Georgia Institute of Technology College of Sciences

We offer several perspectives on the behavior at infinity of solutions of discrete Schroedinger equations. First we study pairs of discrete Schroedinger equations whose potential functions differ by a quantity that can be considered small in a suitable sense as the index n \rightarrow \infty. With simple assumptions on the growth rate of the solutions of the original system, we show that the perturbed system has a fundamental set of solutions with the same behavior at infinity, employing a variation-of-constants scheme to produce a convergent iteration for the solutions of the second equation in terms of those of the original one. We use the relations between the solution sets to derive exponential dichotomy of solutions and elucidate the structure of transfer matrices.Later, we present a sharp discrete analogue of the Liouville-Green (WKB) transformation, making it possible to derive exponential behavior at infinity of a single difference equation, by explicitly constructing a comparison equation to which our perturbation results apply. In addition, we point out an exact relationship connecting the diagonal part of the Green matrix to the asymptotic behavior of solutions. With both of these tools it is possible to identify an Agmon metric, in terms of which, in some situations, any decreasing solution must decrease exponentially. This is joint work with Evans Harrell.

We consider several models from solid state Physics and consider the problem offinding quasi-periodic solutions. We present a KAM theorem that showsthat given an approximate solution with good condition numbers, onecan find a true solution close by. The method of proof leads tovery efficient algorithms. Also it provides a criterion for breakdown.We will present the proof, the algorithms and some conjectures obtainedby computing in some cases. Much of the work was done with R. Calleja and X. Su.

Shape optimization is the study of optimization problems whose unknown is a domain in R^d. The seminar is focused on the understanding of the case where admissible shapes are required to be convex. Such problems arises in various field of applied mathematics, but also in open questions of pure mathematics. We propose an analytical study of the problem. In the case of 2-dimensional shapes, we show some results for a large class of functionals, involving geometric functionals, as well as energies involving PDE. In particular, we give some conditions so that solutions are polygons. We also give results in higher dimension, concerned with the Mahler conjecture in convex geometry and the Polya-Szego conjecture in potential theory. We particularly make the link with the so-called Brunn-Minkowsky inequalities.

We will show that a quantum xy-spin chain which is exposed to a randomexterior magnetic field satisfies a zero-velocity Lieb-Robinson bound. Thiscan be interpreted as dynamical localization for the spin chain or asabsence of information transport. We will also discuss a general result,which says that zero velocity LR-bounds in a quantum spin system implyexponential decay of ground state correlations. This is joint work withRobert Sims and Eman Hamza and motivated by recent works of Burrell-Osborneas well as Hastings.