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Raanan Schul

Publications and source records attributed to Raanan Schul.

25 records · Page 2Linked to original sources
arXiv2012

How to take shortcuts in Euclidean space: making a given set into a short quasi-convex set

For a given connected set $Γ$ in $d-$dimensional Euclidean space, we construct a connected set $\tildeΓ\supset Γ$ such that the two sets have comparable Hausdorff length, and the set $\tildeΓ$ has the property that it is quasiconvex, i.e. any two points $x$ and $y$ in $\tildeΓ$ can be connected via a path, all of which is in $\tildeΓ$, which has length bounded by a fixed constant multiple of the Euclidean distance between $x$ and $y$. Thus, for any set $K$ in $d-$dimensional Euclidean space we have a set $\tildeΓ$ as above such that $\tildeΓ$ has comparable Hausdorff length to a shortest connected set containing $K$. Constants appearing here depend only on the ambient dimension $d$. In the case where $Γ$ is Reifenberg flat, our constants are also independent the dimension $d$, and in this case, our theorem holds for $Γ$ in an infinite dimensional Hilbert space. This work closely related to $k-$spanners, which appear in computer science. Keywords: chord-arc, quasiconvex, k-spanner, traveling salesman.

math.MG↗
arXiv2008

Universal local parametrizations via heat kernels and eigenfunctions of the Laplacian

We use heat kernels or eigenfunctions of the Laplacian to construct local coordinates on large classes of Euclidean domains and Riemannian manifolds (not necessarily smooth, e.g. with $\mathcal{C}^α$ metric). These coordinates are bi-Lipschitz on embedded balls of the domain or manifold, with distortion constants that depend only on natural geometric properties of the domain or manifold. The proof of these results relies on estimates, from above and below, for the heat kernel and its gradient, as well as for the eigenfunctions of the Laplacian and their gradient. These estimates hold in the non-smooth category, and are stable with respect to perturbations within this category. Finally, these coordinate systems are intrinsic and efficiently computable, and are of value in applications.

math.AP↗
arXiv2008

Bi-Lipschitz Decomposition of Lipschitz functions into a Metric space

We prove a quantitative version of the following statement. Given a Lipschitz function f from the k-dimensional unit cube into a general metric space, one can decomposed f into a finite number of BiLipschitz functions f|_{F_i} so that the k-Hausdorff content of f([0,1]^k\setminus \cup F_i) is small. We thus generalize a theorem of P. Jones (1988) from the setting of R^d to the setting of a general metric space. This positively answers problem 11.13 in ``Fractured Fractals and Broken Dreams" by G. David and S. Semmes, or equivalently, question 9 from ``Thirty-three yes or no questions about mappings, measures, and metrics" by J. Heinonen and S. Semmes. Our statements extend to the case of {\it coarse} Lipschitz functions.

math.MG↗
arXiv2007

Subsets of Rectifiable curves in Hilbert Space-The Analyst's TSP

We study one dimensional sets (Hausdorff dimension) lying in a Hilbert space. The aim is to classify subsets of Hilbert spaces that are contained in a connected set of finite Hausdorff length. We do so by extending and improving results of Peter Jones and Kate Okikiolu for sets in $\R^d$. Their results formed the basis of quantitative rectifiability in $\R^d$. We prove a quantitative version of the following statement: a connected set of finite Hausdorff length (or a subset of one), is characterized by the fact that inside balls at most scales around most points of the set, the set lies close to a straight line segment (which depends on the ball). This is done via a quantity, similar to the one introduced in \cite{Jones90}, which is a geometric analog of the Square function. This allows us to conclude that for a given set $K$, the $\ell_2$ norm of this quantity (which is a function of $K$) has size comparable to a shortest (Hausdorff length) connected set containing $K$. In particular, our results imply that, with a correct reformulation of the theorems, the estimates in \cite{Jones90,Okik92} are {\bf independent of the ambient dimension}.

math.MG↗
arXiv2006

Ahlfors-Regular Curves In Metric Spaces

We discuss 1-Ahlfors-regular connected sets in a general metric space and prove that such sets are `flat' on most scales and in most locations. Our result is quantitative, and when combined with work of I. Hahlomaa, gives a characterization of 1-Ahlfors regular subsets of 1-Ahlfors-regular curves in metric spaces. Our result is a generalization to the metric space setting of the Analyst's (Geometric) Traveling Salesman theorems of P. Jones, K. Okikiolu, and G. David and S. Semmes, and it can be stated in terms of average Menger curvature.

math.MG↗