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1. Asymptotic Analysis via Stochastic Differential Equations of Gradient Descent Algorithms in Statistical and Computational Paradigms

   Wang, Yazhen ; Wu, Shang ( September 2020 , Journal of machine learning research)

   Bach, Francis ; Blei, David ; Scholkopf, Bernhard (Ed.)

   This paper investigates the asymptotic behaviors of gradient descent algorithms (particularly accelerated gradient descent and stochastic gradient descent) in the context of stochastic optimization arising in statistics and machine learning, where objective functions are estimated from available data. We show that these algorithms can be computationally modeled by continuous-time ordinary or stochastic differential equations. We establish gradient flow central limit theorems to describe the limiting dynamic behaviors of these computational algorithms and the large-sample performances of the related statistical procedures, as the number of algorithm iterations and data size both go to infinity, where the gradient flow central limit theorems are governed by some linear ordinary or stochastic differential equations, like time-dependent Ornstein-Uhlenbeck processes. We illustrate that our study can provide a novel unified framework for a joint computational and statistical asymptotic analysis, where the computational asymptotic analysis studies the dynamic behaviors of these algorithms with time (or the number of iterations in the algorithms), the statistical asymptotic analysis investigates the large-sample behaviors of the statistical procedures (like estimators and classifiers) that are computed by applying the algorithms; in fact, the statistical procedures are equal to the limits of the random sequences generated from these iterative algorithms, as the number of iterations goes to infinity. The joint analysis results based on the obtained The joint analysis results based on the obtained gradient flow central limit theorems lead to the identification of four factors---learning rate, batch size, gradient covariance, and Hessian---to derive new theories regarding the local minima found by stochastic gradient descent for solving non-convex optimization problems. 

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2. Asymptotic Analysis via Stochastic Differential Equations of Gradient Descent Algorithms in Statistical and Computational Paradigms

   Wang, Yazhen ; Wu, Shang ( September 2021 , Journal of machine learning research)

   Bach, Francis ; Blei, David ; Scholkopf, Bernhard (Ed.)

   This paper investigates the asymptotic behaviors of gradient descent algorithms (particularly accelerated gradient descent and stochastic gradient descent) in the context of stochastic optimization arising in statistics and machine learning, where objective functions are estimated from available data. We show that these algorithms can be computationally modeled by continuous-time ordinary or stochastic differential equations. We establish gradient flow central limit theorems to describe the limiting dynamic behaviors of these computational algorithms and the large-sample performances of the related statistical procedures, as the number of algorithm iterations and data size both go to infinity, where the gradient flow central limit theorems are governed by some linear ordinary or stochastic differential equations, like time-dependent Ornstein-Uhlenbeck processes. We illustrate that our study can provide a novel unified framework for a joint computational and statistical asymptotic analysis, where the computational asymptotic analysis studies the dynamic behaviors of these algorithms with time (or the number of iterations in the algorithms), the statistical asymptotic analysis investigates the large-sample behaviors of the statistical procedures (like estimators and classifiers) that are computed by applying the algorithms; in fact, the statistical procedures are equal to the limits of the random sequences generated from these iterative algorithms, as the number of iterations goes to infinity. The joint analysis results based on the obtained gradient flow central limit theorems lead to the identification of four factors—learning rate, batch size, gradient covariance, and Hessian—to derive new theories regarding the local minima found by stochastic gradient descent for solving non-convex optimization problems. 

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3. Adaptive Gradient Methods for Constrained Convex Optimization and Variational Inequalities

   Ene, Alina ; Nguyen, Huy L ; Vladu, Adrian ( May 2021 , Proceedings of the AAAI Conference on Artificial Intelligence)

   null (Ed.)

   We provide new adaptive first-order methods for constrained convex optimization. Our main algorithms AdaACSA and AdaAGD+ are accelerated methods, which are universal in the sense that they achieve nearly-optimal convergence rates for both smooth and non-smooth functions, even when they only have access to stochastic gradients. In addition, they do not require any prior knowledge on how the objective function is parametrized, since they automatically adjust their per-coordinate learning rate. These can be seen as truly accelerated Adagrad methods for constrained optimization. We complement them with a simpler algorithm AdaGrad+ which enjoys the same features, and achieves the standard non-accelerated convergence rate. We also present a set of new results involving adaptive methods for unconstrained optimization and variational inequalities arising from monotone operators. 

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4. Adaptive Gradient Methods for Constrained Convex Optimization and Variational Inequalities

   Ene, Alina ; Nguyen, Huy L ; Vladu, Adrian ( January 2021 , Proceedings of the AAAI Conference on Artificial Intelligence)

   We provide new adaptive first-order methods for constrained convex optimization. Our main algorithms AdaACSA and AdaAGD+ are accelerated methods, which are universal in the sense that they achieve nearly-optimal convergence rates for both smooth and non-smooth functions, even when they only have access to stochastic gradients. In addition, they do not require any prior knowledge on how the objective function is parametrized, since they automatically adjust their per-coordinate learning rate. These can be seen as truly accelerated Adagrad methods for constrained optimization. We complement them with a simpler algorithm AdaGrad+ which enjoys the same features, and achieves the standard non-accelerated convergence rate. We also present a set of new results involving adaptive methods for unconstrained optimization and variational inequalities arising from monotone operators. 

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5. On the Convergence of AdaGrad(Norm) on R^d: Beyond Convexity, Non-Asymptotic Rate and Acceleration

   Liu, Zijian ; Nguyen, Ta Duy ; Ene, Alina ; Nguyen, Huy ( February 2023 , International Conference on Learning Representations)

   Existing analysis of AdaGrad and other adaptive methods for smooth convex optimization is typically for functions with bounded domain diameter. In unconstrained problems, previous works guarantee an asymptotic convergence rate without an explicit constant factor that holds true for the entire function class. Furthermore, in the stochastic setting, only a modified version of AdaGrad, different from the one commonly used in practice, in which the latest gradient is not used to update the stepsize, has been analyzed. Our paper aims at bridging these gaps and developing a deeper understanding of AdaGrad and its variants in the standard setting of smooth convex functions as well as the more general setting of quasar convex functions. First, we demonstrate new techniques to explicitly bound the convergence rate of the vanilla AdaGrad for unconstrained problems in both deterministic and stochastic settings. Second, we propose a variant of AdaGrad for which we can show the convergence of the last iterate, instead of the average iterate. Finally, we give new accelerated adaptive algorithms and their convergence guarantee in the deterministic setting with explicit dependency on the problem parameters, improving upon the asymptotic rate shown in previous works. 

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