Overlaps, Eigenvalue Gaps, and Pseudospectrum under real Ginibre and Absolutely Continuous Perturbations

Abstract: Let $G_n$ be an $n \times n$ matrix with real i.i.d. $N(0,1/n)$ entries, let $A$ be a real $n \times n$ matrix with $\Vert A \Vert \le 1$, and let $\gamma \in (0,1)$. We show that with probability $0.99$, $A + \gamma G_n$ has all of its eigenvalue condition numbers bounded by $O\left(n<sup>{5/2}/\gamma<sup>{3/2}\right)$ and eigenvector condition number bounded by $O\left(n³ /\gamma<sup>{3/2}\right)$. Furthermore, we show that for any $s &gt; 0$, the probability that $A + \gamma G_n$ has two eigenvalues within distance at most $s$ of each other is $$O\left(n⁴ s<sup>{1/3}/\gamma<sup>{5/2}\right).$$ In fact, we show the above statements hold in the more general setting of non-Gaussian perturbations with real, independent, absolutely continuous entries with a finite moment assumption and appropriate normalization. This extends the previous work [Banks et al. 2019] which proved an eigenvector condition number bound of $O\left(n^{3/2} / \gamma\right)$ for the simpler case of {\em complex} i.i.d. Gaussian matrix perturbations. The case of real perturbations introduces several challenges stemming from the weaker anticoncentration properties of real vs. complex random variables. A key ingredient in our proof is new lower tail bounds on the small singular values of the complex shifts $z-(A+\gamma G_n)$ which recover the tail behavior of the complex Ginibre ensemble when $\Im z\neq 0$. This yields sharp control on the area of the pseudospectrum $\Lambda_\epsilon(A+\gamma G_n)$ in terms of the pseudospectral parameter $\epsilon&gt;0$, which is sufficient to bound the overlaps and eigenvector condition number via a limiting argument.