Diffusion Learning In Non-Convex Environments

Driven by the need to solve increasingly complex optimization problems in signal processing and machine learning, recent years have seen rising interest in the behavior of gradient-descent based algorithms in non-convex environments. Most of the works on distributed non-convex optimization focus on the deterministic setting, where exact gradients are available at each agent. In this work, we consider stochastic cost functions, where exact gradients are replaced by stochastic approximations and the resulting gradient noise persistently seeps into the dynamics of the algorithm. We establish that the diffusion algorithm continues to yield meaningful estimates in these more challenging, non-convex environments, in the sense that (a) despite the distributed implementation, restricted to local interactions, individual agents cluster in a small region around a common and well-defined vector, which will carry the interpretation of a network centroid, and (b) the network centroid inherits many properties of the centralized, stochastic gradient descent recursion, including the return of an O(mu)-mean-square-stationary point in at most O(1/mu(2)) iterations.