高维旋转曲面

本文考虑欧氏空间中一种余一维的高维旋转曲面,通过发展出一种全新的复合映射、维数分解与分块矩阵递推法,我们系统性地研究了同它的面积和曲率有关的一系列问题.当母函数是多元函数时,这种高维旋转曲面的概念尚属首次提出.我们给出了这种高维旋转曲面的面积公式以及它的一些简单应用.我们发现:在任一直径方向上,单位球面的面积分布和低一维单位球体的体积分布完全相同,并且当维数趋于无穷时它们的密度函数的极限都是狄拉克函数.通过研究相应面积泛函的变分问题,我们得到了所谓的极小旋转曲面方程.我们证明了:满足极小旋转曲面方程的母函数对应的旋转曲面的平均曲率等于零.这种极小旋转曲面方程推广了传统的极小曲面方程,并且为非参数极小曲面理论提供了新的更一般的研究框架;通过计算径向对称解对应的常微分方程,我们研究了它的一些简单的特解.我们也简单讨论了相应的预定平均曲率和预定高斯曲率问题.     

高斯曲率估计与常数平均曲率超曲面的稳定性

本文利用活动标架法及Ｌａｐｌａｃｉａｎ特征值方法研究了常数平均曲率超曲面的稳定性．给出了三维拟常曲率流形中具有常数平均曲率超曲面的共形度量的高斯曲率之上界估计．证明了三维拟常曲率流形中具有常数平均曲率超曲面上一个单连通有界区域为稳定的充分条件．     

具有平行平均曲率向量子流形的曲率估计与稳定性

本文估计空间形式中具有平行平均曲率向量子流形上共形度量的数量曲率上界,并利用其研究了具有常平均曲率超曲面的稳定性．     

宇宙时空中具有给定平均曲率的类空超曲面

本文研究了宇宙时空中类空超曲面的推广平均曲率流,通过估计发展超曲面的高度函数、梯度函数和曲率函数等几何量,得到了一类极限类空超曲面,其平均曲率等于给定函数.     

Lorentz-Minkowski空间中给定主曲率函数的旋转超曲面

给出(n,1)型orentz-Minkowski空间中给定主曲率函数的旋转类空超曲面的位置向量场,通过计算超曲面的主曲率,证明了这类超曲面的存在性.     

由仿射平均曲率支配的超曲面的发展

研究由仿射平均曲率支配的严格凸超曲面的发展运动.在假定仿射平均曲率流存在并且曲面保持严格凸的条件下,通过对曲面支撑函数的计算,给出了高斯曲率的发展方程.     

H~3(c)中常平均曲率曲面的稳定性

设M是3维双曲空间H~3(c)中常平均曲率曲面,本文通过估计M上共形度量的Gauss曲率,研究了其稳定性.     

H~3(c)中常平均曲率曲面的稳定性

设M是3维双曲空间H~3(c)中常平均曲率曲面,本文通过估计M上共形度量的Gauss曲率,研究了其稳定性.     

具有共形广义径向场的球对称Finsler流形

共形向量场在Finsler几何的研究特别是共形航海问题中起着重要的作用.本文通过建立和求解相应的偏微分方程,完全确定以广义径向场为共形向量场的所有球对称Finsler度量.这些度量包含常截面曲率的Riemann度量.     

共形空间中有两个共形主曲率的共形等参超曲面

研究了共形空间中正则超曲面的共形几何,并在共形等价意义下对有两个共形主曲率的共形等参超曲面作了分类.     

Evolution equations in Riemannian geometry

A fundamental question in Riemannian geometry is to find canonical metrics on a given smooth manifold. In the 1980s, R.S. Hamilton proposed an approach to this question based on parabolic partial differential equations. The goal is to start from a given initial metric and deform it to a canonical metric by means of an evolution equation. There are various natural evolution equations for Riemannian metrics, including the Ricci flow and the conformal Yamabe flow. In this survey, we discuss the global behavior of the solutions to these equations. In particular, we describe how these techniques can be used to prove the Differentiable Sphere Theorem.     

Conformally invariant differential equations and projective geometry

We combine harmonic analysis on certain pseudo-Riemannian symmetric spaces with results on conformally invariant linear and non-linear differential equations. This gives in many cases part of the decomposition of certain representations of the conformal group of a manifold when restricted to the isometry group.     

Hyperbolic surfaces in the Grassmannian

In this article we study real 2-dimensional surfaces in the Grassmannian of 2-planes in a 4-dimensional vector space. These surfaces occur naturally as the fibers of jet bundles of partial differential equations.On the Grassmannian there is an invariant conformal quadratic form and we will use the structure induced by this quadratic form to study the surfaces. We give a topological classification of compact hyperbolic surfaces similar to the classification by Gluck and Warner [Duke Math. J. 50 (1) (1983)] of compact elliptic surfaces. In contrast with elliptic surfaces there are several topological possibilities for hyperbolic surfaces. We make a calculation of the differential invariants under the action of the group of conformal isometries. Finally, we analyze a class of surfaces called geometrically flat and show that within this class there exist many examples of non-trivial compact surfaces.     

Nonuniqueness for Willmore Surfaces of Revolution Satisfying Dirichlet Boundary Data

In this note Willmore surfaces of revolution with Dirichlet boundary conditions are considered. We show two nonuniqueness results by reformulating the problem in the hyperbolic half plane and solving a suitable initial value problem for the corresponding elastic curves. The behavior of such elastic curves is examined by a method provided by Langer and Singer to reduce the order of the underlying ordinary differential equation. This ensures that these solutions differ from solutions already obtained by Dall’Acqua, Deckelnick and Grunau. We will additionally provide a Bernstein-type result concerning the profile curve of a Willmore surface of revolution. If this curve is a graph on the whole real numbers it has to be a Möbius transformed catenary. We show this by a corollary of the above-mentioned method by Langer and Singer.     

Reduction of the n-dimensional static vacuum Einstein equation and generalized Schwarzschild solutions

We consider the static vacuum Einstein spacetime when the spatial factor is conformal to a n-dimensional pseudo-Euclidean space. The most general ansatz that reduces the resulting system of partial differential equations to a system of ordinary differential equations is completely described. We obtain the entire set of solutions of the reduced system, where the classical Schwarzschild solution arises as a particular solution. In addition, we show that the Riemannian spatial factors associated to these solutions are foliated by parallel hypersurfaces of constant mean curvature.     

On Differential Equations with Nonlocal Switch Functionals

Systems of ordinary differential equations along with nonlocal functionals are considered. These functionals allow us to watch qualitative characteristics of solutions. It is proved that if assumptions are natural, then the switch moment can be found by means of finite difference approximations of differential equations. The results obtained are used to modify the system of equations describing surface waves of an ideal liquid in conformal variables.     

CR-hypersurfaces of a conformal Kenmotsu manifold satisfying certain shape operator conditions

In this paper, conformal Kenmotsu manifolds are introduced. We consider CR-hypersurfaces of a conformal Kenmotsu manifold whose shape operator is parallel, scalar, recurrent or Lie \( \xi \)-parallel. It is proved that if the Lee vector field of a conformal Kenmotsu manifold is tangent and normal to these type of CR-hypersurfaces then the CR-hypersurfaces are totally geodesic and totally umbilic, respectively. An example of a three-dimensional conformal Kenmotsu manifold is constructed for illustration that is not Kenmotsu.     

Large deformations of bodies of revolution made of elastic homogeneous and fiber-reinforced materials 1. Torsion of toroidal bodies

Equations of a mathematical model for bodies of revolution made of elastic homogeneous and fiber-reinforced materials and subjected to large deformations are presented. The volume content of reinforcing fibers is assumed low, and their interaction through the matrix is neglected. The axial lines of the fibers can lie both on surfaces of revolution whose symmetry axes coincide with the axis of the body of revolution and along trajectories directed outside the surfaces. The equations are obtained for the macroscopically axisymmetric problem statement where the parameters of macroscopic deformation of the body vary in its meridional planes, but are constant in the circumferential directions orthogonal to them. The equations also describe the torsion of bodies of revolution and their deformation behavior under the action of inertia forces in rotation around the symmetry axis. The results of a numerical investigation into the large deformations of toroidal bodies made of elastic homogeneous and unidirectionally reinforced materials under torsion caused by a relative rotation of their butt-end sections around the symmetry axis are presented.