Tridiagonal Models for Dyson Brownian Motion

Abstract

In this paper, we consider tridiagonal matrices the eigenvalues of which evolve according to $\beta$-Dyson Brownian motion. This is the stochastic gradient flow on ${\mathbb{R}}^n$ given by, for all $1\le i\le n,$ \[ dλ_{i,t} = \sqrt{\frac{2}β}dZ_{i,t} - \biggl( \frac{V'(λ_i)}{2} - \sum_{j: j \neq i} \frac{1}{λ_i - λ_j} \biggr)\,dt \] where $V$ is a constraining potential and ${\left\{Z_{i,t}\right\}}_1^n$ are independent standard Brownian motions. This flow is stationary with respect to the distribution \[ ρ^β_N(λ) = \frac{1}{Z^β_N} e^{-\fracβ{2} \left( -\sum_{1 \leq i \neq j \leq N} \log|λ_i - λ_j| + \sum_{i=1}^N V(λ_i) \right) }. \] The particular choice of $V(t)=2t^2$ leads to an eigenvalue distribution constrained to lie roughly in $(-\sqrt{n},\sqrt{n}).$ We study evolution of the entries of one choice of tridiagonal flow for this $V$ in the $n\to \infty$ limit. On the way to describing the evolution of the tridiagonal matrices we give the derivative of the Lanczos tridiagonalization algorithm under perturbation.