A Riemannian version of Korn's inequality

We prove a Korn type inequality for vector fields on a Riemann manifold. This inequality includes the special cases proved in the literature for domains in . If the domain is convex, we can considerably weaken the needed assumption on the boundary values. Received: 26 January 2001 / Accepted: 11 May 2001 / Published online: 19 October 2001     

Upper critical field for superconductors with edges and corners

The effect of non-smoothness of sample surfaces on the value of the upper critical field and on the location of superconductivity nucleation is discussed. It is shown that, superconducting samples with edges and corners have higher value of comparing to samples with smooth surfaces. Superconductivity nucleates first at the top and bottom edges in a cylinder with a finite height, and nucleates first at vertices in a cuboid. Received: 10 September 2000 / Accepted: 11 May 2001 / Published online: 19 October 2001     

Semicontinuity of a functional depending on curvature

In this paper we prove a lower semicontinuity result for a functional , defined on a class of bounded subsets of with a piecewise boundary, with respect to the -convergence of the sets. The functional depends on the curvature of in a linear way and contains a penalizing term which prevents the appearance of thin sets in the symmetric difference , where in an -approximating sequence of . Received: 3 January 2001 / Accepted: 11 May 2001 / Published online: 19 October 2001     

Positive semidefinite germs in real analytic surfaces

We find all real analytic surface germs in on which every positive semidefinite function germ is a sum of squares (in fact, of two squares) of analytic function germs. Received: 9 January 2001 / Published online: 24 September 2001     

Coleman integration using the Tannakian formalism

We use a new idea to construct a theory of iterated Coleman functions on overconvergent spaces with good reduction in any dimension. A Coleman function in this theory consists of a unipotent differential equation, a functional on the underlying bundle and a solution to the equation on a residue class. The new idea is to use the theory of Tannakian categories and the action of Frobenius to analytically continue solutions of the differential equation to all residue classes. Received: 22 November 2000 / Revised version: 28 March 2001 / Published online: 24 September 2001     

On uniqueness of vector-valued minimizers of the Ginzburg-Landau functional in annular domains

Consider a class of Sobolev functions satisfying a prescribed degree condition on the boundary of a planar annular domain. It is shown that, within this class, the Ginzburg-Landau functional possesses the unique, radially symmetric minimizer, provided that the annulus is sufficiently narrow. This result is known to be false for wide annuli where vortices are energetically feasible. The estimate for the critical radius below which the uniqueness of the minimizer is guaranteed is obtained as well. Received: 19 July 2000 / Accepted: 23 February 2001/ Published online: 23 July 2001     

On an area-preserving crystalline motion

The asymptotic behavior of solutions to an area-preserving crystalline motion is investigated in this paper. In this equation, the area enclosed by the solution polygon is preserved, while its circumference keeps on shrinking. By geometric consideration, establishing several isoperimetric inequalities and using the theory of dynamical systems, we show that the asymptotic shape of a solution polygon is the boundary of the Wulff shape. Received: 23 February 2000 / Accepted: 23 January 2001 / Published online: 23 April 2001     

Strong versus weak local minimizers for the perturbed Dirichlet functional

Let be a bounded domain and . In this paper we consider functionals of the form where the admissible function belongs to the Sobolev space of vector-valued functions and is such that the integral on the right is well defined. We state and prove a sufficiency theorem for local minimizers of where . The exponent is shown to depend on the dimension and the growth condition of and an exact expression is presented for this dependence. We discuss some examples and applications of this theorem. Received: 20 July 2000 / Accepted: 7 June 2001 / Published online: 19 October 2001     

Arithmetic discriminants and morphisms of curves

This paper deals with upper bounds on arithmetic discriminants of algebraic points on curves over number fields. It is shown, via a result of Zhang, that the arithmetic discriminants of algebraic points that are not pull-backs of rational points on the projective line are smaller than the arithmetic discriminants of families of linearly equivalent algebraic points. It is also shown that bounds on the arithmetic discriminant yield information about how the fields of definition and differ when is an algebraic point on a curve and is a nonconstant morphism of curves. In particular, it is demonstrated that , with at most finitely many exceptions, whenever the degrees of and are sufficiently small, relative to the difference between the genera and . The paper concludes with a detailed analysis of the arithmetic discriminants of quadratic points on bi-elliptic curves of genus 2.

     

Concentration of low energy extremals: Identification of concentration points

We study the variational problem where , is a bounded domain, , F satisfies $0\leq F|t|\leq \alpha |t|^{2^*}$ and is upper semicontinuous. We show that to second order in the value only depends on two ingredients. The geometry of enters through the Robin function (the regular part of the Green's function) and F enters through a quantity which is computed from (radial) maximizers of the problem in . The asymptotic expansion becomes Using this we deduce that a subsequence of (almost) maximizers of must concentrate at a harmonic center of : i.e., , where is a minimum point of . Received: 24 January 2001 / Accepted: 11 May 2001 / Published online: 19 October 2001     

Cohomologies of a double covering of a non-singular algebraic 3-fold

In this paper we study the Hodge numbers of a branched double covering of a smooth, complete algebraic threefold. The involution on the double covering gives a splitting of the Hodge groups into symmetric and skew-symmetric parts. Since the symmetric part is naturally isomorphic to the corresponding Hodge group of the base we study only the skew-symmetric parts and prove that in many cases it can be computed explicitly. Received: 6 March 2001 / in final form: 4 September 2001/ Published online: 4 April 2002     

Note on the spectrum of the Hodge-Laplacian for k-forms on minimal Legendre submanifolds in S^{2n+1}

Given a minimal Legendre immersion L in and we prove that is an eigenvalue of the Hodge-Laplacian acting on k and (k-1)-forms on L. In particular we show that the eigenspaces Eig and Eig are at least of dimension Received: 10 February 2000 / Accepted: 23 January 2001 / Published online: 4 May 2001     

Hypersurfaces of prescribed Gauß curvature in exterior domains

We prove an existence theorem for convex hypersurfaces of prescribed Gau? curvature in the complement of a compact set in Euclidean space which are close to a cone. Received: 23 February 2001 / Accepted: 11 May 2001 / Published online: 19 October 2001     

A matrix-valued Choquet-Deny theorem

Let be a positive matrix-valued measure on a locally compact abelian group such that is the identity matrix. We give a necessary and sufficient condition on for the absence of a bounded non-constant matrix-valued function on satisfying the convolution equation . This extends Choquet and Deny's theorem for real-valued functions on .

     

Equicontinuity of families of plurisubharmonic functions with bounds on their Monge-Ampère masses

It is proved that families of solutions of the complex Monge-Ampère equation with fixed boundary values and certain bounds on the right hand side are equicontinuous. Received: 24 April 2001; in final form: 10 August 2001 / Published online: 28 February 2002     

A Bernstein theorem for special Lagrangian graphs

We obtain a Bernstein theorem for special Lagrangian graphs in for arbitrary n only assuming bounded slope but no quantitative restriction. Received: 18 January 2001 / Accepted: 7 June 2001 / Published online: 12 October 2001 The second-named author is grateful to the Max Planck Institute for Mathematics in the Sciences in Leipzig for its hospitality and support and also 973 project in China.     

Topological horseshoes

When does a continuous map have chaotic dynamics in a set ? More specifically, when does it factor over a shift on symbols? This paper is an attempt to clarify some of the issues when there is no hyperbolicity assumed. We find that the key is to define a ``crossing number' for that set . If that number is and 1$">, then contains a compact invariant set which factors over a shift on symbols.

     

Determinacy of smooth germs with real isolated line singularities

The germ of a smooth real-valued function on Euclidean space is called a real isolated line singularity if its singular set is a nonsingular curve, its Jacobian ideal is ojasiewicz at the singular set, and its Hessian determinant restricted to the singular set is ojasiewicz at 0. Consider the set of all germs whose singular set contains a fixed nonsingular curve . We prove that such a germ is infinitely determined among all such germs with respect to composition by diffeomorphisms preserving if, and only if, the Jacobian ideal of contains all germs which vanish on and are infinitely flat at 0 if, and only if, is a real isolated line singularity.

     

Real groups transitive on complex flag manifolds

Let be a complex flag manifold. The compact real form of is transitive on . If is a noncompact real form, such transitivity is rare but occasionally happens. Here we work out a complete list of Lie subgroups of transitive on and pick out the cases that are noncompact real forms of .