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Robert K. Hladky

Publications and source records attributed to Robert K. Hladky.

13 recordsLinked to original sources
arXiv2024

New Upper Bounds on the Minimal Domination Numbers of High-Dimensional Hypercubes

We briefly review known results on upper bounds for the minimal domination number $γ_n$ of a hypercube of dimension $n$, then present a new method for constructing dominating sets. Write $n =2^{\hat{n}}-1 +{\check{n}}$ with $0\leq {\check{n}}<2^{\hat{n}}$. Our construction applies to all $n$ lying within the expanding wedge $θ({\hat{n}}) \leq {\check{n}} < 2^{\hat{n}}$, where $θ$ is a specific, easily computable function with the asymptotic property $θ(a) \sim 2^{a/2}$. For all $n$ within the smaller wedge $θ({\hat{n}}) \leq {\check{n}} < 2^{\hat{n}-2}$, the resulting upper bound on $γ_n$ betters those previously known.

math.CO↗
arXiv2014

The topology of quaternionic contact manifolds

We explore the consequences of curvature and torsion on the topology of quaternionic contact manifolds with integrable vertical distribution. We prove a general Myers theorem and establish a Cartan-Hadamard result for almost qc-Einstein manifolds.

math.DG↗
arXiv2013

Isometries of complemented subRiemannian manifolds

We show that the group of smooth isometries of a complemented sub-Riemannian manifold form a Lie group and establish dimension estimates based on the torsion of the canonical connection. We explore the interaction of curvature and the structure of isometries and Killing fields and derive a Bochner formula for Killing fields. Sub-Riemannian generalizations of classical results of Bochner and Berger are established. We also apply our theory to common sub-categories of complemented sub-Riemannian geometries and show how to compute the isometry groups for several examples, including SO(n), SL(n) and the rototranslation group.

math.DG↗
arXiv2013

Intrinsic complements of equiregular sub-Riemannian manifolds

Under a nondegeneracy condition, we show that an equiregular sub-Riemannian manifold of step size $r$ admits a canonical, $V$-rigid complement defined from the sub-Riemannian data that is preserved the by action of sub-Riemannian isometries. We explore how the existence of such a complement relates to results from the literature and study the step size 2 case in more detail.

math.DG↗
arXiv2011

Connections and Curvature in sub-Riemannian geometry

For a subRiemannian manifold and a given Riemannian extension of the metric, we define a canonical global connection. This connection coincides with both the Levi-Civita connection on Riemannian manifolds and the Tanaka-Webster connection on strictly pseudoconvex CR manifolds. We define a notion of normality generalizing Tanaka's notion for CR manifolds to the subRiemannian case. Under the assumption of normality, we construct local frames that simplify computations in a manner analogous to Riemannian normal coordinates. We then use these frames to establish Bianchi Identities and symmetries for the associated curvatures. Finally we explore subRiemannian generalizations of the Bonnet-Myers theorem, providing some new results and some new proofs and interpretations of existing results.

math.DG↗
arXiv2008

The \bar{\partial}_b Neumann problem on noncharacteristic domains

We study the $\bar{\partial}_b$-Neumann problem for domains $Ω$ contained in a strictly pseudoconvex manifold M^{2n+1} whose boundaries are noncharacteristic and have defining functions depending solely on the real and imaginary parts of a single CR function w. When the Kohn Laplacian is a priori known to have closed range in L^2, we prove sharp regularity and estimates for solutions. We establish a condition on the boundary which is sufficient for the Kohn Laplacian to be Fredholm on $L^2_{(0,q)}(Ω)$ and show that this condition always holds when M is embedded as a hypersurface in C^{n+1}. We present examples where the inhomogeneous $\bar{\partial}_b$ equation can always be solved smoothly up to the boundary on (p,q)-forms with 0<q<n-1.

math.CV↗
arXiv2007

Variation of Perimeter Measure in sub-Riemannian geometry

We derive a formula for the first variation of horizontal perimeter measure for $C^2$ hypersurfaces of completely general sub-Riemannian manifolds, allowing for the existence of characteristic points. For $C^2$ hypersurfaces in vertically rigid sub-Riemannian manifolds we also produce a second variation formula for variations supported away from the characteristic locus. This variation formula is used to show the bubble sets in \hn{2} are stable under volume preserving variations.

math.DG↗
arXiv2005

Minimal surfaces in the roto-translation group with applications to a neuro-biological image completion model

We investigate solutions to the minimal surface problem with Dirichlet boundary conditions in the roto-translation group equipped with a subRiemannian metric. By work of G. Citti and A. Sarti, such solutions are amodal completions of occluded visual data when using a model of the first layer of the visual cortex. Using a characterization of smooth minimal surfaces as ruled surfaces, we give a method to compute a minimal spanning surface given fixed boundary data presuming such a surface exists. Moreover, we describe a number of obstructions to existence and uniqueness but also show that under suitable conditions, smooth minimal spanning surfaces with good properties exist. Not only does this provide an explicit realization of the disocclusion process for the neurobiological model, but it also has application to contructing disocclusion algorithms in digital image processing.

math.DG↗
arXiv2005

Constant mean curvature surfaces in sub-Riemannian geometry

We investigate the minimal and isoperimetric surface problems in a large class of sub-Riemannian manifolds, the so-called Vertically Rigid spaces. We construct an adapted connection for such spaces and, using the variational tools of Bryant, Griffiths and Grossman, derive succinct forms of the Euler-Lagrange equations for critical points for the associated variational problems. Using the Euler-Lagrange equations, we show that minimal and isoperimetric surfaces satisfy a constant horizontal mean curvature conditions away from characteristic points. Moreover, we use the formalism to construct a horizontal second fundamental form, $II_0$, for vertically rigid spaces and, as a first application, use $II_0$ to show that minimal surfaces cannot have points of horizontal positive curvature and, that minimal surfaces in Carnot groups cannot be locally horizontally geometrically convex. We note that the convexity condition is distinct from others currently in the literature.

math.DG↗
arXiv2004

Boundary Regularity for the \bar{\partial}_b-Neumann Problem, Part 1

We establish sharp regularity and Fredholm theorems for the \bar{\partial}_b-Neumann problem on domains satisfying some non-generic geometric conditions. We use these domains to construct explicit examples of bad behaviour of the Kohn Laplacian: it is not always hypoelliptic up to the boundary, its partial inverse is not compact and it is not globally subelliptic.

math.CV↗
arXiv2004

Boundary Regularity for the \bar{\partial}_b-Neumann problem, Part 2

We adapt the results of Part 1 to include the unit ball in the Heisenberg group, the model domain with characteristic boundary points. In particular, we construct function spaces on which the Kohn Laplacian with the \bar{\partial}_b-Neumann boundary conditions is an isomorphism. As an application, we establish sharp regularity for a canonical solution to the inhomogenous \bar{\partial}_b equation on the unit ball.

math.CV↗