GLOBAL RATES OF CONVERGENCE IN LOG-CONCAVE DENSITY ESTIMATION

摘要

The estimation of a log-concave density on R-d represents a central problem in the area of nonparametric inference under shape constraints. In this paper, we study the performance of log-concave density estimators with respect to global loss functions, and adopt a minimax approach. We first show that no statistical procedure based on a sample of size n can estimate a log-concave density with respect to the squared Hellinger loss function with supremum risk smaller than order n(-4/5), when d = 1, and order n(-2/(d+1)) when d >= 2. In particular, this reveals a sense in which, when d >= 3, log-concave density estimation is fundamentally more challenging than the estimation of a density with two bounded derivatives (a problem to which it has been compared). Second, we show that for d <= 3, the Hellinger e-bracketing entropy of a class of log-concave densities with small mean and covariance matrix close to the identity grows like max {epsilon(-d/2), epsilon(-(d-1))} (up to a logarithmic factor when d = 2). This enables us to prove that when d <= 3 the log-concave maximum likelihood estimator achieves the minimax optimal rate (up to logarithmic factors when d = 2, 3) with respect to squared Hellinger loss..