Piotr Hajlasz
We study a question of density of Lipschitz mappings in the Sobolev class of mappings from a closed manifold into a singular space. The main result of the paper shows that if we change the metric in the target space to a bi-Lipschitz equivalent one, than the property of the density of Lipschitz mappings may be lost.
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We prove that Burenkov's Extension Operator preserves Sobolev spaces built on general Morrey spaces, including classical Morrey spaces. The analysis concerns bounded and unbounded open sets with Lipschitz boundaries in the n-dimensional Euclidean space.
If $M$ is a compact smooth manifold and $X$ is a compact metric space, the Sobolev space $W^{1,p}(M,X)$ is defined through an isometric embedding of $X$ into a Banach space. We prove that the answer to the question whether Lipschitz mappings ${\rm Lip}\,(M,X)$ are dense in $W^{1,p}(M,X)$ may depend on the isometric embedding of the target.
We study properties of the classical fractional Sobolev spaces (or Bessel potential spaces) on non-Lipschitz subsets of $\mathbb{R}^n$. We investigate the extent to which the properties of these spaces, and the relations between them, that hold in the well-studied case of a Lipschitz open set, generalise to non-Lipschitz cases. Our motivation is to develop the functional analytic framework in which to formulate and analyse integral equations on non-Lipschitz sets. In particular we consider an application to boundary integral equations for wave scattering by planar screens that are non-Lipschit…
Extension Operators for Fractional Sobolev Spaces on Lipschitz Submanifolds
本文研究了分数Sobolev空间在Lipschitz子流形上的扩展算子，通过局部坐标图表构造并明确了扩展领域大小对估计的影响，应用于几何简化等数值分析领域。
Let $d$ be a metric on $R^n$ and let $C^{m,(d)}(R^n)$ be the space of $C^m$-function on $R^n$ whose partial derivatives of order $m$ belong to the space $Lip(R^n;d)$. We show that the homogeneous Sobolev space $L^{m+1}_p(R^n),p>n,$ can be represented as a union of $C^{m,(d)}(R^n)$-spaces where $d$ belongs to a family of metrics on $R^n$ with certain "nice" properties. This enables us in several important cases to give intrinsic characterizations of the restrictions of Sobolev spaces to arbitrary closed subsets of $R^n$. In particular, we generalize the classical Whitney extension theorem for t…
We study the question: when are Lipschitz mappings dense in the Sobolev space $W^{1,p}(M,\mathbf{H}^n)$? Here $M$ denotes a compact Riemannian manifold with or without boundary, while $\mathbf{H}^n$ denotes the $n$th Heisenberg group equipped with a sub-Riemannian metric. We show that Lipschitz maps are dense in $W^{1,p}(M,\mathbf{H}^n)$ for all $1\le p<\infty$ if $\dim M \le n$, but that Lipschitz maps are not dense in $W^{1,p}(M,\mathbf{H}^n)$ if $\dim M \ge n+1$ and $n\le p<n+1$. The proofs rely on the construction of smooth horizontal embeddings of the sphere $S^n$ into $\mathbf{H}^n$. We …
We construct a smooth compact n-dimensional manifold Y with one point singularity such that all its Lipschitz homotopy groups are trivial, but Lipschitz mappings Lip(S^n,Y) are not dense in the Sobolev space W^{1,n}(S^n,Y). On the other hand we show that if a metric space Y is Lipschitz (n-1)-connected, then Lipschitz mappings Lip(X,Y) are dense in N^{1,p}(X,Y) whenever the Nagata dimension of X is bounded by n and the space X supports the p-Poincare inequality.
We establish new approximation results, in the sense of Lusin, of Sobolev functions by Lipschitz ones, in some classes of non-doubling metric measure structures. Our proof technique relies upon estimates for heat semigroups and applies to Gaussian and $RCD(K, \infty)$ spaces. As a consequence, we obtain quantitative stability for regular Lagrangian flows in Gaussian settings.
We prove the Sobolev-to-Lipschitz property for metric measure spaces satisfying the quasi curvature dimension condition recently introduced in E. Milman, The Quasi Curvature-Dimension Condition with applications to sub-Riemannian manifolds, Comm. Pure Appl. Math. (to appear, arXiv:1908.01513v5). We provide several applications to properties of the corresponding heat semigroup. In particular, under the additional assumption of infinitesimal Hilbertianity, we show the Varadhan short-time asymptotics for the heat semigroup with respect to the distance, and prove the irreducibility of the heat sem…
Characterization of AC and Sobolev curves via Lipschitz post-compositions
该研究在度量空间框架下，通过Lipschitz函数后复合的性质，分别刻画了p-绝对连续曲线与完备可分度量空间中Sobolev曲线的等价特征，丰富了几何测度与分析理论。
A new characterization of Sobolev spaces on Lipschitz differentiability spaces
本文在不假设双重条件或Poincaré不等式的情况下，通过证明精确的Brezis-Van Schaftingen-Yung型渐进行式，给出了Sobolev范数通过非局部泛函渐进行为的新的刻画，并构造了反例以证明假设的必要性。
We provide sufficient conditions for boundary Hardy inequality to hold in bounded Lipschitz domains, complement of a point (the so-called point Hardy inequality), domain above the graph of a Lipschitz function, the complement of a bounded Lipschitz domain in fractional Orlicz-Sobolev setting. As a consequence, we get sufficient conditions for regional fractional Orlicz Poincaré inequality in bounded Lipschitz domains. Necessary conditions for fractional Orlicz Hardy and regional fractional Orlicz Poincaré inequalities are also given for bounded Lipschitz domains. Various sufficient conditions …
We study in this paper some property of Lipschitz mappings which admit factorization through an operator ideal. We try to construct Lipschitz cross-norms from known tensor norms in order to represent certain classes of Lipschitz mappings. Inspired by the definition of p-summing linear operators we introduce a new concpet in the the category of Lipschitz mappings that is called strictly Lipschitz p-summing.
Characterizations of the Sobolev space $\mathrm{H}^{1}$ on the boundary of a strong Lipschitz domain in 3-D
本文提出通过微分算子与边界弱形式直接刻画三维强Lipschitz域边界上的Sobolev空间$\mathrm{H}^{1}$，与经典坐标卡定义等价，并利于$\mathrm{H}(\operatorname{curl})$向量场的迹处理。
Persistence of Rademacher-type and Sobolev-to-Lipschitz properties
本文研究拟正则强局部Dirichlet空间上Rademacher型与Sobolev-to-Lipschitz性质在多种操作下的保持性，并给出Cheeger能量识别、内蕴距离及Varadhan渐近的张量积结果。
We survey a few trace theorems for Sobolev spaces on $N$-dimensional Euclidean domains. We include known results on linear subspaces, in particular hyperspaces, and smooth boundaries, as well as less known results for Lipschitz boundaries, including Besov's Theorem and other characterizations of traces on planar domains, polygons in particular, in the spirit of the work of P. Grisvard. Finally, we present a recent approach, originally developed by G. Auchmuty in the case of the Sobolev space $H^1(Ω)$ on a Lipschitz domain $Ω$, and which we have further developed for the trace spaces of $H^k(Ω)…
We prove boundary inequalities in arbitrary bounded Lipschitz domains on the trace space of Sobolev spaces. For that, we make use of the trace operator, its Moore-Penrose inverse, and of a special inner product. We show that our trace inequalities are particularly useful to prove harmonic inequalities, which serve as powerful tools to characterize the harmonic functions on Sobolev spaces of non-integer order.
This paper addresses Lipschitzian stability issues that play an important role in both theoretical and numerical aspects of variational analysis, optimization, and their applications. We particularly concentrate on the so-called relaxed one-sided Lipschitz property of set-valued mappings with negative Lipschitz constants. This property has been much less investigated than more conventional Lipschitzian behavior while being well recognized in a variety of applications. Recent work has revealed that set-valued mappings satisfying the relaxed one-sided Lipschitz condition with negative Lipschitz …
We present an overview to the approximation property, paying especial attention to the recent results relating the approximation property to ideals of linear operators and Lipschitz ideals. We complete the paper with some new results on approximation of Lipschitz mappings and their relation to linear operator ideals.
Newtonian spaces generalize first-order Sobolev spaces to abstract metric measure spaces. In this paper, we study regularity of Newtonian functions based on quasi-Banach function lattices. Their (weak) quasi-continuity is established, assuming density of continuous functions. The corresponding Sobolev capacity is shown to be an outer capacity. Assuming sufficiently high integrability of upper gradients, Newtonian functions are shown to be (essentially) bounded and (Hölder) continuous. Particular focus is put on the borderline case when the degree of integrability equals the "dimension of the m…
We extensively discuss the Rademacher and Sobolev-to-Lipschitz properties for generalized intrinsic distances on strongly local Dirichlet spaces possibly without square field operator. We present many non-smooth and infinite-dimensional examples. As an application, we prove the integral Varadhan short-time asymptotic with respect to a given distance function for a large class of strongly local Dirichlet forms.
We are going to show that on bounded Lipschitz domain $D$: both $C_{c}^{\infty}(D)$, the set of smooth functions on $D$ with compact support, and $C_{0}^{\infty}(D)$, the set of smooth functions on $D$ with (extension) zero boundary, are dense in $W^{1,p}\left(D\right)$, $p\in[1,\infty)$. A proof can be found in Nečas's monograph \cite{key-2}, Theorem 4.10, §2.4.3. Our main result in this note is that: we find another proof by showing that both closures is the same as kernel of trace operator $T:\,W^{1,p}(D)\rightarrow L^{p}(\partial D)$ via some change of variables formulas from Evans and Gar…
Orbital Lipschitzian mappings and semigroup actions on metric spaces
本文通过施加弱于传统Lipschitzian的轨道Lipschitzian条件，研究了度量空间上映射族的公共不动点，并在正规结构性质和Lifschitz特征两种经典方法下提供了新结果。
We prove that Stein's Extension Operator preserves Sobolev-Morrey spaces, that is spaces of functions with weak derivatives in Morrey spaces. The analysis concerns classical and generalized Morrey spaces on bounded and unbounded domains with Lipschitz boundaries in the n-dimensional Euclidean space.
Typical Lipschitz mappings are typically non-differentiable
本文证明任意两个Banach空间之间的典型Lipschitz映射在定义域子集的典型点上以最极端形式不可微，该结果对Euclidean空间情形也是新的。
It is known that for Banach valued functions there are several approaches to define a Sobolev class. We compare the usual definition via weak derivatives with the Reshetnyak-Sobolev space and with the Newtonian space; in particular, we provide sufficient conditions when all three agree. As well we revise the difference quotient criterion and the property of Lipschitz mapping to preserve Sobolev space when it acting as a superposition operator.
In this paper, we study a family of general fractional Sobolev spaces $\MsqpOm$ when $\Om=\Rn$ or $\Om$ is a bounded domain, having a compact, Lipschitz boundary $\Bdy$, in $\Rn$ for $n\geq2$. Among other results, some compact embedding results of $\MVsqpRn\hookrightarrow\LqRn$ and $\MVsqpRn\hookrightarrow\LlRn$ for suitable potential functions $V(x)$ are described.
In this paper we give a thorough study of Lipschitz spaces. We obtain the following new results: (1) Sharp Jawerth-Franke-type embeddings between the Besov and Lipschitz spaces extending the classical results for Besov and Sobolev spaces; (2) Sharp embeddings between Lipschitz spaces with different parameters extending the Brézis-Wainger result; (3) Characterizations for Lipschitz spaces norms via Fourier transforms and wavelets; (4) Sharp embeddings from Lipschitz spaces into Lebesgue/Lorentz-Zygmund spaces.
We study a variant of the Whitney extension problem for the space $C^{k,ω}(R^n)$. We identify this space with a space of Lipschitz mappings from $R^n$ into the space $P_k \times R^n$ of polynomial fields on $R^n$ equipped with a certain metric. This identification allows us to reformulate the Whitney problem for $C^{k,ω}(R^n)$ as a Lipschitz selection problem for set-valued mappings into a certain family of subsets of $P_k \times R^n$. We prove a Helly-type criterion for the existence of Lipschitz selections for such set-valued mappings defined on finite sets. With the help of this criterion, …
A generalization of Lipschitz mappings
本文通过连续模引入广泛有界映射以推广 Lipschitz 映射，研究其度量与范数性质、线性化，并应用于算子理想及有限秩与紧映射。
Let ${\mathfrak M}=({\mathcal M},ρ)$ be a metric space and let $X$ be a Banach space. Let $F$ be a set-valued mapping from ${\mathcal M}$ into the family ${\mathcal K}_m(X)$ of all compact convex subsets of $X$ of dimension at most $m$. The main result in our recent joint paper with Charles Fefferman (which is referred to as a "Finiteness Principle for Lipschitz selections") provides efficient conditions for the existence of a Lipschitz selection of $F$, i.e., a Lipschitz mapping $f:{\mathcal M}\to X$ such that $f(x)\in F(x)$ for every $x\in{\mathcal M}$. We give new alternative proofs of this…
Density of subalgebras of Lipschitz functions in metric Sobolev spaces and applications to Wasserstein Sobolev spaces
本文证明了度量Sobolev空间中Lipschitz函数子代数在能量意义下稠密的一般准则，并将其应用于Wasserstein Sobolev空间，证明了其Hilbert性质并给出了Wasserstein梯度的显式特征与微积分法则。
In our companion paper (S.N. Chandler Wilde, D.P. Hewett, A. Moiola, Sobolev spaces on non-Lipschitz subsets of $\mathbb{R}^n$ with application to boundary integral equations on fractal screens, 2016) we studied a number of different Sobolev spaces on a general (non-Lipschitz) open subset $Ω$ of $\mathbb{R}^n$, defined as closed subspaces of the classical Bessel potential spaces $H^s(\mathbb{R}^n)$ for $s\in\mathbb{R}$. These spaces are mapped by the restriction operator to certain spaces of distributions on $Ω$. In this note we make some observations about the relation between these spaces of…
On the structural decomposition of planar Lipschitz quotient mappings
本文修正了Johnson等人关于平面多项式构造Lipschitz商映射的错误，并引入了更强的逐点co-Lipschitz性概念，刻画了其等价性及与高维Lipschitz商映射长期猜想的相关性。
The purpose of this investigation is to extend basic equations and inequalities which hold for functions $f$ in a Bernstein space $B_σ^2$ to larger spaces by adding a remainder term which involves the distance of $f$ from $B_σ^2$. First we present a modification of the classical modulation space $M^{2,1}(\mathbb{R})$, the so-called readapted modulation space $M^{2,1}_\text{a}(\mathbb{R})$. Our approach to the latter space and its role in functional analysis is novel. In fact, we establish several chains of inclusion relations between $M^{2,1}_\text{a}(\mathbb{R})$ and the more common Lipschitz…
Extreme non-differentiability of typical Lipschitz mappings
研究显示，无论给定规范空间的子集如何，典型1-Lipschitz映射在强意义上在集合的典型点处非可导，且在任意小尺度上导数比近似任意线性算子。
Existence Criteria for Lipschitz Selections of Set-Valued Mappings in ${\bf R}^2$
本文提出了度量空间到 $\bf R^2$ 的集值映射存在 Lipschitz 选择的若干构造性准则，并发展了几何方法以提供构造近乎最优选择及计算其 Lipschitz 半范数数量级的高效算法。
We consider Sobolev spaces with values in Banach spaces as they are frequently useful in applied problems. Given two Banach spaces $X\neq\{0\}$ and $Y$, each Lipschitz continuous mapping $F:X\rightarrow Y$ gives rise to a mapping $u\mapsto F\circ u$ from $W^{1,p}(Ω,X)$ to $W^{1,p}(Ω,Y)$ if and only if $Y$ has the Radon-Nikodym Property. But if $F$ is one-sided Gateaux differentiable no condition on the space is needed. We also study when weak properties in the sense of duality imply strong properties. Our results are applied to prove embedding theorems, a multi-dimensional version of the Aubin…
Efficient Algorithms for Lipschitz Selections of Set-Valued Mappings in ${\bf R}^2$: long version
针对平面闭半平面值集值映射，本文提出二次复杂度的Lipschitz选择算法，可判定不存在常数不超过$\lambda$的选择，或构造常数不超过$3\lambda$的选择。
We study the structure of Sobolev spaces on the cartesian/warped products of a given metric measure space and an interval. Our main results are: - the characterization of the Sobolev spaces in such products - the proof that, under natural assumptions, the warped products possess the Sobolev-to-Lipschitz property, which is key for geometric applications. The results of this paper have been needed in the recent proof of the `volume-cone-to-metric-cone' property of ${\sf RCD}$ spaces obtained by the first author and De Philippis.
Chain rule for pointwise Lipschitz mappings
本文提出逐点 Lipschitz 映射的几乎处处链式法则，去除全局可微假设，并推广至无限维空间，同时加强 Rademacher-Stepanov 定理。
Suppose that S is a left amenable semitopological semigroup. We prove that if ${T_{t}: t \in S}$ is a uniformly k-Lipschitzian semigroup on a bounded closed and convex subset C of a Hilbert space and $k<\sqrt{2}$, then the set of fixed points of this semigroup is a Hölder continuous retract of C. This gives a qualitative complement to the Ishihara-Takahashi fixed point existence theorem.
Sobolev spaces via chains in metric measure spaces
本文在度量测度空间上引入链Sobolev空间，证明其在完备情形与经典定义等价，在非完备情形与局部Lipschitz常数松弛定义等价，并应用于Poincaré不等式的等价表述。
Global BMO-Sobolev Estimates for Second-Order Linear Elliptic Equations on Lipschitz Domains
本文在极小正则性假设下，利用 BMO 空间的逐点乘子刻画，建立了 Lipschitz 域上二阶线性椭圆方程弱解的全局 BMO 梯度估计，并由此得到 Hardy 空间数据对应的 $L^1$ 全局估计。
Yet another proof of the density in energy of Lipschitz functions
该研究提出了Lipschitz函数在能量意义下稠密于带重心平面定义的度量Sobolev空间（及Newtonian-Sobolev空间）的新短证明，通过归约到Banach空间框架、结合凸分析工具与正规1-流叠加原理完成，同时得到光滑柱函数稠密性的新证。
In this paper we give characterizations of mappings generate embeddings of Sobolev spaces in the terms of ring capacity inequalities. In addition we prove that such mappings are Lipschitz mappings in the sub-hyperbolic type capacitory metrics.
We study mappings with bounded (p,q)-distortion associated to Sobolev spaces on Carnot groups. Mappings of such type have applications to the Sobolev type embedding theory and classification of manifolds. For this class of mappings, we obtain estimates of linear distortion, and a geometrical description. We prove also Liouville type theorems and give some sufficient conditions for removability of sets.
In this note we study the differentiability with respect to the time-parameter of semigroups consisting of Lipschitzian or smooth self-mappings of a domain in a Banach space.
The coarea formula for projections of Lipschitz mappings on Carnot groups
本文证明了Carnot群上内在Lipschitz映射沿水平投影的余面积不等式，并在有限余畸变映射及Heisenberg群情形下取得等式。
In this paper, we solve the Dirichlet problem for Orlicz-Sobolev maps between singular metric spaces that extends the corresponding result of Guo et al. [arXiv 2021]. As an intermediate step, we develop a version of Rellich-Kondrachov compactness theorem for Orlicz-Sobolev mappings between metric spaces that extends a previous result of Guo and Wenger [Comm. Anal. Geom. 2020]. Another crucial ingredient is an Orlicz-Sobolev extension of the trace theory for metric valued Sobolev maps developed by Korevaar and Schoen [Comm. Anal. Geom. 1993].
Let $Ω\subset\dR^d$ be a bounded or an unbounded Lipschitz domain. In this note we address the problem of continuation of functions from the Sobolev space $H^1(Ω)$ up to functions in the Sobolev space $H^1(\dR^d)$ via a linear operator. The minimal possible norm of such an operator is estimated from below in terms of spectral properties of self-adjoint Robin Laplacians on domains $Ω$ and $\dR^d\setminus\ovΩ$. Another estimate of this norm is also given, where spectral properties of Schrödinger operators with the $δ$-interaction supported on the hypersurface $\partialΩ$ are involved. General re…
Derivations and Sobolev functions on extended metric-measure spaces
本文研究了扩展度量-拓扑测度空间上的一阶微分演算，将Lipschitz导数理论推广至该设定，并通过分部积分公式引入Sobolev空间，证明了其与其他方法的等价性。
We establish two new characterizations of magnetic Sobolev spaces for Lipschitz magnetic fields in terms of nonlocal functionals. The first one is related to the BBM formula, due to Bourgain, Brezis, and Mironescu. The second one is related to the work of the first author on the classical Sobolev spaces. We also study the convergence almost everywhere and the convergence in $L^1$ appearing naturally in these contexts.
This paper deals with the study of parameter dependence of extensions of Lipschitz mappings from the point of view of continuity. We show that if assuming appropriate curvature bounds for the spaces, the multivalued extension operators that assign to every nonexpansive (resp. Lipschitz) mapping all its nonexpansive extensions (resp. Lipschitz extensions with the same Lipschitz constant) are lower semi-continuous and admit continuous selections. Moreover, we prove that Lipschitz mappings can be extended continuously even when imposing the condition that the image of the extension belongs to the…
Homogeneous Sobolev and Besov spaces on special Lipschitz domains and their traces
本文研究了特殊Lipschitz区域上齐次Sobolev和Besov空间的迹算子，证明了在精确正则性范围 $s \in (\frac{1}{p}, 1 + \frac{1}{p})$ 内迹定理成立，并处理了端点情况，填补了齐次函数空间研究的空白。
Korevaar-Schoen Energy and Interpolations of Fractional Sobolev mappings on Carnot groups
该研究将欧氏空间的弱接触方程结果推广到次黎曼框架下的Carnot群，探究分数阶Sobolev映射的Korevaar-Schoen能量与插值性质，证明满足弱接触方程的映射属于特定水平分数阶Sobolev空间，建立插值定理并刻画Korevaar-Schoen能量的极小化子。
Non-standard Sobolev scales and the mapping properties of the X-ray transform on manifolds with strictly convex boundary
本文综述了在严格凸边界黎曼流形上，利用边界凸性获得X射线变换的精细映射估计，并联系反问题、不确定性量化与算子学习。
On the local character of the extension of traces for Sobolev mappings
本文证明Sobolev映射的迹延拓具有局部性质，即延拓存在的障碍在全局情形下纯粹是局部的。
Heterotopic energy for Sobolev mappings
本文提出异质能量的概念，用于研究Sobolev映射在同伦类中的能量特性，通过分析映射在余维一骨架上的同伦性来确定能量是否有限。
The present paper is concerned with Lipschitz properties of convex mappings. One considers the general context of mappings defined on an open convex subset $Ω$ of a locally convex space $X$ and taking values in a locally convex space $Y$ ordered by a normal cone. One proves also equi-Lipschitz properties for pointwise bounded families of continuous convex mappings, provided the source space $X$ is barrelled. Some results on Lipschitz properties of continuous convex functions defined on metrizable topological vector spaces are included as well. The paper has a methodological character - its aim…
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