The Brown measure of a family of free multiplicative Brownian motions

Apr 16, 2021
72 pages
Published in:
  • Probab.Theor.Related Fields 186 (2023) 3-4, 1081-1166,
  • Probab.Theor.Related Fields 186 (2023) 1081
  • Published: Aug 1, 2023
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Abstract: (Springer)
We consider a family of free multiplicative Brownian motions bs,τb_{s,\tau } parametrized by a real variance parameter s and a complex covariance parameter τ.\tau . We compute the Brown measure μs,τ\mu _{s,\tau } of ubs,τ,ub_{s,\tau }, where u is a unitary element freely independent of bs,τ.b_{s,\tau }. We find that μs,τ\mu _{s,\tau } has a simple structure, with a density in logarithmic coordinates that is constant in the τ\tau -direction. These results generalize those of Driver–Hall–Kemp and Ho–Zhong for the case τ=s.\tau =s. We also establish a remarkable “model deformation phenomenon,” stating that all the Brown measures with s fixed and τ\tau varying are related by push-forward under a natural family of maps. Our proofs use a first-order nonlinear PDE of Hamilton–Jacobi type satisfied by the regularized log potential of the Brown measures. Although this approach is inspired by the PDE method introduced by Driver–Hall–Kemp, our methods are substantially different at both the technical and conceptual level.
Note:
  • Author final version. To appear in Probability Theory and Related Fields. 72 pages with 14 figures
  • 46L54 Free probability and free operator algebras
  • 60B20 Random matrices (probabilistic aspects)
  • 35F21 Hamilton-Jacobi equations
  • 58J65 Diffusion processes and stochastic analysis on manifolds
  • boundary condition: unitarity
  • family
  • Brownian motion
  • structure
  • density