Abstract
Stochastic gradient descent (SGD) gives an optimal convergence rate when minimizing convex stochastic objectives f (x). However, in terms of making the gradients small, the original SGD does not give an optimal rate, even when f (x) is convex. If f (x) is convex, to find a point with gradient norm , we design an algorithm SGD3 with a near-optimal rate , improving the best known rate . If f (x) is nonconvex, to find its -approximate local minimum, we design an algorithm SGD5 with rate , where previously SGD variants only achieve [6, 14, 30]. This is no slower than the best known stochastic version of Newton’s method in all parameter regimes [27].