New examples of Willmore submanifolds in the unit sphere via isoparametric functions

An isometric immersion ${x:M^n\rightarrow S^{n+p}}$ is called Willmore if it is an extremal submanifold of the Willmore functional: ${W(x)=\int\nolimits_{M^n} (S-nH^2)^{\frac{n}{2}}dv}$ , where S is the norm square of the second fundamental form and H is the mean curvature. Examples of Willmore submanifolds in the unit sphere are scarce in the literature. This article gives a series of new examples of Willmore submanifolds in the unit sphere via isoparametric functions of FKM-type.     

Willmore submanifolds in the unit sphere via isoparametric functions

In this paper, we show that both focal submanifolds of each isoparametric hypersurface in the sphere with six distinct principal curvatures are Willmore, hence all focal submanifolds of isoparametric hypersurfaces in the sphere are Willmore.     

On the Blaschke isoparametric hypersurfaces in the unit sphere

Given an immersed submanifold x ： M^M → S^n in the unit sphere S^n without umbilics, a Blaschke eigenvalue of x is by definition an eigenvalue of the Blaschke tensor of x. x is called Blaschke isoparametric if its Mobius form vanishes identically and all of its Blaschke eigenvalues are constant. Then the classification of Blaschke isoparametric hypersurfaces is natural and interesting in the MSbius geometry of submanifolds. When n = 4, the corresponding classification theorem was given by the authors. In this paper, we are able to complete the corresponding classification for n = 5. In particular, we shall prove that all the Blaschke isoparametric hypersurfaces in S^5 with more than two distinct Blaschke eigenvalues are necessarily Mobius isoparametric.     

单位球面S~6中的Blaschke等参超曲面

单位球面中的一个无脐点浸入子流形称为Blaschke等参子流形如果它的Mbius形式恒为零并且所有的Blaschke特征值均为常数.维数m4的Blaschke等参超曲面已经有了完全的分类.截止目前,Mbius等参超曲面的所有已知例子都是Blaschke等参的.另一方面,确实存在许多不是Mbius等参的Blaschke等参超曲面,它们都具有不超过两个的不同Blaschke特征值.在已有分类定理的基础上,本文对于5维Blaschke等参超曲面进行了完全的分类.特别地,我们证明了S6中具有多于两个不同Blaschke特征值的Blaschke等参超曲面一定是Mbius等参的,给出了此前一个问题的部分解答.     

Rigidity of compact minimal submanifolds in a unit sphere

LetM be ann-dimensional compact minimal submanifold of a unit sphereS ^n+p (p2); and letS be a square of the length of the second fundamental form. IfS2/3n everywhere onM, thenM must be totally geodesic or a Veronese surface.     

Equivariant harmonic maps into the sphere via isoparametric maps

By using concrete isoparametric maps we obtain some new equivariant harmonic maps between spheres and solve equivariant boundary value problems for harmonic maps from unit open ballB ^m+1 intoS ⁿ. Research partially supported by NNSFC, SFECC and ICTP.     

Classification of irreducible isoparametric submanifolds in CPn

The classification theorem of the isoparametric submanifolds in Pn is obtained and geometry properties of them are discussed.     

Classification of irreducible isoparametric submanifolds in CP n

The classification theorem of the isoparametric submanifolds in CPⁿ is obtained and geometry properties of them are discussed.     

Solving the heat equation on the unit sphere via Laplace transforms and radial basis functions

We propose a method to construct numerical solutions of parabolic equations on the unit sphere. The time discretization uses Laplace transforms and quadrature. The spatial approximation of the solution employs radial basis functions restricted to the sphere. The method allows us to construct high accuracy numerical solutions in parallel. We establish L ₂ error estimates for smooth and nonsmooth initial data, and describe some numerical experiments.     

Polynomial interpolation on the unit sphere II

The problem of interpolation at (n+1)² points on the unit sphere by spherical polynomials of degree at most n is proved to have a unique solution for several sets of points. The points are located on a number of circles on the sphere with even number of points on each circle. The proof is based on a method of factorization of polynomials. Dedicated to Mariano Gasca on the occasion of his 60th birthday The second author was supported by the Graduate Program Applied Algorithmic Mathematics of the Munich University of Technology. The work of the third author was supported in part by the National Science Foundation under Grant DMS-0201669.     

The Minkowski norm and Hessian isometry induced by an isoparametric foliation on the unit sphere

Science China Mathematics - Let Mt be an isoparametric foliation on the unit sphere (Sn?1(1), gst) with d principal curvatures. Using the spherical coordinates induced by Mt, we construct a...     

On spectral characterizations of Willmore hypersurfaces in a sphere

Let M be a closed Willmore hypersurface in the sphere S^n＋1（1） （n ≥ 2） with the same mean curvature of the Willmore torus Wm,n-m, if SpecP（M） = Spec^P（Wm,n-m ） （p = 0, 1,2）, then M is Wm,n-m.     

Homogeneous structures and rigidity of isoparametric submanifolds in Hilbert space

We study isoparametric submanifolds of rank at least two in a separable Hilbert space, which are known to be homogeneous by the main result in [E. Heintze and X. Liu, Ann. of Math. (2), 149 (1999), 149?C181], and with such a submanifold M and a point x in M we associate a canonical homogeneous structure ?? _x (a certain bilinear map defined on a subspace of T _x M × T _x M). We prove that ?? _x , together with the second fundamental form ?? _x , encodes all the information about M, and we deduce from this the rigidity result that M is completely determined by ?? _x and (????) _x , thereby making such submanifolds accessible to classification. As an essential step, we show that the one-parameter groups of isometries constructed in [E. Heintze and X. Liu, Ann. of Math. (2), 149 (1999), 149?C181] to prove their homogeneity induce smooth and hence everywhere defined Killing fields, implying the continuity of ?? (this result also seems to close a gap in [U. Christ, J. Differential Geom., 62 (2002), 1?C15]). Here an important tool is the introduction of affine root systems of isoparametric submanifolds.     

Flatness of CR submanifolds in a sphere

In Euclidean geometry, for a real submanifold M in E n+a , M is a piece of E n if and only if its second fundamental form is identically zero. In projective geometry, for a complex submanifold M in CP n+a , M is a piece of CP n if and only if its projective second fundamental form is identically zero. In CR geometry, we prove the CR analogue of this fact in this paper.     

A characterization of Moebius isoparametric hypersurfaces of the sphere

We consider a proper, umbilic-free immersion of an n-dimensional manifold M in the sphere S ⁿ⁺¹. We show that M is a Moebius isoparametric hypersurface if, and only if, it is a cyclide of Dupin or a Dupin hypersurface with constant Moebius curvature.     

Willmore submanifolds of the Möbius space and a Bernstein-type theorem

We study Willmore immersed submanifoldsf: M ^m →S ⁿ into then-Möbius space, withm≥2, as critical points of a conformally invariant functionalW. We compute the Euler-Lagrange equation and relate this functional with another one applied to the conformal Gauss map of immersions intoS ⁿ . We solve a Bernestein-type problem for compact Willmore hypersurfaces ofS ⁿ , namely, if ?a ∈? ⁿ⁺² such that <γf, a > ≠ 0 onM, whereγ _f is the hyperbolic conformal Gauss map and <, > is the Lorentz inner product of? ⁿ⁺², and iff satisfies an additional condition, thenf(M) is an (n?1)-sphere.     

A characterization of Moebius isoparametric hypersurfaces of the sphere

We consider a proper, umbilic-free immersion of an n-dimensional manifold M in the sphere S ⁿ⁺¹. We show that M is a Moebius isoparametric hypersurface if, and only if, it is a cyclide of Dupin or a Dupin hypersurface with constant Moebius curvature.