## Seminars and Colloquia by Series

Monday, April 26, 2010 - 15:00 , Location: Skiles 255 , Armen Vagharshakyan , School of Mathematics, Georgia Tech , Organizer: Michael Lacey
We improve the lower bound for the L_\infty norm of the discrepancy function. This result makes a partial step towards resolving the Discrepancy Conjecture. Being a theorem in the theory of irregularities of distributions, it also relates to corresponding results in approximation theory (namely, the Kolmogorov entropy of spaces of functions with bounded mixed derivatives) and in probability theory (namely, Small Ball Inequality - small deviation inequality for the Brownian sheet). We also provide sharp bounds for the exponential Orlicz norm and the BMO norm of the discrepancy function in two dimensions. In the second part of the thesis we prove that any sufficiently smooth one-dimensional Calderon-Zygmund convolution operator can be recovered through averaging of Haar shift operators. This allows to generalize the estimates, which had been previously known for Haar shift operators, to Calderon-Zygmund operators. As a result, the A_2 conjecture is settled for this particular type of Calederon-Zygmund operators.
Friday, March 19, 2010 - 09:00 , Location: Skiles 269 , David Howard , School of Math, Georgia Tech , Organizer: Prasad Tetali
In 2001, Fishburn, Tanenbaum, and Trenk published a series of two papers that introduced the notions of linear and weak discrepancy of a partially ordered set or poset. Linear discrepancy for a poset is the least k such that for any ordering of the points in the poset there is a pair of incomparable points at least distance k away in the ordering. Weak discrepancy is similar to linear discrepancy except that the distance is observed over weak labelings (i.e. two points can have the same label if they are incomparable, but order is still preserved). My thesis gives a variety of results pertaining to these properties and other forms of discrepancy in posets.
Thursday, March 11, 2010 - 11:00 , Location: Van Leer Building Room W225 , Shannon Bishop , School of Mathematics, Georgia Tech , Organizer: Christopher Heil
This thesis addresses four topics in the area of applied harmonic analysis. First, we show that the affine densities of separable wavelet frames affect the frame properties. In particular, we describe a new relationship between the affine densities, frame bounds and weighted admissibility constants of the mother wavelets of pairs of separable wavelet frames. This result is also extended to wavelet frame sequences. Second, we consider affine pseudodifferential operators, generalizations of pseudodifferential operators that model wideband wireless communication channels. We find two classes of Banach spaces, characterized by wavelet and ridgelet transforms, so that inclusion of the kernel and symbol in appropriate spaces ensures the operator if Schatten p-class. Third, we examine the Schatten class properties of pseudodifferential operators. Using Gabor frame techniques, we show that if the kernel of a pseudodifferential operator lies in a particular mixed modulation space, then the operator is Schatten p-class. This result improves existing theorems and is sharp in the sense that larger mixed modulation spaces yield operators that are not Schatten class. The implications of this result for the Kohn-Nirenberg symbol of a pseudodifferential operator are also described. Lastly, Fourier integral operators are analyzed with Gabor frame techniques. We show that, given a certain smoothness in the phase function of a Fourier integral operator, the inclusion of the symbol in appropriate mixed modulation spaces is sufficient to guarantee that the operator is Schatten p-class.
Tuesday, October 27, 2009 - 13:30 , Location: Skiles 269 , Selma Yildirim-Yolcu , School of Mathematics, Georgia Tech , Organizer:
In this thesis, some eigenvalue inequalities for Klein-Gordon operators and restricted to a bounded domain in Rd are proved. Such operators become very popular recently as they arise in many problems ranges from mathematical finance to crystal dislocations, especially the relativistic quantum mechanics and \alpha-stable stochastic processes. Many of the results obtained here concern finding bounds for some spectral functions of these operators. The subject, which is well developed for the Laplacian, is examined from the spectral theory perspective through some of the tools used to prove analogues results for the Laplacian. This work highlights some important results, sparking interest in constructing a similar theory for Klein-Gordon operators. For instance, the Weyl asymptotics and semiclassical bounds for the operator Hm, are developed. As a result, a Berezin-Li-Yau type inequality is derived and an improvement of the bound is proved.
Thursday, October 1, 2009 - 14:00 , Location: Skiles 255 , Mitch Keller , School of Mathematics, Georgia Tech , Organizer: William T. Trotter
Tanenbaum, Trenk, and Fishburn introduced the concept of linear discrepancy in 2001, proposing it as a way to measure a partially ordered set's distance from being a linear order. In addition to proving a number of results about linear discrepancy, they posed eight challenges and questions for future work. This dissertation completely resolves one of those challenges and makes contributions on two others. This dissertation has three principal components: 3-discrepancy irreducible posets of width 3, degree bounds, and online algorithms for linear discrepancy. The first principal component of this dissertation provides a forbidden subposet characterization of the posets with linear discrepancy equal to 2 by completing the determination of the posets that are 3-irreducible with respect to linear discrepancy. The second principal component concerns degree bounds for linear discrepancy and weak discrepancy, a parameter similar to linear discrepancy. Specifically, if every point of a poset is incomparable to at most \Delta other points of the poset, we prove three bounds: the linear discrepancy of an interval order is at most \Delta, with equality if and only if it contains an antichain of size \Delta+1; the linear discrepancy of a disconnected poset is at most \lfloor(3\Delta-1)/2\rfloor; and the weak discrepancy of a poset is at most \Delta-1. The third principal component of this dissertation incorporates another large area of research, that of online algorithms. We show that no online algorithm for linear discrepancy can be better than 3-competitive, even for the class of interval orders. We also give a 2-competitive online algorithm for linear discrepancy on semiorders and show that this algorithm is optimal.
Friday, August 14, 2009 - 14:00 , Location: Skiles 114 , Hao Deng , School of Mathematics, Georgia Tech , Organizer:
Monday, August 10, 2009 - 15:00 , Location: Skiles 255 , Kun Zhao , School of Mathematics, Georgia Tech , Organizer:
Thursday, July 2, 2009 - 13:30 , Location: Skiles 255 , Turkay Yolcu , School of Mathematics, Georgia Tech , Organizer:
In this thesis, we extend De Giorgi's interpolation method to a class of parabolic equations which are not gradient flows but possess an entropy functional and an underlying Lagrangian. The new fact in the study is that not only the Lagrangian may depend on spatial variables, but also it does not induce a metric. Assuming the initial condition is a density function, not necessarily smooth, but solely of bounded first moments and finite entropy, we use a variational scheme to discretize the equation in time and construct approximate solutions. Moreover, De Giorgi's interpolation method reveals to be a powerful tool for proving convergence of our algorithm. Finally, we analyze uniqueness and stability of our solution in L^1.
Wednesday, July 1, 2009 - 15:30 , Location: Skiles 255 , Alan J. Michaels , School of Electrical and Computer Engineering, Georgia Tech , Organizer:
This disseratation provides the conceptual development, modeling and simulation, physical implementation and measured hardware results for a procticable digital coherent chaotic communication system.
Monday, May 11, 2009 - 13:00 , Location: Skiles 255 , Evan Borenstein , School of Mathematics, Georgia Tech , Organizer: Ernie Croot