Large deviations for the largest eigenvalue of the sum of two random matrices

Abstract

In this paper, we consider the addition of two matrices in generic position, namely $A+UBU^{*}$, where $U$ is drawn under the Haar measure on the unitary or the orthogonal group. We show that, under mild conditions on the empirical spectral measures of the deterministic matrices $A$ and $B$, the law of the largest eigenvalue satisfies a large deviation principle, in the scale $N$, with an explicit rate function involving the limit of spherical integrals. We cover in particular the case when $A$ and $B$ have no outliers.