2,442 research outputs found
Isomorphism and embedding of Borel systems on full sets
A Borel system consists of a measurable automorphism of a standard Borel
space. We consider Borel embeddings and isomorphisms between such systems
modulo null sets, i.e. sets which have measure zero for every invariant
probability measure. For every t>0 we show that in this category there exists a
unique free Borel system (Y,S) which is strictly t-universal in the sense that
all invariant measures on Y have entropy <t, and if (X,T) is another free
system obeying the same entropy condition then X embeds into Y off a null set.
One gets a strictly t-universal system from mixing shifts of finite type of
entropy at least t by removing the periodic points and "restricting" to the
part of the system of entropy <t. As a consequence, after removing their
periodic points the systems in the following classes are completely classified
by entropy up to Borel isomorphism off null sets: mixing shifts of finite type,
mixing positive-recurrent countable state Markov chains, mixing sofic shifts,
beta shifts, synchronized subshifts, and axiom-A diffeomorphisms. In particular
any two equal-entropy systems from these classes are entropy conjugate in the
sense of Buzzi, answering a question of Boyle, Buzzi and Gomez.Comment: 17 pages, v2: correction to bibliograph
On Factor Universality in Symbolic Spaces
The study of factoring relations between subshifts or cellular automata is
central in symbolic dynamics. Besides, a notion of intrinsic universality for
cellular automata based on an operation of rescaling is receiving more and more
attention in the literature. In this paper, we propose to study the factoring
relation up to rescalings, and ask for the existence of universal objects for
that simulation relation. In classical simulations of a system S by a system T,
the simulation takes place on a specific subset of configurations of T
depending on S (this is the case for intrinsic universality). Our setting,
however, asks for every configurations of T to have a meaningful interpretation
in S. Despite this strong requirement, we show that there exists a cellular
automaton able to simulate any other in a large class containing arbitrarily
complex ones. We also consider the case of subshifts and, using arguments from
recursion theory, we give negative results about the existence of universal
objects in some classes
On the zero-temperature limit of Gibbs states
We exhibit Lipschitz (and hence H\"older) potentials on the full shift
such that the associated Gibbs measures fail to converge
as the temperature goes to zero. Thus there are "exponentially decaying"
interactions on the configuration space for which the
zero-temperature limit of the associated Gibbs measures does not exist. In
higher dimension, namely on the configuration space ,
, we show that this non-convergence behavior can occur for finite-range
interactions, that is, for locally constant potentials.Comment: The statement of Theorem 1.2 is more accurate and some new comment
follow i
Subshifts, MSO Logic, and Collapsing Hierarchies
We use monadic second-order logic to define two-dimensional subshifts, or
sets of colorings of the infinite plane. We present a natural family of
quantifier alternation hierarchies, and show that they all collapse to the
third level. In particular, this solves an open problem of [Jeandel & Theyssier
2013]. The results are in stark contrast with picture languages, where such
hierarchies are usually infinite.Comment: 12 pages, 5 figures. To appear in conference proceedings of TCS 2014,
published by Springe
Online Popularity and Topical Interests through the Lens of Instagram
Online socio-technical systems can be studied as proxy of the real world to
investigate human behavior and social interactions at scale. Here we focus on
Instagram, a media-sharing online platform whose popularity has been rising up
to gathering hundred millions users. Instagram exhibits a mixture of features
including social structure, social tagging and media sharing. The network of
social interactions among users models various dynamics including
follower/followee relations and users' communication by means of
posts/comments. Users can upload and tag media such as photos and pictures, and
they can "like" and comment each piece of information on the platform. In this
work we investigate three major aspects on our Instagram dataset: (i) the
structural characteristics of its network of heterogeneous interactions, to
unveil the emergence of self organization and topically-induced community
structure; (ii) the dynamics of content production and consumption, to
understand how global trends and popular users emerge; (iii) the behavior of
users labeling media with tags, to determine how they devote their attention
and to explore the variety of their topical interests. Our analysis provides
clues to understand human behavior dynamics on socio-technical systems,
specifically users and content popularity, the mechanisms of users'
interactions in online environments and how collective trends emerge from
individuals' topical interests.Comment: 11 pages, 11 figures, Proceedings of ACM Hypertext 201
The Assouad dimensions of projections of planar sets
We consider the Assouad dimensions of orthogonal projections of planar sets
onto lines. Our investigation covers both general and self-similar sets.
For general sets, the main result is the following: if a set in the plane has
Assouad dimension , then the projections have Assouad dimension at
least almost surely. Compared to the famous analogue for
Hausdorff dimension -- namely \emph{Marstrand's Projection Theorem} -- a
striking difference is that the words `at least' cannot be dispensed with: in
fact, for many planar self-similar sets of dimension , we prove that the
Assouad dimension of projections can attain both values and for a set
of directions of positive measure.
For self-similar sets, our investigation splits naturally into two cases:
when the group of rotations is discrete, and when it is dense. In the `discrete
rotations' case we prove the following dichotomy for any given projection:
either the Hausdorff measure is positive in the Hausdorff dimension, in which
case the Hausdorff and Assouad dimensions coincide; or the Hausdorff measure is
zero in the Hausdorff dimension, in which case the Assouad dimension is equal
to 1. In the `dense rotations' case we prove that every projection has Assouad
dimension equal to one, assuming that the planar set is not a singleton.
As another application of our results, we show that there is no
\emph{Falconer's Theorem} for Assouad dimension. More precisely, the Assouad
dimension of a self-similar (or self-affine) set is not in general almost
surely constant when one randomises the translation vectors.Comment: 29 pages, 2 figures, to appear in Proc. Lond. Math. So
Return times, recurrence densities and entropy for actions of some discrete amenable groups
Results of Wyner and Ziv and of Ornstein and Weiss show that if one observes
the first k outputs of a finite-valued ergodic process, then the waiting time
until this block appears again is almost surely asymptotic to , where
is the entropy of the process. We examine this phenomenon when the allowed
return times are restricted to some subset of times, and generalize the results
to processes parameterized by other discrete amenable groups.
We also obtain a uniform density version of the waiting time results: For a
process on symbols, within a given realization, the density of the initial
-block within larger -blocks approaches , uniformly in ,
as tends to infinity. Again, similar results hold for processes with other
indexing groups.Comment: To appear in Journal d'Analyse Mathematiqu
Small union with large set of centers
Let be a fixed set. By a scaled copy of around
we mean a set of the form for some .
In this survey paper we study results about the following type of problems:
How small can a set be if it contains a scaled copy of around every point
of a set of given size? We will consider the cases when is circle or sphere
centered at the origin, Cantor set in , the boundary of a square
centered at the origin, or more generally the -skeleton () of an
-dimensional cube centered at the origin or the -skeleton of a more
general polytope of .
We also study the case when we allow not only scaled copies but also scaled
and rotated copies and also the case when we allow only rotated copies
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