The Brown measure of the free multiplicative Brownian motion

Mar 26, 2019
64 pages
Published in:
  • Probab.Theor.Related Fields 184 (2022) 1-2, 209-273,
  • Probab.Theor.Related Fields 184 (2022) 209-273
  • Published: Oct 1, 2022
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Abstract: (Springer)
The free multiplicative Brownian motion btb_{t} is the large-N limit of the Brownian motion on GL(N;C),\mathsf {GL}(N;\mathbb {C}), in the sense of * -distributions. The natural candidate for the large-N limit of the empirical distribution of eigenvalues is thus the Brown measure of btb_{t} . In previous work, the second and third authors showed that this Brown measure is supported in the closure of a region Σt\Sigma _{t} that appeared in the work of Biane. In the present paper, we compute the Brown measure completely. It has a continuous density WtW_{t} on Σt,\overline{\Sigma }_{t}, which is strictly positive and real analytic on Σt\Sigma _{t} . This density has a simple form in polar coordinates: Wt(r,θ)=1r2wt(θ),\begin{aligned} W_{t}(r,\theta )=\frac{1}{r^{2}}w_{t}(\theta ), \end{aligned} where wtw_{t} is an analytic function determined by the geometry of the region Σt\Sigma _{t} . We show also that the spectral measure of free unitary Brownian motion utu_{t} is a “shadow” of the Brown measure of btb_{t} , precisely mirroring the relationship between the circular and semicircular laws. We develop several new methods, based on stochastic differential equations and PDE, to prove these results.
Note:
  • Added references to subsequent works building on these results. Made a notational change, replacing the regularization parameter x with epsilon
  • 60B20
  • 46L54
  • 35F21
  • measure: spectral
  • differential equations: stochastic
  • Brownian motion
  • density
  • unitarity
  • geometry
  • Wigner