First Order Stochastic Optimization with Oblivious Noise

We initiate the study of stochastic optimization with oblivious noise, broadly generalizing the standard heavy-tailed noise setup. In our setting, in addition to random observation noise, the stochastic gradient may be subject to independent oblivious noise, which may not have bounded moments and is not necessarily centered. Specifically, we assume access to a noisy oracle for the stochastic gradient of $f$ at $x$, which returns a vector $\nabla f(γ, x) + ξ$, where $γ$ is the bounded variance observation noise and $ξ$ is the oblivious noise that is independent of $γ$ and $x$. The only assumption we make on the oblivious noise $ξ$ is that $\mathbf{Pr}[ξ= 0] \ge α$ for some $α\in (0, 1)$. In this setting, it is not information-theoretically possible to recover a single solution close to the target when the fraction of inliers $α$ is less than $1/2$. Our main result is an efficient list-decodable learner that recovers a small list of candidates, at least one of which is close to the true solution. On the other hand, if $α= 1-ε$, where $0< ε< 1/2$ is sufficiently small constant, the algorithm recovers a single solution. Along the way, we develop a rejection-sampling-based algorithm to perform noisy location estimation, which may be of independent interest.