Codimension two Umbilic points on Surfaces Immersed in R^3

The umbilic set of Willmore surfaces

It is well known that the umbilic points of minimal surfaces in spaces of constant sectional curvature consist only of isolated points unless the surface is totally umbilic on some connected component, as for example the Hopf form is holomorphic. In this note, we prove that on Willmore surfaces in codimension one the umbilic set is locally a one dimensional real-analytic manifold without boundary or an isolated point.

Optimal rigidity estimates for nearly umbilical surfaces in arbitrary codimension

In [dLMu05], DeLellis and Müller proved a quantitative version of Codazzi's theorem, namely for a smooth embedded surface $\ Σ\subseteq \mathbb{R}^3\ $ with area normalized to $\ {\cal H}^2(Σ) = 4 π $, it was shown that $\ \parallel A_Σ- id \parallel_{L^2(Σ)} \leq C \parallel A^0_Σ\parallel_{L^2(Σ)}\ $, and building on this, closeness of $\ Σ $ to a round sphere in $\ W^{2,2}\ $ was established, when $\ \parallel A^0_Σ\parallel_{L^2(Σ)}\ $ is small. This was supplemented in [dLMu06] by giving a conformal parametrization $\ S^2 \stackrel{\approx}{\longrightarrow} Σ $ with small conformal factor…

Geometric Mean Curvature Lines on Surfaces Immersed in R3

Here are studied pairs of transversal foliations with singularities, defined on the Elliptic region (where the Gaussian curvature $\mathcal K$ is positive) of an oriented surface immersed in $\mathbb R^3$. The leaves of the foliations are the lines of geometric mean curvature, along which the normal curvature is given by $\sqrt {\mathcal K}$, which is the geometric mean curvature of the principal curvatures $ k_1, k_2$ of the immersion. The singularities of the foliations are the umbilic points and parabolic curves}, where $ k_1 = k_2$ and ${\mathcal K} = 0$, respectively. Here are determined …

Umbilic hypersurfaces of constant sigma-k curvature in the Heisenberg group

We study immersed, connected, umbilic hypersurfaces in the Heisenberg group $H_{n}$ with $n$ $\geq $ $2.$ We show that such a hypersurface, if closed, must be rotationally invariant up to a Heisenberg translation. Moreover, we prove that, among others, Pansu spheres are the only such spheres with positive constant sigma-k curvature up to Heisenberg translations.

Affine focal sets of codimension $2$ submanifolds contained in hyper surfaces

In this paper we study the affine focal set, which is the bifurcation set of the affine distance to submanifolds $N^n$ contained in hypersurfaces $M^{n+1}$ of the $(n+2)$-space. We give condition under which this affine focal set is a regular hypersurface and, for curves in $3$-space, we describe its stable singularities. For a given Darboux vector field $ξ$ of the immersion $N\subset M$, one can define the affine metric $g$ and the affine normal plane bundle $\mathcal{A}$. We prove that the $g$-Laplacian of the position vector belongs to $\mathcal{A}$ if and only if $ξ$ is parallel. For umbil…

Principal configurations around umbilics of spacelike surfaces in null hypersurfaces of $\mathbb{R}_1^4$

We study the principal configurations around an isolated $η$-umbilical point on a generic spacelike surface $S$ immersed in a null hypersurface $M$ of Minkowski space $\mathbb{R}_1^4$ relative to a well-defined null vector field $η$ orthogonal to the surface $S$. In the particular case of $M$ being a null rotation hypersurface of $\mathbb{R}_1^4$ we also recover the local Darbouxian principal configurations around that kind of $η$-umbilical points.

Umbilical submanifolds of $\mathbb{S}^n\times \mathbb{R}$

We give a complete classification of umbilical submanifolds of arbitrary dimension and codimension of $\Sf^n\times \R$, extending the classification of umbilical surfaces in $\Sf^2\times \R$ by Rabah-Souam and Toubiana as well as the local description of umbilical hypersurfaces in $\Sf^n\times \R$ by Van der Veken and Vrancken. We prove that, besides small spheres in a slice, up to isometries of the ambient space they come in a two-parameter family of rotational submanifolds whose substantial codimension is either one or two and whose profile is a curve in a totally geodesic $\Sf^1\times \R$ o…

Partially umbilic singularities of hypersurfaces of $\mathbb R^4$

This paper establishes the geometric structure of the lines of principal curvature of a hypersurface immersed in ${\mathbb R}^4$ in a neighborhood of the set $\mathcal{S}$ of its principal curvature singularities, consisting of the points at which atF least two principal curvatures are equal. Under generic conditions defined by appropriate transversality hypotheses it is proved that $\mathcal{S}$ is the union of regular smooth curves $\mathcal{S}_{12}$ and $\mathcal{S}_{23}$, consisting of partially umbilic points, where only two principal curvatures coincide. This curve is partitioned into re…

Lines of Mean Curvature on Surfaces Immersed in R3

Associated to oriented surfaces immersed in R^3 here are studied pairs of transversal foliations with singularities, defined on the Elliptic region, where the Gaussian curvature K, given by the product of the principal curvatures k_1, k_2 of the immersion, is positive. The leaves of the foliations are the lines of M- mean curvature, along which the normal curvature of the immersion is given by a function M = M(k_1,k_2) in [k_1, k_2], called a M- mean curvature, whose properties extend and unify those of the arithmetic H=(k_1+k_2)/2, the geometric K^{1/2} and harmonic K/H=((1/{k_1} + 1/{k_2})/2…

Spacelike capillary surfaces in the Lorentz-Minkowski space

For a compact spacelike constant mean curvature surface with nonempty boundary in the three-dimensional Lorentz-Minkowski space, we introduce a rotation index of the lines of curvature at the boundary umbilic point, which was developed by Choe \cite{Choe}. Using the concept of the rotation index at the interior and boundary umbilic points and applying the Poincaré-Hopf index formula, we prove that a compact immersed spacelike disk type capillary surface with less than $4$ vertices in a domain of $\Bbb L^3$ bounded by (spacelike or timelike) totally umbilic surfaces is part of a (spacelike) pla…

浸入最小曲面在任意余维数中的李维定理

Liouville theorem for immersed minimal surfaces in any codimension

研究了浸入最小盘在余维数任意情况下的李维定理，证明了谐函数的负部分以亚线性速率增长时为常数，从而得出更高余维数的伯恩斯坦定理，并通过猫眼曲面、螺旋面和恩内珀族曲面证明结果最优，同时展示了谐函数的均匀霍尔德正则性。

3-球面中Willmore浸入脐点的若干全局性质

Some global properties of umbilic points of Willmore immersions in the $3$-sphere

该文从共形高斯映射视角研究3-球面中余维1的Willmore曲面脐点，刻画了脐曲线经共形变换为测地线的等价条件，证明含脐曲线长度的共形高斯映射Gauss–Bonnet公式，并将其作为各空间形式中共形极小曲面Willmore能量表达式的统一版本。

脐斜率与三次Weingarten曲面

Umbilic slopes and cubic Weingarten surfaces

本文研究脐斜率并分类满足κ₂=cκ₁³的紧致浸入曲面，证明其每个连通分量均为旋转椭球，方法涉及齐次Hessian恒等式、唯一延拓及四阶增长估计。

Harmonic Mean Curvature Lines on Surfaces Immersed in R3

Consider oriented surfaces immersed in $\mathbb R^3.$ Associated to them, here are studied pairs of transversal foliations with singularities, defined on the Elliptic region, where the Gaussian curvature $\mathcal K$, given by the product of the principal curvatures $k_1, k_2$ is positive. The leaves of the foliations are the lines of harmonic mean curvature, also called characteristic or diagonal lines, along which the normal curvature of the immersion is given by ${\mathcal K}/{\mathcal H}$, where $ {\mathcal H}=({k_1}+k_2)/2$ is the arithmetic mean curvature. That is, ${\mathcal K}/{\mathca…

洛伦兹-闵可夫斯基时空中包含于光锥的、具有平行平均曲率向量场的余维二类空子流形

Spacelike Submanifolds of Codimension Two with Parallel Mean Curvature Vector Field in Lorentz-Minkowski Spacetime Contained in the Light Cone

该研究针对洛伦兹-闵可夫斯基时空中余维二、具平行平均曲率向量场的紧致类空子流形建立积分不等式，推导出刚性结论，完整刻画了光锥内这类子流形必为类空超平面中的全脐球面。

Properly immersed surfaces in hyperbolic 3-manifolds

We study complete finite topology immersed surfaces $Σ$ in complete Riemannian $3$-manifolds $N$ with sectional curvature $K_N\leq -a^2\leq 0$, such that the absolute mean curvature function of $Σ$ is bounded from above by $a$ and its injectivity radius function is not bounded away from zero on each of its annular end representatives. We prove that such a surface $Σ$ must be proper in $N$ and its total curvature must be equal to $2πχ(Σ)$. If $N$ is a hyperbolic $3$-manifold of finite volume and $Σ$ is a properly immersed surface of finite topology with nonnegative constant mean curvature less …

Umbilical-Type Surfaces in Spacetime

A spacelike surface S immersed in a 4-dimensional Lorentzian manifold will be said to be umbilical along a direction N normal to S if the second fundamental form along N is proportional to the first fundamental form of S. In particular, S is pseudo-umbilical if it is umbilical along the mean curvature vector field H, and (totally) umbilical if it is umbilical along all possible normal directions. The possibility that the surface be umbilical along the unique normal direction orthogonal to H --- "ortho-umbilical" surface--- is also considered. I prove that the necessary and sufficient condition…

平面伪测地线与全脐子流形

Planar pseudo-geodesics and totally umbilic submanifolds

本文研究 Riemannian 流形间的全脐等距浸入，通过引入平面伪测地线这一自然外蕴推广，给出了具有平行单位平均曲率向量的全脐等距浸入的新刻画，并证明了余维为一时浸入全脐当且仅当所有测地线的外蕴形状为平面，推广了三维欧氏空间曲面的经典结果。

包含在类光超柱面 $\mathcal{LC}^n\times\mathbb R$ 中的余维二子流形

Codimensions-Two Submanifolds Contained in the Light-like Hypercylinder $\mathcal{LC}^n\times\mathbb R$

本文研究洛伦兹-闵可夫斯基空间中类光超柱面上的类空余维二子流形，通过构造全局标架场分析外蕴不变量，并给出迷向、伪脐及平坦法丛子流形的局部分类定理。

Gap phenomena for a class of fourth-order geometric differential operators on surfaces with boundary

In this paper we establish a gap phenomenon for immersed surfaces with arbitrary codimension, topology and boundaries that satisfy one of a family of systems of fourth-order anisotropic geometric partial differential equations. Examples include Willmore surfaces, stationary solitons for the surface diffusion flow, and biharmonic immersed surfaces in the sense of Chen. On the boundary we enforce either umbilic or flat boundary conditions: that the tracefree second fundamental form and its derivative or the full second fundamental form and its derivative vanish. For the umbilic boundary conditio…

在三维球面与双曲空间中具有圆形边界的常平均曲率圆盘

Constant mean curvature disks with circular boundary in the $3$-sphere and in hyperbolic space

该文证明三维球面或双曲空间中边界为圆的紧致浸入常平均曲率圆盘必为嵌入的全脐圆盘。

Classification results for biharmonic submanifolds in spheres

We classify biharmonic submanifolds with certain geometric properties in Euclidean spheres. For codimension 1, we determine the biharmonic hypersurfaces with at most two distinct principal curvatures and the conformally flat biharmonic hypersurfaces. We obtain some rigidity results for pseudo-umbilical biharmonic submanifolds of codimension 2 and for biharmonic surfaces with parallel mean curvature vector field. We also study the type, in the sense of B-Y. Chen, of compact proper biharmonic submanifolds with constant mean curvature in spheres.

Axiumbilic Singular Points on Surfaces Immersed in R4 and their Generic Bifurcations

Here are described the axiumbilic points that appear in generic one parameter families of surfaces immersed in R4. At these points the ellipse of curvature of the immersion, Little, Garcia - Sotomayor has equal axes. A review is made on the basic preliminaries on axial curvature lines and the associated axiumbilic points which are the singularities of the fields of principal, mean axial lines}, axial crossings and the quartic differential equation defining them. The Lie-Cartan vector field suspension of the quartic differential equation, giving a line field tangent to the Lie-Cartan surface (i…

具有常接触角的平行平均曲率曲面

Parallel mean curvature surfaces with constant contact angle along free boundaries

本文研究了具有常接触角的平行平均曲率曲面，通过分类和构造例子验证了相关定理的准确性。

Minkowski formulae and Alexandrov theorems in spacetime

The classical Minkowski formula is extended to spacelike codimension-two submanifolds in spacetimes which admit "hidden symmetry" from conformal Killing-Yano two-forms. As an application, we obtain an Alexandrov type theorem for spacelike codimension-two submanifolds in a static spherically symmetric spacetime: a codimension-two submanifold with constant normalized null expansion (null mean curvature) must lie in a shear-free (umbilical) null hypersurface. These results are generalized for higher order curvature invariants. In particular, the notion of mixed higher order mean curvature is intr…

渐近锥形高余维自收缩子与自扩张子的唯一性

Uniqueness of Asymptotically Conical Higher Codimension Self-Shrinkers and Self-Expanders

本文证明了渐近于同一锥的高余维自收缩子唯一且以单位重数收敛，并给出自扩张子在此条件下像重合的唯一性结果。

Quartic differentials and harmonic maps in conformal surface geometry

We consider codimension 2 sphere congruences in pseudo-conformal geometry that are harmonic with respect to the conformal structure of an orthogonal surface. We characterise the orthogonal surfaces of such congruences as either $S$-Willmore surfaces, quasi-umbilical surfaces, constant mean curvature surfaces in 3-dimensional space forms or surfaces of constant lightlike mean curvature in 3-dimensional lightcones. We then investigate Bryant's quartic differential in this context and show that generically this is divergence free if and only if the surface under consideration is either superconfo…

Homotopy Groups, Focal Points and Totally Geodesic Immersions

In this paper we consider on a complete Riemannian manifold $M$ an immersed totally geodesic hypersurface $\Si$ existing together with an immersed submanifold $N$ without focal points. No curvature condition is needed. We obtained several connectedness results relating the topologies of $M$ and $\Si$ which depend on the codimension of $N$.

A proof of the multiplicity one conjecture for min-max minimal surfaces in arbitrary codimension

Given any admissible $k$-dimensional family of immersions of a given closed oriented surface into an arbitrary closed Riemannian manifold, we prove that the corresponding min-max width for the area is achieved by a smooth (possibly branched) immersed minimal surface with multiplicity one and Morse index bounded by $k$.

Uniqueness and Pseudolocality Theorems of the Mean Curvature Flow

Mean curvature flow evolves isometrically immersed base manifolds $M$ in the direction of their mean curvatures in an ambient manifold $\bar{M}$. If the base manifold $M$ is compact, the short time existence and uniqueness of the mean curvature flow are well-known. For complete isometrically immersed submanifolds of arbitrary codimensions, the existence and uniqueness are still unsettled even in the Euclidean space. In this paper, we solve the uniqueness problem affirmatively for the mean curvature flow of general codimensions and general ambient manifolds. In the second part of the paper, ins…

任意余维数下曲线的Li-Yau型不等式

Li-Yau type inequality for curves in any codimension

本研究针对任意余维数欧氏空间中的浸入曲线建立Li-Yau型不等式，给出弯曲能下界，证明多数情形下的最优性及奇数重数平面闭曲线的非最优性，采用变分法结合相关分类与定理，还探讨了相关应用。

任意余维数中具有给定平均曲率和接触角的圆盘的存在性

Existence of disks with prescribed mean curvature and contact angle in arbitrary codimension

本文在任意余维数下建立极小极大理论，证明在适当假设下存在具有给定平均曲率和接触角的非恒定分支浸入圆盘或球面，并在欧氏空间中给出自由边界圆盘及维数相关的莫尔斯指标界。

黎曼流形中具有常加权平均曲率的曲面的Hopf型定理

Hopf type theorem for surfaces of constant weighted mean curvature in Riemannian manifolds

该研究针对三维光滑度量测度空间中的亏格零紧致浸入曲面，引入新的加权平均曲率，证明满足特定条件时该类曲面若加权平均曲率为常数则必为全脐曲面，并将结果应用于对偶Christoffel-Minkowski问题的唯一性研究。

On skew loops, skew branes and quadratic hypersurfaces

A skew brane is an immersed codimension 2 submanifold in affine space, free from pairs of parallel tangent spaces. Using Morse theory, we prove that a skew brane cannot lie on a quadratic hypersurface. We also prove that there are no skew loops on embedded ruled developable discs in 3-space. The paper extends recent work by M. Ghomi and B. Solomon.

Surface diffusion flow near spheres

We consider closed immersed hypersurfaces evolving by surface diffusion flow, and perform an analysis based on local and global integral estimates. First we show that a properly immersed stationary (ΔH \equiv 0) hypersurface in \R^3 or \R^4 with restricted growth of the curvature at infinity and small total tracefree curvature must be an embedded union of umbilic hypersurfaces. Then we prove for surfaces that if the L^2 norm of the tracefree curvature is globally initially small it is monotonic nonincreasing along the flow. We also derive pointwise estimates for all derivatives of the curvatur…

整体图的余维二平均曲率流

Codimension two mean curvature flow of entire graphs

研究整体图的余维二平均曲率流，通过推广Ecker-Huisken最大原理，证明一致面积递减映射的图性与面积递减性保持，并证明渐近锥形初始图归一化流收敛至自展开子流形。

极小极大理论与任意余维数H球面的存在性

Min-max theory and existence of H-spheres with arbitrary codimensions

本文通过极小极大理论，在闭黎曼流形中证明了具有指定平均曲率、受控莫尔斯指标和任意余维数的分支浸入2-球面的存在性，并在特定曲率条件下推广至所有平均曲率选择，进而构造平行平均曲率球面并部分解决同伦问题。

Ancient solutions of geometric flows with curvature pinching

We prove rigidity theorems for ancient solutions of geometric flows of immersed submanifolds. Specifically, we find pinching conditions on the second fundamental form that characterize the shrinking sphere among compact ancient solutions for the mean curvature flow in codimension greater than one, and for some nonlinear curvature flows of hypersurfaces.

Gaussian Harmonic Forms and Two-Dimensional Self-Shrinkers

We consider 2-dimensional orientable self-shrinkers $Σ$ for the Mean Curvature Flow of polynomial volume growth immersed in $\mathbb R^n$. We look at closed one forms minimizing the norm $\int_Σ\eterm |ω|^2$ in their cohomology class. Any closed form satisfying the Euler-Lagrange equation for this minimization will be called a Gaussian Harmonic one Form (GHF). We then use these forms to show that if such a $Σ$ has genus $\geq 1,$ then we have a lower bound on the supremum norm of $A^2$. GHF's may also be applied to create an upperbound for the lowest eigenvalue of the operator $L$. In the codi…

4维共形不变曲率能量的临界点的结构方程

Structural Equations for Critical Points of Conformally Invariant Curvature Energies in 4d

本文研究4维流形浸入欧氏空间中共形不变曲率能量的临界点，利用不变性和诺特定理将欧拉-拉格朗日方程化为有利结构，推广Rivière的二维方法至四维。

Rotational Surfaces in S^3 with constant mean curvature

Very recently Ben Andrews and Haizhong Li showed that every embedded cmc torus in the three dimensional sphere is axially symmetric. There is a two-parametric family of axially symmetric cmc surfaces; more precisely, for every real number H and every C > 2 (H+\sqrt{1+H^2}) there is an axially symmetry surface Σ_{H,C} with mean curvature H. In 2010, Perdomo showed that for every H between cot(π/m) and (m^2-2)/(2(m^2-1)^1/2), there exists an embedded axially symmetric example with non constant principal curvatures that is invariant under the ciclic group Z_m. Andrews and Li, showed that these ex…

Variational Principles for immersed Surfaces with $L^2$-bounded Second Fundamental Form

In this work we present new fundamental tools for studying the variations of the Willmore functional of immersed surfaces into $R^m$. This approach gives for instance a new proof of the existence of a Willmore minimizing embedding of an arbitrary closed surface in arbitrary codimension. We explain how the same approach can solve constraint minimization problems for the Willmore functional. We show in particular that, for a given closed surface and a given conformal class for this surface, there is an immersion in $R^m$, away possibly from isolated branched points, which minimizes the Willmore …

Surfaces with parallel mean curvature in $\mathbb{C}P^n\times\mathbb{R}$ and $\mathbb{C}H^n\times\mathbb{R}$

We consider surfaces with parallel mean curvature vector (pmc surfaces) in $\mathbb{C}P^n\times\mathbb{R}$ and $\mathbb{C}H^n\times\mathbb{R}$, and, more generally, in cosymplectic space forms. We introduce a holomorphic quadratic differential on such surfaces. This is then used in order to show that the anti-invariant pmc $2$-spheres of a $5$-dimensional non-flat cosymplectic space form of product type are actually the embedded rotational spheres $S_H^2\subset\bar M^2\times\mathbb{R}$ of Hsiang and Pedrosa, where $\bar M^2$ is a complete simply-connected surface with constant curvature. When …

具有常长度曲率法向的高秩子流形

Submanifolds of higher rank with curvature normals of constant length

本文研究高秩欧几里得子流形，其曲率法向长度恒定，证明其由超曲面经平行流形生成，并推广了曲率齐性结果。

$W^{2,2}$-conformal immersions of a closed Riemann surface into $\R^n$

We study sequences $f_k:Σ_k \to \R^n$ of conformally immersed, compact Riemann surfaces with fixed genus and Willmore energy ${\cal W}(f) \leq Λ$. Assume that $Σ_k$ converges to $Σ$ in moduli space, i.e. $ϕ_k^\ast(Σ_k) \to Σ$ as complex structures for diffeomorphisms $ϕ_k$. Then we construct a branched conformal immersion $f:Σ\to \R^n$ and Möbius transformations $σ_k$, such that for a subsequence $σ_k \circ f_k \circ ϕ_k \to f$ weakly in $W^{2,2}_{loc}$ away from finitely many points. For $Λ< 8π$ the map $f$ is unbranched. If the $Σ_k$ diverge in moduli space, then we show $\liminf_{k \to \inf…

Compactification of the space of Hamiltonian stationary Lagrangian submanifolds with bounded total extrinsic curvature and volume

For a sequence of immersed connected closed Hamiltonian stationary Lagrangian submaniolds in $\mathbb{C}^{n}$ with uniform bounds on their volumes and the total extrinsic curvatures, we prove that a subsequence converges either to a point or to a Hamiltonian stationary Lagrangian $n$-varifold locally uniformly in $C^{k}$ for any nonnegative integer $k$ away from a finite set of points, and the limit is Hamiltonian stationary in ${\mathbb{C}}^{n}$. We also obtain a theorem on extending Hamiltonian stationary Lagrangian submanifolds $L$ across a compact set $N$ of Hausdorff codimension at least …

Variational characterizations of $ξ$-submanifolds in the Eulicdean space $\bbr^{m+p}$

$ξ$-submanifold in the Euclidean space $\bbr^{m+p}$ is a natural extension of the concept of self-shrinker to the mean curvature flow in $\bbr^{m+p}$. It is also a generalization of the $λ$-hypersurface defined by Q.-M. Cheng et al to arbitrary codimensions. In this paper, some characterizations for $ξ$-submanifolds are established. First, it is shown that a submanifold in $\bbr^{m+p}$ is a $ξ$-submanifold if and only if its modified mean curvature is parallel when viewed as a submanifold in the Gaussian space $(\bbr^{m+p},e^{-\fr{|x|^2}{m}}\lagl\cdot,\cdot\ragl)$; Then, two weighted volume fu…

双调和共形浸入到三维反德西特空间：刚性、局部存在性和抛物旋转族

Making Surfaces Biharmonic by Conformal Reparametrization in Anti-de Sitter Three-Space

研究非退化曲面到三维反德西特空间的双调和共形浸入，通过特定符号约定和相关量表示双调和方程，证明类空曲面相关性质，推导解析系统并证明局部存在性，给出零坐标重构及具体初始数据，记录类时抛物约化。