_i) + \ln\left(1 + \exp\left(\max_{j \ne b_i} f_j(x + a_i) - f_{b_i}(x + a_i)\right)\right) \right) where fj(x+ai)f_j(x+a_i) is the jj-th pre-softmax logit, and ε\varepsilon is set to 0.40.4 (MNIST), 0.30.3 (FashionMNIST), 0.10.1 (CIFAR-10), and 0.20.2 (SVHN). Batch size was fixed to 10.
    • Acc-ZOM parameters: γ=0.1\gamma = 0.1, k=1k = 1, m=3m = 3, c=3c = 3.

    Findings:

    1. Query Efficiency: Acc-ZOM reduced attack loss with significantly fewer function queries than baseline zeroth-order methods (ZO-AdaMM, ZO-SPIDER-Coord, SPIDER-SZO, and ZO-SFW) across all four datasets.
    2. Batch Size Robustness: When tested across batch sizes of 5, 10, and 20, Acc-ZOM exhibited consistent convergence trajectories and query efficiency, validating its batch-size independence.
  11. Knowl 11 — Empirical Performance on Data Poisoning Attacks to Logistic Regression

    empirical result

    Acc-ZOMDA (two-sided black-box), Acc-Semi-ZOMDA (one-sided black-box w.r.t. attacker), and Acc-MDA (transparent) were evaluated on data poisoning attacks against ℓ2\ell_2-regularized binary logistic regression.

    Formulation: max⁡∥x∥∞≤εmin⁡∥y∥22≤λregf(x,y)=h(x,y;Dp)+h(0,y;Dt)\max_{\|x\|_\infty \le \varepsilon} \min_{\|y\|_2^2 \le \lambda_{\text{reg}}} f(x, y) = h(x, y; \mathcal{D}_p) + h(0, y; \mathcal{D}_t) where h(x,y;D)=−1∣D∣∑(ai,bi)∈D[biln⁡g(x,y;ai)+(1−bi)ln⁡(1−g(x,y;ai))]h(x, y; \mathcal{D}) = -\frac{1}{|\mathcal{D}|}\sum_{(a_i, b_i) \in \mathcal{D}} [b_i \ln g(x, y; a_i) + (1 - b_i) \ln(1 - g(x, y; a_i))] with logistic sigmoid g(x,y;ai)=(1+e−(x+ai)Ty)−1g(x, y; a_i) = (1 + e^{-(x+a_i)^T y})^{-1}. Synthetic data consisted of n=1000n = 1000 samples in d=100d = 100 dimensions from N(0,1)\mathcal{N}(0, 1), corruption rate ∣Dp∣/(∣Dt∣+∣Dp∣)=0.15|\mathcal{D}_p| / (|\mathcal{D}_t| + |\mathcal{D}_p|) = 0.15, ε=2\varepsilon = 2, and λreg=0.001\lambda_{\text{reg}} = 0.001. Acc-ZOMDA hyperparameters were γ=0.2\gamma = 0.2, λ=0.08\lambda = 0.08, k=1k = 1, m=3m = 3, c1=3c_1 = 3, c2=3c_2 = 3.

    Findings:

    1. In the two-side black-box setting, Acc-ZOMDA achieved the fastest convergence and the smallest final stationary gap compared to ZO-Min-Max, ZO-SGDMSA, and ZO-SREDA-Boost.
    2. In the one-side black-box setting, Acc-Semi-ZOMDA substantially outperformed ZO-Min-Max in convergence speed and stationary gap reduction.
    3. In the transparent setting, Acc-MDA reached a lower stationary gap in fewer iterations than SGDA and SREDA-Boost.
    4. Parameter sensitivity visualization across γ∈[0.04,0.36]\gamma \in [0.04, 0.36] and λ∈[0.02,0.18]\lambda \in [0.02, 0.18] showed that all proposed momentum descent-ascent variants maintain stable convergence across a broad range of step-size pairs.

Coverage note — Intermediate algebraic derivations, supporting Lyapunov decrease inequalities (Lemmas 21, 22, 24, 25, 27, 28, 29, 31, 32, 33, 35, 36, 37, 39, 40, 41), and Appendix C's re-derivation of the ZO-Min-Max query complexity were omitted as supporting proof apparatus.

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Citation

MLA
Huang, F., et al. “Accelerated Zeroth-Order and First-Order Momentum Methods from Mini to Minimax Optimization”. Journal of Machine Learning Research, vol. 23, no. 36, 2022, pp. 1–0, https://www.jmlr.org/papers/v23/20-924.html.
APA
Huang, F., Gao, S., Pei, J., & Huang, H. (2022). Accelerated Zeroth-Order and First-Order Momentum Methods from Mini to Minimax Optimization. Journal of Machine Learning Research, 23(36), 1–70. https://www.jmlr.org/papers/v23/20-924.html
Chicago
Huang, F., S. Gao, J. Pei, and H. Huang. 2022. “Accelerated Zeroth-Order and First-Order Momentum Methods from Mini to Minimax Optimization”. Journal of Machine Learning Research 23 (36): 1–70. https://www.jmlr.org/papers/v23/20-924.html.
Harvard
Huang, F. et al. (2022) “Accelerated Zeroth-Order and First-Order Momentum Methods from Mini to Minimax Optimization”, Journal of Machine Learning Research, 23(36), pp. 1–70. Available at: https://www.jmlr.org/papers/v23/20-924.html.
Vancouver
1. Huang F, Gao S, Pei J, Huang H (2022) Accelerated Zeroth-Order and First-Order Momentum Methods from Mini to Minimax Optimization. Journal of Machine Learning Research 23:1–70

BibTeX

@article{JMLR:v23:20-924,
  author  = {Feihu Huang and Shangqian Gao and Jian Pei and Heng Huang},
  title   = {Accelerated Zeroth-Order and First-Order Momentum Methods from Mini to Minimax Optimization},
  journal = {Journal of Machine Learning Research},
  year    = {2022},
  volume  = {23},
  number  = {36},
  pages   = {1--70},
  url     = {http://jmlr.org/papers/v23/20-924.html}
}
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