GTAdam: Gradient Tracking With Adaptive Momentum for Distributed Online Optimization

This article deals with a network of computing agents aiming to solve an online optimization problem in a distributed fashion, i.e., by means of local computation and communication, without any central coordinator. We propose the gradient tracking with an adaptive momentum estimation (GTAdam) distributed algorithm, which combines a gradient tracking mechanism with first- and second-order momentum estimates of the gradient. The algorithm is analyzed in the online setting for strongly convex cost functions with Lipschitz continuous gradients. We provide an upper bound for the dynamic regret given by a term related to the initial conditions and another term related to the temporal variations of the objective functions. Moreover, a linear convergence rate is guaranteed in the static setup. The algorithm is tested on a time-varying classification problem, on a (moving) target localization problem, and in a stochastic optimization setup from image classification. In these numerical experiments from multiagent learning, GTAdam outperforms state-of-the-art distributed optimization methods.