We discuss new work in a very classical field: the study of branched covers of the Riemann sphere. We first recall the classical picture as developed by Hurwitz, including the relationship between branched covers and group-theoretic monodromy data, and the Hurwitz spaces which parametrize branched covers. We then give two new results: a connectedness result, joint with Fu Liu, for certain Hurwitz spaces in the classical setting, and a result which can be viewed as an analogue of the Riemann existence theorem for certain tamely branched covers of the projective line over fields of positive characteristic.
Spaces of sections of tensor powers of the theta line bundle on moduli spaces of semistable arbitrary rank bundles on a compact Riemann surface are subject to a level-rank duality: each space of sections is geometrically isomorphic to the dual of the space of sections obtained by interchanging the tensor power (level) of the theta bundle on the moduli space and the rank of the bundles that make up the moduli space.
This corresponds in representation theory to an isomorphism of conformal blocks of representations of affine Lie algebras, when the rank of the algebra and the level of the representation are interchanged.
Dr. Marian will sketch a proof of the geometric statement, which is the result of joint work with Dragos Oprea, and draws inspiration from work by Prakash Belkale who established the isomorphism for a generic Riemann surface.
Sulcal fundi are 3D curves that lie in the depths of the cerebral cortex and are often used as landmarks for downstream computations in brain image processing. In this talk, a sequence of geometric algorithms is introduced to automatically extract the sulcal fundi from magnetic resonance images (MRI) and represent sulcal fundi as smooth polylines lying on the cortical surface. The automatic sulcal extraction can improve the quality and reproducibility of the process as well as yielding considerable time savings. This makes the large number of high-resolution MRI datasets available for analysis.
Benjamin Peirce Assistant Professor Joachim Krieger
Institution:
Princeton
Time:
Wednesday, November 29, 2006 - 4:00pm
Location:
MSTB 254
Dr. Krieger will discuss a recent result, joint with W. Schlag and D. Tataru, which establishes reguarity breakdown for wave maps with suitable initial data and target S^{2} in the energy critical dimension. The breakdown occurs via the bubbling off of a ground state harmonic map.
I will discuss local Holder and W^{1,p} estimates for solutions of some fully nonlinear equations in conformal geometry, and analyze the behavior of singular solutions in punctured balls. I will then show how these estimates are used in the solution of the \sigma_k-Yamabe problem for k > n/2
Many remarkable classical questions about prime numbers have natural analogues in the context of elliptic curves. In this talk I will give a brief introduction to the theory of elliptic curves, and discuss how "higher dimensional" analogues of open questions about prime numbers appear naturally in the study of the reductions of an elliptic curve. In particular, I will discuss progress made towards the resolution of variations of a Lang-Trotter conjecture from 1976.
We consider a system of N bosons, interacting through a repulsive short range mean field potential. In the limit of large N, we prove rigorously that the macroscopic dynamics of the system can be described in terms of the one-particle nonlinear Schroedinger equation.
In this talk I will review some recent developments in the area of reaction-diffusion-advection equations. I will concentrate on the phenomenon of quenching (extinction) of flames by a strong flow. These questions naturally lead to the related problem of estimating the relaxation speed for the solution of a corresponding passive scalar equation, which will also be discussed.
We discuss a recently discovered connection between the discrete spectrum and the essential spectrum of Schr"odinger operators in one or two space dimensions. The situation is particularly interesting on the half-line since new phenomena occur in this case due to boundary effects. For example, we show that the existence of singular spectrum embedded in the essential spectrum implies that the discrete spectrum is infinite. The proof starts out by relating the problem at hand to the theory of orthogonal polynomials on the unit circle via the Szeg"o and Geronimus transformation. This transformation yields estimates on the potential, which can then be fed into an analysis of the non-linear Fourier transform arising in the Pr"ufer reformulation of the time-independent Schr"odinger equation.
The main focus of my talk shall be on the well-posedness for the interface problem between a viscous fluid and an elastic solid. This is a two-phases problem, where each phase satisfies its own natural equation of evolution, and where the interaction between the two phases comes from the natural continuity of velocity field and normal stress across the unknown moving
interface. The methods known in fluid moving boundary problems (viscous or inviscid) cannot handle the apparent incompatibility between the regularity of the two phases, which has led previous authors to consider the case where the solid satisfies a simplified law where the difficulties are not present. I shall present the new methods that where required in order to allow the treatment of classical elasticity laws in this moving
interface problem.
I shall then briefly explain how some of these ideas and some new tools preserving the transport structure of the Euler equations can provide the well-posedness for the free surface Euler equations with (or without) surface tension, without any restriction on the curl of the initial velocity.