First cohomology, rigidity and deformations of isometric group actions

A note on Hang-Wang's hemisphere rigidity theorem

Let $(M,g)$ be a compact manifold with boundary and $Ric_g\geq (n-1)g$, Hang and Wang proved that $(M,g)$ is isometric to the standard hemisphere if $\partial M$ is convex and isometric to $\mathbb{S}^{n-1}(1)$. We prove some rigidity theorems when $\partial M $ is isometric to a product manifold where one factor is the standard sphere.

Rigidity of Almost-Isometric Universal Covers

Almost-isometries are quasi-isometries with multiplicative constant one. Lifting a pair of metrics on a compact space gives quasi-isometric metrics on the universal cover. Under some additional hypotheses on the metrics, we show that there is no almost-isometry between the universal covers. We show that Riemannian manifolds which are almost-isometric have the same volume growth entropy. We establish various rigidity results as applications.

Local and global rigidity for isometric actions of simple Lie groups on pseudo-Riemannian manifolds

Let $M$ be a finite volume analytic pseudo-Riemannian manifold that admits an isometric $G$-action with a dense orbit, where $G$ is a connected non-compact simple Lie group. For low-dimensional $M$, i.e. $\dim(M) < 2\dim(G)$, when the normal bundle to the $G$-orbits is non-integrable and for suitable conditions, we prove that $M$ has a $G$-invariant metric which is locally isometric to a Lie group with a bi-invariant metric (local rigidity theorem). The latter does not require $M$ to be complete as in previous works. We also prove a general result showing that $M$ is, up to a finite covering, …

Rigidity and regularity of co-dimension one Sobolev isometric immersions

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 $W^{2,2}$.

关于Wasserstein空间的度量构造的等距刚性

Isometric Rigidity of Metric Constructions with respect to Wasserstein Spaces

本文研究度量空间关于p-Wasserstein空间的等距刚性，证明可分裂Hilbert空间的空间非刚性，并证明无限射线、紧致非分支测地空间上的无限半柱面及小直径紧致空间的球面纬悬在相应条件下具有等距刚性。

完备流形中余维数为1的等距浸入的刚性

Rigidity of codimension-1 isometric immersions in complete manifolds

研究了完备流形中余维数为1的等距浸入的渐进行为，通过弹性能量趋于零的浸入序列，证明其子序列收敛于等距浸入，扩展了Alpern等人结果，避免了Young测度的使用。

Compensated Compactness in Banach Spaces and Weak Rigidity of Isometric Immersions of Manifolds

We present a compensated compactness theorem in Banach spaces established recently, whose formulation is originally motivated by the weak rigidity problem for isometric immersions of manifolds with lower regularity. As a corollary, a geometrically intrinsic div-curl lemma for tensor fields on Riemannian manifolds is obtained. Then we show how this intrinsic div-curl lemma can be employed to establish the global weak rigidity of the Gauss-Codazzi-Ricci equations, the Cartan formalism, and the corresponding isometric immersions of Riemannian submanifolds.

Convexity, Rigidity, and Reduction of Codimension of Isometric Immersions into Space Forms

We consider isometric immersions of complete connected Riemannian manifolds into space forms of nonzero constant curvature. We prove that if such an immersion is compact and has semi-definite second fundamental form, then it is an embedding with codimension one, its image bounds a convex set, and it is rigid. This result generalizes previous ones by M. do Carmo and E. Lima, as well as by M. do Carmo and F. Warner. It also settles affirmatively a conjecture by do Carmo and Warner. We establish a similar result for complete isometric immersions satisfying a stronger condition on the second funda…

A Rigidity Theorem for the Hemisphere

We prove the following rigidity theorem: For an n-dimensional compact Riemannian manifold with boundary whose Ricci curvature is bounded by n-1 from below, if its boundary is isometric to the standard sphere of dimension n-1 and totally geodesic, then the manifold is isometric to the standard hemisphere.

等距延拓的刚性与柔性

Rigidity and Flexibility of Isometric Extensions

本文研究$C^{1,θ}$等距延拓的刚性与柔性，证明赫尔德指数1/2为临界值，大于该值时切联络与Levi-Civita联络沿子流形一致，小于时可通过凸积分构造反例，并得到余维数更优的紧黎曼流形等距嵌入存在性定理。

具有流形目标的 L^2-空间的等距刚性

Isometric rigidity of $L^2$-spaces with manifold targets

本文研究以黎曼流形为目标的 L^2 空间的等距刚性，证明其等距由底空间自同构和流形等距族生成，并给出空间等距与底流形等距的等价条件。

Global Weak Rigidity of the Gauss-Codazzi-Ricci Equations and Isometric Immersions of Riemannian Manifolds with Lower Regularity

We are concerned with the global weak rigidity of the Gauss-Codazzi-Ricci (GCR) equations on Riemannian manifolds and the corresponding isometric immersions of Riemannian manifolds into the Euclidean spaces. We develop a unified intrinsic approach to establish the global weak rigidity of both the GCR equations and isometric immersions of the Riemannian manifolds, independent of the local coordinates, and provide further insights of the previous local results and arguments. The critical case has also been analyzed. To achieve this, we first reformulate the GCR equations with div-curl structure …

黎曼流形上Wasserstein空间的刚性

Rigidity of Wasserstein spaces over Riemannian manifolds

研究黎曼流形上L2 Wasserstein空间的等距刚性，证明其刚性等价于流形无欧氏de Rham因子，并刻画等距映射的形状保持性质。

The Cayley hyperbolic space and volume entropy rigidity

Let $M$ be a Riemannian manifold with dimension greater or equal to $3$ which admits a complete, finite-volume Riemannian metric $g_0$ locally isometric to a rank-1 symmetric space of non-compact type. The volume entropy rigidity theorem asserts that $g_0$ minimizes a normalized volume growth entropy among all complete, finite-volume, Riemannian metric on $M$. We will repair a gap in the proof when $g_0$ is locally isometric to the Cayley hyperbolic space.

余维数为2的等距形变问题注记

Notes in the isometric deformations problem in codimension 2

本文通过具体例子研究余维数为2的局部等距刚性问题，指出为保证传递性需扩展真正刚性与诚实刚性概念，并证明必须借助半欧几里得空间中的等距浸入，且该扩展自然引入内积奇异性。

Rigidity and $L^2$ cohomology of hyperbolic manifolds

When $X=Γ\backslash \H^n$ is a real hyperbolic manifold, it is already known that if the critical exponent is small enough then some cohomology spaces and some spaces of $L^2$ harmonic forms vanish. In this paper, we show rigidity results in the borderline case of these vanishing results.

Rigidity for local holomorphic isometric embeddings from ${\BB}^n$ into ${\BB}^{N_1}\times... \times{\BB}^{N_m}$ up to conformal factors

In this article, we study local holomorphic isometric embeddings from ${\BB}^n$ into ${\BB}^{N_1}\times... \times{\BB}^{N_m}$ with respect to the normalized Bergman metrics up to conformal factors. Assume that each conformal factor is smooth Nash algebraic. Then each component of the map is a multi-valued holomorphic map between complex Euclidean spaces by the algebraic extension theorem derived along the lines of Mok and Mok-Ng. Applying holomorphic continuation and analyzing real analytic subvarieties carefully, we show that each component is either a constant map or a proper holomorphic map…

h-Principle and Rigidity for $C^{1,α}$ Isometric Embeddings

In this paper we study the embedding of Riemannian manifolds in low codimension. The well-known result of Nash and Kuiper says that any short embedding in codimension one can be uniformly approximated by $C^1$ isometric embeddings. This statement clearly cannot be true for $C^2$ embeddings in general, due to the classical rigidity in the Weyl problem. In fact Borisov extended the latter to embeddings of class $C^{1,α}$ with $α>2/3$. On the other hand he announced in that the Nash-Kuiper statement can be extended to local $C^{1,α}$ embeddings with $α<(1+n+n^2)^{-1}$, where $n$ is the dimension …

A geometric study of Wasserstein spaces: isometric rigidity in negative curvature

Given a metric space X, one defines its Wasserstein space W2(X) as a set of sufficiently decaying probability measures on X endowed with a metric defined from optimal transportation. In this article, we continue the geometric study of W2(X) when X is a simply connected, nonpositively curved metric spaces by considering its isometry group. When X is Euclidean, the second named author proved that this isometry group is larger than the isometry group of X. In contrast, we prove here a rigidity result: when X is negatively curved, any isometry of W2(X) comes from an isometry of X.

局部自由等距李群作用的谱序列

A Spectral Sequence for locally free isometric Lie group actions

该研究提出适用于自由等距李代数及局部自由等距李群作用的谱序列，关联流形德拉姆上同调、李代数上同调与基本上同调，还引入新的德拉姆上同调描述，并将Wang长正合序列推广到低余维李代数作用。

A rigidity theorem for surfaces in Schwarzschild manifold

In this article, we prove a rigidity theorem for isometric embeddings into the Schwarzschild manifold, by using the variational formula of quasi-local mass.

Rigidity of Convex Surfaces in Homogeneous Spaces

We prove rigidity of oriented isometric immersions of complete surfaces in the homo- geneous 3-manifolds E(k; τ) (different from the space forms) having the same positive extrinsic curvature.

Anosov 型曲面的标记边界刚性

Marked boundary rigidity for surfaces of Anosov type

本文证明 Anosov 型曲面由标记边界距离决定度量，并借助传递原理将闭流形刚性结果推广至开曲面，恢复简单圆盘情形。

Rigidity estimates for isometric and conformal maps from $\mathbb{S}^{n-1}$ to $\mathbb{R}^n$

We investigate both linear and nonlinear stability aspects of rigid motions (resp. Möbius transformations) of $\mathbb{S}^{n-1}$ among Sobolev maps from $\mathbb{S}^{n-1}$ into $\mathbb{R}^n$. Unlike similar in flavour results for maps defined on domains of $\mathbb{R}^n$ and mapping into $\mathbb{R}^n$, not only an isometric (resp. conformal) deficit is necessary in this more flexible setting, but also a deficit measuring the distortion of $\mathbb{S}^{n-1}$ under the maps in consideration. The latter is defined as an associated isoperimetric type of deficit. We mostly focus on the case $n=3$…

Rigidity for Bach-flat metrics on manifolds with boundary and applications

In the article we consider Bach-flat metrics on four-manifolds with boundary, with conformally invariant boundary conditions. We show that such metrics arise naturally as critical points of the Weyl energy under a constraint. We then prove a rigidity result: if a Yamabe metric associated to a critical metric when restricted to the boundary is isometric to the round three-sphere, then the critical metric must be isometric to the standard upper hemisphere.

A Rigidity Theorem for Spacelike Hypersurfaces in de Sitter Space

In this paper we present a rigidity theorem for locally isometric hypersurfaces with a curvature restriction in de Sitter space. This is an analogue to the case for Riemannian space forms given by Guan and Shen in [5].

Lorentz-Minkowski 空间中类时极小曲面的等距类与反等距类

Isometric and anti-isometric classes of timelike minimal surfaces in Lorentz-Minkowski space

本文证明洛伦兹-闵可夫斯基空间中存在等距但非关联族全等的类时极小曲面，并在无平坦点假设下建立等距与反等距刚性定理，且探讨对称性在变形中的传播及Goursat变换的守恒律。

Isometric Rigidity of compact Wasserstein spaces

Let $(X,d,\mathfrak{m})$ be a metric measure space. The study of the Wasserstein space $(\mathbb{P}_p(X),\mathbb{W}_p)$ associated to $X$ has proved useful in describing several geometrical properties of $X.$ In this paper we focus on the study of isometries of $\mathbb{P}_p(X)$ for $p \in (1,\infty)$ under the assumption that there is some characterization of optimal maps between measures, the so called Good transport behaviour $GTB_p$. Our first result states that the set of Dirac deltas is invariant under isometries of the Wasserstein space. Additionally we obtain that the isometry groups o…

Spherical rank rigidity and Blaschke manifolds

Let M be a complete Riemannian manifold whose sectional curvature is bounded above by 1. We say that M has positive spherical rank if along every geodesic one hits a conjugate point at t=π. The following theorem is then proved: If M is a complete, simply connected Riemannian manifold with upper curvature bound 1 and positive spherical rank, then M is isometric to a compact, rank one symmetric space (CROSS) i.e., isometric to a sphere, complex projective space, quaternionic projective space or to the Cayley plane. The notion of spherical rank is analogous to the notions of Euclidean rank and hy…

Isospectral simply-connected homogeneous spaces and the spectral rigidity of group actions

We generalize Sunada's method to produce new examples of closed, locally non-isometric manifolds which are isospectral. In particular, we produce pairs of isospectral, simply-connected, locally non-isometric normal homogeneous spaces. These pairs also allow us to see that in general group actions with discrete spectra are not determined up to measurable conjugacy by their spectra. In particular, we show this for lattice actions.

A local rigidity theorem for minimal two-spheres in an electrovacuum spacetime

The purpose of this article is to prove that, under suitable constrains on the electrovacuum spacetime $M$, if $Σ\subset M$ is an embedded strictly stable minimal two-sphere which locally maximizes the charged Hawking mass, then there exist a neighborhood of it in $M$ isometric to the Reissner-Nordström-de Sitter space. At the same time, motived by \cite{Brendle}, we will deduce an estimate for area of a two-sphere which is locally area minimizing in an electrovacuum spacetime. Moreover, if the equality holds, then there exist a neighborhood of it in $M$ isometric to the charged Nariai space.

Strong rigidity of constant curvature Finsler manifolds

Here, an extension of the Obata-Tanno's theorem to Finsler geometry is established and the following rigidity result is obtained; Every complete connected Finsler manifold of positive constant flag curvature is isometrically homeomorphic to an $n$-sphere equipped with a certain Finsler metric, and vise versa.

Large-scale conformal rigidity in dimension three

We define a complete Riemannian manifold X to be large-scale conformally rigid if all groups that are quasi-isometric to some complete Riemannian manifold of bounded geometry conformal to X are quasi-isometric to X. We prove that many 3-manifolds, including Euclidean 3-space, hyperbolic 3-space and the product of the hyperbolic plane with the real line are large-scale conformally rigid. This implies new characterizations of groups that can act properly, cocompactly by isometries on those manifolds.

具有局部对称叶的李叶状结构的刚性

Rigidity of Lie foliations with locally symmetric leaves

本文证明极小李叶状结构的叶若局部等距于无庞加莱圆盘因子的非紧型对称空间，则光滑共轭于齐次李叶状结构，推广了Zimmer定理，并应用于和乐群算术性与黎曼叶状结构刚性。

Rigidity Theorem for integral pinched shrinking Ricci solitons

We prove that an $n$-dimensional, $n\geq4$, compact gradient shrinking Ricci soliton satisfying a $L^{\frac n2}$-pinching condition is isometric to a quotient of the round $\mathbb{S}^n$, which improves the rigidity theorem given by G. Catino (arXiv:1509.07416vl).

A Rigidity Theorem for Hypersurfaces in Higher Dimensional Space Forms

The classical Cohn-Vossen theorem states that two isometric compact convex surfaces in $\mathbb{R}^{3}$ are congruent. In this short note, we generalize the classical Cohn-Vossen Theorem to higher dimensional surfaces in space form $N^{n+1}(K)$ for $n\ge 2$.

Exterior differential systems, Lie algebra cohomology, and the rigidity of homogenous varieties

These are expository notes from the 2008 Srni Winter School. They have two purposes: (1) to give a quick introduction to exterior differential systems (EDS), which is a collection of techniques for determining local existence to systems of partial differential equations, and (2) to give an exposition of recent work (joint with C. Robles) on the study of the Fubini-Griffiths-Harris rigidity of rational homogeneous varieties, which also involves an advance in the EDS technology.

Mass rigidity for hyperbolic manifolds

We prove the rigidity of positive mass theorem for asymptotically hyperbolic manifolds. Namely, if the mass equality holds, then the manifold is isometric to hyperbolic space. The result was previously proven for spin manifolds or under special asymptotics.

Some rigidity results for the Hawking mass and a lower bound for the Bartnik capacity

We prove rigidity results involving the Hawking mass for surfaces immersed in a $3$-dimensional, complete Riemannian manifold $(M,g)$ with non-negative scalar curvature (resp. with scalar curvature bounded below by $-6$). Roughly, the main result states that if an open subset $Ω\subset M$ satisfies that every point has a neighbourhood $U\subset Ω$ such that the supremum of the Hawking mass of surfaces contained in $U$ is non-positive, then $Ω$ is locally isometric to Euclidean ${\mathbb R}^3$ (resp. locally isometric to the Hyperbolic 3-space ${\mathbb H}^3$). Under mild asymptotic conditions …

Quasi-Isometric Embeddings of Symmetric Spaces

We prove a rigidity theorem that shows that, under many circumstances, quasi-isometric embeddings of equal rank, higher rank symmetric spaces are close to isometric embeddings. We also produce some surprising examples of quasi-isometric embeddings of higher rank symmetric spaces. In particular, we produce embeddings of $SL(n,\mathbb R)$ into $Sp(2(n-1),\mathbb R)$ when no isometric embeddings exist. A key ingredient in our proofs of rigidity results is a direct generalization of the Mostow-Morse Lemma in higher rank. Typically this lemma is replaced by the quasi-flat theorem which says that ma…

Rigidity for the isoperimetric inequality of negative effective dimension on weighted Riemannian manifolds

We study, on a weighted Riemannian manifold of Ric$_{N} \geq K > 0$ for $N < -1$, when equality holds in the isoperimetric inequality. Our main theorem asserts that such a manifold is necessarily isometric to the warped product $\mathbb{R} \times_{\cosh(\sqrt{K/(1-N)}t)} Σ^{n-1}$ of hyperbolic nature, where $Σ^{n-1}$ is an $(n-1)$-dimensional manifold with lower weighted Ricci curvature bound and $\mathbb{R}$ is equipped with a hyperbolic cosine measure. This is a similar phenomenon to the equality condition of Poincaré inequality. Moreover, every isoperimetric minimizer set is isometric to a …

Rigidity of Secondary Characteristic Classes

In this paper we study the variability and rigidity of secondary characteristic classes which arise from flat connections on a manifold. Considering the connection as a Lie-algebra valued one-form, we study the characteristic map from Lie algebra cohomology to de Rham cohomology of the manifold, and prove that if the Leibniz cohomology of the Lie algebra vanishes, then all secondary characteristic classes are rigid. In the case of codimension one foliations (with trivial normal bundle), we show that a characteristic map from the Leibniz cohomology of formal vector fields to the de Rham cohomol…

Morse-Novikov 上同调与李仿射叶状结构的刚性

Morse-Novikov cohomology and rigidity of Lie affine foliations

研究二维非阿贝尔李代数仿射(1)建模的李叶状结构的刚性，通过Morse-Novikov上同调给出消失条件，证明3或4维紧致可定向流形上的李仿射叶状结构在形变下是刚性的。

Horowitz-Myers猜想中的刚性陈述

The rigidity statement in the Horowitz-Myers conjecture

本文通过另一种方法证明了Horowitz-Myers猜想在特定维度下的成立，并展示了达到等式的度量与Horowitz-Myers度量的局部等距关系。

四维哈密顿 $\mathbf{S^1}$-流形的等变上同调刚性

Equivariant cohomological rigidity for four-dimensional Hamiltonian $\mathbf{S^1}$-manifolds

本文解决了紧致连通辛四流形上哈密顿圆作用的等变上同调决定等变微分同胚型的问题，证明了等变上同调环同构与等变微分同胚的充要条件及更强的诱导关系。

An elementary approach to some rigidity theorems

Using elementary comparison geometry, we prove: Let $(M,g)$ be a simply-connected complete Riemannian manifold of dimension $\ge 3$. Suppose that the sectional curvature $K$ satisfies $ -1-s(r) \le K \le -1$, where $r$ denotes distance to a fixed point in $M$. If $\lim_{r \rt \infty} e^{2r}s(r) =0$, then $(M,g)$ has to be isometric to ${\mathbb H}^n$. The same proof also yields that if $K$ satisfies $-s(r) \le K \le 0$ where $\lim_{r \rt \infty} r^2s(r)=0$, then $(M,g)$ is isometric to $\R^n$, a result due to Greene and Wu. Our second result is a local one: Let $(M,g)$ be any Riemannian manifo…

低Sobolev正则性下等距嵌入的Cartan方程、Gauss方程及刚性定理

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。

De Rham L^p-上同调对于高秩空间与群：临界指数与刚性

De Rham $L^p$-Cohomology For Higher Rank Spaces And Groups: Critical Exponents And Rigidity

研究高秩李群和相关流形的L^p上同调临界指数及刚性特性，通过谱序列论证揭示非正曲率可解李群的连续准等距类。

Isometric deformations of pseudoholomorphic curves in the nearly K{ä}hler sphere $\mathbb{S}^6$

The aim of the paper is to investigate the rigidity and the deformability of pseudoholomorphic curves in the nearly K{ä}hler sphere $\mathbb{S}^6,$ among minimal surfaces in spheres. Under various assumptions we describe the moduli space of all noncongruent minimal surfaces $f\colon M\to\mathbb{S}^n$ that are isometric to a pseudoholomorphic curve in $\mathbb{S}^6.$ Moreover, we prove a Schur type theorem (see \cite[p. 36]{Chnew}) for minimal surfaces in spheres.

Hadamard 空间等距群的 $L^p$-上同调

$L^p$-cohomology for groups of isometries of Hadamard spaces

本文研究 Hadamard 流形等距作用与 $L^p$-上同调的联系，证明非初等作用可诱导非平凡的一阶 $L^p$-上同调。

Fubini-Griffiths-Harris rigidity and Lie algebra cohomology

We prove a general extrinsic rigidity theorem for homogeneous varieties in $\mathbb{CP}^N$. The theorem is used to show that the adjoint variety of a complex simple Lie algebra $\mathfrak{g}$ (the unique minimal G orbit in $\mathbb{P}\mathfrak{g}$) is extrinsically rigid to third order. In contrast, we show that the adjoint variety of $SL_3\mathbb{C}$, and the Segre product $\mathit{Seg}(\mathbb{P}^1\times \mathbb{P}^n)$, both varieties with osculating sequences of length two, are flexible at order two. In the $SL_3\mathbb{C}$ example we discuss the relationship between the extrinsic projectiv…

The Radius Rigidity Theorem for Manifolds of Positive Curvature

Recall that the radius of a compact metric space $(X, dist)$ is given by $rad\ X = \min_{x\in X} \max_{y\in X} dist(x,y)$. In this paper we generalize Berger's $\frac{1}{4}$-pinched rigidity theorem and show that a closed, simply connected, Riemannian manifold with sectional curvature $\geq 1$ and radius $\geq \fracπ{2}$ is either homeomorphic to the sphere or isometric to a compact rank one symmetric space.

Quantum rigidity of negatively curved manifolds

We show that an isometric action of a compact quantum group on the underlying geodesic metric space of a compact connected Riemannian manifold $(M,g)$ with strictly negative curvature is automatically classical, in the sense that it factors through the action of the isometry group of $(M,g)$. This partially answers a question by D. Goswami.

Reshetnyak rigidity for Riemannian manifolds

We prove two rigidity theorems for maps between Riemannian manifolds. First, we prove that a Lipschitz map $f:M\to N$ between two oriented Riemannian manifolds, whose differential is almost everywhere an orientation-preserving isometry, is an isometric immersion. This theorem was previously proved using regularity theory for conformal maps; we give a new, simple proof, by generalizing the Piola identity for the cofactor operator. Second, we prove that if there exists a sequence of mapping $f_n:M\to N$, whose differentials converge in $L^p$ to the set of orientation-preserving isometries, then …