## Seminars and Colloquia by Series

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).
Monday, March 14, 2011 - 11:00 , Location: Skiles 005 , , University of Kansas , , Organizer: Shui-Nee Chow
They may flow like fluids but under constraints of mechanical energies from their crystal aspects. As a result, they exhibit very rich phenomena that grant them  tremendous applications in modern technology.  Based on works of Oseen, Z\"ocher,  Frank and others, a continuum theory (not most general but satisfactory to a great extent) for liquid-crystals was formulated by Ericksen and Leslie in 1960s.  We will  first give a brief introduction to this classical theory and then focus on various important special settings in both static and dynamic cases. These special flows are rather simple for classical fluids but are quite  nonlinear for liquid-crystals.  We are able to apply abstract theory of nonlinear dynamical systems upon  revealing specific structures of the problems at hands.
Friday, March 11, 2011 - 11:00 , Location: Skiles 005 , Weiping Li , Oklahoma State University , Organizer: Haomin Zhou
In this talk, I will explain the correspondence between the Lorenz periodic solution and the topological knot in 3-space.The effect of small random perturbation on the Lorenz flow will lead to a certain nature order developed previously by Chow-Li-Liu-Zhou. This work provides an answer to an puzzle why the Lorenz periodics are only geometrically simple knots.
Monday, March 7, 2011 - 11:00 , Location: Skiles 005 , Qinglan Xia , University of California Davis , Organizer: Haomin Zhou
An optimal transport path may be viewed as a geodesic in the space of probability measures under a suitable family of metrics. This geodesic may exhibit a tree-shaped branching structure in many applications such as trees, blood vessels, draining and irrigation systems. Here, we extend the study of ramified optimal transportation between probability measures from Euclidean spaces to a geodesic metric space. We investigate the existence as well as the behavior of optimal transport paths under various properties of the metric such as completeness, doubling, or curvature upper boundedness. We also introduce the transport dimension of a probability measure on a complete geodesic metric space, and show that the transport dimension of a probability measure is bounded above by the Minkowski dimension and below by the Hausdorff dimension of the measure. Moreover, we introduce a metric, called "the dimensional distance", on the space of probability measures. This metric gives a geometric meaning to the transport dimension: with respect to this metric, the transport dimension of a probability measure equals to the distance from it to any finite atomic probability measure.