Ultralimits, Amenable actions and Entropy
Abstract
In this paper we show that the minimal value of Furstenberg entropy (along all measures, not restricting to stationary ones) for any amenable action is the same as for the action of the group on itself. Using the boundary amenability result of Adams, this allows us to compute the minimal value of the entropy over all the measure classes in the boundary of the free group. Similar results are proved for the action of a hyperbolic group on its Gromov boundary. Our main tool is an ultralimit realization of the Poisson boundary of a time dependent matrix-valued random walk on the group. This extends and refines the results and tools of previous paper of the author with Y. Shalom.
Keywords
Cite
@article{arxiv.2204.11112,
title = {Ultralimits, Amenable actions and Entropy},
author = {Elad Sayag},
journal= {arXiv preprint arXiv:2204.11112},
year = {2024}
}
Comments
50 pages. Changes include few typos removed, two sections moved into appendices and a section of "further problems" added