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## Fundamenta Mathematicae

2002 | 173 | 2 | 191-199
Tytuł artykułu

### Dispersing cocycles and mixing flows under functions

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Let T be a measure-preserving and mixing action of a countable abelian group G on a probability space (X,𝒮,μ) and A a locally compact second countable abelian group. A cocycle c: G × X → A for T disperses if $lim_{g→∞}c(g,·) - α(g) = ∞$ in measure for every map α: G → A. We prove that such a cocycle c does not disperse if and only if there exists a compact subgroup A₀ ⊂ A such that the composition θ ∘ c: G × X → A/A₀ of c with the quotient map θ: A → A/A₀ is trivial (i.e. cohomologous to a homomorphism η: G → A/A₀).
This result extends a number of earlier characterizations of coboundaries and trivial cocycles by tightness conditions on the distributions of the maps {c(g,·):g ∈ G} and has implications for flows under functions: let T be a measure-preserving ergodic automorphism of a probability space (X,𝒮,μ), f: X → ℝ be a nonnegative Borel map with ∫fdμ = 1, and $T^f$ be the flow under the function f with base T. Our main result implies that, if T is mixing and $T^f$ is weakly mixing, or if T is ergodic and $T^f$ is mixing, then the cocycle f: ℤ × X → ℝ defined by f disperses. The latter statement answers a question raised by Mariusz Lemańczyk in [7].
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Tom
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191-199
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wydano
2002
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autor
• Mathematics Institute, University of Vienna, Strudlhofgasse 4, A-1090 Wien, Austria
• Erwin Schrödinger Institute, for Mathematical Physics, Boltzmanngasse 9, A-1090 Vienna, Austria
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