## Seminars and Colloquia by Series

Monday, August 10, 2015 - 11:00 , Location: Skiles 005 , Shangjiang Guo , College of Mathematics and Econometrics, Hunan University , Organizer: Rafael de la Llave
We study the existence and branching patterns of wave trains in a two-dimensional lattice with linear and nonlinear coupling between nearest particles and a nonlinear substrate potential. The wave train equation of the corresponding discrete nonlinear equation is formulated as an advanced-delay differential equation which is reduced by a Lyapunov-Schmidt reduction to a finite-dimensional bifurcation equation with certain symmetries and an inherited Hamiltonian structure. By means of invariant theory and singularity theory, we obtain the small amplitude solutions in the Hamiltonian system near equilibria in non-resonance and $p:q$ resonance, respectively. We show the impact of the direction $\theta$ of propagation and obtain the existence and branching patterns of wave trains in a one-dimensional lattice by investigating the existence of travelling waves of the original two-dimensional lattice in the direction $\theta$ of propagation satisfying $\tan\theta$ is rational
Wednesday, April 29, 2015 - 11:00 , Location: Skiles 005 , Jason Mireles-James , University of Florida Atlantic , Organizer: Rafael de la Llave
I will discuss a two dimensional spatial pattern formation problem proposed by Doelman, Sandstede, Scheel, and Schneider in 2003 as a phenomenological model of convective fluid flow .  In the same work the authors just mentioned use geometric singular perturbation theory to show that the coexistence of certain spatial patterns is equivalent to the existence of some heteroclinic orbits between equilibrium solutions in a four dimensional vector field.  More recently Andrea Deschenes, Jean-Philippe Lessard, Jan Bouwe van den Berg and the speaker have shown, via a computer assisted argument, that these heteroclinic orbits exist.  Taken together these arguments provide mathematical proof of the existence of some non-trivial patterns in the original planar PDE.  I will present some of the ingredients of this computer assisted proof.
Thursday, April 23, 2015 - 13:30 , Location: Skiles 005 , Stuart S. Antman , University of Maryland , Organizer:

This is the 3rd Jorge Ize Memorial lecture, at IIMAS, Mexico City. We will join a videoconference of the event.

The equations governing the motion of a system consisting of a deformable body attached to a rigid body are the partial differential equations for the deformable body subject to boundary conditions that are the equations of motion for the rigid body. (For the ostensibly elementary problem of a mass point on a light spring, the dynamics of the spring itself is typically ignored: The spring is reckoned merely as a feedback device to transmit force to the mass point.) If the inertia of a deformable body is small with respect to that of a rigid body to which it is attached, then the governing equations admit an asymptotic expansion in a small inertia parameter. Even for the simple problem of the spring considered as a continuum, the asymptotics is tricky: The leading term of the regular expansion is not the usual equation for a mass on a massless spring, but is a curious evolution equation with memory. Under very special physical circumstances, an elementary but not obvious process shows that the solution of this equation has an attractor governed by a second-order ordinary differential equation. (This survey of background material is based upon joint work with Michael Wiegner, J. Patrick Wilber, and Shui Cheung Yip.) This lecture describes the rigorous asymptotics and the dimensions of attractors for the motion in space of light nonlinearly viscoelastic rods carrying heavy rigid bodies and subjected to interesting loads. (The motion of the rod is governed by an 18th-order quasilinear parabolic-hyperbolic system.) The justification of the full expansion and the determination of the dimensions of attractors, which gives meaning to these curious equations, employ some simple techniques, which are briefly described (together with some complicated techniques, which are not described). These results come from work with Suleyman Ulusoy.
Monday, April 13, 2015 - 11:00 , Location: Skiles 005 , Alex Haro , Univ. of Barcelona , Organizer: Rafael de la Llave
We present a method to find KAM tori with fixed frequency in degenerate cases, in which the Birkhoff normal form is singular. The method provides a natural classification of KAM tori which is based on Singularity Theory. The method also leads to effective algorithms of computation, and we present some numerical results up to the verge of breakdown. This is a joint work with Alejandra Gonzalez and Rafael de la Llave.
Monday, April 6, 2015 - 11:00 , Location: Skiles 005 , Alex Haro , Univ. of Barcelona , Organizer: Rafael de la Llave
We present a methodology to rigorously validate a given approximation of a quasi-periodic Lagrangian torus of a symplectic map. The approach consists in verifying the hypotheses of a-posteriori KAM theory based of the parameterization method (following Rafael de la Llave and collaborators). A crucial point of our imprementation is an analytic Lemma that allows us to control the norm of periodic functions using their discrete Fourier transform. An outstanding consequence of this approach it that the computational cost of the validation is assymptotically equivalent of the cost of the numerical computation of invariant tori using the parametererization method. We pretend to describe some technical aspects of our implementation. This is a work in progress joint with Jordi-Lluis Figueras and Alejandro Luque.
Monday, March 30, 2015 - 11:00 , Location: Slikes 005 , Chungen Liu , Nankai University, China , Organizer: Chongchun Zeng
The iteration theory for Lagrangian Maslov index is a very useful tool    in studying the multiplicity of brake orbits of Hamiltonian systems.  In  this talk, we show how to use this theory to prove that there exist at    least $n$ geometrically distinct brake orbits on every $C^2$ compact convex symmetric hypersurface in $\R^{2n}$ satisfying the reversible condition. As a consequence, we show that if    the Hamiltonian function is convex and even, then Seifert conjecture of 1948 on the multiplicity of brake orbits holds for any positive integer $n$.
Monday, March 23, 2015 - 11:00 , Location: Skiles 005 , Adam Fox , Western New England Univ. , Organizer: Rafael de la Llave
The Standard Map is a discrete time area-preserving dynamical system and is one of the simplest of such systems to exhibit chaotic dynamics.  Traditional studies of the Standard Map have employed symmetric forcing functions that do not induce a net flux.  Although the dynamics of these maps is rich there are many systems which cannot be modeled with these restrictions.  In this talk we will explore the dynamics of the Standard Map when the forcing is asymmetric and induces a positive flux on the system.  We will introduce new numerical methods to study these dynamics and give an overview of how transport in the system changes under these new forces.
Monday, March 9, 2015 - 11:00 , Location: Skiles 005 , Rodrigo Trevino , Courant Inst. of Mathematical Sciences, NYU , Organizer: Rafael de la Llave
A Penrose tiling is an example of an aperiodic tiling and its vertex set is an example of an aperiodic point set (sometimes known as a quasicrystal). There are higher rank dynamical systems associated with any aperiodic tiling or point set, and in many cases they define a uniquely ergodic action on a compact metric space. I will talk about the ergodic theory of these systems. In particular, I will state the results of an ongoing work with S. Schmieding on the deviations of ergodic averages of such actions for point sets, where cohomology plays a big role. I'll relate the results to the diffraction spectrum of the associated quasicrystals.
Monday, February 23, 2015 - 11:00 , Location: Skiles 005 , Rafael Tiedra de Aldecoa , Pontificia Univ. Catolica de Chile , Organizer: Rafael de la Llave
We show that all time changes of the horocycle flow on compact surfaces of constant negative curvature have purely absolutely continuous spectrum in the orthocomplement of the constant functions. This provides an answer to a question of A. Katok and J.-P. Thouvenot on the spectral nature of time changes of horocycle flows. Our proofs rely on positive commutator methods for self-adjoint operators and the unique ergodicity of the horocycle flow. www.mat.uc.cl/~rtiedra/download/Horocycles_Bordeaux_2014.pdf <http://www.mat.uc.cl/%7Ertiedra/download/Horocycles_Bordeaux_2014.pdf <http://www.mat.uc.cl/~rtiedra/download/Horocycles_Bordeaux_2014.pdf>>
Thursday, February 19, 2015 - 13:00 , Location: Skiles 006 , Yannan Shen , Univ. of Texas at Dallas , Organizer: Rafael de la Llave
We develop a mathematical model for  ultra-short pulse propagation in nonlinear metamaterials characterized by a weak Kerr-type nonlinearity in their dielectric response. The fundamental equation in the model is the short-pulse equation (SPE) which will be derived in frequency band gaps. We use a multi-scale ansatz to relate the SPE to the nonlinear Schroedinger equation, thereby characterizing the change of width of the pulse from the ultra short regime to the classical slow varying envelope approximation. We will discuss families of solutions of the SPE in characteristic coordinates, as well as discussing the global wellposedness of generalizations of the model that describe uni- and bi-directional nonlinear waves.