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Strict Ergodicity and Transformation of the Torus

Strict ergodicity and transformation of the torus
Authors: Furstenberg, H.;

Strict Ergodicity and Transformation of the Torus

Abstract

Introduction. If T is a measure preserving transformation ofl a probability space Q with measure Iu, the ergodic theorem assures the existence N-1 almost everywhere with respect to /i of the average limN-'Ef(T i ), where Noo n=O f is an integrable function. Can this statement be improved in case Q is a compact topological space, T a suitable homeomorphism of ?2 with itself, and f a continuous function on Q? In particular, can convergence almost everywhere be replaced by convergence everywhere? In what follows we shall examine this question for the case that Q is an r-dimensional torus. When r = 1, i. e. Q is a circle, the answer is in the affirmative and the averages in question always exist (a fact implicit in the results of Denjoy [1] and van Kampen [5]). For r > 1, however, further restrictions must be imposed on the transformation T and part of our objective will be to exhibit a class of T for which this sharpened form of the ergodic theorem holds. The question we are considering is closely tied up with that of the "strict ergodicity" of a transformation. A transformation T of a compact Hausdorff space Q is strictly ergodic if it leaves invariant a unique probability measure on the borel field of ?. This notion was first introduced in conlnection with the theory of dynamical systems by Kryloff and Bogoliuboff ([6]; cf. also [8], [9]). When T is a strictly ergodic transformation, then (Theorem N-1 1. 1) the limits of N-1 E f(TAw) necessarily exist for f continuous and all n=O X C EQ, and moreover, this limit is independent of w. In the case of a strictly ergodic transformation, these conclusions are in fact a good deal more elementary than the usual ergodic theorem. Thus it is quite natural to inquire when a transformation of a given space will be strictly ergodic. As we will see, an important condition for the validity of some of our conclusions is that the transformation T not be homotopic to the identity transformation. This implies that the transformation T cannot be embedded in a continuous transformation group T(t) and so, in particular, could :not arise from the consideration of dynamical systems on the torus. The homotopy

Keywords

differentiation and integration, measure theory

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
296
Top 1%
Top 0.1%
Average
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