Seminars and Colloquia by Series

Monday, April 27, 2009 - 15:00 , Location: Skiles 255 , Tin Yau Tam , Department of Mathematics, Auburn University , Organizer:
Let A be a Hilbert space operator. If A = UP is the polar decomposition of A, and 0 < \lambda < 1, the \lambda-Aluthge transform of A is defined to be the operator \Delta_\lambda = P^\lambda UP^{1-\lambda}. We will discuss the recent progress on the convergence of the iteration. Infinite and finite dimensional cases will be discussed.
Friday, April 24, 2009 - 15:00 , Location: Skiles 255 , Mokshay Madiman , Department of Statistics, Yale University , Organizer: Prasad Tetali
We develop an information-theoretic foundation for compound Poisson approximation and limit theorems (analogous to the corresponding developments for the central limit theorem and for simple Poisson approximation). First, sufficient conditions are given under which the compound Poisson distribution has maximal entropy within a natural class of probability measures on the nonnegative integers. In particular, it is shown that a maximum entropy property is valid if the measures under consideration are log-concave, but that it fails in general. Second, approximation bounds in the (strong) relative entropy sense are given for distributional approximation of sums of independent nonnegative integer valued random variables by compound Poisson distributions. The proof techniques involve the use of a notion of local information quantities that generalize the classical Fisher information used for normal approximation, as well as the use of ingredients from Stein's method for compound Poisson approximation. This work is joint with Andrew Barbour (Zurich), Oliver Johnson (Bristol) and Ioannis Kontoyiannis (AUEB).
Friday, April 24, 2009 - 15:00 , Location: Skiles 269 , Thang Le , School of Mathematics, Georgia Tech , Organizer: John Etnyre

These are two hour lectures.

We will develop general theory of quantum invariants based on sl_2 (the simplest Lie algebra): The Jones polynomials, the colored Jones polynomials, quantum sl_2 groups, operator invariants of tangles, and relations with the Alexander polynomial and the A-polynomials. Optional: Finite type invariants and the Kontsevich integral.
Thursday, April 23, 2009 - 15:00 , Location: Skiles 269 , Yichuan Zhao , Department of Mathematics, Georgia State University , Organizer: Heinrich Matzinger
It is of interest that researchers study competing risks in which subjects may fail from any one of k causes. Comparing any two competing risks with covariate effects is very important in medical studies. In this talk, we develop omnibus tests for comparing cause-specific hazard rates and cumulative incidence functions at specified covariate levels. The omnibus tests are derived under the additive risk model by a weighted difference of estimates of cumulative cause-specific hazard rates. Simultaneous confidence bands for the difference of two conditional cumulative incidence functions are also constructed. A simulation procedure is used to sample from the null distribution of the test process in which the graphical and numerical techniques are used to detect the significant difference in the risks. In addition, we conduct a simulation study, and the simulation result shows that the proposed procedure has a good finite sample performance. A melanoma data set in clinical trial is used for the purpose of illustration.
Thursday, April 23, 2009 - 15:00 , Location: Skiles 255 , Tom Tucker , Univ. of Rochester , Organizer: Matt Baker
Let S be a group or semigroup acting on a variety V, let x be a point on V, and let W be a subvariety of V. What can be said about the structure of the intersection of the S-orbit of x with W? Does it have the structure of a union of cosets of subgroups of S? The Mordell-Lang theorem of Laurent, Faltings, and Vojta shows that this is the case for certain groups of translations (the Mordell conjecture is a consequence of this). On the other hand, Pell's equation shows that it is not true for additive translations of the Cartesian plane. We will see that this question relates to issues in complex dynamics, simple questions from linear algebra, and techniques from the study of linear recurrence sequences.
Thursday, April 23, 2009 - 13:00 , Location: Skiles 255 , Per-Gunnar Martinsson , Dept of Applied Mathematics, University of Colorado , Organizer: Haomin Zhou

Note special day

Linear boundary value problems occur ubiquitously in many areas of science and engineering, and the cost of computing approximate solutions to such equations is often what determines which problems can, and which cannot, be modelled computationally. Due to advances in the last few decades (multigrid, FFT, fast multipole methods, etc), we today have at our disposal numerical methods for most linear boundary value problems that are "fast" in the sense that their computational cost grows almost linearly with problem size. Most existing "fast" schemes are based on iterative techniques in which a sequence of incrementally more accurate solutions is constructed. In contrast, we propose the use of recently developed methods that are capable of directly inverting large systems of linear equations in almost linear time. Such "fast direct methods" have several advantages over existing iterative methods: (1) Dramatic speed-ups in applications involving the repeated solution of similar problems (e.g. optimal design, molecular dynamics). (2) The ability to solve inherently ill-conditioned problems (such as scattering problems) without the use of custom designed preconditioners. (3) The ability to construct spectral decompositions of differential and integral operators. (4) Improved robustness and stability. In the talk, we will also describe how randomized sampling can be used to rapidly and accurately construct low rank approximations to matrices. The cost of constructing a rank k approximation to an m x n matrix A for which an O(m+n) matrix-vector multiplication scheme is available is O((m+n)*k). This cost is the same as that of the well-established Lanczos scheme, but the randomized scheme is significantly more robust. For a general matrix A, the cost of the randomized scheme is O(m*n*log(k)), which should be compared to the O(m*n*k) cost of existing deterministic methods.
Thursday, April 23, 2009 - 12:05 , Location: Skiles 255 , Jie Ma , School of Mathematics, Georgia Tech , Organizer: Robin Thomas
A well know theorem of Kuratowski states that a graph is planar graph iff it contains no TK_5 or TK_{3,3}. In 1970s Seymour conjectured that every 5-connected nonplanar graph contains a TK_5. In the talk we will discuss several special cases of the conjecture, for example the graphs containing K_4^- (K_4 withour an edge). A related independent paths theorem also will be covered.
Wednesday, April 22, 2009 - 14:00 , Location: Skiles 255 , Peter D. Miller , University of Michigan , Organizer: Jeff Geronimo
We will discuss a new method of asymptotic analysis of matrix-valued Riemann-Hilbert problems that involves dispensing with analyticity in favor of measured deviation therefrom. This method allows the large-degree analysis of orthogonal polynomials on the real line with respect to varying nonanalytic weights with external fields having two Lipschitz-continuous derivatives, as long as the corresponding equilibrium measure has typical support properties. Universality of local eigenvalue statistics of unitary-invariant ensembles in random matrix theory follows under the same conditions. This is joint work with Ken McLaughlin.
Wednesday, April 22, 2009 - 13:30 , Location: ISyE Executive Classroom , David Cash , Computer Science, Georgia Tech , Organizer: Annette Rohrs
We construct efficient and natural encryption schemes that remain secure (in the standard model) even when used to encrypt messages that may depend upon their secret keys. Our schemes are based on well-studied "noisy learning" problems. In particular, we design 1) A symmetric-key cryptosystem based on the "learning parity with noise" (LPN) problem, and 2) A public-key cryptosystem based on the "learning with errors" (LWE) problem, a generalization of LPN that is at least as hard as certain worst-case lattice problems (Regev, STOC 2005; Peikert, STOC 2009). Remarkably, our constructions are close (but non-trivial) relatives of prior schemes based on the same assumptions --- which were proved secure only in the usual key-independent sense --- and are nearly as efficient. For example, our most efficient public-key scheme encrypts and decrypts in amortized O-tilde(n) time per message bit, and has only a constant ciphertext expansion factor. This stands in contrast to the only other known standard-model schemes with provable security for key-dependent messages (Boneh et al., CRYPTO 2008), which incur a significant extra cost over other semantically secure schemes based on the same assumption. Our constructions and security proofs are simple and quite natural, and use new techniques that may be of independent interest. This is joint work with Chris Peikert and Amit Sahai.
Wednesday, April 22, 2009 - 12:00 , Location: Skiles 255 , Evans Harrell , School of Mathematics, Georgia Tech , Organizer:
The eigenvalues of the Laplacian are the squares of the frequencies of the normal modes of vibration, according to the wave equation. For this reason, Bers and Kac referred to the problem of determining the shape of a domain from the eigenvalue spectrum of the Laplacian as the question of whether one can "hear" the shape. It turns out that in general the answer is "no." Sometimes, however, one can, for instance in extremal cases where a domain, or a manifold, is round. There are many "isoperimetric" theorems that allow us to conclude that a domain, curve, or a manifold, is round, when enough information about the spectrum of the Laplacian or a similar operator is known. I'll describe a few of these theorems and show how to prove them by linking geometry with functional analysis.

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