Seminars and Colloquia by Series

Monday, February 27, 2012 - 11:05 , Location: Skiles 006 , Andrew Torok , Univ. of Houston , Organizer: Rafael de la Llave
Consider a hyperbolic basic set of a smooth diffeomorphism. We are interested in the transitivity of Holder skew-extensions with fiber a non-compact connected Lie group. In the case of compact fibers, the transitive extensions contain an open and dense set. For the non-compact case, we conjectured that this is still true within the set of extensions that avoid the obvious obstructions to transitivity. Within this class of cocycles, we proved generic transitivity for extensions with fiber the special Euclidean group SE(2n+1) (the case SE(2n) was known earlier), general Euclidean-type groups, and some nilpotent groups. We will discuss the "correct" result for extensions by the Heisenberg group: if the induced extension into its abelinization is transitive, then so is the original extension. Based on earlier results, this implies the conjecture for Heisenberg groups. The results for nilpotent groups involve questions about Diophantine approximations. This is joint work with Ian Melbourne and Viorel Nitica.
Monday, February 20, 2012 - 11:05 , Location: Skiles 006 , Diego Del Castillo-Negrete , Oak Ridge National Lab , Organizer: Rafael de la Llave
The study of transport is an active area of applied mathematics of interest to fluid mechanics, plasma physics, geophysics, engineering, and biology among other areas. A considerable amount of work has been done in the context of diffusion models in which, according to the Fourier-­‐Fick’s prescription, the flux is assumed to depend on the instantaneous, local spatial gradient of the transported field. However, despiteits relative success, experimental, numerical, and theoretical results indicate that the diffusion paradigm fails to apply in the case of anomalous transport. Following an  overview of anomalous transport we present an alternative(non-­‐diffusive) class of models in which the flux and the gradient are related non-­‐locally through integro-­differential operators, of which fractional Laplacians are a particularly important special case. We discuss the statistical foundations of these models in the context of generalized random walks with memory (modeling non-­‐locality in time) and jump statistics corresponding to general Levy processes (modeling non-­‐locality in space). We discuss several applications including: (i) Turbulent transport in the presence of coherent structures; (ii) chaotic transport in rapidly rotating fluids; (iii) non-­‐local fast heat transport in high temperature plasmas; (iv) front acceleration in the non-­‐local Fisher-­‐Kolmogorov equation, and (v) non-­‐Gaussian fluctuation-­‐driven transport in the non-­‐local Fokker-­‐Planck equation.
Monday, February 13, 2012 - 11:05 , Location: Skiles 006 , Hector Lomeli , Univ. of Texas at Austin/ITAM , Organizer: Rafael de la Llave
We generalize some notions that have played an important role in dynamics, namely invariant manifolds, to the more general context of difference equations. In particular, we study Lagrangian systems in discrete time. We define invariant manifolds, even if the corresponding difference equations can not be transformed in a dynamical system. The results apply to several examples in the Physics literature: the Frenkel-Kontorova model with long-range interactions and the Heisenberg model of spin chains with a perturbation. We use a modification of the parametrization method to show the existence of Lagrangian stable manifolds. This method also leads to efficient algorithms that we present with their implementations. (Joint work with Rafael de la Llave.)
Monday, January 30, 2012 - 11:05 , Location: Skiles 006 , Alex Haro , Univ.. of Barcelona , Organizer: Rafael de la Llave
We present a novel method to find KAM tori in degenerate (nontwist) cases. We also require that the tori thus constructed have a singular Birkhoff normal form. 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 preliminary numerical results. This work is in collaboration with R. de la Llave and A. Gonzalez.
Monday, January 23, 2012 - 11:05 , Location: Skiles 006 , Jordi Lluis Figueras , Uppsala University , Organizer: Rafael de la Llave
In this talk we will present a numerical algorithm for the computation of (hyperbolic) periodic orbits of the 1-D K-S equation           u_t+v*u_xxxx+u_xx+u*u_x = 0, with v>0. This numerical algorithm consists on apply a suitable Newton scheme for a given approximate solution. In order to do this, we need to rewrite the invariance equation that must satisfy a periodic orbit in a form that its linearization around an approximate solution is a bounded operator. We will show also how this methodology can be used to compute rigorous estimates of the errors of the solutions computed.
Monday, January 9, 2012 - 11:05 , Location: Skiles 006 , Marcel Guardia , Institute for Advanced Studies , Organizer: Rafael de la Llave
We consider the restricted planar elliptic 3 body problem, which models the Sun, Jupiter and an Asteroid (which we assume that has negligible mass). We take a realistic value of the mass ratio between Jupiter and the Sun and their eccentricity arbitrarily small and we study the regime of the mean motion resonance 1:7, namely when the period of the Asteroid is approximately seven times the period of Jupiter. It is well known that if one neglects the influence of Jupiter on the Asteroid, the orbit of the latter is an ellipse. In this talk we will show how the influence of Jupiter may cause a substantial change on the shape of Asteriod's orbit. This instability mechanism may give an explanation of the existence of the Kirkwood gaps in the Asteroid belt. This is a joint work with J. Fejoz, V. Kaloshin and P. Roldan.
Monday, December 5, 2011 - 11:00 , Location: Skiles 006 , John Mallet-Paret , Division of Applied Mathematics, Brown University , , Organizer: Shui-Nee Chow
We study a class of linear delay-differential equations, with a singledelay, of the form$$\dot x(t) = -a(t) x(t-1).\eqno(*)$$Such equations occur as linearizations of the nonlinear delay equation$\dot x(t) = -f(x(t-1))$ around certain solutions (often around periodicsolutions), and are key for understanding the stability of such solutions.Such nonlinear equations occur in a variety of scientific models, anddespite their simple appearance, can lead to a rather difficultmathematical analysis.We develop an associated linear theory to equation (*) by taking the$m$-fold wedge product (in the infinite dimensional sense of tensorproducts) of the dynamical system generated by (*). Remarkably, in the caseof a ``signed feedback'' where $(-1)^m a(t) > 0$ for some integer $m$, theassociated linear system is given by an operator which is positive withrespect to a certain cone in a Banach space. This leads to very detailedinformation about stability properties of (*), in particular, informationabout characteristic multipliers.
Monday, November 7, 2011 - 11:00 , Location: Skiles 006 , Phil Morrison , Univ. of Texas at Austin , Organizer: Rafael de la Llave
The Vlasov-Poisson and Vlasov-Maxwell equations possess variousvariational formulations1 or action principles, as they are generallytermed by physicists. I will discuss a particular variational principlethat is based on a Hamiltonian-Jacobi formulation of Vlasov theory,a formulation that is not widely known. I will show how this formu-lation can be reduced for describing the Vlasov-Poisson system. Theresulting system is of Hamilton-Jacobi form, but with nonlinear globalcoupling to the Poisson equation. A description of phase (function)space geometry will be given and comments about Hamilton-Jacobipde methods and weak KAM will be made.Supported by the US Department of Energy Contract No. DE-FG03-96ER-54346.H. Ye and P. J. Morrison Phys. Fluids 4B 771 (1992).D. Prsch, Z. Naturforsch. 39a, 1 (1984); D. Prsch and P. J. Morrison, Phys. Rev.32A, 1714 (1985).
Monday, September 26, 2011 - 11:00 , Location: Skiles 006 , Teresa Martinez-Seara , Univ. Polit. de Catalunya , Organizer: Rafael de la Llave
Friday, April 1, 2011 - 11:00 , Location: Skiles 005 , Genevieve Raugel , Universite Paris-Sud , Organizer: Yingfei Yi
In this talk, we generalize the classical Kupka-Smale theorem for ordinary differential equations on R^n to the case of scalar parabolic equations. More precisely, we show that, generically with respect to the non-linearity, the semi-flow of a reaction-diffusion equation defined on a bounded domain in R^n or on the torus T^n has the "Kupka-Smale" property, that is, all the critical elements (i.e. the equilibrium points and periodic orbits) are hyperbolic and the stable and unstable manifolds of the critical elements intersect transversally. In the particular case of T1, the semi-flow is generically Morse-Smale, that is, it has the Kupka-Smale property and, moreover, the non-wandering set is finite and is only composed of critical elements. This is an important property, since Morse-Smale semi-flows are structurally stable. (Joint work with P. Brunovsky and R. Joly).