Furstenberg entropy realizations for virtually free groups and lamplighter groups


Let (G,µ) be a discrete group with a generating probability measure. Nevo showed that if G has Kazhdan’s property (T), then there exists ɛ > 0 such that the Furstenberg entropy of any (G,µ)-stationary ergodic space is either 0 or larger than ɛ. Virtually free groups, such as SL 2(ℤ), do not have property (T), and neither do their extensions, such as surface groups. For virtually free groups, we construct stationary actions with arbitrarily small, positive entropy. The construction involves building and lifting spaces of lamplighter groups. For some classical lamplighter gropus, these spaces realize a dense set of entropies between 0 and the Poisson boundary entropy.

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Corresponding author

Correspondence to Yair Hartman.

Additional information

Y. Hartman is supported by the European Research Council, grant 239885.

O. Tamuz is supported by ISF grant 1300/08, and is a recipient of the Google Europe Fellowship in Social Computing. This research is supported in part by this Google Fellowship.

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Hartman, Y., Tamuz, O. Furstenberg entropy realizations for virtually free groups and lamplighter groups. JAMA 126, 227–257 (2015). https://doi.org/10.1007/s11854-015-0016-2

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  • Markov Chain
  • Stationary Space
  • Random Walk
  • Discrete Group
  • Large Neighborhood