Dimensional Hausdorff Measure Research Articles

Hausdorff dimension of intersections with planes and general sets

We give conditions on a general family P_{\lambda}\colon \R^n\to\R^m , \lambda \in \Lambda , of orthogonal projections which guarantee that the Hausdorff dimension formula \dim A\cap P_{\lambda}^{-1}\{u\}=s-m holds generically for measurable sets A\subset\R^n with positive and finite s -dimensional Hausdorff measure, s>m , and with positive lower density. As an application we prove for measurable sets A,B\subset\R^n with positive s - and t -dimensional measures, and with positive lower density that if s + (n-1)t/n > n , then \dim A\cap (g(B)+z) = s+t - n for almost all rotations g and for positively many z\in\R^n .

Rectifiability of singular sets of noncollapsed limit spaces with Ricci curvature bounded below

This paper is concerned with the structure of Gromov-Hausdorff limit spaces $(M^n_i,g_i,p_i)\stackrel{d_{GH}}{\longrightarrow} (X^n,d,p)$ of Riemannian manifolds satisfying a uniform lower Ricci curvature bound $\mathrm{Ric}_{M^n_i}\geq -(n-1)$ as well as the noncollapsing assumption $\mathrm{Vol}(B_1(p_i))>\mathrm{v}>0$. In such cases, there is a filtration of the singular set, $S^0\subset S^1\cdots S^{n-1}:= S$, where $S^k:=${$x\in X:$ no tangent cone at $x$ is $(k+1)$-symmetric}. Equivalently, $S^k$ is the set of points such that no tangent cone splits off a Euclidean factor $\mathbb{R}^{k+1}$. It is classical from Cheeger-Colding that the Hausdorff dimension of $S^k$ satisfies $\mathrm{dim}\, S^k\leq k$ and $S=S^{n-2}$, i.e., $S^{n-1}\setminus S^{n-2}=\emptyset$. However, little else has been understood about the structure of the singular set $S$. Our first result for such limit spaces $X^n$ states that $S^k$ is $k$-rectifiable for all $k$. In fact, we will show for $\mathcal{H}^k$-a.e.\ $x\in S^k$ that \textit{every} tangent cone $X_x$ at $x$ is $k$-symmetric, i.e., that $X_x= \mathbb{R}^k\times C(Y)$ where $C(Y)$ might depend on the particular $X_x$. Here $\mathcal{H}^k$ denotes the $k$-dimensional Hausdorff measure. As an application we show for all $0\lt \epsilon\lt \epsilon(n,\mathrm{v})$ there exists an $(n-2)$-rectifiable closed set $S^{n-2}_\epsilon$ with $\mathcal{H}^{n-2}(S_{\epsilon}^{n-2}) \lt C(n,\mathrm{v},\epsilon)$, such that $X^n\setminus S^{n-2}_\epsilon$ is $\epsilon$-bi-Hölder equivalent to a smooth Riemannian manifold. Moreover, $S=\bigcup_\epsilon S^{n-2}_\epsilon$. As another application, we show that tangent cones are unique $\mathcal H^{n-2}$-a.e. In the case of limit spaces $X^n$ satisfying a $2$-sided Ricci curvature bound $|\mathrm{Ric}_{M^n_i}|\leq n-1$, we can use these structural results to give a new proof of a conjecture from Cheeger-Colding stating that $S$ is $(n-4)$-rectifiable with uniformly bounded measure. We can also conclude from this structure that tangent cones are unique $\mathcal H^{n-4}$-a.e. Our analysis builds on the notion of quantitative stratification introduced by Cheeger-Naber, and the neck region analysis developed by Jiang-Naber-Valtorta. Several new ideas and new estimates are required, including a sharp cone-splitting theorem and a geometric transformation theorem, which will allow us to control the degeneration of harmonic functions on these neck regions.

Anisotropic Gaussian random fields: criteria for hitting probabilities and applications

We develop criteria for hitting probabilities of anisotropic Gaussian random fields with associated canonical pseudo-metric given by a class of gauge functions. This yields lower and upper bounds in terms of general notions of capacity and Hausdorff measure, respectively, therefore extending the classical estimates with the Bessel-Riesz capacity and the $\gamma$-dimensional Hausdorff measure. We apply the criteria to a system of linear stochastic partial differential equations driven by space-time noises that are fractional in time and either white or colored in space.

Uniqueness criteria for the Oseen vortex in the 3d Navier-Stokes equations

In this paper, we consider the uniqueness of solutions to the 3d Navier-Stokes equations with initial vorticity given by where is the one dimensional Hausdorff measure of an infinite, vertical line and is an arbitrary circulation. This initial data corresponds to an idealized, infinite vortex filament. One smooth, mild solution is given by the self-similar Oseen vortex column, which coincides with the heat evolution. Previous work by Germain, Harrop-Griffiths, and the first author implies that this solution is unique within a class of mild solutions that converge to the Oseen vortex in suitable self-similar weighted spaces. In this paper, the uniqueness class of the Oseen vortex is expanded to include any solution that converges to the initial data in a sufficiently strong sense. This gives further evidence in support of the expectation that the Oseen vortex is the only possible mild solution that is identifiable as a vortex filament. The proof is a 3d variation of a 2d compactness/rigidity argument in originally due to Gallagher and Gallay.

Partially Regular Weak Solutions of the Navier–Stokes Equations in $$\mathbb {R}^4 \times [0,\infty [$$

We show that for any given solenoidal initial data in $L^2$ and any solenoidal external force in $L_{\text{loc}}^q \bigcap L^{3/2}$ with $q>3$, there exist partially regular weak solutions of the Navier-Stokes equations in $\R^4 \times [0,\infty[$ which satisfy certain local energy inequalities and whose singular sets have locally finite $2$-dimensional parabolic Hausdorff measure. With the help of a parabolic concentration-compactness theorem we are able to overcome the possible lack of compactness arising in the spatially $4$-dimensional setting by using defect measures, which we then incorporate into the partial regularity theory.

On the partial regularity of suitable weak solutions in the non-Newtonian shear-thinning case

We study the partial regularity of suitable weak solutions to incompressible non-Newtonian fluids in the shear-thinning case p < 2. For the shear-thickening case p > 2 this problem was previously considered in 2002 by Guo and Zhu (J. Differ. Equ. 178 281--97). By partially appealing to some of their ideas, we show that in the p < 2 case the singular points are concentrated on a closed set whose one dimensional Hausdorff measure is zero.

The Maximum Distance Problem and Minimum Spanning Trees

Given a compact E⊂Rn and s&gt;0, the maximum distance problem seeks a compact and connected subset of Rn of smallest one dimensional Hausdorff measure whose s-neighborhood covers E. For E⊂R2, we prove that minimizing over minimum spanning trees that connect the centers of balls of radius s, which cover E, solves the maximum distance problem. The main difficulty in proving this result is overcome by the proof of a key lemma which states that one is able to cover the s-neighborhood of a Lipschitz curve Γ in R2 with a finite number of balls of radius s, and connect their centers with another Lipschitz curve Γ*, where H1 (Γ*) is arbitrarily close to H1(Γ). We also present an open source package for computational exploration of the maximum distance problem using minimum spanning trees, available at github.com/mtdaydream/MDP_MST.

Hausdorff measure and Assouad dimension of generic self-conformal IFS on the line

This paper considers self-conformal iterated function systems (IFSs) on the real line whose first level cylinders overlap. In the space of self-conformal IFSs, we show that generically (in topological sense) if the attractor of such a system has Hausdorff dimension less than 1 then it has zero appropriate dimensional Hausdorff measure and its Assouad dimension is equal to 1. Our main contribution is in showing that if the cylinders intersect then the IFS generically does not satisfy the weak separation property and hence, we may apply a recent result of Angelevska, Käenmäki and Troscheit. This phenomenon holds for transversal families (in particular for the translation family) typically, in the self-similar case, in both topological and in measure theoretical sense, and in the more general self-conformal case in the topological sense.

Existence and regularity theorems of one-dimensional Brakke flows

Given a closed countably 1-rectifiable set in \mathbb R^2 with locally finite 1 -dimensional Hausdorff measure, we prove that there exists a Brakke flow starting from the given set with the following regularity property. For almost all time, the flow locally consists of a finite number of embedded curves of class W^{2,2} whose endpoints meet at junctions with angles of either 0, 60 or 120 degrees.

Removable Singularities of Solutions of the Symmetric Minimal Surface Equation

We show that compact sets of (n − 1)-dimensional Hausdorff measure zero constitute removable singularities for weak solutions of the singular and symmetric minimal surface equation. These results comprise minimal surfaces in hyperbolic space and “heavy” minimal surfaces in gravitational fields.

Epsilon-regularity for p-harmonic maps at a free boundary on a sphere

We prove an $\epsilon$-regularity theorem for vector-valued p-harmonic maps, which are critical with respect to a partially free boundary condition, namely that they map the boundary into a round sphere. This does not seem to follow from the reflection method that Scheven used for harmonic maps with free boundary (i.e., the case $p=2$): the reflected equation can be interpreted as a $p$-harmonic map equation into a manifold, but the regularity theory for such equations is only known for round targets. Instead, we follow the spirit of the last-named author's recent work on free boundary harmonic maps and choose a good frame directly at the free boundary. This leads to growth estimates, which, in the critical regime $p=n$, imply H\"older regularity of solutions. In the supercritical regime, $p < n$, we combine the growth estimate with the geometric reflection argument: the reflected equation is super-critical, but, under the assumption of growth estimates, solutions are regular. In the case $p<n$, for stationary $p$-harmonic maps with free boundary, as a consequence of a monotonicity formula we obtain partial regularity up to the boundary away from a set of $(n-p)$-dimensional Hausdorff measure.

A note on topological dimension, Hausdorff measure, and rectifiability

The purpose of this note is to record a consequence, for general metric spaces, of a recent result of David Bate. We prove the following fact: Let $X$ be a compact metric space of topological dimension $n$. Suppose that the $n$-dimensional Hausdorff measure of $X$, $\mathcal H^n(X)$, is finite. Suppose further that the lower n-density of the measure $\mathcal H^n$ is positive, $\mathcal H^n$-almost everywhere in $X$. Then $X$ contains an $n$-rectifiable subset of positive $\mathcal H^n$-measure. Moreover, the assumption on the lower density is unnecessary if one uses recently announced results of Csornyei-Jones.

Necessary Condition for Rectifiability Involving Wasserstein Distance W2

Abstract A Radon measure $\mu $ is $n$-rectifiable if it is absolutely continuous with respect to $n$-dimensional Hausdorff measure and $\mu $-almost all of ${\operatorname{supp}}\mu $ can be covered by Lipschitz images of $\mathbb{R}^n$. In this paper, we give a necessary condition for rectifiability in terms of the so-called $\alpha _2$ numbers — coefficients quantifying flatness using Wasserstein distance $W_2$. In a recent article, we showed that the same condition is also sufficient for rectifiability, and so we get a new characterization of rectifiable measures.

Projection inequalities for antichains

A set $A \subseteq {\mathbb{R}}^n$ is called an antichain (resp. antichain) if it does not contain two distinct elements ${\mathbf x}=(x_1,\ldots, x_n)$ and ${\mathbf y}=(y_1,\ldots, y_n)$ satisfying $x_i\le y_i$ (resp. $x_i < y_i$) for all $i\in \{1,\ldots,n\}$. We show that the Hausdorff dimension of a weak antichain $A$ in the $n$-dimensional unit cube $[0,1]^n$ is at most $n-1$ and that the $(n-1)$-dimensional Hausdorff measure of $A$ is at most $n$, which are the best possible bounds. This result is derived as a corollary of the following {\it projection inequality}, which may be of independent interest: The $(n-1)$-dimensional Hausdorff measure of a (weak) antichain $A\subseteq [0, 1]^n$ cannot exceed the sum of the $(n-1)$-dimensional Hausdorff measures of the $n$ orthogonal projections of $A$ onto the facets of the unit $n$-cube containing the origin. For the proof of this result we establish a discrete variant of the projection inequality applicable to weak antichains in ${\mathbb Z}^n$ and combine it with ideas from geometric measure theory.

HAUSDORFF MEASURES OF A CLASS OF MORAN SETS

Let [Formula: see text] be the class of Moran sets with integer [Formula: see text] and real [Formula: see text] satisfying [Formula: see text]. It is well known that the Hausdorff dimension of any set in this class is [Formula: see text]. We show that for any [Formula: see text], [Formula: see text] where [Formula: see text] denotes [Formula: see text]-dimensional Hausdorff measure of [Formula: see text]. For any [Formula: see text] with [Formula: see text] there exists a self-similar set [Formula: see text] such that [Formula: see text].

Bubble tree convergence for conformal metrics with [formula omitted] bounds

Given a sequence of conformal metrics {gk=uk4n−2g0} on a smooth compact boundaryless Riemannian manifold (Mn,g0). Assume the volume of gk and Ln2 norm of scalar curvatures both are bounded. We prove that, after passing to a subsequence, uk weakly converges to the bubble tree limit (u,u1,1,…,ui,α,…,ul,αl),1≤i≤l<∞,1≤α≤αi<∞ in W2,p, for some p<n2, where u∈W2,p(M,g0) and ui,α∈W2,p(Rn,gRn). Moreover, after passing to a subsequence,the sequence of metric spaces (M,dk) defined by gk converges to a connected metric space (M∞,d∞) in the Gromov-Hausdorff topology sense and limk→∞Vol(M,gk)=Hn(M∞,d∞), where Hn is the n dimensional Hausdorff measure defined by d∞.

On $k$-antichains in the unit $n$-cube

A \emph{chain} in the unit $n$-cube is a set $C\subset [0,1]^n$ such that for every $\mathbf{x}=(x_1,\ldots,x_n)$ and $\mathbf{y}=(y_1,\ldots,y_n)$ in $C$ we either have $x_i\le y_i$ for all $i\in [n]$, or $x_i\ge y_i$ for all $i\in [n]$. We consider subsets, $A$, of the unit $n$-cube $[0,1]^n$ that satisfy \[ \text{card}(A \cap C) \le k, \, \text{ for all chains } \, C \subset [0,1]^n \, , \] where $k$ is a fixed positive integer. We refer to such a set $A$ as a $k$-antichain. We show that the $(n-1)$-dimensional Hausdorff measure of a $k$-antichain in $[0,1]^n$ is at most $kn$ and that the bound is asymptotically sharp. Moreover, we conjecture that there exist $k$-antichains in $[0,1]^n$ whose $(n-1)$-dimensional Hausdorff measure equals $kn$ and we verify the validity of this conjecture when $n=2$.

On $k$-antichains in the unit $n$-cube

A \emph{chain} in the unit $n$-cube is a set $C\subset [0,1]^n$ such that for every $\mathbf{x}=(x_1,\ldots,x_n)$ and $\mathbf{y}=(y_1,\ldots,y_n)$ in $C$ we either have $x_i\le y_i$ for all $i\in [n]$, or $x_i\ge y_i$ for all $i\in [n]$. We consider subsets, $A$, of the unit $n$-cube $[0,1]^n$ that satisfy \[ \text{card}(A \cap C) \le k, \, \text{ for all chains } \, C \subset [0,1]^n \, , \] where $k$ is a fixed positive integer. We refer to such a set $A$ as a $k$-antichain. We show that the $(n-1)$-dimensional Hausdorff measure of a $k$-antichain in $[0,1]^n$ is at most $kn$ and that the bound is asymptotically sharp. Moreover, we conjecture that there exist $k$-antichains in $[0,1]^n$ whose $(n-1)$-dimensional Hausdorff measure equals $kn$ and we verify the validity of this conjecture when $n=2$.

A NOTE ON ANTICHAINS IN THE CONTINUOUS CUBE

It is well-known that an antichain in the poset $[0,1]^n$ must have measure zero. Engel, Mitsis, Pelekis and Reiher showed that in fact it must have $(n-1)$-dimensional Hausdorff measure at most $n$, and they conjectured that this bound can be attained. In this note we show that, for every $n$, such an antichain does indeed exist.

The Generalized Baker–Schmidt Problem on Hypersurfaces

Abstract The generalized Baker–Schmidt problem (1970) concerns the $f$-dimensional Hausdorff measure of the set of $\psi $-approximable points on a nondegenerate manifold. There are two variants of this problem concerning simultaneous and dual approximation. Beresnevich–Dickinson–Velani (in 2006, for the homogeneous setting) and Badziahin–Beresnevich–Velani (in 2013, for the inhomogeneous setting) proved the divergence part of this problem for dual approximation on arbitrary nondegenerate manifolds. The corresponding convergence counterpart represents a major challenging open question and the progress thus far has only been attained over planar curves. In this paper, we settle this problem for hypersurfaces in a more general setting, that is, for inhomogeneous approximations and with a non-monotonic multivariable approximating function.