Forthcoming events in this series


Mon, 07 Jun 2004
17:00
L1

Convexity on Grassmannians and calculus of variations

Berck Gautier
(Universite catholique de louvain)
Abstract

The talk will discuss the variationnal problem on finite

dimensional normed spaces and Finsler manifolds.

We first review different notions of ellipticity (convexity) for

parametric integrands (densities) on normed spaces and compare them with

different minimality properties of affine subspaces. Special attention will

be given to Busemann and Holmes-Thompson k-area. If time permits, we will

then present the first variation formula on Finsler manifolds and exhibit a

class of minimal submanifolds.

Mon, 24 May 2004
17:00
L1

Currents in metric spaces, isoperimetric inequalities, and applications to area minimization problems

Stefan Wenger
(ETH-Zurich)
Abstract

Integral currents were introduced by H. Federer and W. H. Fleming in 1960

as a suitable generalization of surfaces in connection with the study of area

minimization problems in Euclidean space. L. Ambrosio and B. Kirchheim have

recently extended the theory of currents to arbitrary metric spaces. The new

theory provides a suitable framework to formulate and study area minimization

and isoperimetric problems in metric spaces.

The aim of the talk is to discuss such problems for Banach spaces and for

spaces with an upper curvature bound in the sense of Alexandrov. We present

some techniques which lead to isoperimetric inequalities, solutions to

Plateau's problem, and to other results such as the equivalence of flat and

weak convergence for integral currents.

Mon, 17 May 2004
17:00
L1

Half-eigenvalues and semilinear problems with jumping nonlinearities

Bryan Rynne
(Heriot-Watt)
Abstract

We consider semilinear Sturm-Liouville and elliptic problems with jumping

nonlinearities. We show how `half-eigenvalues' can be used to describe the

solvability of such problems and consider the structure of the set of

half-eigenvalues. It will be seen that for Sturm-Liouville problems the

structure of this set can be considerably more complicated for periodic than

for separated boundary conditions, while for elliptic partial differential

operators only partial results are known about the structure in general.

Mon, 08 Mar 2004
17:00
L1

Ideal Knots

Friedemann Schuricht
(Cologne)
Abstract

Let gamma be a closed knotted curve in R^3 such that the tubular

neighborhood U_r (gamma) with given radius r>0 does not intersect

itself. The length minimizing curve gamma_0 within a prescribed knot class is

called ideal knot. We use a special representation of curves and tools from

nonsmooth analysis to derive a characterization of ideal knots. Analogous

methods can be used for the treatment of self contact of elastic rods.

Mon, 01 Mar 2004
17:00
L1

Elliptic systems, integral functionals and singular sets

Guiseppe Mingione
(Parma)
Abstract

I shall give a brief overview of the partial regularity results for minima

of integral functionals and solutions to elliptic systems, concentrating my

attention on possible estimates for the Hausdorff dimension of the singular

sets; I shall also include more general variational objects called almost

minimizers or omega-minima. Open questions will be discussed at the end.

Mon, 23 Feb 2004
17:00
L1

Adaptive finite elements for relaxed methods (FERM) in computational microstructures

Carsten Carstensen
(Bristol)
Abstract
Nonconvex minimisation problems are encountered in many applications such as phase transitions in solids (1) or liquids but also in optimal design tasks (2) or micromagnetism (3). In contrast to rubber-type elastic materials and many other variational problems in continuum mechanics, the minimal energy may be not attained. In the sense of (Sobolev) functions, the non-rank-one convex minimisation problem (M) is ill-posed: As illustrated in the introduction of FERM, the gradients of infimising sequences are enforced to develop finer and finer oscillations called microstructures. Some macroscopic or effective quantities, however, are well-posed and the target of an efficient numerical treatment. The presentation proposes adaptive mesh-refining algorithms for the finite element method for the effective equations (R), i.e. the macroscopic problem obtained from relaxation theory. For some class of convexified model problems, a~priori and a~posteriori error control is available with an reliability-efficiency gap. Nevertheless, convergence of some adaptive finite element schemes is guaranteed. Applications involve model situations for (1), (2), and (3) where the relaxation is provided by a simple convexification.
Mon, 02 Feb 2004
17:00
L1

Geometric rigidity of conformal matrices

Daniel Faraco
(Max Planck Leipzig)
Abstract

Recently Friesecke, James and Muller established the following

quantitative version of the rigidity of SO(n) the group of special orthogonal

matrices. Let U be a bounded Lipschitz domain. Then there exists a constant

C(U) such that for any mapping v in the L2-Sobelev space the L^2-distance of

the gradient controlls the distance of v a a single roation.

This interesting inequality is fundamental in several problems concerning

dimension reduction in nonlinear elasticity.

In this talk, we will present a joint work with Muller and Zhong where we

investigate an analagous quantitative estimate where we replace SO(n) by an

arbitrary smooth, compact and SO(n) invariant subset of the conformal

matrices E. The main novelty is that exact solutions to the differential

inclusion Df(x) in E a.e.x in U are not necessarily affine mappings.

Mon, 26 Jan 2004
17:00
L1

Polyconvexity and counterexamples to regularity in the calculus of variations

Jonathan Bevan
(Oxford)
Abstract

Using a technique explored in unpublished work of Ball and Mizel I shall

show that already in 2 and 3 dimensions there are vectorfields which are

singular minimizers of integral functionals whose integrand is strictly

polyconvex and depends on the gradient of the map only. The analysis behind

these results gives rise to an interesting question about the relationship

between the regularity of a polyconvex function and that of its possible

convex representatives. I shall indicate why this question is interesting in

the context of the regularity results above and I shall answer it in certain

cases.

Mon, 17 Nov 2003
17:00
L1

The Aviles Giga functional

Dr Andrew Lorent
(Oxford)
Abstract

Take any region omega and let function u defined inside omega be the

distance from the boundary, u solves the iconal equation \lt|Du\rt|=1 with

boundary condition zero. Functional u is also conjectured (in some cases

proved) to be the "limiting minimiser" of various functionals that

arise models of blistering and micro magnetics. The precise formulation of

these problems involves the notion of gamma convergence. The Aviles Giga

functional is a natural "second order" generalisation of the Cahn

Hilliard model which was one of the early success of the theory of gamma

convergence. These problems turn out to be surprisingly rich with connections

to a number of areas of pdes. We will survey some of the more elementary

results, describe in detail of one main problems in field and state some

partial results.