TY - JOUR

T1 - Finite generators for countable group actions in the Borel and Baire category settings

AU - Tserunyan, Anush

N1 - Publisher Copyright:
© 2014 Published by Elsevier Inc.

PY - 2015/1/1

Y1 - 2015/1/1

N2 - For a continuous action of a countable discrete group G on a Polish space X, a countable Borel partition P of X is called a generator if GP:={gP:g∈G,P∈P} generates the Borel σ-algebra of X. For G=Z, the Kolmogorov-Sinai theorem gives a measure-theoretic obstruction to the existence of finite generators: they do not exist in the presence of an invariant probability measure with infinite entropy. It was asked by Benjamin Weiss in the late 1980s whether the nonexistence of any invariant probability measure guarantees the existence of a finite generator. We show that the answer is positive (in fact, there is a 32-generator) for an arbitrary countable group G and σ-compact X (in particular, for locally compact X). We also show that for aperiodic actions there is always a 4-generator on a comeager set, thus giving a positive answer to a question of Alexander Kechris asked the mid-1990s.Furthermore, assuming a positive answer to Weiss's question for arbitrary Polish spaces and G=Z, we prove the following dichotomy: every aperiodic Borel action of Z on a Polish space X admits either an invariant probability measure of infinite entropy or a finite generator. As an auxiliary lemma, we prove the following statement, which may be of independent interest: every aperiodic Borel action of a countable group G on a Polish space X admits a G-equivariant Borel map to the aperiodic part of the shift action G{right curved arrow}2G. We also obtain a number of other related results, among which is a criterion for the nonexistence of non-meager weakly wandering sets for continuous actions of Z. A consequence of this is a negative answer to a question asked by Eigen-Hajian-Nadkarni, which was also independently answered by Benjamin Miller.

AB - For a continuous action of a countable discrete group G on a Polish space X, a countable Borel partition P of X is called a generator if GP:={gP:g∈G,P∈P} generates the Borel σ-algebra of X. For G=Z, the Kolmogorov-Sinai theorem gives a measure-theoretic obstruction to the existence of finite generators: they do not exist in the presence of an invariant probability measure with infinite entropy. It was asked by Benjamin Weiss in the late 1980s whether the nonexistence of any invariant probability measure guarantees the existence of a finite generator. We show that the answer is positive (in fact, there is a 32-generator) for an arbitrary countable group G and σ-compact X (in particular, for locally compact X). We also show that for aperiodic actions there is always a 4-generator on a comeager set, thus giving a positive answer to a question of Alexander Kechris asked the mid-1990s.Furthermore, assuming a positive answer to Weiss's question for arbitrary Polish spaces and G=Z, we prove the following dichotomy: every aperiodic Borel action of Z on a Polish space X admits either an invariant probability measure of infinite entropy or a finite generator. As an auxiliary lemma, we prove the following statement, which may be of independent interest: every aperiodic Borel action of a countable group G on a Polish space X admits a G-equivariant Borel map to the aperiodic part of the shift action G{right curved arrow}2G. We also obtain a number of other related results, among which is a criterion for the nonexistence of non-meager weakly wandering sets for continuous actions of Z. A consequence of this is a negative answer to a question asked by Eigen-Hajian-Nadkarni, which was also independently answered by Benjamin Miller.

KW - Compressibility

KW - Entropy

KW - Finite generator

KW - Generating partition

KW - Group action

KW - Shift action

KW - Weakly wandering set

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U2 - 10.1016/j.aim.2014.10.013

DO - 10.1016/j.aim.2014.10.013

M3 - Article

AN - SCOPUS:84909594846

VL - 269

SP - 585

EP - 646

JO - Advances in Mathematics

JF - Advances in Mathematics

SN - 0001-8708

ER -