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Magdalena Nowak

Publications and source records attributed to Magdalena Nowak.

14 recordsLinked to original sources
arXiv2024

Characterizing Lipschitz images of injective metric spaces

A metric space $X$ is {\em injective} if every non-expanding map $f:B\to X$ defined on a subspace $B$ of a metric space $A$ can be extended to a non-expanding map $\bar f:A\to X$. We prove that a metric space $X$ is a Lipschitz image of an injective metric space if and only if $X$ is Lipschitz connected in the sense that for every points $x,y\in X$, there exists a Lipschitz map $f:[0,1]\to X$ such that $f(0)=x$ and $f(1)=y$. In this case the metric space $X$ carries a well-defined intrinsic metric. A metric space $X$ is a Lipschitz image of a compact injective metric space if and only if $X$ is compact, Lipschitz connected and its intrinsic metric is totally bounded. A metric space $X$ is a Lipschitz image of a separable injective metric space if and only if $X$ is a Lipschitz image of the Urysohn universal metric space if and only if $X$ is analytic, Lipschitz connected and its intrinsic metric is separable.

math.GN↗
arXiv2023

A Controlled Hahn-Mazurkiewicz Theorem and its Applications

For a metric Peano continuum $X$, let $S_X$ be a Sierpiński function assigning to each $\varepsilon>0$ the smallest cardinality of a cover of $X$ by connected subsets of diameter $\le \varepsilon$. We prove that for any increasing function $Ω:\mathbb R_+\to\mathbb R_+$ with $(0,1]\subseteqΩ[\mathbb R_+]$ and $s:=\sum_{n=1}^\infty S_X(2^{-n})\sum_{m=n}^\infty S_X(2^{-m})\,Ω^{-1}(\min\{1,2^{6-m}\})<\infty$ there exists a continuous surjective function $f:[0,s]\to X$ with continuity modulus $ω_f\leΩ$. This controlled version of the classical Hahn-Mazurkiewicz Theorem implies that $SDim(X)\le HDim(X)\le 2{\cdot}SDim(X)$, where $SDim(X)=\limsup_{\varepsilon\to 0}\frac{\ln(S_X(\varepsilon))}{\ln(1/\varepsilon)}$ is the $S$-dimension of $X$, and $HDim(X)=\inf\{α\in (0,\infty]:$ there is a~surjective $\frac1α$-Hölder map $f:[0,1]\to X\}$ is the $H\ddot older$ $dimension$ of $X$.

math.MG↗
arXiv2022

Pointwise attractors which are not strict

We deal with the finite family $\mathcal{F}$ of continuous maps on the Hausdorff space. A nonempty compact subset $A$ of such space is called a strict attractor if it has an open neighborhood $U$ such that $A=\lim_{n\to\infty}\mathcal{F}^n(S)$ for every nonempty compact $S\subset U$. Every strict attractor is a pointwise attractor, which means that the set $\{x\in X ; \lim_{n\to\infty}\mathcal{F}^n(x)=A\}$ contains $A$ in its interior. We present a class of examples of pointwise attractors - from the finite set to the Sierpi\'nski carpet - which are not strict when we add to the system one nonexpansive map.

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arXiv2021

Peano continua with self regenerating fractals

We deal with the question of Masayoshi Hata: is every Peano continuum a topological fractal? A compact space $X$ is a topological fractal if there exists $\mathcal{F}$ a finite family of self-maps on $X$ such that $X=\bigcup_{f\in\mathcal{F}}f(X)$ and for every open cover $\mathcal{U}$ of $X$ there is $n\in\mathbb{N}$ such that for all maps $f_1,\dots,f_n\in\mathcal{F}$ the set $f_1\circ\dots\circ f_n(X)$ is contained in some set $U\in\mathcal{U}$. In the paper we present some idea how to extend a topological fractal and we show that a Peano continuum is a topological fractal if it contains so-called self regenerating fractal with nonempty interior. A Hausdorff topological space $A$ is a self regenerating fractal if for every non-empty open subset $U$, $A$ is a topological fractal for some family of maps constant on $A\setminus U$. The notion of self regenerating fractal much better reflects the intuitive perception of self-similarity. We present some classical fractals which are self regenerating.

math.DS↗
arXiv2018

Embedding fractals in Banach, Hilbert or Euclidean spaces

By a metric fractal we understand a compact metric space $K$ endowed with a finite family $\mathcal F$ of contracting self-maps of $K$ such that $K=\bigcup_{f\in\mathcal F}f(K)$. If $K$ is a subset of a metric space $X$ and each $f\in\mathcal F$ extends to a contracting self-map of $X$, then we say that $(K,\mathcal F)$ is a fractal in $X$. We prove that each metric fractal $(K,\mathcal F)$ is $\bullet$ isometrically equivalent to a fractal in the Banach spaces $C[0,1]$ and $\ell_\infty$; $\bullet$ bi-Lipschitz equivalent to a fractal in the Banach space $c_0$; $\bullet$ isometrically equivalent to a fractal in the Hilbert space $\ell_2$ if $K$ is an ultrametric space. We prove that for a metric fractal $(K,\mathcal F)$ with the doubling property there exists $k\in\mathbb N$ such that the metric fractal $(K,\mathcal F^{\circ k})$ endowed with the fractal structure $\mathcal F^{\circ k}=\{f_1\circ\dots\circ f_k:f_1,\dots,f_k\in\mathcal F\}$ is equi-Hölder equivalent to a fractal in a Euclidean space $\mathbb R^d$. This result is used to prove our main result saying that each finite-dimensional compact metrizable space $K$ containing an open uncountable zero-dimensional space $Z$ is homeomorphic to a fractal in a Euclidean space $\mathbb R^d$. For $Z$, being a copy of the Cantor set, this embedding result was proved by Duvall and Husch in 1992.

math.MG↗
arXiv2015

Counterexamples in theory of fractal dimension for fractal structures

Fractal dimension constitutes the main tool to test for fractal patterns in Euclidean contexts. For this purpose, it is always used the box dimension, since it is easy to calculate, though the Hausdorff dimension, which is the oldest and also the most accurate fractal dimension, presents the best analytical properties. Additionally, fractal structures provide an appropriate topological context where new models of fractal dimension for a fractal structure could be developed in order to generalize the classical models of fractal dimension. In this paper, we provide some counterexamples regarding these new models of fractal dimension in order to show the reader how they behave mathematically with respect to the classical models, and also to point out which features of such models can be exploited to powerful effect in applications.

math.DS↗
arXiv2015

Counterexamples for IFS-attractors

In this paper, we deal with the part of Fractal Theory related to finite families of (weak) contractions, called iterated function systems (IFS, herein). An attractor is a compact set which remains invariant for such a family. Thus, we consider spaces homeomorphic to attractors of either IFS or weak IFS, as well, which we will refer to as Banach and topological fractals, respectively. We present a collection of counterexamples in order to show that all the presented definitions are essential, though they are not equivalent in general.

math.DS↗
arXiv2015

Detecting topological and Banach fractals among zero-dimensional spaces

A topological space $X$ is called a topological fractal if $X=\bigcup_{f\in\mathcal F}f(X)$ for a finite system $\mathcal F$ of continuous self-maps of $X$, which is topologically contracting in the sense that for every open cover $\mathcal U$ of $X$ there is a number $n\in\mathbb N$ such that for any functions $f_1,\dots,f_n\in \mathcal F$, the set $f_1\circ\dots\circ f_n(X)$ is contained in some set $U\in\mathcal U$. If, in addition, all functions $f\in\mathcal F$ have Lipschitz constant $<1$ with respect to some metric generating the topology of $X$, then the space $X$ is called a Banach fractal. It is known that each topological fractal is compact and metrizable. We prove that a zero-dimensional compact metrizable space $X$ is a topological fractal if and only if $X$ is a Banach fractal if and only if $X$ is either uncountable or $X$ is countable and its scattered height $\hbar(X)$ is a successor ordinal. For countable compact spaces this classification was recently proved by M.Nowak.

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arXiv2014

Contractive function systems, their attractors and metrization

In this paper we study the Hutchinson-Barnsley theory of fractals in the setting of multimetric spaces (which are sets endowed with point separating families of pseudometrics) and in the setting of topological spaces. We find natural connections between these two approaches.

math.GN↗
arXiv2013

Topological classification of scattered IFS-attractors

We study countable compact spaces as potential attractors of iterated function systems. We give an example of a convergent sequence in the real line which is not an IFS-attractor and for each countable ordinal $δ$ we show that a countable compact space of height $δ+1$ can be embedded in the real line so that it becomes the attractor of an IFS. On the other hand, we show that a scattered compact metric space of limit height is never an IFS-attractor.

math.DS↗
arXiv2013

The shark teeth is a topological IFS-attractor

We show that the space called shark teeth is a topological IFS-attractor, that is for every open cover of $X=\bigcup_{i=1}^nf_i(X)$, its image under every suitable large composition from the family of continuous functions $\{f_1,...,f_n\}$ lies in some set from the cover. In particular, there exists a space which is not homeomorphic to any IFS-attractor but is a topological IFS-attractor.

math.DS↗
arXiv2012

A class of continua that are not attractors of any IFS

This paper presents a sufficient condition for a continuum in $R^n$ to be embeddable in $R^n$ in such a way that its image is not an attractor of any iterated function system. An example of a continuum in $R^2$ that is not an attractor of any weakly contracting iterated function system is also given.

math.DS↗