### Brown bag seminar on mathematical challenges in astrophysics

- Series
- Other Talks
- Time
- Wednesday, July 1, 2009 - 12:00 for 1 hour (actually 50 minutes)
- Location
- Skiles 255
- Speaker
- Pablo Laguna – School of Physics, Georgia Tech

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- Series
- Other Talks
- Time
- Wednesday, July 1, 2009 - 12:00 for 1 hour (actually 50 minutes)
- Location
- Skiles 255
- Speaker
- Pablo Laguna – School of Physics, Georgia Tech

This will be an informal seminar with a discussion on some mathematical problems in relativistic astrophysics, and discuss plans for future joint seminars between the Schools of Mathematics and Physics.

- Series
- Graph Theory Seminar
- Time
- Thursday, June 11, 2009 - 11:05 for 1 hour (actually 50 minutes)
- Location
- Skiles 255
- Speaker
- Daniel Kral – ITI, Charles University, Prague

We study several parameters of cubic graphs with large girth. In particular, we prove that every n-vertex cubic graph with sufficiently large girth satisfies the following:

- has a dominating set of size at most 0.29987n (which improves the previous bound of 0.32122n of Rautenbach and Reed)
- has fractional chromatic number at most 2.37547 (which improves the previous bound of 2.66881 of Hatami and Zhu)
- has independent set of size at least 0.42097n (which improves the previous bound of 0.41391n of Shearer), and
- has fractional total chromatic number arbitrarily close to 4 (which answers in the affirmative a conjecture of Reed). More strongly, there exists g such that the fractional total chromatic number of every bridgeless graph with girth at least g is equal to 4.

The presentation is based on results obtained jointly with Tomas Kaiser, Andrew King, Petr Skoda and Jan Volec.

- Series
- Graph Theory Seminar
- Time
- Thursday, June 4, 2009 - 11:00 for 1 hour (actually 50 minutes)
- Location
- Skiles 255
- Speaker
- Zdenek Dvorak – Simon Fraser University

Richter and Salazar conjectured that graphs that are critical for a fixed crossing number k have bounded bandwidth. A weaker well-known conjecture of Richter is that their maximum degree is bounded in terms of k. We disprove these conjectures for every k >170, by providing examples of k-crossing-critical graphs with arbitrarily large maximum degree, and explore the structure of such graphs.

- Series
- Combinatorics Seminar
- Time
- Thursday, May 21, 2009 - 11:00 for 1 hour (actually 50 minutes)
- Location
- Skiles 255
- Speaker
- Joshua Cooper – Department of Mathematics, University of South Carolina

We consider the Ulam "liar" and "pathological liar" games, natural and well-studied variants of "20 questions" in which the adversarial respondent is permitted to lie some fraction of the time. We give an improved upper bound for the optimal strategy (aka minimum-size covering code), coming within a triply iterated log factor of the so-called "sphere covering" lower bound. The approach is twofold: (1) use a greedy-type strategy until the game is nearly over, then (2) switch to applying the "liar machine" to the remaining Berlekamp position vector. The liar machine is a deterministic (countable) automaton which we show to be very close in behavior to a simple random walk, and this resemblance translates into a nearly optimal strategy for the pathological liar game.

- Series
- Dissertation Defense
- Time
- Monday, May 11, 2009 - 13:00 for 2 hours
- Location
- Skiles 255
- Speaker
- Evan Borenstein – School of Mathematics, Georgia Tech

- Series
- PDE Seminar
- Time
- Tuesday, May 5, 2009 - 15:00 for 1 hour (actually 50 minutes)
- Location
- Skiles 255
- Speaker
- Giuseppe Savare – Università degli Studi di Pavia, Italy

Some interesting nonlinear fourth-order parabolic equations, including the "thin-film" equation with linear mobility and the quantum drift-diffusion equation, can be seen as gradient flows of first-order integral functionals in the Wasserstein space of probability measures. We will present some general tools of the metric-variational approach to gradient flows which are useful to study this kind of equations and their asymptotic behavior. (Joint works in collaboration with U.Gianazza, R.J. McCann, D. Matthes, G. Toscani)

- Series
- Analysis Seminar
- Time
- Wednesday, April 29, 2009 - 14:00 for 1 hour (actually 50 minutes)
- Location
- Skiles 255
- Speaker
- Francisco Marcellan – Universidad Carlos III de Madrid

In this contribution we study the asymptotic behaviour of polynomials orthogonal with respect to a Sobolev-Type inner product

\langle p, q\rangle_S = \int^\infty_0 p(x)q(x)x^\alpha e^{-x} dx + IP(0)^t AQ(0), \alpha > -1,

where p and q are polynomials with real coefficients,

A = \pmatrix{M_0 & \lambda\\ \lambda & M_1},
IP(0) = \pmatrix{p(0)\\ p'(0)}, Q(0) = \pmatrix{q(0)\\ q'(0)},

and A is a positive semidefinite matrix.

First, we analyze some algebraic properties of these polynomials. More precisely, the connection relations between the polynomials orthogonal with respect to the above inner product and the standard Laguerre polynomials are deduced. On the other hand, the symmetry of the multiplication operator by x^2 yields a five term recurrence relation that such polynomials satisfy.

Second, we focus the attention on their outer relative asymptotics with respect to the standard Laguerre polynomials as well as on an analog of the Mehler-Heine formula for the rescaled polynomials.

Third, we find the raising and lowering operators associated with these orthogonal polynomials. As a consequence, we deduce the holonomic equation that they satisfy. Finally, some open problems will be considered.

- Series
- ACO Seminar
- Time
- Wednesday, April 29, 2009 - 11:00 for 1 hour (actually 50 minutes)
- Location
- Skiles 255
- Speaker
- Jeong Han Kim – Yonsei University and NIMS, South Korea

We consider the problem of finding an unknown graph by using two types of queries with an additive property. Given a graph, an additive query asks the number of edges in a set of vertices while a cross-additive query asks the number of edges crossing between two disjoint sets of vertices. The queries ask sum of weights for the weighted graphs. These types of queries were partially motivated in DNA shotgun sequencing and linkage discovery problem of artificial intelligence. For a given unknown weighted graph G with n vertices, m edges, and a certain mild condition on weights, we prove that there exists a non-adaptive algorithm to find the edges of G using O\left(\frac{m\log n }{\log m}\right) queries of both types provided that m \geq n^{\epsilon} for any constant \epsilon> 0. For a graph, it is shown that the same bound holds for all range of m. This settles a conjecture of Grebinski for finding an unweighted graph using additive queries. We also consider the problem of finding the Fourier coefficients of a certain class of pseudo-Boolean functions. A similar coin weighing problem is also considered. (This is joint work with S. Choi)

- Series
- Dissertation Defense
- Time
- Tuesday, April 28, 2009 - 14:30 for 3 hours
- Location
- Skiles 269
- Speaker
- Alexander Grigo – School of Mathematics, Georgia Tech

- Series
- Analysis Seminar
- Time
- Monday, April 27, 2009 - 15:00 for 1 hour (actually 50 minutes)
- Location
- Skiles 255
- Speaker
- Tin Yau Tam – Department of Mathematics, Auburn University

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.