CSCAMM Archives for Academic Year 2019

Nonlocal-interaction equations on graphs and their continuum limits

When: Wed, September 25, 2019 - 2:00pm
Where: CSIC 4122
Speaker: Prof. Dejan Slepcev (Department of Mathematical Sciences, Carnegie Mellon University) -
Abstract: We consider transport equations on graphs, where mass is distributed over vertices and is transported along the edges. The first part of the talk will deal with the graph analogue of the Wasserstein distance, in the particular case where the notion of density along edges is inspired by the upwind numerical schemes. This natural notion of interpolation however leads to the fact that Wasserstein distance is only a quasi-metric. In the second part of the talk we will interpret the nonlocal-interaction equation equations on graphs as gradient flows with respect to the graph-Wasserstain quasi-metric of the nonlocal-interaction energy. We show that for graphs representing data sampled from a manifold, the solutions of the nonlocal-interaction equations on graphs converge to solutions of an integral equation on the manifold. We also show that the limiting equation is a gradient flow of the nonlocal-interaction energy with respect to a nonlocal analogue of the Wasserstein metric.

Periodic Solutions to Conservation Laws

When: Wed, October 2, 2019 - 2:00pm
Where: CSIC 4122
Speaker: Prof. Constantine Dafermos (Division of Applied Mathematics, Brown University) -
Abstract: The lecture will discuss the decay to equilibrium of space-periodic solutions to scalar conservation laws, in several space dimensions, under minimal assumptions on the nonlinearity of the flux. The proof will be self-contained, combining basic topological dynamics with elementary integral geometry.

Recognizing Emergent Behaviors from Short-time Trajectory Data

When: Wed, October 16, 2019 - 2:00pm
Where: CSIC 4122
Speaker: Dr. Ming Zhong (Department of Applied Mathematics & Statistics, Johns Hopkins University) -
Abstract: The study of emergent behaviors in collective dynamics is a fundamental challenge in a wide variety of disciplines. Classical approaches focus mainly on inducing the emergent behaviors from known interaction laws. We, on the other hand, developed a nonparametric inference approach to learn the interaction laws from trajectory data in [1], and use it to recover the desired dynamics. Having theoretically and computationally examined the convergence properties of our estimators in [1], we employ them to predict the corresponding emergent behaviors. We investigate the prediction capability of our estimators by testing them on a wide range of collective dynamics: opinion dynamics, flocking dynamics, milling dynamics, synchronized oscillator dynamics, and planetary motion in our Solar system.

[1]: F. Lu, M. Zhong, S. Tang, and M. Maggioni, Nonparametric inference of interaction laws in systems of agents from trajectory data, Proceedings of the National Academy of Sciences, 116 (29), 14424 - 14433, 2019.

Spectral Problems in Inverse Scattering for Inhomogeneous Media

When: Wed, October 30, 2019 - 2:00pm
Where: CSIC 4122
Speaker: Prof. Fioralba Cakoni (Department of Mathematics, Rutgers University) -
Abstract: The inverse scattering problem for inhomogeneous media amounts to inverting a locally compact nonlinear operator, thus presenting difficulties in arriving at a solution. Initial efforts to deal with the nonlinear and ill-posed nature of the inverse scattering problem focused on the use of nonlinear optimization methods. Although efficient in many situations, their use suffers from the need for strong a priori information in order to implement such an approach. In addition, recent advances in material science and nanostructure fabrications have introduced new exotic materials for which full reconstruction of the constitutive parameters from scattering data is challenging or even impossible. In order to circumvent these difficulties, a recent trend in inverse scattering theory has focused on the development of new methods, in which the amount of a priori information needed is drastically reduced but at the expense of obtaining only limited information of the scatterers. Such methods come under the general title of qualitative approach in inverse scattering theory; they yield mathematically justified and computationally simple reconstruction algorithms by investigating properties of the linear scattering operator to decode non-linear information about the scattering object. In this spirit, a possible approach is to exploit spectral properties of operators associated with scattering phenomena which carry essential information about the media. The identified eigenvalues must satisfy two important properties: 1) can be determined from the scattering operator, and 2) are related to geometrical and physical properties of the media in an understandable way.

In this talk we will discuss some old and new eigenvalue problems arising in scattering theory for inhomogeneous media. We will present a two-fold discussion: on one hand relating the eigenvalues to the measurement operator (to address the first property) and on the other hand viewing them as the spectrum of appropriate (possibly non-self-adjoint) partial differential operators (to address the second property). Numerical examples will be presented to show what kind of information these eigenvalues, and more generally the qualitative approach, yield on the unknown inhomogeneity.

Computational methods for Quantifying Gerrymandering and other computational statistical mechanics problems

When: Wed, November 6, 2019 - 2:00pm
Where: CSIC 4122
Speaker: Prof. Jonathan Mattingly (Department of Mathematics, Duke University) -
Abstract: To Be Announced

Low-Rank Solution Algorithms for Parameter-Dependent Partial Differential Equations

When: Wed, November 13, 2019 - 2:00pm
Where: CSIC 4122
Speaker: Prof. Howard Elman (Department of Computer Science and UMIACS, University of Maryland) -
Abstract: The collection of solutions of discrete parameter-dependent partial differential equations often takes the form of a low-rank object. We show that in this scenario, iterative algorithms for computing these solutions can take advantage of this structure to reduce both computational effort and memory requirements. Implementation of such solvers requires that explicit rank-compression computations be done to truncate the ranks of intermediate quantities that must be computed. We prove that when truncation strategies are used as part of a multigrid solver, the resulting algorithms retain "textbook" (grid-independent) convergence rates and we demonstrate how the truncation criteria affect convergence behavior. In addition, we show that these techniques can be used to construct efficient solution algorithms for computing the eigenvalues of parameter-dependent operators.

This is joint work with Tengfei Su.

Uniqueness of the solution of Vlasov-Maxwell linearized system without a Cauchy datum: the response of a plasma

When: Wed, November 20, 2019 - 2:00pm
Where: CSIC 4122
Speaker: Prof. Olivier Lafitte (Institut Galilee, University Paris 13) -
Abstract: Generally, Vlasov equation and Vlasov-Maxwell equations are considered as kinetic equations and are addressed as Cauchy problems. Though, when one listens to the Physics community, they often describe their results as 'the response' of a plasma, stressing on the uniqueness of a solution without prescribing the Cauchy data.

This is only an apparent contradiction, and in this talk (joint work with Omar Maj, NMPP, IPP, Max Planck Institute Garching) describes the obtention of this response for the linearized Vlasov-Maxwell equations around a Maxwellian-type density function using distribution theory.

Regression of functions on low-dimensional manifolds by deep ReLU networks

When: Wed, December 4, 2019 - 2:00pm
Where: CSIC 4122
Speaker: Prof. Wenjing Liao (School of Mathematics, Georgia Institute of Technology) -
Abstract: Many data in real-world applications lie in a high-dimensional space but are concentrated on or near a low-dimensional manifold. Our goal is to estimate functions on the manifold from finite samples of data. This talk focuses on an efficient approximation theory of deep ReLU networks for functions supported on low-dimensional manifolds. We construct a ReLU network for such function approximation where the size of the network grows exponentially with respect to the intrinsic dimension of the manifold. When the function is estimated from finite samples, we proved that the mean squared error for the function approximation converges as the training samples increases with a rate depending on the intrinsic dimension of the manifold. These results demonstrate that deep neural networks are adaptive to low-dimensional geometric structures of data. This is a joint work with Minshuo Chen, Haoming Jiang, Tuo Zhao at Georgia Institute of Technology.

Propagation of regularity for solutions of the transport equation with non-Lipschitz velocity field and their applications

When: Wed, February 5, 2020 - 2:00pm
Where: CSIC 4122
Speaker: Prof. Quoc Hung Nguyen (Institute of Mathematical Sciences, Shanghaitech University) -
Abstract: Since the work by Di Perna and Lions (’89) the continuity and transport equation under mild regularity assumptions on the vector field have been extensively studied, becoming a florid research field.

The aim of this talk is to give an overview of the quantitative side of the theory, focusing mostly on the problem of propagation of regularity for ODE flows and solutions to the transport equation and the advection diffusion equation. We present recent results, examples, applications and open problems. This is based on joint works in collaboration with Elia Bruè.

Solving the wave equation: acoustic scattering in the time domain

When: Wed, February 12, 2020 - 2:00pm
Where: CSIC 4122
Speaker: Prof. Paul Martin (Department of Applied Mathematics and Statistics, Colorado School of Mines) -
Abstract: Transient acoustic waves are generated or scattered by an obstacle. This leads to initial-boundary value problems for the wave equation. Recent studies usually assume that the solution is smooth. However, many interesting physical problems lead to non-smooth solutions: there are moving wavefronts. These situations are usually handled by seeking weak solutions, but care is needed to ensure that constraints imposed by the underlying continuum mechanics are respected. We investigate some of the consequences, with a focus on the benchmark problem of scattering by a sphere.

Patterning through growth

When: Wed, February 19, 2020 - 2:00pm
Where: CSIC 4122
Speaker: Prof. Arnd Scheel (School of Mathematics, University of Minnesota) -
Abstract: I will survey recent work on pattern formation in the context of growth. Descriptions of pattern formation often start in a scenario of uniform quenching, where spatial structure emerges from a spatially constant state which is destabilized by a sudden change of a parameter. Patterns are often incoherent, marred by many defects, and difficult to predict in this scenario. In many scenarios, the formation of patterns is accompanied by a growth process, where for domain occupied by patterns grows in space, or the spatial domain itself grows in time. I will demonstrate how such growth processes act as a selection mechanism for patterns, how patterns arising in such contexts are often much more coherent, and how very simple systems that lack the ability to provide long-range spatial order form coherent crystalline phases in such a growth scenario.

Boltzmann-type collision operators for Bogoliubov excitations of Bose-Einstein condensates

When: Wed, March 4, 2020 - 2:00pm
Where: CSIC 4122
Speaker: Prof. Minh Binh Tran (Department of Mathematics, Southern Methodist University) -
Abstract: Starting from the Bogoliubov diagonalization for the Hamiltonian of a weakly interacting Bose gas under the presence of a Bose-Enstein Condensate (BEC), we formally derive the kinetic equation for the Bogoliubov excitations. Without dropping any of the commutators, we find three collisional processes. One of them describes the 1 ↔ 2 interactions between the condensate and the excited atoms. The other two describe the 2 ↔ 2 and 1 ↔ 3 interactions between the excited atoms themselves. This is joint work with Yves Pomeau (ENS Paris).