**Date:** December 11, 2018

**Speaker:** Yasumasa Nishiura, Advanced Institute for Materials Research, Tohoku University

**Title:** What is a good mathematical descriptor for the toughness of
heterogeneous materials

**Abstract:**
One of the dreams of materials scientists is to make a novel materials
through the design of atomistic scale, however most of the modern
composite materials is very heterogeneous in order to make it strong
via network structure like epoxy resin matrix with carbon fibers used in
the aircraft. Those are far from crystal structure nor completely random
so that it is not apriori clear what type of mathematical concepts is
appropriate to describe it, especially medium-range structure. As for
the static profile, recent statistical methods as well as topological
approach TDA clarify some aspects of it.
On the other hand, the performance of the materials, especially its
dynamic robusness against mechanical stress remains open and still
heavily depends on trials and errors in the laboratories. The difficulty
lies in that, firstly the lack of good macroscopic mathematical model to
describe dynamical processes, secondly it is not clear how to implement
the microscopic heterogeneity into the macroscopic model, thirdly how to
extract an appropriate mathematical descriptor that can measure and
predict the strength of it and even allows us to design novel materials.
I would like to present a case study in this direction in the context of
cracking phenomena for brittle materials. The stage is still early phase, however
it suggests many interesting questions and challenge not only for for
materials scientists but for mathematicians.

**Date:** February 1, 2019

**Speaker:** Heather Harrington, Oxford University

**Title:** TBA

**Abstract:**
TBA

**Date:** November 16, 2018

**Speaker:** Alexander Vladimirsky, Cornell University

**Title:** Surveillance-Evasion games under uncertainty

**Abstract:**
Adversarial path planning problems are important in robotics applications and in modeling the behavior of humans in dangerous environments. Surveillance-Evasion (SE) games form an important subset of such problems and require a blend of numerical techniques from multiobjective dynamic programming, game theory, numerics for Hamilton-Jacobi PDEs, and convex optimization. We model the basic SE problem as a semi-infinite zero-sum game between two players: an Observer (O) and an Evader (E) traveling through a domain with occluding obstacles.
O chooses a pdf over a finite set of predefined surveillance plans, while E chooses a pdf over an infinite set of trajectories that bring it to a target location. The focus of this game is on "E's expected cumulative exposure to O", and we have recently developed an algorithm for finding the Nash Equilibrium open-loop policies for both players. I will use numerical experiments to illustrate algorithmic extensions to handle multiple Evaders, moving Observes, and anisotropic observation sensors. Time permitting, I will also show preliminary results for a very large number of selfish/independent Evaders modeled via Mean Field Games.
Joint work with M.Gilles, E.Cartee, and REU-2018 participants.

**Date:** November 2, 2018

**Speaker:** Abner Salgado, University of Tennessee, Knoxville

**Title:** Regularity and rate of approximation for obstacle problems for a class of integro-differential operators

**Abstract:**
We consider obstacle problems for three nonlocal operators:

A) The integral fractional Laplacian

B) The integral fractional Laplacian with drift

C) A second order elliptic operator plus the integral fractional
Laplacian

For the solution of the problem in Case A, we derive regularity results
in weighted Sobolev spaces, where the weight is a power of the distance
to the boundary. For cases B and C we derive, via a Lewy-Stampacchia
type argument, regularity results in standard Sobolev spaces. We use
these regularity results to derive error estimates for finite element
schemes. The error estimates turn out to be optimal in Case A, whereas
there is a loss of optimality in cases B and C, depending on the order
of the integral operator.

**Date:** October 26, 2018

**Speaker:** Bill Kalies, Florida Atlantic University

**Title:** Order Theory in Dynamics

**Abstract:**
Recurrent versus gradient-like behavior in global dynamics
can be characterized via a surjective lattice homomorphism between
certain bounded, distributive lattices, that is, between attracting blocks
(or neighborhoods) and attractors. In this lecture we explain the basic
order and lattice theory for dynamical systems which lays a foundation
for a computational theory for dynamical systems that focuses on Morse
decompositions and index lattices. We build combinatorial order-theoretic
models for global dynamics. We give computational examples that
illustrate the theory for both maps and flows.

**Date:** October 24, 2018, 5:00-6:00pm

**Room:** Hill 005

**Speaker:** Prof. Wojciech Chacholski, Department of Mathematics, KTH

**Title:** What is persistence

**Abstract:**
It is not surprising that different units and scales are used to measure different phenomena. So why the Gromov-Hausdorff and bottleneck distances are the only one used to measure inputs and outcomes of topological data analysis applied to a variety of different data sets? My aim is to explain and illustrate a new approach to persistence. I will present both mathematical and real life data examples illustrating effectiveness of our approach to improve various classification tasks.

**Date:** October 19, 2018

**Speaker:** Francisco Sayas, University of Delaware

**Title:** Waves in viscoelastic solids

**Abstract:**
I will first explain a transfer function based framework collecting well-known models of wave propagation in viscoelastic solids, fractional derivative extensions, and their couplings.
I will briefly explain the associated Laplace domain stability results and their time domain counterparts, as well as how they are affected by finite element discretization in space.
Finally, I will discuss a semigroup approach to a non-strictly diffusive Zener model. This is joint work with Tom Brown, Shukai Du, and Hasan Eruslu.

**Date:** October 12, 2018

**Speaker:** Vladimir Itskov, The Pennsylvania State University

**Title:** Directed complexes, sequence dimension and inverting a neural network.

**Abstract:**
What is the embedding dimension, and more generally, the geometry of a set of sequences? This problem arises in the context of neural coding and neural networks. Here one would like to infer the geometry of a space that is measured by unknown quasiconvex functions. A natural object that captures all the inferable geometric information is the directed complexes (a.k.a. semi-simplicial sets). It turns out that the embedding dimension as well as some other geometric properties of data can be estimated from the homology of an associated directed complex. Moreover each such directed complex gives rise to a multi-parameter filtration that provides a dual topological description of the underlying space. I will also illustrate these methods in the neuroscience context of understanding the "olfactory space".

**Date:** September 28, 2018

**Speaker:** Carina Curto, The Pennsylvania State University

**Title:** Graph rules for inhibitory network dynamics

**Abstract:**
Many networks in the nervous system possess an abundance of inhibition, which serves to shape and stabilize neural dynamics. The neurons in such networks exhibit intricate patterns of connectivity, whose structure controls the allowed patterns of neural activity. In this work, we examine inhibitory threshold-linear networks whose dynamics are dictated by an underlying directed graph. We develop a set of parameter-independent graph rules that enable us to predict features of the dynamics from properties of the graph. These rules provide a direct link between the structure and function of these networks, and provides new insights into how connectivity may shape dynamics in real neural circuits.

**Date:** December 1, 2017

**Speaker:** Harbir Antil, George Mason University

**Title:** Fractional Operators with Inhomogeneous Boundary Conditions: Analysis, Control, and Discretization

**Abstract:**
In this talk we introduce new characterizations of spectral fractional Laplacian to incorporate non homogeneous Dirichlet and Neumann boundary conditions. The classical cases with homogeneous boundary conditions arise as a special case. We apply our definition to fractional elliptic equations of order $s \in (0,1)$ with nonzero Dirichlet and Neumann boundary conditions. Here the domain $\Omega$ is assumed to be a bounded, quasi-convex.

**Date:** November 3, 2017

**Speaker:** Marcio Gameiro, University of Sao Paulo at Sao Carlos, Brazil

**Title:** Rigorous Multi-parameter Continuation of Solutions of Differential Equations

**Abstract:**
We present a rigorous multi-parameter continuation method to compute solutions
of differential equations depending on parameters. The method combines classical
numerical methods, analytic estimates and the uniform contraction principle to prove
the existence of solutions of nonlinear differential equations. The method is applied to
the computation of equilibria for the Cahn-Hilliard equation and periodic solutions
of the Kuramoto-Sivashinsky equation.

**Date:** September 22, 2017

**Speaker:** Qi Wang, University of South Carolina

**Title:** Energy quadratization strategy for numerical approximations of nonequilibrium models

**Abstract:**
There are three fundamental laws in equilibrium thermodynamics. But, what are the laws in nonequilibrium thermodynamics that guides the development of theories/models to describe nonequilibrium phenomena? Continued efforts have been invested in the past on developing a general framework for nonequilibrium thermodynamic models, which include Onsager's maximum entropy theory, Prigogine's minimum entropy production rate theory, Poisson bracket formulation of Beris and Edwards, as well as the GENERIC formalism promoted by Ottinger and Grmela. To some extent, they are equivalent and all give practical means to develop nonequilibrium dynamic models. In this talk, I will focus on the Onsager approach, termed the Generalized Onsager Principle (GOP). I will review how one can derive thermodynamic and generalized hydrodynamic models using the generalized Onsager principle coupled with the variational principle. Then, I will discuss how we can exploit the mathematical structure of the models derived using GOP to design structure and property preserving numerical approximations to the governing system of partial differential equations. Since the approach is valid near equilibrium as pointed it out by Onsager, an energy quadratization strategy is proposed to arrive linear numerical schemes. This approach is so general that in principle we can use it to any nonequilibrium model so long as it has the desired variational and dissipative structure. Some numerical examples will be given to illustrate the usefulness of this approach.

**Date:** April 20, 2017

**Speaker:** Michael Neilan, University of Pittsburgh

**Title:** Discrete theories for elliptic problems in non--divergence
form

**Abstract:** In this talk, two discrete theories for elliptic problems in
non-divergence form are presented. The first, which is applicable to problems
with continuous coefficients and is motivated by the strong solution concept,
is based on discrete Calderon-Zygmund-type estimates. The second theory relies
on discrete Miranda-Talenti estimates for elliptic problems with discontinuous
coefficients satisfying the Cordes condition. Both theories lead to simple,
efficient, and convergent finite element methods. We provide numerical
experiments which confirm the theoretical results, and we discuss possible
extensions to fully nonlinear second order PDEs.

**Date:** March 3, 2017

**Speaker:** Ridgway Scott, University of Chicago

**Title:** Electron correlation in van der Waals interactions

**Abstract:** We examine a technique of Slater and Kirkwood which provides an
exact resolution of the asymptotic behavior of the van der Waals attraction
between two hydrogens atoms. We modify their technique to make the problem
more tractable analytically and more easily solvable by numerical
methods. Moreover, we prove rigorously that this approach provides an exact
solution for the asymptotic electron correlation. The proof makes use of
recent results that utilize the Feshbach-Schur perturbation technique. We
provide visual representations of the asymptotic electron correlation
(entanglement) based on the use of Laguerre approximations.We also describe an
a computational approach using the Feshbach-Schur perturbation and
tensor-contraction techniques that make a standard finite difference approach
tractable.

**Date:** April 22, 2016

**Speaker:** Guillaume Bal, Columbia University

**Title:** Boundary control in transport and diffusion equations

**Abstract:** Consider a prescribed solution to a diffusion equation in a
small domain embedded in a larger one. Can one (approximately) control such a
solution from the boundary of the larger domain? The answer is positive and
this form of Runge approximation is a corollary of the unique continuation
property (UCP) that holds for such equations. Now consider a (phase space,
kinetic) transport equation, which models a large class of scattering
phenomena, and whose vanishing mean free path limit is the above diffusion
model. This talk will present positive as well as negative results on the
control of transport solutions from the boundary. In particular, we will show
that internal transport solutions can indeed be controlled from the boundary
of a larger domain under sufficient convexity conditions. Such results are not
based on a UCP. In fact, UCP does not hold for any positive mean free path
even though it does apply in the (diffusion) limit of vanishing mean free
path. These controls find applications in inverse problems that model a large
class of coupled-physics medical imaging modalities. The stability of the
reconstructions is enhanced when the answer to the control problem is
positive.

**Date:** April 8, 2016

**Speaker:** John Sylvester, University of Washington

**Title:** Evanescence, Translation, and Uncertainty Principles in the
Inverse Source Problem

**Abstract:** The inverse source problem for the Helmholtz equation (time
harmonic wave equation) seeks to recover information about a radiating source
from remote observations of a monochromatic (single frequency) radiated wave
measured far from the source (the far field). The two properties of far fields
that we use to deduce information about shape and location of sources depend
on the physical phenomenon of evanescence, which limits imaging resolution to
the size of a wavelength, and the formula for calculating how a far field
changes when the source is translated. We will show how adaptations of
"uncertainty principles", as described by Donoho and Stark [1] provide a very
useful and simple tool for this kind of analysis.

**Date:**March 24, 2016

**Speaker:** Qi Wang , Interdisciplinary Mathematics Institute and
NanoCenter at University of South Carolina

**Title:** Onsager principle, generalized hydrodynamic theories and
energy stable numerical schemes

**Abstract:** In this talk, I will discuss the Onsager principle for
nonequilibrium thermodynamics and present the generalized Onsager principle
for deriving generalized hydrodynamic theories for complex fluids and active
matter. For closed matter systems, the generalized Onsager principle combines
variational principle with the dissipative property of the system to give a
hydrodynamic system that dissipates the total energy. I will illustrate the
idea using a few examples in complex fluids. For the hydrodynamic system of
equations derived from the generalized Onsager principle, dissipation property
preserving numerical schemes can be devised , known as energy stable
schemes. These schemes are unconditional stable in time. Several applications
of generalized hydrodynamic theories to active matter systems, like cell
migration on solid substrates and cytokinesis of animal cells will be
presented.

**Date:** February 26, 2016

**Speaker:** Andrea Bonito, Texas A&M University

**Title:** Bilayer Plates: From Model Reduction to Gamma-Convergent
Finite Element Approximation

**Abstract:** The bending of bilayer plates is a mechanism which allows for
large deformations via small externally induced lattice mismatches of the
underlying materials. Its mathematical modeling consists of a geometric
nonlinear fourth order problem with a nonlinear pointwise isometry constraint
and where the lattice mismatches act as a spontaneous curvature. A gradient
flow is proposed to decrease the system energy and is coupled with finite
element approximations of the plate deformations based on Kirchhoff
quadrilaterals. In this talk, we give a general overview on the model
reduction procedure, discuss to the convergence of the iterative algorithm
towards stationary configurations and the Gamma-convergence of their finite
element approximations. We also explore the performances of the numerical
algorithm as well as the reduced model capabilities via several insightful
numerical experiments involving large (geometrically nonlinear)
deformations. Finally, we briefly discuss applications to drug delivery, which
requires replacing the gradient flow relaxation by a physical flow.

**Date:** February 26, 2016

**Speaker:** Lou Kondic, New Jersey Institute of Technology

**Title:** Force networks in particulate-based systems: persistence,
percolation, and universality

**Abstract:** Force networks are mesoscale structures that form
spontaneously as particulate-based systems (such as granulars, emulsions,
colloids, foams) are exposed to shear, compression, or impact. The
presentation will focus on few different but closely related questions
involving properties of these networks:

(i) Are the networks universal, with their properties independent of those of
the underlying particles?

(ii) What are percolation properties of these networks, and can we use the
tools of percolation theory to explain their features?

(iii) How to use topological tools, and in particular persistence approach to
quantify the properties of these networks?

The presentation will focus on the results of molecular dynamics/discrete
element simulations to discuss these questions and (currently known) answers,
but I will also comment and discuss how to relate and apply these results to
physical experiments.