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Eigenvectors of some large sample covariance matrix ensembles

Abstract

We consider sample covariance matrices SN=1pΣN1/2XNXNΣN1/2S_N=\frac{1}{p}\Sigma_N^{1/2}X_NX_N^* \Sigma_N^{1/2} where XNX_N is a N×pN \times p real or complex matrix with i.i.d. entries with finite 12th12^{\rm th} moment and ΣN\Sigma_N is a N×NN \times N positive definite matrix. In addition we assume that the spectral measure of ΣN\Sigma_N almost surely converges to some limiting probability distribution as NN \to \infty and p/Nγ>0.p/N \to \gamma >0. We quantify the relationship between sample and population eigenvectors by studying the asymptotics of functionals of the type 1NTr(g(ΣN)(SNzI)1)),\frac{1}{N} \text{Tr} (g(\Sigma_N) (S_N-zI)^{-1})), where II is the identity matrix, gg is a bounded function and zz is a complex number. This is then used to compute the asymptotically optimal bias correction for sample eigenvalues, paving the way for a new generation of improved estimators of the covariance matrix and its inverse.

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