The One World Seminar on The Mathematics of Machine Learning
Date: Wednesday, June 9, 2021
Time: 12 noon EST (5PM BST / 6PM CEST / 9AM PDT / 10AM MDT / 11AM CDT).
Speaker: Sebastian Kassing (University of Münster)
The link to the seminar will be sent on Tuesday (and will also be visible on the website just before the talk).
Title: : Convergence of Stochastic Gradient Descent for analytic target functions
Abstract: In this talk we discuss almost sure convergence of Stochastic Gradient Descent in discrete and continuous time for a given twice continuously-differentiable target function F. In a first step we give assumptions on the step-sizes and perturbation size to ensure convergence of the target value F and gradient f=DF assuming that f is locally Hölder-continuous. This result entails convergence of the iterates itself in the case where F does not possess a continuum of critical points.
In a general non-convex setting with F possibly containing a rich set of critical points, convergence of the process itself is sometimes taken for granted, but actually is a non-trivial issue as there are solutions to the gradient flow ODE for smooth target functions that stay in a compact set but do not converge. Using the Lojasiewicz-inequality we give sharp bounds on the step-sizes and the size of the perturbation in order to guarantee convergence of the SGD scheme for analytic target functions. Also, we derive the convergence rate of the function value under the assumptions that F satisfies a particular Lojasiewicz-inequality with exponent in [1/2,1). Finally, we compare the discrete and continuous time results and discuss optimality of the assumptions. This is joint work with Steffen Dereich (WWU Münster).