On the consistent estimators of the population covariance matrix and its reparameterizations

For the high-dimensional covariance estimation problem, when limn→ ∞p/n=c ∈ (0,1) the orthogonally equivariant estimator of the population covariance matrix proposed by Tsai and Tsai (2024b) enjoys some optimal properties. Under some regularity conditions, they showed that their novel estimators of eigenvalues are consistent for the eigenvalues of the population covariance matrix. In this note, first, we show that their novel estimator is consistent estimator of the population covariance matrix under a high-dimensional asymptotic setup. Moreover, we also show that the novel estimator is the MLE of population covariance matrix when c ∈ (0, 1). The novel estimator is incorporated to establish the optimal decomposite TT²-test for a high-dimensional statistical hypothesis testing problem.