Non universality of fluctuations of outlier eigenvectors for block diagonal deformations of Wigner matrices

TL;DR

This paper studies the fluctuations of outlier eigenvectors in deformed Wigner matrices and demonstrates that these fluctuations are not universal, depending on specific matrix details.

Contribution

It provides a rigorous analysis showing the non-universality of outlier eigenvector fluctuations in block deformed Wigner matrices.

Findings

Fluctuations of outlier eigenvectors are non-universal.

The non-universality depends on the specific matrix structure.

Results apply to block diagonal deformations of Wigner matrices.

Abstract

In this paper, we investigate the fluctuations of a unit eigenvector associated to an outlier in the spectrum of a spiked $N\times N$ complex Deformed Wigner matrix $M_N$: $M_N =W_N/\sqrt{N} + A_N$ where $W_N$ is an $N \times N$ Hermitian Wigner matrix whose entries have a law $\mu$ satisfying a Poincar\'e inequality and the matrix $A_N$ is a block diagonal matrix, with an eigenvalue $\theta$ of multiplicity one, generating an outlier in the spectrum of $M_N$. We prove that the fluctuations of the norm of the projection of a unit eigenvector corresponding to the outlier of $M_N$ onto a unit eigenvector corresponding to $\theta$ are not universal.