In 1956, the Dutch graphic artist M.C. Escher made an unusual
lithograph with the title `Print Gallery'. It shows a young man
viewing a print in an exhibition gallery. Amongst the buildings
depicted on the print, he sees paradoxically the very same gallery
that he is standing in. A lot is known about the way in which
Escher made his lithograph. It is not nearly as well known that it
contains a hidden `Droste effect', or infinite repetition; but
this is brought to light by a mathematical analysis of the studies
used by Escher. On the basis of this discovery, a team of
mathematicians at Leiden produced a series of hallucinating
computer animations. These show, among others, what happens
inside the mysterious spot in the middle of the lithograph that
Escher left blank.
It has been a challenging problem to studying the existence of Kahler-Einstein metrics on Fano manifolds. A Fano manifold is a compact Kahler manifold with positive first Chern class. There are obstructions to the existence of Kahler-Einstein metrics on Fano manifolds. In these lectures, I will report on recent progresses on the study of Kahler-Einstein metrics on Fano manifolds. The first lecture will be a general one. I will discuss approaches to studying the existence problem. I will discuss the difficulties and tools in these approaches and results we have for studying them. In the second lecture, I will discuss the partial C^0-estimate which plays a crucial role in recent progresses on the existence of Kahler-Einstein metrics. I will show main technical aspects of proving such an estimate.
I will first discuss why Hamilton-Jacobi equations for nonconvex Hamiltonians are so interesting, and then explain some recent progress in characterizing the geometric structure and other properties of viscosity solutions.
We consider the behavior of trajectories for the billiard on a regular polygon. In three special cases which give rise to lattice tilings of the plane (the triangle, the square and the hexagon), the behavior of trajectories is very simple to analyze: they are either periodic or quasiperiodic. Can quasiperiodicity be found in the other cases? Our discussion will take us to the analysis of the renormalization flow for Veech surfaces which are non-arithmetic in the sense that the trace field is a non-trivial finite extension of $\Q$. We will see that the typical behavior presents no remains of quasiperiodicity, but exceptional behavior can appear (with positive Hausdorff dimension) if the Veech group contains a Salem element.
One-Frequency Schrödinger operators give one of the simplest models where fast transport and localization phenomena are possible. From a dynamical perspective, they can be studied in terms of certain one-parameter families of quasi-periodic co-cycles, which are similarly distinguished as simplest classes of dynamical systems compatible with both KAM phenomena and non-uniform hyperbolicity (NUH). While much studied since the 1970's, until recently the analysis was mostly confined to ''local theories'' describing the KAM and the NUH regimes in detail. In this talk we will describe some of the main aspects of the global theory that has been developed in the last few years.
An important question is to non-invasively find the volume of each phase in body, by only probing its response at the boundary. Here we consider a body containing two phases arranged in any configuration, and address the inverse problem of bounding the volume fraction of each phase from electrical tomography measurements at the boundary, i.e. measurements of the current flux through the boundary produced by potentials applied at the boundary. It turns out that this problem is closely related to the extensively studied problem of bounding the effective conductivity of periodic composite materials. Those bounds can be used to bound the response of an arbitrarily shaped body, and if this response has been measured, they can be used to extract information about the volume fraction.
Numerical experiments show that for a wide range of inclusion shapes one of the bounds turns out to be close to the actual volume fraction. The bounds extend those obtained by Capdeboscq and Vogelius for asymptotically small inclusions. The same ideas can be extended to elasticity and used to incorporate thermal measurements as well as electrical measurements. The translation method for obtaining bounds on the effective conductivity can also be applied directly to bound the volume fraction of inclusions in a body. This is joint work with Hyeonbae Kang and Eunjoo Kim.