arXiv subjects

Tom Meyerovitch

Publications and source records attributed to Tom Meyerovitch.

At least 19 recordsLinked to original sources

arXiv2026

Extensions of invariant random orders on groups

In this paper we study the action of a countable group $Γ$ on the space of orders on the group. In particular, we are concerned with the invariant probability measures on this space, known as invariant random orders. We show that for any countable group the space of random invariant orders is rich enough to contain an isomorphic copy of any free ergodic action, and characterize the non-free actions realizable. We prove a Glasner-Weiss dichotomy regarding the simplex of invariant random orders. We also show that the invariant partial order on $\mathrm{SL}_3(\mathbf{Z})$ corresponding to the semigroup of positive matrices cannot be extended to an invariant random total order. We thus provide the first example for a partial order (deterministic or random) that cannot be randomly extended.

math.DS↗

arXiv2026

A Krieger Embedding Theorem for Near Markov Sofic Shifts

Krieger's classical embedding theorem gives necessary and sufficient conditions for embedding a subshift into a mixing shift of finite type (SFT) as a proper subshift. The same result does not hold if one replaces mixing SFT by a mixing sofic shift. In this paper, we generalize Krieger's conditions to give necessary and sufficient conditions for embedding a subshift into a mixing (in fact irreducible) near Markov sofic shift (a special conjugacy-invariant class of sofic shifts). We also show that if the subshift to be embedded is irreducible sofic, then the conditions are finitely decidable.

math.DS↗

arXiv2026

A new notion of dimension for dynamical systems and shift embeddability

A dynamical system $(X,T)$ is \emph{shift embeddable} if $(X,T)$ embeds continuously and equivariantly in the shift over $[0,1]^d$ for some finite $d$. Refuting a major conjecture in the field, in a recent result of Dranishnikov and Levin it was shown that Gromov's mean dimension and Lebesgue covering dimension of finite orbits are not the only obstructions for shift embeddability. We present a new notion of dimension for dynamical systems over any countable group. We show that this new notion of dimension accounts for all known obstructions for shift embeddability.

math.DS↗

arXiv2026

Rationality and computability of the covering radius for sofic shifts

The covering radius of a shift space is a quantity of interest for information-theoretic applications of data transmission over noisy channels. We prove that the covering radius of a primitive sofic shift is a rational number, and describe an algorithm to compute the covering radius from a labeled graph presentation.

math.DS↗

arXiv2025

Kac's Lemma and countable generators for actions of countable groups

Kac's lemma determines the expected return time to a set of positive measure under iterations of an ergodic probability preserving transformations. We introduce the notion of an \emph{allocation} for a probability preserving action of a countable group. Using this notion, we formulate and prove generalization of Kac's lemma for an action of a general countable group, and another generalization that applies to probability preserving equivalence relations. As an application, we provide a short proof for the existence of countable generating partitions for any ergodic action of a countable group.

math.DS↗

arXiv2025

Factorizable embeddings and the period of an irreducible sofic shift

Generalizing a result of MacDonald we give necessary and sufficient conditions for an arbitrary subshift to embed into an irreducible sofic shift factoring through a given cover by an irreducible subshift of finite type (SFT). We obtain also necessary and sufficient conditions for an arbitrary subshift to embed into an irreducible sofic shift factoring through \emph{some} sliding block code out of an irreducible SFT. We do that when the code is required to be surjective, and hence a factor code, and when it is required to be injective or almost invertible, or is allowed to be arbitrary. These results require concepts of the period of an irreducible sofic shift as well as a concept of a $p$-periodic subshift. Several equivalent formulations of the period are developed.

math.DS↗

arXiv2025

Non-alternating mean payoff games

We present and study a variant of the mean payoff games introduced by A. Ehrenfeucht and J. Mycielski. In this version, the second player makes an infinite sequence of moves only after the first player's sequence of moves has been decided and revealed. Such games occur in the computation of the covering radius of constrained systems, a quantity of interest in coding theory.

cs.IT↗

arXiv2024

An embedding theorem for multidimensional subshifts

Krieger's embedding theorem provides necessary and sufficient conditions for an arbitrary subshift to embed in a given topologically mixing $\mathbb{Z}$-subshift of finite type. For some $\mathbb{Z}^d$-subshifts of finite type, Lightwood characterized the \emph{aperiodic} subsystems. In the current paper we prove a new embedding theorem for a class of subshifts of finite type over any countable abelian group. Our main theorem provides necessary and sufficient conditions for an arbitrary subshift $X$ to embed inside a given subshift of finite type $Y$ that satisfies a certain condition. For the particular case of $\mathbb{Z}$-subshifts, our new theorem coincides with Krieger's theorem. In particular, our result gives the first complete characterization of the subsystems of the multidimensional full shift $Y= A^{\mathbb{Z}^d}$. The natural condition on the target subshift $Y$, introduced explicitly for the first time in the current paper, is called the map extension property. It was introduced implicitly by Mike Boyle in the early 1980's for $\mathbb{Z}$-subshifts, and is closely related to the notion of an absolute retract, introduced by Borsuk in the 1930's. A $\mathbb{Z}$-subshift has the map extension property if and only if it is a topologically mixing subshift of finite type. Over abelian groups, a subshift has the map extension property if and only if it is a contractible SFT as shown in work of Poirier and Salo. We also establish a new theorem regarding lower entropy factors of multidimensional subshifts, that extends Boyle's lower entropy factor theorem from the one-dimensional case.

math.DS↗

arXiv2024

Periodicity of joint co-tiles in $\mathbb{Z}^d$

An old theorem of Newman asserts that any tiling of $\mathbb{Z}$ by a finite set is periodic. A few years ago, Bhattacharya proved the periodic tiling conjecture in $\mathbb{Z}^2$. Namely, he proved that for a finite subset $F$ of $\mathbb{Z}^2$, if there exists $A \subseteq \mathbb{Z}^2$ such that $F \oplus A = \mathbb{Z}^2$ then there exists a periodic $A' \subseteq \mathbb{Z}^2$ such that $F \oplus A' = \mathbb{Z}^2$. The recent refutation of the periodic tiling conjecture in high dimensions due to Greenfeld and Tao motivates finding different generalizations of Newman's theorem and of Bhattacharya's theorem that hold in arbitrary dimension $d$. In this paper, we formulate and prove such generalizations. We do so by studying the structure of joint co-tiles in $\mathbb{Z}^d$. Our generalization of Newman's theorem states that for any $d \ge 1$, any joint co-tile for $d$ independent tiles is periodic. For a $(d-1)$-tuple of finite subsets of $\mathbb{Z}^d$ that satisfy a certain technical condition that we call property $(\star)$, we prove that any joint co-tile decomposes into disjoint $(d-1)$-periodic sets. Consequently, we show that for a $(d-1)$-tuple of finite subsets of $\mathbb{Z}^d$ that satisfy property $(\star)$, the existence of a joint co-tile implies the existence of periodic joint co-tile. Conversely, we prove that if a finite subset $F$ in $\mathbb{Z}^d$ admits a periodic co-tile $A$, then there exist $(d-1)$ additional tiles that together with $F$ are independent and admit $A$ as a joint co-tile, so that the first $(d-2)$ of these tiles together with $F$ satisfy property $(\star)$. Combined, our results give a new necessary and sufficient condition for a subset of $\mathbb{Z}^d$ to tile periodically. We also discuss tilings and joint tilings in other countable abelian groups.

math.DS↗

arXiv2024

Automatic continuity of Polynomial maps and cocycles

Classical theorems from the early 20th century state that any Haar measurable homomorphism between locally compact groups is continuous. In particular, any Lebesgue-measurable homomorphism $ϕ:\mathbb{R} \to \mathbb{R}$ is of the form $ϕ(x)=ax$ for some $a \in \mathbb{R}$. In this short note, we prove that any Lebesgue measurable function $ϕ:\mathbb{R} \to \mathbb{R}$ that vanishes under any $d+1$ ``difference operators'' is a polynomial of degree at most $d$. More generally, we prove the continuity of any Haar measurable polynomial map between locally compact groups, in the sense of Leibman. We deduce the above result as a direct consequence of a theorem about the automatic continuity of cocycles.

math.GT↗

arXiv2024

Equivariant embedding of finite-dimensional dynamical systems

We prove an equivariant version of the classical Menger-Nobeling theorem regarding topological embeddings: Whenever a group $G$ acts on a finite-dimensional compact metric space $X$, a generic continuous equivariant function from $X$ into $([0,1]^r)^G$ is a topological embedding, provided that for every positive integer $N$ the space of points in $X$ with orbit size at most $N$ has topological dimension strictly less than $\frac{rN}{2}$. We emphasize that the result imposes no restrictions whatsoever on the acting group $G$ (beyond the existence of an action on a finite-dimensional space). Moreover, if $G$ is finitely generated then there exists a finite subset $F\subset G$ so that for a generic continuous map $h:X\to [0,1]^{r}$, the map $h^{F}:X\to ([0,1]^{r})^{F}$ given by $x\mapsto (f(gx))_{g\in F}$ is an embedding. This constitutes a generalization of the Takens delay embedding theorem into the topological category.

math.DS↗

arXiv2023

Well-distribution of Polynomial maps on locally compact groups

Weyl's classical equidistribution theorem states that real-valued polynomial sequences are uniformly distributed modulo 1, unless all non-constant coefficients are rational. A continuous function between two topological groups is called a \emph{polynomial map} of degree at most $d$ if it vanishes under any $d+1$ difference operators. Leibman, and subsequently Green and Tao, formulated and proved equidistribution theorems about polynomial sequences that take values in a nilmanifold. We formulate and prove some general equidistribution theorems regarding polynomial maps from a locally compact group into a compact abelian group.

math.DS↗

arXiv2023

Quantized-Constraint Concatenation and the Covering Radius of Constrained Systems

We introduce a novel framework for implementing error-correction in constrained systems. The main idea of our scheme, called Quantized-Constraint Concatenation (QCC), is to employ a process of embedding the codewords of an error-correcting code in a constrained system as a (noisy, irreversible) quantization process. This is in contrast to traditional methods, such as concatenation and reverse concatenation, where the encoding into the constrained system is reversible. The possible number of channel errors QCC is capable of correcting is linear in the block length $n$, improving upon the $O(\sqrt{n})$ possible with the state-of-the-art known schemes. For a given constrained system, the performance of QCC depends on a new fundamental parameter of the constrained system - its covering radius. Motivated by QCC, we study the covering radius of constrained systems in both combinatorial and probabilistic settings. We reveal an intriguing characterization of the covering radius of a constrained system using ergodic theory. We use this equivalent characterization in order to establish efficiently computable upper bounds on the covering radius.

cs.IT↗

arXiv2022

A note on reduction of tiling problems

We show that translational tiling problems in a quotient of $\mathbb{Z}^d$ can be effectively reduced or ``simulated'' by translational tiling problems in $\mathbb{Z}^d$. In particular, for any $d \in \mathbb{N}$, $k < d$ and $N_1,\ldots,N_k \in \mathbb{N}$ the existence of an aperiodic tile in $\mathbb{Z}^{d-k} \times (\mathbb{Z} / N_1\mathbb{Z} \times \ldots \times \mathbb{Z} / N_k \mathbb{Z})$ implies the existence of an aperiodic tile in $\mathbb{Z}^d$. Greenfeld and Tao have recently disproved the well-known periodic tiling conjecture in $\mathbb{Z}^d$ for sufficiently large $d \in \mathbb{N}$ by constructing an aperiodic tile in $\mathbb{Z}^{d-k} \times (\mathbb{Z} / N_1\mathbb{Z} \times \ldots \times \mathbb{Z} / N_k \mathbb{Z})$ for suitable $d,N_1,\ldots,N_k \in \mathbb{N}$.

math.CO↗

arXiv2022

The Lanford-Ruelle theorem for actions of sofic groups

Let $Γ$ be a sofic group, $Σ$ be a sofic approximation sequence of $Γ$ and $X$ be a $Γ$-subshift with nonnegative sofic topological entropy with respect to $Σ$. Further assume that $X$ is a shift of finite type, or more generally, that $X$ satisfies the topological Markov property. We show that for any sufficiently regular potential $f \colon X \to \mathbb{R}$, any translation-invariant Borel probability measure on $X$ which maximizes the measure-theoretical sofic pressure of $f$ with respect to $Σ$, is a Gibbs state with respect to $f$. This extends a classical theorem of Lanford and Ruelle, as well as previous generalizations of Moulin Ollagnier, Pinchon, Tempelman and others, to the case where the group is sofic. As applications of our main result we present a criterion for uniqueness of an equilibrium measure, as well as sufficient conditions for having that the equilibrium states do not depend upon the chosen sofic approximation sequence. We also prove that for any group-shift over a sofic group, the Haar measure is the unique measure of maximal sofic entropy for every sofic approximation sequence, as long as the homoclinic group is dense. On the expository side, we present a short proof of Chung's variational principle for sofic topological pressure.

math.DS↗

arXiv2022

What does a typical metric space look like?

The collection $\mathcal{M}_n$ of all metric spaces on $n$ points whose diameter is at most $2$ can naturally be viewed as a compact convex subset of $\mathbb{R}^{\binom{n}{2}}$, known as the metric polytope. In this paper, we study the metric polytope for large $n$ and show that it is close to the cube $[1,2]^{\binom{n}{2}} \subseteq \mathcal{M}_n$ in the following two senses. First, the volume of the polytope is not much larger than that of the cube, with the following quantitative estimates: \[ \left(\tfrac{1}{6}+o(1)\right)n^{3/2} \le \log \mathrm{Vol}(\mathcal{M}_n)\le O(n^{3/2}). \] Second, when sampling a metric space from $\mathcal{M}_n$ uniformly at random, the minimum distance is at least $1 - n^{-c}$ with high probability, for some $c > 0$. Our proof is based on entropy techniques. We discuss alternative approaches to estimating the volume of $\mathcal{M}_n$ using exchangeability, Szemerédi's regularity lemma, the hypergraph container method, and the Kővári--Sós--Turán theorem.

math.PR↗

arXiv2022

Entropy-efficient finitary codings

We show that any finite-entropy, countable-valued finitary factor of an i.i.d process can also be expressed as a finitary factor of a finite-valued i.i.d process whose entropy is arbitrarily close to the target process. As an application, we give an affirmative answer to a question of van den Berg and Steif about the critical Ising model on $\mathbb{Z}^d$. En route, we prove several results about finitary isomorphisms and finitary factors. Our results are developed in a new framework for processes invariant to a permutation group of a countable set satisfying specific properties. This new framework includes all ``classical'' processes over countable amenable groups and all invariant processes on transitive amenable graphs with ``uniquely centered balls''. Some of our results are new already for $\mathbb{Z}$-processes. We prove a relative version of Smorodinsky's isomorphism theorem for finitely dependent $\mathbb{Z}$-processes. We also extend the Keane--Smorodinsky finitary isomorphism theorem to countable-valued i.i.d processes and to i.i.d processes taking values in a Polish space.

math.PR↗

arXiv2020

Iterated Minkowski sums, horoballs and north-south dynamics

Given a finite generating set $A$ for a group $Γ$, we study the map $W \mapsto WA$ as a topological dynamical system -- a continuous self-map of the compact metrizable space of subsets of $Γ$. If the set $A$ generates $Γ$ as a semigroup and contains the identity, there are precisely two fixed points, one of which is attracting. This supports the initial impression that the dynamics of this map is rather trivial. Indeed, at least when $Γ= \mathbb{Z}^d$ and $A \subseteq \mathbb{Z}^d$ a finite positively generating set containing the natural invertible extension of the map $W \mapsto W+A$ is always topologically conjugate to the unique "north-south" dynamics on the Cantor set. In contrast to this, we show that various natural "geometric" properties of the finitely generated group $(Γ,A)$ can be recovered from the dynamics of this map, in particular, the growth type and amenability of $Γ$. When $Γ= \mathbb{Z}^d$, we show that the volume of the convex hull of the generating set $A$ is also an invariant of topological conjugacy. Our study introduces, utilizes and develops a certain convexity structure on subsets of the group $Γ$, related to a new concept which we call the sheltered hull of a set. We also relate this study to the structure of horoballs in finitely generated groups, focusing on the abelian case.

math.DS↗