Title: Topology of ends of finite volume, nonpositively curved manifolds

Abstract: The structure of ends of a nonpositively curved, locally symmetric manifold M is very well understood. By Borel-Serre, the thin part of the universal cover of such a manifold is homotopy equivalent to a rational Tits building. This is a simplicial complex built out of the algebra of the locally symmetric space which turns out to have dimension = dim M/2.

Speaker: Gitta Kutyniok

Title: Applied Harmonic Analysis meets Sparse Regularization of Inverse Problems

Abstract:
Sparse regularization of inverse problems has already shown its effectiveness both theoretically and practically. The area of applied harmonic analysis offers a variety of systems such as wavelet systems which provide sparse approximations within certain model situations which then allows to apply this general approach provided that the solution belongs to this model class. However, many important problem classes in the multivariate situation are governed by anisotropic structures such as singularities concentrated on lower dimensional embedded manifolds, for instance, edges in images or shear layers in solutions of transport dominated equations. Since it was shown that the (isotropic) wavelet systems are not capable of sparsely approximating such anisotropic features, the need arose to introduce appropriate anisotropic representation systems. Among various suggestions, shearlets are the most widely used today. Main reasons for this are their optimal sparse approximation properties within a suitable model situation in combination with their unified treatment of the continuum and digital realm, leading to faithful implementations.

In this talk, we will first provide an introduction to sparse regularization of inverse problems, followed by an introduction to the area of applied harmonic analysis, in particular, discussing the anisotropic representation system of shearlets and presenting the main theoretical results. We will then analyze the effectiveness of using shearlets for sparse regularization of exemplary inverse problems both theoretically and numerically.

Title: Some applications of logic to additive number theory

Abstract: I will review the Loeb measure construction; I will
assume some exposure to nonstandard analysis, or at least 1st order logic,
comparable to the review I gave last semester in my seminars on fixed
points. Time permitting I will give the Loeb-measure proof of Szemeredi’s
Theorem.

Logic seminar: David Ross
Title: Some applications of logic to additive number theory (cont.)
Room: Keller 404.

Abstract:
I will continue with some examples of results about sets of positive upper Banach density proved using Loeb measures.

Speaker: Evan Gawlik (UCSD)

Title: Numerical Methods for Partial Differential Equations on Evolving Domains

Abstract: Many important and challenging problems in computational science and
engineering involve partial differential equations with a high level
of geometric complexity. Examples include moving-boundary problems,
where the domain on which a PDE is posed evolves with time in a
prescribed fashion; free-boundary problems, where the domain is one of
the unknowns in and of itself; and geometric evolution equations,
where the domain is an evolving Riemannian manifold. Such problems are
inherently challenging to solve numerically, owing not only to the
difficulty of discretizing functions defined on evolving geometries,
but also to the coupling, if any, between the geometry’s evolution and
the underlying PDE. Similar difficulties, which are in some sense dual
to those just mentioned, are faced when the goal is to numerically
approximate functions taking values in a manifold. This talk will
focus on tackling these unique challenges that lie at the intersection
of numerical analysis, PDEs, and geometry.

Speaker: Tam Nguyen Phan (Binghamton U.)

Title: Examples of negatively curved and nonpositively curved manifolds

Abstract: Let M be a noncompact, complete, Riemannian manifold. Gromov proved that if the sectional curvature of M negative and bounded, and if the volume of M is finite, then M is homeomorphic to the interior of a compact manifold overline{M} with boundary B. In other words, M has finitely many ends, and each end of M is topologically a product of a closed manifold C with a ray. A natural question is how the geometry (i.e. in terms of the curvature) of M controls the topology of C. The same question is interesting in nonpositive curvature settings. I will discuss what topological restrictions there are on each end and give old and new constructions of such manifolds.