Gauss equation and injectivity radii for subspaces in spaces of curvature bounded above

Injectivity of minimal immersions and homeomorphic extensions to space

We study a recent general criterion for the injectivity of the conformal immersion of a Riemannian manifold into higher dimensional Euclidean space, and show how it gives rise to important conditions for Weierstrass-Ennerper lifts defined in the unit disk $\mathbb{D}$ endowed with a conformal metric. Among the corollaries, we obtain a Becker type condition and a sharp condition depending on the Gaussian curvature and the diameter for an immersed geodesically convex minimal disk in $\mathbb{R}^3$ to be embedded. Extremal configurations for the criteria are also determined, and can only occur on…

驯黎曼度量凸和的单射半径下界及其在辛拓扑中的应用

Injectivity radius lower bound of convex sum of tame Riemannian metrics and applications to symplectic topology

该研究证明了一对曲率有界、单射半径有正下界的完备驯黎曼度量的凸和仍保持曲率有界且单射半径有一致下界，进而证明驯度量拟等距类的凸性与可缩性，以及对应$C^3$-驯殆复结构集合的可缩性，为辛拓扑研究提供了黎曼几何工具。

Every conformal class contains a metric of bounded geometry

We show that on every manifold, every conformal class of semi-Riemannian metrics contains a metric $g$ such that each $k$-th-order covariant derivative of the Riemann tensor of $g$ has bounded absolute value $a_k$. This result is new also in the Riemannian case, where one can arrange in addition that $g$ is complete with injectivity and convexity radius greater than 1. One can even make the radii rapidly increasing and the functions $a_k$ rapidly decreasing at infinity. We prove generalizations to foliated manifolds, where curvature, second fundamental form and injectivity radius of the leaves…

Yau有界平均曲率嵌入问题的一个反例

A counterexample to Yau's bounded-mean-curvature embedding question

针对Yau关于有界Ricci曲率与正单射半径流形是否存在有界平均曲率等距嵌入的问题，构造了四维及以上维度的反例，证明此类嵌入不存在。

The geodesic ray transform on two-dimensional Cartan-Hadamard manifolds

We prove two injectivity theorems for the geodesic ray transform on two-dimensional, complete, simply connected Riemannian manifolds with non-positive Gaussian curvature, also known as Cartan-Hadamard manifolds. The first theorem is concerned with bounded non-positive curvature and the second with decaying non-positive curvature.

Higher order distance-like functions and Sobolev spaces

We consider complete Riemannian manifolds with a controlled growth of the covariant derivatives of Ricci curvatures up to order $k-2$ and a controlled decay of the injectivity radii. On such manifolds we construct distance-like functions with a control on covariant derivatives up to order $k$. Alternatively, the assumption on the injectivity radii can be replaced with the request of a controlled growth of the full curvature tensor at order $0$. The control in the assumptions occur via non-necessarily polynomial growth functions. This construction largely extend previously known results in vari…

Hypersurfaces in H^{n+1} and conformally invariant equations: the generalized Christoffel and Nirenberg problems

Our first objective in this paper is to give a natural formulation of the Christoffel problem for hypersurfaces in $H^{n+1}$, by means of the hyperbolic Gauss map and the notion of hyperbolic curvature radii for hypersurfaces. Our second objective is to provide an explicit equivalence of this Christoffel problem with the famous problem of prescribing scalar curvature on $§^n$ for conformal metrics, posed by Nirenberg and Kazdan-Warner. This construction lets us translate into the hyperbolic setting the known results for the scalar curvature problem, and also provides a hypersurface theory inte…

Parabolic Classical Curvature Flows

We consider classical curvature flows: 1-parameter families of convex embeddings of the 2-sphere into Euclidean 3-space which evolve by an arbitrary (non-homogeneous) function of the radii of curvature. The associated flow of the radii of curvature is a second order system of partial differential equations which we show decouples to highest order. We determine the conditions for this system to be parabolic and investigate the lower order terms. The zeroth order terms are shown to form a Hamiltonian system, which is therefore completely integrable. We find conditions on parabolic curvature flow…

Bounds on harmonic radius and limits of manifolds with bounded Bakry-Émery Ricci curvature

Under the usual condition that the volume of a geodesic ball is close to the Euclidean one or the injectivity radii is bounded from below, we prove a lower bound of the $C^α W^{1, q}$ harmonic radius for manifolds with bounded Bakry-Émery Ricci curvature when the gradient of the potential is bounded. Under these conditions, the regularity that can be imposed on the metrics under harmonic coordinates is only $C^αW^{1,q}$, where $q>2n$ and $n$ is the dimension of the manifolds. This is almost 1 order lower than that in the classical $C^{1,α} W^{2, p}$ harmonic coordinates under bounded Ricci cur…