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1
Content available remote Two commuting maps without common minimal points
100%
Downarowicz T.
Colloquium Mathematicum
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2011
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tom 123
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nr 2
205-209
EN
We construct an example of two commuting homeomorphisms S, T of a compact metric space X such that the union of all minimal sets for S is disjoint from the union of all minimal sets for T. In other words, there are no common minimal points. This answers negatively a question posed in [C-L]. We remark that Furstenberg proved the existence of "doubly recurrent" points (see [F]). Not only are these points recurrent under both S and T, but they recur along the same sequence of powers. Our example shows that nothing similar holds if recurrence is replaced by the stronger notion of uniform recurrence.
2
Content available remote Odometers and Toeplitz systems revisited in the context of Sarnak's conjecture
63%
Downarowicz T. , Kasjan S.
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tom 229
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nr 1
45-72
EN
Although Sarnak's conjecture holds for compact group rotations (irrational rotations, odometers), it is not even known whether it holds for all Jewett-Krieger models of such rotations. In this paper we show that it does, as long as the model is at the same a topological extension, via the same map that establishes the isomorphism, of an equicontinuous model. In particular, we recover (after [AKL]) that regular Toeplitz systems satisfy Sarnak's conjecture, and, as another consequence, so do all generalized Sturmian subshifts (not only the classical Sturmian subshift). We also give an example of an irregular Toeplitz subshift which meets our criterion. We give an example of a model of an odometer which is not even Toeplitz (it is weakly mixing), hence does not meet our criterion. However, for this example, we manage to produce a separate proof of Sarnak's conjecture. Next, we provide a class of Toeplitz sequences which fail Sarnak's conjecture (in a weak sense); all these examples have positive entropy. Finally, we examine the example of a Toeplitz sequence from [AKL] (which fails Sarnak's conjecture in the strong sense) and prove that it also has positive entropy (this proof has been announced in [AKL]). This paper can be considered a sequel to [AKL], it also fills some gaps of [D].
3
Content available remote Faithful zero-dimensional principal extensions
63%
Downarowicz T. , Huczek D.
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2012
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tom 212
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nr 1
1-19
EN
We prove that every topological dynamical system (X,T) has a faithful zero-dimensional principal extension, i.e. a zero-dimensional extension (Y,S) such that for every S-invariant measure ν on Y the conditional entropy h(ν | X) is zero, and, in addition, every invariant measure on X has exactly one preimage on Y. This is a strengthening of the authors' result in Acta Appl. Math. [to appear] (where the extension was principal, but not necessarily faithful).
4
Content available remote Phenomena in rank-one ℤ²-actions
63%
Downarowicz T. , Serafin J.
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nr 3
281-294
EN
We present an example of a rank-one partially mixing ℤ²-action which possesses a non-rigid factor and for which the Weak Closure Theorem fails. This is in sharp contrast to one-dimensional actions, which cannot display this type of behavior.
5
Content available remote When every point is either transitive or periodic
63%
Downarowicz T. , Ye X.
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nr 1
137-150
EN
We study transitive non-minimal ℕ-actions and ℤ-actions. We show that there are such actions whose non-transitive points are periodic and whose topological entropy is positive. It turns out that such actions can be obtained by perturbing minimal systems under some reasonable assumptions.
6
Content available remote Large sets of integers and hierarchy of mixing properties of measure preserving systems
63%
Bergelson V. , Downarowicz T.
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nr 1
117-150
EN
We consider a hierarchy of notions of largeness for subsets of ℤ (such as thick sets, syndetic sets, IP-sets, etc., as well as some new classes) and study them in conjunction with recurrence in topological dynamics and ergodic theory. We use topological dynamics and topological algebra in βℤ to establish connections between various notions of largeness and apply those results to the study of the sets $R^{ε}_{A,B} = {n ∈ ℤ: μ(A ∩ TⁿB) > μ(A)μ(B) - ε}$ of times of "fat intersection". Among other things we show that the sets $R^{ε}_{A,B}$ allow one to distinguish between various notions of mixing and introduce an interesting class of weakly but not mildly mixing systems. Some of our results on fat intersections are established in a more general context of unitary ℤ-actions.
7
Content available remote Propriétés topologiques et combinatoires des échelles de numération
51%
Barat G. , Downarowicz T. , Iwanik A. , Liardet P.
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nr 2
285-306
EN
Topological and combinatorial properties of dynamical systems called odometers and arising from number systems are investigated. First, a topological classification is obtained. Then a rooted tree describing the carries in the addition of 1 is introduced and extensively studied. It yields a description of points of discontinuity and a notion of low scale, which is helpful in producing examples of what the dynamics of an odometer can look like. Density of the orbits is also discussed.
8
Content available remote Dynamiques associees a une echelle de numeration
51%
Barat G. , Downarowicz T. , Liardet P.
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tom 103
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nr 1
41-78
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