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Eigenvectors of some large sample covariance matrix ensembles

16 November 2009
Olivier Ledoit
S. Péché
ArXiv (abs)PDFHTML
Abstract

We consider sample covariance matrices SN=1pΣN1/2XNXN∗ΣN1/2S_N=\frac{1}{p}\Sigma_N^{1/2}X_NX_N^* \Sigma_N^{1/2}SN​=p1​ΣN1/2​XN​XN∗​ΣN1/2​ where XNX_NXN​ is a N×pN \times pN×p real or complex matrix with i.i.d. entries with finite 12th12^{\rm th}12th moment and ΣN\Sigma_NΣN​ is a N×NN \times NN×N positive definite matrix. In addition we assume that the spectral measure of ΣN\Sigma_NΣN​ almost surely converges to some limiting probability distribution as N→∞N \to \inftyN→∞ and p/N→γ>0.p/N \to \gamma >0.p/N→γ>0. We quantify the relationship between sample and population eigenvectors by studying the asymptotics of functionals of the type 1NTr(g(ΣN)(SN−zI)−1)),\frac{1}{N} \text{Tr} (g(\Sigma_N) (S_N-zI)^{-1})),N1​Tr(g(ΣN​)(SN​−zI)−1)), where III is the identity matrix, ggg is a bounded function and zzz 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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