Algorithms for Smooth Nonconvex Optimization with Worst-case Guarantees


Book Description

The nature of global convergence guarantees for nonconvex optimization algorithms has changed significantly in recent years. New results characterize the maximum computational cost required for algorithms to satisfy approximate optimality conditions, instead of focusing on the limiting behavior of the iterates. In many contexts, such as those arising from machine learning, convergence to approximate second order points is desired. Algorithms designed for these problems must avoid saddle points efficiently to achieve optimal worst-case guarantees. In this dissertation, we develop and analyze a number of nonconvex optimization algorithms. First, we focus on accelerated gradient algorithms and provide results related to the avoidance of "strict saddle points''. In addition, the rate of divergence these accelerated gradient algorithms exhibit when in a neighborhood of strict saddle points is proven. Subsequently, we propose three new algorithms for smooth, nonconvex optimization with worst-case complexity guarantees. The first algorithm is developed for unconstrained optimization and is based on the classical Newton Conjugate Gradient method. This approach is then extended to bound constrained optimization by modifying the primal-log barrier method. Finally, we present a method for a special class of ``strict saddle functions'' which does not require knowledge of the parameters defining the optimization landscape. These algorithms converge to approximate second-order points in the best known computational complexity for their respective problem classes.




Evaluation Complexity of Algorithms for Nonconvex Optimization


Book Description

A popular way to assess the “effort” needed to solve a problem is to count how many evaluations of the problem functions (and their derivatives) are required. In many cases, this is often the dominating computational cost. Given an optimization problem satisfying reasonable assumptions—and given access to problem-function values and derivatives of various degrees—how many evaluations might be required to approximately solve the problem? Evaluation Complexity of Algorithms for Nonconvex Optimization: Theory, Computation, and Perspectives addresses this question for nonconvex optimization problems, those that may have local minimizers and appear most often in practice. This is the first book on complexity to cover topics such as composite and constrained optimization, derivative-free optimization, subproblem solution, and optimal (lower and sharpness) bounds for nonconvex problems. It is also the first to address the disadvantages of traditional optimality measures and propose useful surrogates leading to algorithms that compute approximate high-order critical points, and to compare traditional and new methods, highlighting the advantages of the latter from a complexity point of view. This is the go-to book for those interested in solving nonconvex optimization problems. It is suitable for advanced undergraduate and graduate students in courses on advanced numerical analysis, data science, numerical optimization, and approximation theory.







Trust Region Methods


Book Description

Mathematics of Computing -- General.










Proceedings Of The International Congress Of Mathematicians 2018 (Icm 2018) (In 4 Volumes)


Book Description

The Proceedings of the ICM publishes the talks, by invited speakers, at the conference organized by the International Mathematical Union every 4 years. It covers several areas of Mathematics and it includes the Fields Medal and Nevanlinna, Gauss and Leelavati Prizes and the Chern Medal laudatios.




Optimization for Machine Learning


Book Description

An up-to-date account of the interplay between optimization and machine learning, accessible to students and researchers in both communities. The interplay between optimization and machine learning is one of the most important developments in modern computational science. Optimization formulations and methods are proving to be vital in designing algorithms to extract essential knowledge from huge volumes of data. Machine learning, however, is not simply a consumer of optimization technology but a rapidly evolving field that is itself generating new optimization ideas. This book captures the state of the art of the interaction between optimization and machine learning in a way that is accessible to researchers in both fields. Optimization approaches have enjoyed prominence in machine learning because of their wide applicability and attractive theoretical properties. The increasing complexity, size, and variety of today's machine learning models call for the reassessment of existing assumptions. This book starts the process of reassessment. It describes the resurgence in novel contexts of established frameworks such as first-order methods, stochastic approximations, convex relaxations, interior-point methods, and proximal methods. It also devotes attention to newer themes such as regularized optimization, robust optimization, gradient and subgradient methods, splitting techniques, and second-order methods. Many of these techniques draw inspiration from other fields, including operations research, theoretical computer science, and subfields of optimization. The book will enrich the ongoing cross-fertilization between the machine learning community and these other fields, and within the broader optimization community.