# Seminars & Events for 2007-2008

March 27, 2008
12:30pm - 2:30pm
##### Counting Circles and Other Things

Take a circle of radius one and inscribe in it two circles that are tangent to each other. Now add another circle into the original one so that it's tangent to all three. If we repeat this process over and over, we get an old picture known as the Apollonian circle packing. What radii will we get from one such packing?

Speaker: Elena Fuchs, Princeton University
Location:
Fine Hall 214
March 27, 2008
2:00pm - 4:00pm
##### Logarithm laws for horocycles
###### Ergodic Theory & Statistical Mechanics

In joint work with G. Margulis, we prove a logarithm law for unipotent flows on the space of unimodular lattices in $R^n$.

Location:
Fine Hall 401
March 27, 2008
4:30pm - 6:30pm
##### Sequences of Hyperbolic $3$-Manifolds with Unfaithful Markings
###### Topology Seminar

Let $\Gamma$ be a finitely generated group. To every representation $\rho : \Gamma\to Isom (BH3)$ with discrete and torsion-free image there corresponds a hyperbolic $3$-manifold $M_\rho = BH3 / \rho (\Gamma)$. I will present some new results linking the pointwise convergence of a sequence of such representations with Gromov-Hausdorff convergence of the corresponding quotient manifolds.

Speaker: Ian Biringer, University of Chicago
Location:
Fine Hall 314
March 27, 2008
4:30pm - 6:30pm
##### On dimensionality of mean structure from a single data matrix
###### Operations Research and Financial Engineering

This presentation compares various total return world stock indices based on daily data. Due to diversification these indices are noticeably similar. A diversification theorem identifies any diversified portfolio as a proxy for the growth optimal portfolio.

Speaker: Eckhard Platen, University of Technology, Sydney
Location:
March 28, 2008
4:00pm - 6:00pm
##### A priori estimates for special Lagrangian equations
###### Differential Geometry & Geometric Analysis Seminar

We discuss recent a priori interior Hessian estimates for solutions of the special Lagrangian equation, when the equation has phase at least a certain value, or when the solution is convex. These equations include the sigma-2 equation in dimension three. The gradient graph of any solution is a minimizing Lagrangian surface.

Speaker: Micah Warren, University of Washington
Location:
Fine Hall 314
March 31, 2008
4:00pm - 6:00pm
##### Mathematical and Computational Challenges in Shear Stiffness Imaging of Tissue: Can cancerous and benign lesions be distinguished?
###### PACM/Applied Mathematics Colloquium

For centuries doctors have palpated tissue to detect abnormalities. We target imaging the stiffness the doctor feels in the palpation exam, including imaging deeper than what can be felt in this exam and distinguishing between benign and cancerous lesions. Current applications include breast and prostate cancer. Current experimentalists with whom we collaborate are: Dr.

Speaker: Joyce McLaughlin, Rensselaer Polytechnic Institute
Location:
Fine Hall 214
April 1, 2008
4:30pm - 6:30pm
##### Modelling high dimensional daily volatilities based on high-frequency data
###### Operations Research and Financial Engineering

It is increasingly popular in financial economics to estimate volatilities of asset returns by the methods based on realized volatility and bipower realized volatility from high-frequency data. However the most available methods are not directly relevant when the number of assets involved is large, due to the lack of accuracy in estimating high dimensional matrices.

Speaker: Qiwei Yao, London School of Economics
Location:
April 1, 2008
4:30pm - 6:30pm
##### Localisation in the Anderson tight binding model with several particles
###### Mathematical Physics Seminar

The Anderson model (which will celebrate its 50th anniversary in 2008) is among most popular topics in the random matrix and operator theory. However, so far the attention here was concentrated on single-particle models, where the random external potential is either IID or has a rapid decay of spatial correlations.

Speaker: Y. Suhov, Cambridge University
Location:
April 1, 2008
4:30pm - 6:30pm
##### Real singular Del Pezzo surfaces and rationally connected threefolds
###### Algebraic Geometry Seminar

Recent results on classification of real algebraic threefolds will be described. Let W -> X be a real smooth projective threefold fibred by rational curves. J. Kollár proved that if the set of real points W(R) is orientable, then a connected component N of W(R) is essentially either a Seifert fibred manifold or a connected sum of lens spaces.

Speaker: Frédéric Mangolte, Université de Savoie
Location:
Fine Hall 322
April 2, 2008
4:30pm - 6:30pm
##### Entropy and the localization of eigenfunctions
###### Department Colloquium

We study the behaviour of the eigenfunctions of the laplacian, on a compact negatively curved manifold, and for large eigenvalues. The Quantum Unique Ergodicity conjecture predicts that the probability measures defined by these eigenfunctions should converge weakly to the Riemannian volume.

Speaker: Nalini Anantharaman, Ecole Polytechnique
Location:
Fine Hall 314
April 3, 2008
12:30pm - 2:30pm
##### The Moment Map and Delzant Polytopes

In symplectic geometry, the moment map (or momentum map) is a tool associated with a Hamiltonian action of a Lie group on a symplectic manifold, used to construct conserved quantities for the action. The moment map generalizes the classical notions of linear and angular momentum.

Location:
Fine Hall 214
April 3, 2008
2:00pm - 4:00pm
##### Trigonometric sums and continued fractions with even partial quotients
###### Ergodic Theory & Statistical Mechanics

I will talk about the geometric features ("curlicues") of quadratic trigonometric sums and discuss how the renormalization of such sums is connected with continued fraction expansions with even partial quotients. I will also explain a recent renewal-time limit theorem for the sequence of denominators generated by such expansions.

Speaker: Francesco Cellarosi, Princeton University
Location:
Fine Hall 401
April 3, 2008
2:15pm - 4:15pm
##### Points Surrounding the Origin
###### Discrete Mathematics Seminar
Speaker: János Pach, Courant Institute, NYU
Location:
Fine Hall 314
April 3, 2008
4:30pm - 6:30pm
##### Length Spectrum of a Flat Metric
###### Topology Seminar

I'll discuss joint work-in-progress with M. Duchin and K. Rafi on the geometry of flat structures on surfaces via the lengths of its closed geodesics.

Speaker: Chris Leininger, University of Illinois, Urbana-Champaign
Location:
Fine Hall 314
April 3, 2008
4:30pm - 6:30pm
##### Cohen-Lenstra heuristics and the negative Pell equation
###### Princeton University/IAS Number Theory Seminar

For a squarefree integer $d$ we ask, if the negative Pell equation $x2-dy2 = -1$ is solvable over the integers. By easy considerations we see that in this case $d>0$ and that all odd prime divisors of $d$ are congruent to 1 modulo 4. Now we call a $d$ special, if it satisfies those two conditions.

Speaker: J.Klueners, Universität Duesseldorf
Location:
IAS - Simonyi Hall Seminar Room SH-101
April 4, 2008
2:00pm - 4:00pm
##### The action gap and periodic orbits of Hamiltonian systems
###### Joint PU/IAS Symplectic Geometry Seminar

The action and index spectra of a Hamiltonian diffeomorphism and their behavior under iterations carry important information about the periodic orbits of the diffeomorphism.

Speaker: Basak Gurel, Centre de Recherches Mathématiques
Location:
Fine Hall 314
April 8, 2008
3:30pm - 5:30pm
##### Pricing American Contingent Claims by Stochastic Linear Programming
###### Operations Research and Financial Engineering

In this talk I consider pricing of American contingent claims (ACC) as well as theirspecial cases, in a multi-period, discrete time, discrete state space setting. Determining the buyer's price for ACC's requires solving an integer program unlike European contingent claims for which solving a linear program is sufficient.

Speaker: Mustafa Pinar, Bilkent University
Location:
April 8, 2008
4:30pm - 6:30pm
##### Wallcrossing for K-theoretic Donaldson invariants and computations for rational surfaces
###### Algebraic Geometry Seminar

Let $(X,H)$ be a polarized algebraic surface. Let $M=M^H_X(c_1,c_2)$ be the moduli spaces of $H$-semistable rank 2 sheaves on $X$ with Chern classes $c_1, c_2$.  K-theoretic Donaldson invariants of $X$ are holomorphic Euler characteristics of determinant line bundles on $M$. These invariants are subject to wallcrossing when $H$ varies.

Speaker: Lothar Goettsche, International Centre for Theoretical Physics (ICTP)
Location:
Fine Hall 322
April 8, 2008
4:30pm - 6:30pm
##### Photon localization and Dicke superradiance : a crossover to small world networks
###### Mathematical Physics Seminar

We study photon localization in a gas of cold atoms, using a Dicke Hamiltonian that accounts for photon mediated atomic dipolar interactions. The photon escape rates are obtained from a new class of random matrices. A scaling behavior is observed for photons escape rates as a function of disorder and system size.

Speaker: E. Akkermans, Technion (Israel Institute of Technology)
Location: