The Busemann-Petty problem in hyperbolic and spherical spaces

The Busemann-Petty problem asks whether origin-symmetric convex bodies in Rⁿ with smaller central hyperplane sections necessarily have smaller n-dimensional volume. It is known that the answer to this problem is affirmative if n?4 and negative if n?5. We study this problem in hyperbolic and spherical spaces.     

Eigenvalue estimates for minimal hypersurfaces in hyperbolic space

Recently Candel [A. Candel, Eigenvalue estimates for minimal surfaces in hyperbolic space, Trans. Amer. Math. Soc. 359 (2007) 3567-3575] proved that if M is a simply-connected stable minimal surface isometrically immersed in H³, then the first eigenvalue of M satisfies 1/4?λ(M)?4/3 and he asked whether the bound is sharp and gave an example such that the lower bound is attained. In this note, we prove that the upper bound can never be attained. Also we extend the result by proving that if M is compact stable minimal hypersurface isometrically immersed in Hn+1 where n?3 such that its smooth Yamabe invariant is negative, then (n−1)/4?λ(M)?n²(n−2)/(7n−6).     

Antipodal convex hypersurfaces

Motivated by a conjecture of Steinhaus, we consider the mapping F, associating to each point x of a convex hypersurface the set of all points at maximal intrinsic distance from x. We first provide two large classes of hypersurfaces with the mapping F single-valued and involutive. Afterwards we show that a convex body is smooth and has constant width if its double has the above properties of F, and we prove a partial converse to this result. Additional conditions are given, to characterize the (doubly covered) balls.     

Complete minimal hypersurfaces in complex hyperbolic space

We construct new examples of embedded, complete, minimal hypersurfaces in complex hyperbolic space, including deformations of bisectors and some minimal foliations. Received: 20 March 2000 / Revised version: 21 July 2000     

Complete hypersurfaces with constant mean curvature and finite L p norm curvature in Euclidean spaces

In this paper, we consider complete hypersurfaces in R ⁿ⁺¹ with constant mean curvature H and prove that M ⁿ is a hyperplane if the L ² norm curvature of M ⁿ satisfies some growth condition and M ⁿ is stable. It is an improvement of a theorem proved by H. Alencar and M. do Carmo in 1994. In addition, we obtain that M ⁿ is a hyperplane (or a round sphere) under the condition that M ⁿ is strongly stable (or weakly stable) and has some finite L ^p norm curvature. Received: 14 July 2007     

Antipodality in hyperbolic space

An antipodal set in Euclidean n-space is a set of points with the property that through any two of them there is a pair of parallel hyperplanes supporting the set. In this paper we discuss the various possible ways to translate this notion to hyperbolic space and find the maximal cardinality of a hyperbolic antipodal set (according to the different definitions). The first two authors were partially supported by the Hung. Nat. Sci. Found. (OTKA), grant no. T043556 and T037752 and the first author was partially supported also by a Natural Sciences and Engineering Research Council of Canada Discovery Grant.     

Canonization theorems for finite affine and linear spaces

In this paper we prove a canonical (i.e. unrestricted) version of the Graham—Leeb—Rothschild partition theorem for finite affine and linear spaces [3]. We also mention some other kind of canonization results for finite affine and linear spaces.     

Multiplicities of Dupin hypersurfaces

All known examples of isoparametric hypersurfaces of spheres are of the following two types:?(1) The homogeneous examples, which are principal orbits of isotropy representations of symmetric spaces of rank 2.?(2) The Clifford examples of Ferus–Karcher–Münzner, which are constructed from modules over Clifford algebras.?Our main result, which extends work of Münzner, Thorbergsson and Grove–Halperin, is that any isoparametric hypersurface has the same number of principal curvatures and the same multiplicities as some hypersurface of type (1) or (2). More generally, this holds for any Dupin hypersurface. The proof is homotopy theoretic; the main tools are the Hopf invariant and the EHP–sequence. Oblatum 19-III-1997 & 9-III-1999 / Published online: 5 August 1999     

A finiteness theorem for isoparametric hypersurfaces

In this note we prove that for eachn there are only finitely many diffeomorphism classes of compact isoparametric hypersurfaces ofS ⁿ⁺¹ with four distinct principal curvatures.     

A Ricci inequality for hypersurfaces in the sphere

Let Mⁿ be a complete Riemannian manifold immersed isometrically in the unity Euclidean sphere In [9], B. Smyth proved that if Mⁿ, n ≧ 3, has sectional curvature K and Ricci curvature Ric, with inf K > −∞, then sup Ric ≧ (n − 2) unless the universal covering of Mⁿ is homeomorphic to Rn or homeomorphic to an odd-dimensional sphere. In this paper, we improve the result of Smyth. Moreover, we obtain the classification of complete hypersurfaces of with nonnegative sectional curvature.Received: 11 November 2003     

Integral geometry and geometric inequalities in hyperbolic space

Using results from integral geometry, we find inequalities involving mean curvature integrals of convex hypersurfaces in hyperbolic space. Such inequalities generalize the Minkowski formulas for euclidean convex sets.     

Rigidity theorems for complete spacelike hypersurfaces with constant scalar curvature in De Sitter space

In this paper we give a partially affirmative answer to the following question posed by Haizhong Li: is a complete spacelike hypersurface in De Sitter space , n?3, with constant normalized scalar curvature R satisfying totally umbilical?     

A new expression for the density of totally geodesic submanifolds in space forms, with stereological applications

Integral section formulae for totally geodesic submanifolds (planes) intersecting a compact submanifold in a space form are available from appropriate representations of the motion invariant density (measure) of these planes. Here we present a new decomposition of the invariant density of planes in space forms. We apply the new decomposition to rewrite Santaló's sectioning formula and thereby to obtain new mean values for lines meeting a convex body. In particular we extend to space forms a recently published stereological formula valid for isotropic plane sections through a fixed point of a convex body in R³.     

Diametric theorems in sequence spaces

We determine in almost all Manhattan lattices configurations, which for specified diameter have maximal cardinality. Cases, in which those configurations are spheres, have been studied recently by Kleitman and Fellows.For Hamming spaces we present a partial result supplementing a result of Frankl and Füredi and we formulate a general conjecture.     

Contact hypersurfaces of Kaehler manifolds

Contact hypersurfaces of a Kaehler manifold have been characterized and classified, assuming the second fundamental form to be Codazzi (in particular, parallel). We have also discussed the special cases when the ambient space is a (i) Calabi-Yau manifold and (ii) a complex space-form.     

Farkas-type theorems for positively homogeneous systems in ordered topological vector spaces

In this paper, we present versions of the Farkas Lemma and the Gale Lemma for a semi-infinite system involving positively homogeneous functions in a topological vector space. In particular, we present two such versions for a semi-infinite system containing min-type functions. Our main theoretical tool is abstract convexity.