We consider the twin problems of estimating the effective rank and the Schatten norms $\|{ \bf A}\|_{ s}$ of a rectangular $p\times q$ matrix ${ \bf A}$ from noisy observations. When $s$ is an even integer, we introduce a polynomial-time estimator of $\|{ \bf A}\|_s$ that achieves the minimax rate $(pq)^{ 1/4}$. Interestingly, this optimal rate does not depend on the underlying rank of the matrix. When $s$ is not an even integer, the optimal rate is much slower. A simple thresholding estimator of the singular values achieves the rate $(q\wedge p)(pq)^{ 1/4}$, which turns out to be optimal up to a logarithmic multiplicative term. The tight minimax rate is achieved by a more involved polynomial approximation method. This allows us to build estimators for a class of effective rank indices. As a byproduct, we also characterize the minimax rate for estimating the sequence of singular values of a matrix.
Categories: math.ST stat.TH