arXiv subjects

Ábel Farkas

Publications and source records attributed to Ábel Farkas.

7 recordsLinked to original sources

arXiv2021

Schmidt's game on Hausdorff metric and function spaces: generic dimension of sets and images

We consider Schmidt's game on the space of compact subsets of a given metric space equipped with the Hausdorff metric, and the space of continuous functions equipped with the supremum norm. We are interested in determining the generic behaviour of objects in a metric space, mostly in the context of fractal dimensions, and the notion of `generic' we adopt is that of being winning for Schmidt's game. We find properties whose corresponding sets are winning for Schmidt's game that are starkly different from previously established, and well-known, properties which are generic in other contexts, such as being residual or of full measure.

math.MG↗

arXiv2019

Conditional measure on the Brownian path and other random sets

Let $B$ denote the range of the Brownian motion in $\mathbb{R}^{d}$ ($d\geq3$). For a deterministic Borel measure $ν$ on $\mathbb{R}^{d}$ we wish to find a random measure $μ$ such that the support of $μ$ is contained in $B$ and it is a solution to the equation $E(μ(A))=ν(A)$ for every Borel set $A$. We discuss when it is possible to find a solution $μ$ and in that case we construct the solution. We study several properties of $μ$ such as the probability of $μ\neq0$ and we establish a formula for the expectation of the double integral with respect to $μ\timesμ$. We calculate $μ$ in terms of the occupation measure when $ν$ is the Lebesgue measure, i.e. we provide an explicit deterministic density function of $μ$ with respect to the occupation measure. As a conclusion we calculate an explicit formula for the expectation of the double integral with respect to the occupation measure. We generalise the theory for more general random sets in separable, metric, Radon spaces. As an additional example, we also apply our results to percolation limit sets on boundaries of trees.

math.PR↗

arXiv2019

Dimension and measure for generic continuous images

We consider the Banach space consisting of continuous functions from an arbitrary uncountable compact metric space, $X$, into $\mathbb{R}^n$. The key question is `what is the generic dimension of $f(X)$?' and we consider two different approaches to answering it: Baire category and prevalence. In the Baire category setting we prove that typically the packing and upper box dimensions are as large as possible, $n$, but find that the behaviour of the Hausdorff, lower box and topological dimensions is considerably more subtle. In fact, they are typically equal to the minimum of $n$ and the topological dimension of $X$. We also study the typical Hausdorff and packing measures of $f(X)$ and, in particular, give necessary and sufficient conditions for them to be zero, positive and finite, or infinite. It is interesting to compare the Baire category results with results in the prevalence setting. As such we also discuss a result of Dougherty on the prevalent topological dimension of $f(X)$ and give some simple applications concerning the prevalent dimensions of graphs of real-valued continuous functions on compact metric spaces, allowing us to extend a recent result of Bayart and Heurteaux.

math.CA↗

arXiv2018

Dimension approximation of attractors of graph directed IFSs by self-similar sets

We show that for the attractor $(K_{1},\dots,K_{q})$ of a graph directed iterated function system, for each $1\leq j\leq q$ and $\varepsilon>0$ there exits a self-similar set $K\subseteq K_{j}$ that satisfies the strong separation condition and $\dim_{H}K_{j}-\varepsilon<\dim_{H}K$. We show that we can further assume convenient conditions on the orthogonal parts and similarity ratios of the defining similarities of $K$. Using this property as a `black box' we obtain results on a range of topics including on dimensions of projections, intersections, distance sets and sums and products of sets.

math.DS↗

arXiv2018

Interval projections of self-similar sets

We show that if $K$ is a self-similar $1$-set that is not contained in a line and either satisfies the strong separation condition or is defined via homotheties then there are at most finitely many lines through the origin such that the projection of $K$ onto them is an interval.

math.DS↗

arXiv2016

Projections of self-similar sets with no separation condition

We investigate how the Hausdorff dimension and measure of a self-similar set $K\subseteq\mathbb{R}^{d}$ behave under linear images. This depends on the nature of the group $\mathcal{T}$ generated by the orthogonal parts of the defining maps of $K$. We show that if $\mathcal{T}$ is finite then every linear image of $K$ is a graph directed attractor and there exists at least one projection of $K$ such that the dimension drops under the image of the projection. In general, with no restrictions on $\mathcal{T}$ we establish that $\mathcal{H}^{t}(L\circ O(K))=\mathcal{H}^{t}(L(K))$ for every element $O$ of the closure of $\mathcal{T}$, where $L$ is a linear map and $t=\dim_{H}K$. We also prove that for disjoint subsets $A$ and \textbf{$B$} of $K$ we have that $\mathcal{H}^{t}(L(A)\cap L(B))=0$. Hochman and Shmerkin showed that if $\mathcal{T}$ is dense in $SO(d,\mathbb{R})$ and the strong separation condition is satisfied then $\dim_{H}(g(K))=\min\{\dim_{H}K,l\} $ where $g$ is a continuously differentiable map of rank $l$. We deduce the same result without any separation condition and we generalize a result of Ero$\breve{\mathrm{g}}$lu by obtaining that $\mathcal{H}^{t}(g(K))=0$.

math.DS↗

arXiv2015

On the equality of Hausdorff measure and Hausdorff content

We are interested in situations where the Hausdorff measure and Hausdorff content of a set are equal in the critical dimension. Our main result shows that this equality holds for any subset of a self-similar set corresponding to a nontrivial cylinder of an irreducible subshift of finite type, and thus also for any self-similar or graph-directed self-similar set, regardless of separation conditions. The main tool in the proof is an exhaustion lemma for Hausdorff measure based on the Vitali Covering Theorem. We also give several examples showing that one cannot hope for the equality to hold in general if one moves in a number of the natural directions away from `self-similar'. For example, it fails in general for self-conformal sets, self-affine sets and Julia sets. We also give applications of our results concerning Ahlfors regularity. Finally we consider an analogous version of the problem for packing measure. In this case we need the strong separation condition and can only prove that the packing measure and $δ$-approximate packing pre-measure coincide for sufficiently small $δ>0$.

math.MG↗