Exact minimax risk for linear least squares, and the lower tail of sample covariance matrices
线性最小二乘的精确极小极大风险与样本协方差矩阵的下尾
We consider random-design linear prediction and related questions on the lower tail of random matrices. It is known that, under boundedness constraints, the minimax risk is of order d/n in dimension d with n samples. Here, we study the minimax expected excess risk over the full linear class, depending on the distribution of covariates. First, the least squares estimator is exactly minimax optimal in the well-specified case, for every distribution of covariates. We express…