Markus Biegert, Robin Nittka
Given bounded domains $Ω_1$ and $Ω_2$ in $\mathds{R}^N$ and an isometry $T$ from $W^{1,p}(Ω_1)$ to $W^{1,p}(Ω_2)$, we give sufficient conditions ensuring that $T$ corresponds to a rigid motion of the space, i.e., $Tu = \pm (u \circ ξ)$ for an isometry $ξ$, and that the domains are congruent. More general versions of the involved results are obtained along the way.
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We prove the $C^{1}$ regularity and developability of $W^{2,p}$ isometric immersions of $n$-dimensional flat domains into ${\mathbb R}^{n+k}$ where $p\ge \min\{2k, n\}$. Another parallel consequence of our methods is a similar regularity and rigidity result for the $W^{2,n}$ solutions of the degenerate Monge-Ampère equations in $n$ dimensions. The analysis also applies to the situations when the degeneracy is extended to $(k+1)\times (k+1)$ minors of the Hessian matrix and the solution is $W^{2,p}$, with $p\ge \min\{2k, n\}$.
We show that every vector lattice homomorphism $T$ between Sobolev spaces can be represented by a composition and a multiplication, that is, $T$ is of the form $Tu(x)=u(h(x))g(x)$ for quasi every/almost every $x$ and all $u$.
We prove the developability and $C^{1,1/2}$ regularity of $W^{2,2}$ isometric immersions of $n$-dimensional domains into $R^{n+1}$. As a conclusion we show that any such Sobolev isometry can be approximated by smooth isometries in the $W^{2,2}$ strong norm, provided the domain is $C^1$ and convex. Both results fail to be true if the Sobolev regularity is weaker than $W^{2,2}$.
Improved Fractional Sobolev Embeddings on Closed Riemannian Manifolds under Isometric Group Actions
本文研究在等距群作用下闭合黎曼流形上的对称改进分数Sobolev嵌入，证明了G不变分数Sobolev空间嵌入到更高L^p空间，并探讨了有限轨道对称下的最优常数。
Fractional Sobolev isometric immersions of planar domains
本文研究了平面域在特定分数Sobolev空间下的等距浸入正则性和可展开性，扩展了Sobolev和Hölder情形的结果，并分析了Jacobian行列式的行为。
Cartan's and Gauss's equations and rigidity theorems for isometric embeddings in low Sobolev regularity
本文证明在低正则性（C^0 ∩ H^{1/2}）下Cartan结构方程仍成立，并推导出Gauss方程在C^1 ∩ W^{1+2/3,3}或c^{1,1/2} ∩ BV^2正则性下成立，应用于闭曲面和凸帽的等距嵌入的正则性与凸性。
We define the notion of higher-order colocally weakly differentiable maps from a manifold $M$ to a manifold $N$. When $M$ and $N$ are endowed with Riemannian metrics, $p\ge 1$ and $k\ge 2$, this allows us to define the intrinsic higher-order homogeneous Sobolev space $\dot{W}^{k,p}(M,N)$. We show that this new intrinsic definition is not equivalent in general with the definition by an isometric embedding of $N$ in a Euclidean space; if the manifolds $M$ and $N$ are compact, the intrinsic space is a larger space than the one obtained by embedding. We show that a necessary condition for the dens…
A fundamental theorem of Wolfe isometrically identifies the space of flat differential forms of dimension $m$ in $\mathbb{R}^n$ with the space of flat $m$-cochains, that is, the dual space of flat chains of dimension $m$ in $\mathbb{R}^n$. The main purpose of the present paper is to generalize Wolfe's theorem to the setting of Sobolev differential forms and Sobolev cochains in $\mathbb{R}^n$. A suitable theory of Sobolev cochains has recently been initiated by the second and third author. It is based on the concept of upper norm and upper gradient of a cochain, introduced in analogy with Heino…
In this article we study Sobolev metrics of order one on diffeomorphism groups on the real line. We prove that the space $\operatorname{Diff}_{1}(\mathbb R)$ equipped with the homogenous Sobolev metric of order one is a flat space in the sense of Riemannian geometry, as it is isometric to an open subset of a mapping space equipped with the flat $L^2$-metric. Here $\operatorname{Diff}_{1}(\mathbb R)$ denotes the extension of the group of all either compactly supported, rapidly decreasing or $H^\infty$-diffeomorphisms, that allows for a shift towards infinity. In particular this result provides …
By using optimal mass transport theory we prove a sharp isoperimetric inequality in ${\sf CD} (0,N)$ metric measure spaces assuming an asymptotic volume growth at infinity. Our result extends recently proven isoperimetric inequalities for normed spaces and Riemannian manifolds to a nonsmooth framework. In the case of $n$-dimensional Riemannian manifolds with nonnegative Ricci curvature, we outline an alternative proof of the rigidity result of S. Brendle (2021). As applications of the isoperimetric inequality, we establish Sobolev and Rayleigh-Faber-Krahn inequalities with explicit sharp const…
The present paper is devoted to analysis of the lack of compactness of bounded sequences in \emph{inhomogeneous} Sobolev spaces, where bounded sequences might fail to be compact due to an isometric group action, that is, \emph{translation}. It will be proved that every bounded sequence $(u_n)$ has (possibly infinitely many) \emph{profiles}, and then the sequence is asymptotically decomposed into a sum of translated profiles and a double-suffixed residual term, where the residual term becomes arbitrarily small in appropriate Lebesgue or Sobolev spaces of lower order. To this end, functional ana…
Korevaar-Schoen and heat kernel characterizations of Sobolev and BV spaces on local trees
本文研究满足局部一致体积增长的局部树上 Sobolev 与 BV 空间，利用测地弱梯度给出 Korevaar-Schoen 能量和热核两种刻画，并导出插值、临界指数、Nash 不等式及热半群梯度界。
We characterize all (signed) measures in $BV_{\frac{n}{n-1}}(\mathbb{R}^n)^*$, where $BV_{\frac{n}{n-1}}(\mathbb{R}^n)$ is defined as the space of all functions $u$ in $L^{\frac{n}{n-1}}(\mathbb{R}^n)$ such that $Du$ is a finite vector-valued measure. We also show that $BV_{\frac{n}{n-1}}(\mathbb{R}^n)^*$ and $BV(\mathbb{R}^n)^*$ are isometrically isomorphic, where $BV(\mathbb{R}^n)$ is defined as the space of all functions $u$ in $L^{1}(\mathbb{R}^n)$ such that $Du$ is a finite vector-valued measure. As a consequence of our characterizations, an old issue raised in Meyers-Ziemer [MZ] is resol…
Let $(Σ,g)$ be a closed Riemannian surface, $W^{1,2}(Σ,g)$ be the usual Sobolev space, $\textbf{G}$ be a finite isometric group acting on $(Σ,g)$, and $\mathscr{H}_\textbf{G}$ be a function space including all functions $u\in W^{1,2}(Σ,g)$ with $\int_Σudv_g=0$ and $u(σ(x))=u(x)$ for all $σ\in \textbf{G}$ and all $x\inΣ$. Denote the number of distinct points of the set $\{σ(x): σ\in \textbf{G}\}$ by $I(x)$ and $\ell=\inf_{x\in Σ}I(x)$. Let $λ_1^\textbf{G}$ be the first eigenvalue of the Laplace-Beltrami operator on the space $\mathscr{H}_\textbf{G}$. Using blow-up analysis, we prove that if $α<…
We prove that if $M$ is a closed $n$-dimensional Riemannian manifold, $n \ge 3$, with ${\rm Ric}\ge n-1$ and for which the optimal constant in the critical Sobolev inequality equals the one of the $n$-dimensional sphere $\mathbb{S}^n$, then $M$ is isometric to $\mathbb{S}^n$. An almost-rigidity result is also established, saying that if equality is almost achieved, then $M$ is close in the measure Gromov-Hausdorff sense to a spherical suspension. These statements are obtained in the ${\rm RCD}$-setting of (possibly non-smooth) metric measure spaces satisfying synthetic lower Ricci curvature bo…
Singular extension of critical Sobolev mappings with values into complete Riemannian manifolds
基于Petrunin的判据，将临界Sobolev映射的奇异延拓结果从欧氏空间推广到具有正触及完备非紧黎曼流形，并证明延拓映射满足指数弱型Sobolev-Marcinkiewicz估计。
Sobolev mappings between RCD spaces and applications to harmonic maps: a heat kernel approach
本文利用热核嵌入方法研究RCD空间之间的Sobolev映射，建立拉回度量、弱上梯度与能量的关系，并刻画了到标准球面的极小等距浸入与特征映射的等价条件。
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。
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