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preprint2017arXiv

Rational Polygons: Odd Compression Ratio and Odd Plane Coverings

Let P be a polygon with rational vertices in the plane. We show that for any finite odd-sized collection of translates of P, the area of the set of points lying in an odd number of these translates is bounded away from 0 by a constant depending on P alone. The key ingredient of the proof is a construction of an odd cover of the plane by translates of P. That is, we establish a family F of translates of P covering (almost) every point in the plane a uniformly bounded odd number of times.

preprint2014arXiv

Reflection groups in non-negative curvature

We provide an equivariant description/classification of all complete (compact or not) non-negatively curved manifolds M together with a co-compact action by a reflection group W, and moreover, classify such W. In particular, we show that the building blocks consist of the classical constant curvature models and generalized open books with non negatively curved bundle pages, and derive a corresponding splitting theorem for the universal cover.

preprint2016arXiv

A criterion for a locally distributive semilattice to have CAT(0) orthoscheme complex

In this paper, we give a simple criterion for a locally distributive semilattice to have CAT(0) orthoscheme complex. Namely, the orthoscheme complex of a locally distributive semilattice S is CAT(0) if and only if S is a flag semilattice, that is, any pairwise bounded triple of S is bounded.

preprint2017arXiv

A characterization of Gromov hyperbolicity via quasigeodesic subspaces

By a geodesic subspace of a metric space $X$ we mean a subset $A$ of $X$ such that any two points in $A$ can be connected by a geodesic in $A$ . It is easy to check that a geodesic metric space $X$ is an $\mathbb{R}$ -tree (that is, a $0$ -hyperbolic space in the sense of Gromov) if and only if the union of any two intersecting geodesic subspaces is again a geodesic subspace. In this paper, we prove an analogous characterization of general Gromov hyperbolic spaces, where we replace geodesic subspaces by quasigeodesic subspaces.

preprint2015arXiv

Extensions of isometric embeddings of Pseudo-Euclidean metric polyhedra

We extend the results of B. Minemyer by showing that any indefinite metric polyhedron (either compact or not) with the vertex degree bounded from above admits an isometric simplicial embedding into a Minkowski space of the lowest possible dimension. We provide a simple algorithm of constructing such embeddings. We also show that every partial simplicial isometric embedding of such space in general position extends to a simplicial isometric embedding of the whole space.

preprint2016arXiv

Tangents, rectifiability, and corkscrew domains

In a recent paper, Csörnyei and Wilson prove that curves in Euclidean space of $σ$ -finite length have tangents on a set of positive $\mathscr{H}^{1}$ -measure. They also show that a higher dimensional analogue of this result is not possible without some additional assumptions. In this note, we show that if $Σ\subseteq \mathbb{R}^{d+1}$ has the property that each ball centered on $Σ$ contains two large balls in different components of $Σ^{c}$ and $Σ$ has $σ$ -finite $\mathscr{H}^{d}$ -measure, then it has $d$ -dimensional tangent points in a set of positive $\mathscr{H}^{d}$ -measure. We also give shorter proofs that Semmes surfaces are uniformly rectifiable and, if $Ω\subseteq \mathbb{R}^{d+1}$ is an exterior corkscrew domain whose boundary has locally finite $\mathscr{H}^{d}$ -measure, one can find a Lipschitz subdomain intersecting a large portion of the boundary.

preprint2016arXiv

Geometric and spectral consequences of curvature bounds on tessellations

This is a chapter of a forthcoming Lecture Notes in Mathematics "Modern Approaches to Discrete Curvature" edited by L. Najman and P. Romon. It provides a survey on geometric and spectral consequences of curvature bounds. The geometric setting are tessellations of surfaces with finite and vanishing genus. We consider a curvature arising as an angular defect. Several of the results presented here have analogues in Riemannian geometry. In some cases one can go even beyond the Riemannian results and there also striking differences which shall be highlighted.

preprint2014arXiv

Simple game induced manifolds

Starting by a simple game $Q$ as a combinatorial data, we build up a cell complex $M(Q)$ , whose construction resembles combinatorics of the permutohedron. The cell complex proves to be a combinatorial manifold; we call it the \textit{ simple game induced manifold.} By some motivations coming from polygonal linkages, we think of $Q$ and of $M(Q)$ as of\textit{ a quasilinkage} and the \textit{moduli space of the quasilinkage} respectively. We present some examples of quasilinkages and show that the moduli space retains many properties of moduli space of polygonal linkages. In particular, we show that the moduli space $M(Q)$ is homeomorphic to the space of stable point configurations on $S^1$ , for an associated with a quasilinkage notion of stability.

preprint2016arXiv

On the Beer index of convexity and its variants

Let $S$ be a subset of $\mathbb{R}^d$ with finite positive Lebesgue measure. The Beer index of convexity $\operatorname{b}(S)$ of $S$ is the probability that two points of $S$ chosen uniformly independently at random see each other in $S$ . The convexity ratio $\operatorname{c}(S)$ of $S$ is the Lebesgue measure of the largest convex subset of $S$ divided by the Lebesgue measure of $S$ . We investigate the relationship between these two natural measures of convexity. We show that every set $S\subseteq\mathbb{R}^2$ with simply connected components satisfies $\operatorname{b}(S)\leqα\operatorname{c}(S)$ for an absolute constant $α$ , provided $\operatorname{b}(S)$ is defined. This implies an affirmative answer to the conjecture of Cabello et al. that this estimate holds for simple polygons. We also consider higher-order generalizations of $\operatorname{b}(S)$ . For $1\leq k\leq d$ , the $k$ -index of convexity $\operatorname{b}_k(S)$ of a set $S\subseteq\mathbb{R}^d$ is the probability that the convex hull of a $(k+1)$ -tuple of points chosen uniformly independently at random from $S$ is contained in $S$ . We show that for every $d\geq 2$ there is a constant $β(d)>0$ such that every set $S\

preprint2016arXiv

Transport-entropy inequalities and curvature in discrete-space Markov chains

We show that if the random walk on a graph has positive coarse Ricci curvature in the sense of Ollivier, then the stationary measure satisfies a W^1 transport-entropy inequality. Peres and Tetali have conjectured a stronger consequence, that a modified log-Sobolev inequality (MLSI) should hold, in analogy with the setting of Markov diffusions. We discuss how our entropy interpolation approach suggests a natural attack on the MLSI conjecture.

preprint2016arXiv

Kinematic formulae for tensorial curvature measures

Tensorial curvature measures are tensor-valued generalizations of the curvature measures of convex bodies. We prove a complete set of kinematic formulae for such tensorial curvature measures on convex bodies and for their (nonsmooth) generalizations on convex polytopes. These formulae express the integral mean of the tensorial curvature measure of the intersection of two given convex bodies (resp.~polytopes), one of which is uniformly moved by a proper rigid motion, in terms of linear combinations of tensorial curvature measures of the given convex bodies (resp.~polytopes). We prove these results in a more direct way than in the classical proof of the principal kinematic formula for curvature measures, which uses the connection to Crofton formulae to determine the involved constants explicitly.

preprint2016arXiv

A Federer-style characterization of sets of finite perimeter on metric spaces

In the setting of a metric space equipped with a doubling measure that supports a Poincaré inequality, we show that a set $E$ is of finite perimeter if and only if $\mathcal H(\partial^1 I_E)<\infty$ , that is, if and only if the codimension one Hausdorff measure of the \emph{ $1$ -fine boundary} of the set's measure theoretic interior $I_E$ is finite.

preprint2016arXiv

Non-minimality of corners in subriemannian geometry

We give a short solution to one of the main open problems in subriemannian geometry. Namely, we prove that length minimizers do not have corner-type singularities. With this result we solve Problem II of Agrachev's list, and provide the first general result toward the 30-year-old open problem of regularity of subriemannian geodesics.

preprint2016arXiv

Bounded geometry and leaves

The main theorem states that any complete connected Riemannian manifold of bounded geometry can be isometrically realized as a leaf with trivial holonomy in a compact Riemannian foliated space.

preprint2016arXiv

Minkowski concentricity and complete simplices

This paper considers the radii functionals (circumradius, inradius, and diameter) as well as the Minkowski asymmetry for general (possibly non-symmetric) gauge bodies. A generalization of the concentricity inequality (which states that the sum of the inradius and circumradius is not greater than the diameter in general Minkowski spaces) for non-symmetric gauge bodies is derived and a strong connection between this new inequality, extremal sets of the generalized Bohnenblust inequality, and completeness of simplices is revealed.

preprint2016arXiv

Trimming of finite metric spaces

We define a class of trim metric spaces and show that every finite metric space is the leaf space of a metric forest with trim base.

preprint2016arXiv

Strict and pointwise convergence of BV functions in metric spaces

In the setting of a metric space $X$ equipped with a doubling measure that supports a Poincaré inequality, we show that if $u_i\to u$ strictly in $BV(X)$ , i.e. if $u_i\to u$ in $L^1(X)$ and $\Vert Du_i\Vert(X)\to\Vert Du\Vert(X)$ , then for a subsequence (not relabeled) we have $\widetilde{u}_i(x)\to \widetilde{u}(x)$ for $\mathcal H$ -almost every $x\in X\setminus S_u$ .

preprint2015arXiv

Sard Property for the endpoint map on some Carnot groups

In Carnot-Caratheodory or sub-Riemannian geometry, one of the major open problems is whether the conclusions of Sard's theorem holds for the endpoint map, a canonical map from an infinite-dimensional path space to the underlying finite-dimensional manifold. The set of critical values for the endpoint map is also known as abnormal set, being the set of endpoints of abnormal extremals leaving the base point. We prove that a strong version of Sard's property holds for all step-2 Carnot groups and several other classes of Lie groups endowed with left-invariant distributions. Namely, we prove that the abnormal set lies in a proper analytic subvariety. In doing so we examine several characterizations of the abnormal set in the case of Lie groups.

preprint2016arXiv

Top dense hyperbolic ball packings and coverings for complete Coxeter orthoscheme groups

In $n$ -dimensional hyperbolic space $\mathbf{H}^n$ $(n\ge2)$ there are $3$ -types of spheres (balls): the sphere, horosphere and hypersphere. If $n=2,3$ we know an universal upper bound of the ball packing densities, where each ball volume is related to the volume of the corresponding Dirichlet-Voronoi (D-V) cell. E.g. in $\mathbf{H}^3$ the densest horoball packing is derived from the $\{3,3,6\}$ Coxeter tiling consisting of ideal regular simplices $T_{reg}^\infty$ with dihedral angles $\fracπ{3}$ . The density of this packing is $δ_3^\infty\approx 0.85328$ and this provides a very rough upper bound for the ball packing densities as well. However, there are no "essential" results regarding the "classical" ball packings with congruent balls, and for ball coverings either. The goal of this paper to find the extremal ball arrangements in $\mathbf{H}^3$ with "classical balls". We consider only periodic congruent ball arrangements (for simplicity) related to the generalized, so-called {\it complete Coxeter orthoschemes} and their extended groups. In Theorems 1.1-1.2 we formulate also conjectures for the densest ball packing with density $0.77147\dots$ and the loose

preprint2016arXiv

Cyclic Cohomology Groups of Some Self-similar Sets

We define a variant of the Young integration on some kinds of self-similar sets which are called cellular self-similar sets. This variant is an analogue of the Young integration defined on the unit interval. We give the criteria of the variant on cellular self-similar sets, and also show that the variant is a cyclic 1-cocycle of the algebra of complex-valued Hölder continuous functions on the cellular self-similar sets. This suggests that the cocycle is a variant of currents.

preprint2016arXiv

Notions of Dirichlet problem for functions of least gradient in metric measure spaces

We study two notions of Dirichlet problem associated with BV energy minimizers (also called functions of least gradient) in bounded domains in metric measure spaces whose measure is doubling and supports a $(1,1)$ -Poincaré inequality. Since one of the two notions is not amenable to the direct method of the calculus of variations, we construct, based on an approach of [23, 29], solutions by considering the Dirichlet problem for $p$ -harmonic functions, $p>1$ , and letting $p\to 1$ . Tools developed and used in this paper include the inner perimeter measure of a domain.

preprint2016arXiv

Beyond traditional Curvature-Dimension I: new model spaces for isoperimetric and concentration inequalities in negative dimension

We study the isoperimetric, functional and concentration properties of $n$ -dimensional weighted Riemannian manifolds satisfying the Curvature-Dimension condition, when the generalized dimension $N$ is negative, and more generally, is in the range $N \in (-\infty,1)$ , extending the scope from the traditional range $N \in [n,\infty]$ . In particular, we identify the correct one-dimensional model-spaces under an additional diameter upper bound, and discover a new case yielding a \emph{single} model space (besides the previously known $N$ -sphere and Gaussian measure when $N \in [n,\infty]$ ): a (positively curved) sphere of (possibly negative) dimension $N \in (-\infty,1)$ . When curvature is non-negative, we show that arbitrarily weak concentration implies an $N$ -dimensional Cheeger isoperimetric inequality, and derive various weak Sobolev and Nash-type inequalities on such spaces. When curvature is strictly positive, we observe that such spaces satisfy a Poincaré inequality uniformly for all $N \in (-\infty,1-ε]$ , and enjoy a two-level concentration of the type $\exp(-\min(t,t^2))$ . Our main technical tool is a generalized version of the Heintze--Karcher theorem, which we extend to the

preprint2016arXiv

Distinct distances between a collinear set and an arbitrary set of points

We consider the number of distinct distances between two finite sets of points in ${\bf R}^k$ , for any constant dimension $k\ge 2$ , where one set $P_1$ consists of $n$ points on a line $l$ , and the other set $P_2$ consists of $m$ arbitrary points, such that no hyperplane orthogonal to $l$ and no hypercylinder having $l$ as its axis contains more than $O(1)$ points of $P_2$ . The number of distinct distances between $P_1$ and $P_2$ is then $Ω\left(\min\left\{ n^{2/3}m^{2/3},\; \frac{n^{10/11}m^{4/11}}{\log^{2/11}m},\; n^2,\; m^2\right\}\right) .$ Without the assumption on $P_2$ , there exist sets $P_1$ , $P_2$ as above, with only $O(m+n)$ distinct distances between them.

preprint2016arXiv

The measures with an associated square function operator bounded in $L^2$

In this paper we provide an extension of a theorem of David and Semmes ('91) to general non-atomic measures. The result provides a geometric characterization of the non-atomic measures for which a certain class of square function operators, or singular integral operators, are bounded in $L^2(μ)$ . The description is given in terms of a modification of Jones' $β$ -coefficients.

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