Recent years have seen a flurry of activities in designing provably efficient nonconvex procedures for solving statistical estimation problems. Due to the highly nonconvex nature of the empirical loss, state-of-the-art procedures often require proper regularization (e.g. trimming, regularized cost, projection) in order to guarantee fast convergence. For vanilla procedures such as gradient descent, however, prior theory either recommends highly conservative step sizes to avoid overshooting, or completely lacks performance guarantees. This paper uncovers a striking phenomenon in nonconvex optimization: even in the absence of explicit regularization, gradient descent enforces proper regularization implicitly under various statistical models. In fact, gradient descent follows a trajectory staying within a basin that enjoys nice geometry, consisting of points incoherent with the sampling mechanism. This "implicit regularization" feature allows gradient descent to proceed in a far more aggressive fashion without overshooting, which in turn results in substantial computational savings. Focusing on three fundamental statistical estimation problems, i.e. phase retrieval, low-rank matrix completion, and blind deconvolution, we establish that gradient descent achieves near-optimal statistical and computational guarantees without explicit regularization. In particular, by marrying statistical modeling with generic optimization theory, we develop a general recipe for analyzing the trajectories of iterative algorithms via a leave-one-out perturbation argument. As a byproduct, for noisy matrix completion, we demonstrate that gradient descent achieves near-optimal error control --- measured entrywise and by the spectral norm --- which might be of independent interest.

研究非凸优化问题中梯度下降算法的隐式正则化特性，证明在多种统计模型中，梯度下降算法在没有显式正则化的情况下也能够实现正则化，并在相位恢复、低秩矩阵补全和盲反卷积等三个基本统计估计问题中实现近乎最优的统计和计算保证。

Implicit Regularization in Nonconvex Statistical Estimation: Gradient
  Descent Converges Linearly for Phase Retrieval, Matrix Completion and Blind
  Deconvolution

非凸统计估计中的隐式正则化：梯度下降在相位恢复、矩阵补全和盲源分离问题中线性收敛