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Martha Łącka

Publications and source records attributed to Martha Łącka.

8 recordsLinked to original sources
arXiv2026

Entropy Density of Uniquely Ergodic Measures for Full Shifts over Amenable Residually Finite Groups

We study entropy density for full shifts over amenable residually finite groups. Entropy density means that every invariant measure can be approximated in the weak$^*$ topology, together with its entropy, by measures from a distinguished family. In symbolic dynamics it is known, by a result of Weiss, that uniquely ergodic measures are entropy dense among ergodic measures for the shift action of $\mathbb Z$. We extend this result to full shifts over amenable residually finite groups: invariant measures supported on uniquely ergodic subsystems are entropy dense in the collection of ergodic invariant measures. This applies in particular to full shifts over finitely generated abelian groups. The proof uses Cortez--Petite Følner tilings and a block-replacement construction adapted to finite-index subgroups.

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arXiv2025

On the closedness of ergodic measures in a characteristic class

We endow the set of all invariant measures of a topological dynamical system with a metric $\barρ$, which induces a topology stronger than the the weak$^*$-topology. Then, we study the closedness of ergodic measures within a characteristic class under this metric. Specifically, we show that if a sequence of generic points associated with ergodic measures from a fixed characteristic class converges in the Besicovitch pseudometric, then the limit point is generic for an ergodic measure in the same class. This implies that the set of ergodic measures belonging to a fixed characteristic class is closed in $\barρ$ (by a result of Babel, Can, Kwietniak, and Oprocha in [1]).

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arXiv2025

Feldman-Katok pseudometric and the GIKN construction of nonhyperbolic ergodic measures

The GIKN construction was introduced by Gorodetski, Ilyashenko, Kleptsyn, and Nalsky in [Functional Analysis and its Applications, 39 (2005), 21--30]. It gives a nonhyperbolic ergodic measure which is a weak$^*$ limit of a special sequence of measures supported on periodic orbits. This method was later adapted by numerous authors and provided examples of nonhyperbolic invariant measures in various settings. We prove that the result of the GIKN construction is always a loosely Kronecker measure in the sense of Ornstein, Rudolph, and Weiss (equivalently, standard measure in the sense of Katok, another name is loosely Bernoulli measure with zero entropy). For a proof we introduce and study the Feldman-Katok pseudometric $\bar{F_{K}}$. The pseudodistance $\bar{F_{K}}$ is a topological counterpart of the $\bar f$ metric for finite-state stationary stochastic processes introduced by Feldman and, independently, by Katok, later developed by Ornstein, Rudolph, and Weiss. We show that every measure given by the GIKN construction is the $\bar{F_{K}}$-limit of a sequence of periodic measures. On the other hand we prove that a measure which is the $\bar{F_{K}}$-limit of a sequence of ergodic measures is ergodic and its entropy is smaller or equal than the lower limit of entropies of measures in the sequence. Furthermore we demonstrate that $\bar{F_{K}}$-Cauchy sequence of periodic measures tends in the weak$^*$ topology either to a periodic measure or to a loosely Kronecker measure.

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arXiv2021

Uniform continuity of entropy rate with respect to the $\bar f$-pseudometric

Assume that a sequence $x=x_0x_1\ldots$ is frequency-typical for a finite-valued stationary stochastic process $\textbf X$. We prove that the function associating to $x$ the entropy-rate $\bar H(\textbf X)$ of $\textbf X$ is uniformly continuous when one endows the set of all frequency-typical sequences with the $\bar f$ pseudometric. As a consequence, we obtain the same result for the $\bar d$ pseudometric. We also give an alternative proof of the Abramov formula for the Kolmogorov-Sinai entropy of the induced measure-preserving transformation.

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arXiv2018

Quasi-uniform Convergence in Dynamical Systems Generated by an Amenable Group Action

We study properties of the Weyl pseudometric associated with an action of a countable amenable group on a compact metric space. We prove that the topological entropy and the number of minimal subsets of the closure of an orbit are both lower semicontinuous with respect to the Weyl pseudometric. Furthermore, the simplex of invariant measures supported on the orbit closure varies continuously. We apply the Weyl pseudometric to Toeplitz sequences for amenable residually finite groups. We introduce the notion of a regular Toeplitz sequence for an arbitrary amenable residually finite group and demonstrate that all regular Toeplitz sequences define minimal and uniquely ergodic systems. We prove path-connectivity of this family with respect to the Weyl pseudometric. This leads to a new proof of a theorem of Fabrice Krieger, which says that the values of entropy of Toeplitz sequences can have arbitrary finite entropy.

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arXiv2016

Generic Points for Dynamical Systems with Average Shadowing

It is proved that to every invariant measure of a compact dynamical system one can associate a certain asymptotic pseudo orbit such that any point asymptotically tracing in average that pseudo orbit is generic for the measure. It follows that the asymptotic average shadowing property implies that every invariant measure has a generic point. The proof is based on the properties of the Besicovitch pseudometric DB which are of independent interest. It is proved among the other things that the set of generic points of ergodic measures is a closed set with respect to DB. It is also showed that the weak specification property implies the average asymptotic shadowing property thus the theory presented generalizes most known results on the existence of generic points for arbitrary invariant measures.

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arXiv2015

A panorama of specification-like properties and their consequences

We offer an overview of the specification property, its relatives and their consequences. We examine relations between specification-like properties and such notions as: mixing, entropy, the structure of the simplex of invariant measures, and various types of the shadowing property. We pay special attention to these connections in the context of symbolic dynamics.

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