S. Bezuglyi, J. Kwiatkowski, K. Medynets
This survey is focused on the results related to topologies on the groups of transformations in ergodic theory, Borel, and Cantor dynamics. Various topological properties (density, connectedness, genericity) of these groups and their subsets (subgroups) are studied.
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Using the notion of proper Cantor colorings we prove the following theorem. For any countably infinite group $Γ$, there exists a free continuous action $ζ: Γ\curvearrowright C$ on the Cantor set, which is universal in the following sense: for any free Borel action $α: Γ\curvearrowright X$ on the standard Borel space, there exists an injective Borel map $Θ_α: X\to C$ such that $Θ_α\circ α=ζ\circ Θ_α$. We extend our theorem for (nonfree) Borel $(Γ,Z)$-actions, where $Z$ is a uniformly recurrent subgroup.
The Borel complexity of conjugacy for Cantor minimal systems
本文证明 Cantor 极小系统共轭的 Borel 复杂度与可数图同构相同，通过编码 pro-有限群和逆向极限构造回答 Foreman 问题。
We introduce the fundamental group ${\mathcal F}(\mathcal{R}_{G, ϕ})$ of a uniquely ergodic Cantor minimal $G$-system $\mathcal{R}_{G, ϕ}$ where $G$ is a countable discrete group. We compute fundamental groups of several uniquely ergodic Cantor minimal $G$-systems. We show that if $\mathcal{R}_{G, ϕ}$ arises from a free action $ϕ$ of a finitely generated abelian group, then there exists a unital countable subring $R$ of $\mathbb{R}$ such that $\mathcal{F}(\mathcal{R}_{G, ϕ})=R_{+}^\times$. We also consider the relation between fundamental groups of uniquely ergodic Cantor minimal $\mathbb{Z}^n…
In this paper, we analyze the complexity of topological conjugacy of pointed Cantor minimal systems from the point of view of descriptive set theory. We prove that the topological conjugacy relation on pointed Cantor minimal systems is Borel bireducible with the Borel equivalence relation $Δ_{\mathbb{R}}^+$ on $\mathbb{R}^{\mathbb{N}}$ defined by $x Δ_{\mathbb{R}}^+ y \Leftrightarrow \{x_i:i \in \mathbb{N}\}=\{y_i:i \in \mathbb{N}\}$. Moreover, we show that $Δ_{\mathbb{R}}^+$ is a lower bound for the Borel complexity of topological conjugacy of Cantor minimal systems. Finally, we interpret our…
A Borel system $(X,S)$ is `almost Borel universal' if any free Borel dynamical system $(Y,T)$ of strictly lower entropy is isomorphic to a Borel subsystem of $(X,S)$, after removing a null set. We obtain and exploit a new sufficient condition for a topological dynamical system to be almost Borel universal. We use our main result to deduce various conclusions and answer a number of questions. Along with additional results, we prove that a `generic' homeomorphism of a compact manifold of topological dimension at least two can model any ergodic transformation, that non-uniform specification impli…
Downarowicz and Maass (2008) proposed topological ranks for all homeomorphic Cantor minimal dynamical systems using properly ordered Bratteli diagrams. In this study, we adopt this definition to the case of all essentially minimal zero-dimensional systems. We consider the cases in which topological ranks are 2 and unique minimal sets are fixed points. Akin and Kolyada (2003), in their study of Li--Yorke sensitivity, showed that if the unique minimal set of an essentially minimal system is a fixed point, then the system must be proximal. However, a finite topological rank implies expansiveness;…
In this survey we give a concise introduction to a continuous version of Borel combinatorics. Our approach will have a certain algorithm-theoretic nature and we will give special emphasis to the notion of almost finiteness introduced by Matui as a continuous analogue of Borel hyperfiniteness. We also show how the theory can be used to study spectral convergence for graph Laplacians.
In the first part of our generalized ergodic theory we introduced Cantor-systems, when we managed to prove the generalized ergodic theorem 3.3. The first component of a Cantor-system is a group of the flow and its second component is a set of sets covering the phase-space. Now we continue and we first come across the resistence of the term of metric sensitivity against further generalization. Finding a way of generalization of sensitivity, we understand, that generalized chaos can come out of special sources of sensitivity, which we call ultrasensitive. However there are conditions of generali…
Let $P$ be a Borel probability measure on $\mathbb R^2$ supported by the Cantor dusts generated by a set of $4^u,\ u\geq 1$, contractive similarity mappings satisfying the strong separation condition. For this probability measure, we determine the optimal sets of $n$-means and the $n$th quantization errors for all $n\geq 2$. In addition, it is shown that though the quantization dimension of the measure $P$ is known, the quantization coefficient for $P$ does not exist.
Orbit equivalence of Cantor minimal systems
本文研究了Cantor最小系统轨道等价性的描述复杂性，证明了不同Borel类的轨道等价关系的可约性及性质。
Strong Orbit Equivalence in Cantor dynamics and simple locally finite groups
本文研究极小同胚相关的可数局部有限群，证明其同构关系为Borel $S_\infty$-作用的泛关系，并为强轨道等价的刻画提供动力学途径。
This paper is a survey devoted to the study of probability and infinite ergodic invariant measures for aperiodic homeomorphisms of a Cantor set. We focus mostly on the cases when a homeomorphism has either a unique ergodic invariant measure or finitely many such measures (finitely ergodic homeomorphisms). Since every Cantor dynamical system $(X,T)$ can be realized as a Vershik map acting on the path space of a Bratteli diagram, we use combinatorial methods developed in symbolic dynamics and Bratteli diagrams during the last decade to study the simplex of invariant measures.
In this note we consider dynamical systems $(X,G)$ on a Cantor set $X$ satisfying some mild technical conditions. The considered class includes, in particular, minimal and transitive aperiodic systems. We prove that two such systems $(X_1,G_1)$ and $(X_2,G_2)$ are orbit equivalent if and only if their full groups are isomorphic as abstract groups. This result is a topological version of the well-known Dye's theorem established originally for ergodic measure-preserving actions.
The aim of this paper is to prove ergodic decomposition theorems for probability measures quasi-invariant under Borel actions of inductively compact groups (Theorem 1) as well as for sigma-finite invariant measures (Corollary 1). For infinite measures the ergodic decomposition is not unique, but the measure class of the decomposing measure on the space of projective measures is uniquely defined by the initial invariant measure (Theorem 2).
A Backward Ergodic Theorem for Uncountable-to-one Transformations
通过去除可数对一假设并利用Jankov-von Neumann一致化定理及Borel与测度场结构，将Anush Tserunyan和Jenna Zomback的2024年后向遍历定理推广到任意保测变换。
We prove that the family of measured dynamical systems which can be realised as uniquely ergodic minimal homeomorphisms on a given manifold (of dimension at least two) is stable under measured extension. As a corollary, any ergodic system with an irrational eigenvalue is isomorphic to a uniquely ergodic minimal homeomorphism on the two-torus. The proof uses the following improvement of Weiss relative version of Jewett-Krieger theorem: any extension between two ergodic systems is isomorphic to a skew-product on Cantor sets.
We prove that commutator subgroups of topological full groups arising from minimal subshifts have exponential growth. We also prove that the measurable full group associated to the countable, measure-preserving, ergodic and hyperfinite equivalence relation is topologically generated by two elements.
We examine some of the properties of uniformly rigid transformations, and analyze the compatibility of uniform rigidity and (measurable) weak mixing along with some of their asymptotic convergence properties. We show that on Cantor space, there does not exist a finite measure-preserving, totally ergodic, uniformly rigid transformation. We briefly discuss general group actions and show that (measurable) weak mixing and uniform rigidity can coexist in a more general setting.
Decompositions and measures on countable Borel equivalence relations
本文证明可数Borel等价关系的测度论遍历分解可由生成它的群作用的拓扑遍历分解实现，并应用于基数代数与弱等价性研究。
We introduce concepts of Radon MSJ and Radon disjointness for infinite Radon measure preserving homeomorphisms of the locally compact Cantor space. We construct an uncountable family of pairwise Radon disjoint infinite Chacon like transformations. Every such transformation is Radon strictly ergodic, totally ergodic, asymmetric (not isomorphic to its inverse), has Radon MSJ and possesses Radon joinings whose ergodic components are not joinings.
Let $K$ be the Cantor set. We prove that arbitrarily close to a homeomorphism $T:K\rightarrow K$ there exists a homeomorphism $\widetilde T:K\rightarrow K$ such that the $α$-limit and the $ω$-limit of every orbit is a periodic orbit. We also prove that arbitrarily close to an endomorphism $T:K\rightarrow K$ there exists an endomorphism $\widetilde T:K\rightarrow K$ close to $T$ such that every orbit is finally periodic.
We construct a minimal topological action $\wp$ of a non-amenable group on a Cantor set $C$, which is non-uniquely ergodic and furthermore there exist ergodic invariant measures $μ_1$ and $μ_2$ such that $(\wp,C,μ_1)$ and $(\wp,C,μ_2)$ are not orbit equivalent measurable equivalence relations.
We describe the approximation of a continuous dynamical system on a p. l. manifold or Cantor set by a tractable system. A system is tractable when it has a finite number of chain components and, with respect to a given full background measure, almost every point is generic for one of a finite number of ergodic invariant measures. The approximations use non-degenerate simplicial dynamical systems for p. l. manifolds and shift-like dynamical systems for Cantor Sets.
We consider finitely generated groups of real-analytic circle diffeomorphisms. We show that if such a group admits an exceptional minimal set (i.e., a minimal invariant Cantor set), then its Lebesgue measure is zero; moreover, there are only finitely many orbits of connected components of its complement. For the case of minimal actions, we show that if the underlying group is (algebraically) free, then the action is ergodic with respect to the Lebesgue measure. This provides first answers to questions due to É. Ghys, G. Hector and D. Sullivan.
The purpose of this note is twofold. In the first part we observe that two finitely generated non-amenable groups are quasi-isometric if and only if they admit topologically orbit equivalent Cantor minimal actions. In particular, free groups if different rank admit topologically orbit equivalent Cantor minimal actions unlike in the measurable setting. In the second part we introduce the measured orbit equivalence category of a Cantor minimal system and construct (in certain cases) a representation of this category on the category of finite-dimensional vector spaces. This gives rise to novel fu…
Pointwise Convergence of Ergodic Averages Along Integer Cantor Sets
研究沿整数康托集的遍历平均的逐点收敛，通过证明相关遍历平均在特定条件下几乎处处收敛，解决了康托测度在自相似尺度下的稀疏微分问题。
Suppose $L$ is a semisimple Levi subgroup of a connected Lie group~$G$, $X$ is a Borel $G$-space with finite invariant measure, and $α\colon X \times G \to \GL_n(\real)$ is a Borel cocycle. Assume $L$ has finite center, and that the real rank of every simple factor of~$L$ is at least two. We show that if $L$ is ergodic on~$X$, and the restriction of~$α$ to~$X \times L$ is cohomologous to a homomorphism (modulo a compact group), then, after passing to a finite cover of~$X$, the cocycle $α$ itself is cohomologous to a homomorphism (modulo a compact group).
Let $X$ be a zero-dimensional compact metrizable space endowed with a strictly positive continuous Borel $σ$-additive measure $μ$ which is good in the sense that for any clopen subsets $U,V\subset X$ with $μ(U)<μ(V)$ there is a clopen set $W\subset V$ with $μ(W)=μ(U)$. We study $σ$-ideals with Borel base on $X$ which are invariant under the action of the group $H_μ(X)$ of measure-preserving homeomorphisms of $(X,μ)$, and show that any such $σ$-ideal $\mathcal I$ is equal to one of seven $σ$-ideals: $\{\emptyset\}$, $[X]^{\leω}$, $\mathcal E$, $\mathcal M\cap\mathcal N$, $\mathcal M$, $\mathcal…
The paper is devoted to the study of topologies on the group Aut(X,B) of all Borel automorphisms of a standard Borel space $(X, B)$. Several topologies are introduced and all possible relations between them are found. One of these topologies, $τ$, is a direct analogue of the uniform topology widely used in ergodic theory. We consider the most natural subsets of $Aut(X, B)$ and find their closures. In particular, we describe closures of subsets formed by odometers, periodic, aperiodic, incompressible, and smooth automorphisms with respect to the defined topologies. It is proved that the set of …
We show that every uniquely ergodic minimal Cantor system is topological orbit equivalent to the natural extension of a numeration scale associated to a logistic map.
There exist uniquely ergodic affine interval exchange transformations of [0,1] with flips having wandering intervals and such that the support of the invariant measure is a Cantor set.
Automorphism groups of measures on the Cantor space. Part II: Abstract homogeneous measures
本文引入了一类在 Cantor 空间 clopen 子集 Boolean 代数上超齐次的半群值测度，通过三值谱刻画了其自同构群的传递性与极小性，并证明了存在不可数个满足极小作用且具稠密共轭类的全非原子概率 Borel 测度。
A ratio ergodic theorem via tiling and uniformly syndetic markers
本文通过铺砌方法和一致辛迪提标记，在连续到一的 Borel 变换上证明了 Dowker 比率遍历定理的无测度版本，并精确刻画了局部遍历比率的极限，同时推广了消失标记引理。
Non-strong ergodicity of canonical actions of the Thompson groups
证明Thompson群V及其推广在Cantor集上的典范作用不是强遍历的，导致交叉积von Neumann代数不饱满，并得到Thompson群的非嵌入结果。
Given any amenable group $G$ (with a left Haar measure $|\cdot|$ or $dg$), we can select out a \textit{Følner subnet} $\{F_θ,θ\inΘ\}$ from any left Følner net in $G$, which is \textit{$L^\infty$-admissible}, namely, for any Borel $G$-space $(X,\mathscr{X})$ and any $φ\in L^\infty(X,\mathscr{X})$, \begin{gather*} \lim_{θ\inΘ}\frac{1}{|F_θ|}\int_{F_θ}φ(gx)dg=φ^*(x)\ \forall x\in X\quad {\textrm{and}}\quad φ^*=(gφ)^*\ \forall g\in G. \end{gather*} Moreover, if $G$ is $σ$-compact such as a locally compact second countable Hausdorff amenable group, then $φ^*\in L^\infty(X,\mathscr{X})$, $φ^*(gx)=φ^…
We characterize the points that satisfy Birkhoff's ergodic theorem under certain computability conditions in terms of algorithmic randomness. First, we use the method of cutting and stacking to show that if an element x of the Cantor space is not Martin-Lof random, there is a computable measure-preserving transformation and a computable set that witness that x is not typical with respect to the ergodic theorem, which gives us the converse of a theorem by V'yugin. We further show that if x is weakly 2-random, then it satisfies the ergodic theorem for all computable measure-preserving transforma…
Each topological group $G$ admits a unique universal minimal dynamical system $(M(G),G)$. When $G$ is a non-compact locally compact group the phase space $M(G)$ of this universal system is non-metrizable. There are however topological groups for which $M(G)$ is the trivial one point system (extremely amenable groups), as well as topological groups $G$ for which $M(G)$ is a metrizable space and for which there is an explicit description of the dynamical system $(M(G),G)$. One such group is the topological group $S_\infty$ of all permutations of the integers ${\mathbb Z}$, with the topology of p…
Bratteli diagrams in Borel dynamics
本文研究非紧Borel动力系统的广义Bratteli图模型，利用无限矩阵Perron-Frobenius与随机矩阵理论刻画尾不变测度、传递性、唯一遍历性及Vershik映射。
We introduce computable actions of computable groups and prove the following versions of effective Birkhoff's ergodic theorem. Let $Γ$ be a computable amenable group, then there always exists a canonically computable tempered two-sided Følner sequence $(F_n)_{n \geq 1}$ in $Γ$. For a computable, measure-preserving, ergodic action of $Γ$ on a Cantor space $\{0,1\}^{\mathbb N}$ endowed with a computable probability measure $μ$, it is shown that for every bounded lower semicomputable function $f$ on $\{0,1\}^{\mathbb N}$ and for every Martin-Löf random $ω\in \{0,1\}^{\mathbb N}$ the equality \[ \…
On the entropy of processes generated by quasifactors
研究在动力系统中，若测度的熵大于零，则其准因子测度的熵也大于零。
Suppose that $X$ is a Polish space, $E$ is a countable Borel equivalence relation on $X$, and $μ$ is an $E$-invariant Borel probability measure on $X$. We consider the circumstances under which for every countable non-abelian free group $Γ$, there is a Borel sequence $(\cdot_r)_{r \in \mathbb{R}}$ of free actions of $Γ$ on $X$, generating subequivalence relations $E_r$ of $E$ with respect to which $μ$ is ergodic, with the further property that $(E_r)_{r \in \mathbb{R}}$ is an increasing sequence of relations which are pairwise incomparable under $μ$-reducibility. In particular, we show that if…
The isomorphism problem for group actions
本文研究局部紧群遍历作用的同构问题，证明可指标群的保测遍历作用的共轭关系非Borel，揭示了该分类问题的复杂性。
Let $α: G\curvearrowright X$ be a minimal free continuous action of an infinite countable amenable group on an infinite compact metrizable space. In this paper, under the hypothesis that the invariant ergodic probability Borel measure space $E_G(X)$ is compact and zero-dimensional, we show that the action $α$ has the small boundary property. This partially answers an open problem in dynamical systems that asks whether a minimal free action of an amenable group has the small boundary property if its space $M_G(X)$ of invariant Borel probability measures forms a Bauer simplex. In addition, under…
Measurable boundary maps and Patterson--Sullivan measures for non-Borel Anosov groups on the Furstenberg boundary
本文研究非博雷尔阿诺斯群在弗斯滕伯格边界上的帕特森-苏利文测度，探讨其存在性、唯一性和ergodic性，以及临界指数的严格凸性，并建立阿诺斯群的熵刚性结果。
The unbalanced graph of the conjugacy relation of ergodic measure-preserving transformations
本研究证明遍历保测变换的同构关系无法Borel归约到其满群自共轭作用诱导的关系，否定回答Le Maître的一个问题，支持Sabok的猜想，还证实该同构关系非平衡，正面回答Le Maître的另一问题。
On the spectrum of non-ergodic measures
研究非遍历测度的动力谱性质，证明其在特定条件下几乎所有遍历测度具有相同谱特征，并应用该结果展示均势几乎周期测度在外壳中的周期性结构。
We prove that for a countable discrete group $Γ$ containing a copy of the free group $\F_n$, for some $2\leq n\leq\infty$, as a normal subgroup, the equivalence relations of conjugacy, orbit equivalence and von Neumann equivalence of the ergodic a.e. free actions of $Γ$ are analytic non-Borel equivalence relations in the Polish space of probability measure preserving $Γ$ actions. As a consequence we obtain that the isomorphism relation in the spaces of separably acting factors of type $\II_1$, $\II_\infty$ and $\III_λ$, $0\leqλ\leq 1$, are analytic and not Borel when these spaces are given the…
Let R a be countable ergodic equivalence relation of type II_1 on a standard probability space (X,m). The group Out(R) of outer automorphisms of R consists of all invertible Borel measure preserving maps of the space which map R-classes to R-classes modulo those which preserve almost every R-class. We analyze the group Out(R) for relations R generated by actions of higher rank lattices, providing general conditions on finiteness and triviality of Out(R) and explicitly computing this group for the standard actions. The method is based on Zimmer's superrigidity for measurable cocycles, Ratner's …
We study equivalence relations $\mathcal R(Γ\curvearrowright G)$ that arise from left translation actions of countable groups on their profinite completions. Under the assumption that the action $Γ\curvearrowright G$ is free and has spectral gap, we describe precisely when $\mathcal R(Γ\curvearrowright G)$ is orbit equivalent or Borel reducible to another such equivalence relation $\mathcal R(Λ\curvearrowright H)$. As a consequence, we provide explicit uncountable families of free ergodic probability measure preserving (p.m.p.) profinite actions of $SL_2(\mathbb Z)$ and its non-amenable subgro…
Non-Archimedean TSI Polish groups and their potential Borel complexity spectrum
本文将 Scott 分析变体应用于非阿基米德 TSI 波兰群的连续作用，研究了其潜在 Borel 复杂度谱，定义了通用轨道等价关系，识别了可分类性障碍，并推广了 Ding 和 Gao 的等价关系刻画结果。
A numerical study of infinitely renormalizable area-preserving maps
通过数值方法研究无限可重整化保面积映射的不变Cantor集上最大Lyapunov指数为零时的Holder指数α，并验证导数余循环的遍历性及重整化谱的实性。
Building on work of Popa, Ioana, and Epstein--Törnquist, we show that, for every nonamenable countable discrete group $Γ$, the relations of conjugacy, orbit equivalence, and von Neumann equivalence of free ergodic (or weak mixing) measure preserving actions of $Γ$ on the standard atomless probability space are not Borel, thus answering questions of Kechris. This is an optimal and definitive result, which establishes a neat dichotomy with the amenable case, since any two free ergodic actions of an amenable group on the standard atomless probability space are orbit equivalent by classical result…
Almost 1-1 extensions of Furstenberg-Weiss type and test for amenability
该研究针对无限剩余有限群，证明极小等度连续Cantor系统的极小扩张存在诱导不变测度空间仿射双射的极小几乎1-1扩张，并据此得出Toeplitz子移位族可检验剩余有限群的顺从性。
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