Revisiting Stochastic Approximation and Stochastic Gradient Descent

In this paper, we take a fresh look at stochastic approximation (SA) and Stochastic Gradient Descent (SGD). We derive new sufficient conditions for the convergence of SA. In particular, the "noise" or measurement error need not have a finite second moment, and under suitable conditions, not even a finite mean. By adapting this method of proof, we also derive sufficient conditions for the convergence of zero-order SGD, wherein the stochastic gradient is computed using only two function evaluations, and no gradient computations. The sufficient conditions derived here are the weakest to date, thus leading to a considerable expansion of the applicability of SA and SGD theory.