# All Seminars

Title: Structured Matrix Approximation by Separation and Hierarchy
Seminar: Numerical Analysis and Scientific Computing
Speaker: Difeng Cai of Emory University
Contact: Yuanzhe Xi, yxi26@emory.edu
Date: 2019-09-13 at 2:00PM
Venue: MSC W303
Abstract:
The past few years have seen the advent of big data, which brings unprecedented convenience to our daily life. Meanwhile, from a computational point of view, a central question arises amid the exploding amount of data: how to tame big data in an economic and efficient way. In the context of matrix computations, the question consists in the ability to handle large dense matrices. In this talk, I will first introduce data-sparse hierarchical representations for dense matrices. Then I will present recent development of a versatile algorithm called SMASH to operate dense matrices with optimal complexity in the most general setting. Various applications will be presented to demonstrate the advantage of SMASH over traditional approaches.
Title: Analytic representations of large discrete structures
Seminar: Combinatorics
Speaker: Daniel Kral of Masaryk University and the University of Warwick
Contact: Dwight Duffus, dwightduffus@emory.edu
Date: 2019-09-13 at 4:00PM
Venue: MSC W301
Abstract:
The theory of combinatorial limits aims to provide analytic models representing large graphs and other discrete structures. Such analytic models have found applications in various areas of computer science and mathematics, for example, in relation to the study of large networks in computer science. We will provide a brief introduction to this rapidly developing area of combinatorics and we will then focus on several questions motivated by problems from extremal combinatorics and computer science. The two topics that we will particularly discuss include quasirandomness of discrete structures and a counterexample to a a conjecture of Lovasz, which was was one of the two most cited conjectures in the area and which informally says that optimal solutions to extremal graph theory problems can be made asymptotically unique by introducing finitely many additional constraints.
Title: Computing unit groups
Seminar: Algebra
Speaker: Justin Chen of Georgia Tech
Contact: David Zureick-Brown, dzb@mathcs.emory.edu
Date: 2019-09-10 at 4:00PM
Venue: MSC W303
Abstract:
The group of units of a ring is one of the most basic, yet mysterious, invariants of the ring. Little is known about the structure of the unit group in general, much less explicit algorithms for computation, although the need for these do arise in applications such as tropical geometry. I will discuss some general questions about unit groups, and then specialize to the case of coordinate rings of classical algebraic varieties - in particular, describing explicit algorithms for computation in the case of smooth curves of low genus (rational and elliptic). This is based on joint work with Sameera Vemulapalli and Leon Zhang.
Title: A Step in the Right Dimension: Tensor Algebra and Applications
Seminar: Numerical Analysis and Scientific Computing
Speaker: Elizabeth Newman of Emory University
Contact: Yuanzhe Xi, yxi26@emory.edu
Date: 2019-09-06 at 2:00PM
Venue: MSC W303
Abstract:
As data have become more complex to reflect multi-way relationships in the real world, tensors have become essential to reveal latent content in multidimensional data. In this talk, we will focus on a tensor framework based on the M-product, a general class of tensor-tensor products which imposes algebraic structure in a high-dimensional space (Kilmer and Martin, 2011; Kernfeld et al., 2015). The induced M-product algebra inherits matrix-mimetic properties and offers provably optimal, compressed representations. To demonstrate the efficacy of working in an algebraic tensor framework, we will explore two applications: classifying data using tensor neural networks and forming sparse representations using tensor dictionaries.
Title: Spanning subgraphs in uniformly dense and inseparable graphs
Seminar: Combinatorics
Speaker: Mathias Schacht of The University of Hamburg and Yale University
Contact: Dwight Duffus, dwightduffus@emory.edu
Date: 2019-09-06 at 4:00PM
Venue: MSC W301
Abstract:
We consider sufficient conditions for the existence of k-th powers of Hamiltonian cycles in n-vertex graphs G with minimum degree cn for arbitrarily small c >0 . About 20 years ago Komlós, Sarközy, and Szemerédi resolved the conjectures of Pósa and Seymour and obtained optimal minimum degree conditions for this problem by showing that c=k/k+1 suffices for large n. For smaller values of c the given graph G must satisfy additional assumptions. We show that inducing subgraphs of density d>0 on linear subsets of vertices and being inseparable, in the sense that every cut has density at least c, are sufficient assumptions for this problem and, in fact, for a variant of the bandwidth theorem. This generalises recent results of Staden and Treglown.
Title: Modular linear differential equations
Seminar: Algebra
Speaker: Kiyokazu Nagatomo of Osaka University
Contact: David Zureick-Brown, dzb@mathcs.emory.edu
Date: 2019-09-03 at 4:00PM
Venue: MSC W303
Abstract:
The most naive definition of \textit{modular linear differential equations} (MLDEs) would be linear differential equations whose space of solutions are invariant under the weight $k$ slash action of $\Gamma_1=SL_2(\mathbb{Z})$, for some $k$. Then under an analytic condition for coefficients functions and the Wronskians of a~basis of the space of solutions of equations, we have (obvious) expressions of MLDEs as: $L(f) \,=\,\mathfrak{d}_k^n(f)+\sum_{i=2}^nP_{2i}\mathfrak{d}_k^{n-i}(f)$ where $P_{2i}$ is a modular form of of weight $2i$ on $SL_2(\mathbb{Z})$ and $\mathfrak{d}_k(f)$ is the \textit{Serre derivative}. (We could replace $\Gamma$ by a Fuchsian subgroup of $SL_2(\mathbb{R})$ and allow the modular forms $P_{2i}$ to be meromorphic.) However, the iterated Serre derivative $\mathfrak{d}_k^n(f)$ (called a higher Serre derivation'' because as an operator it preserves modulality) is very complicated since it involves the Eisenstein series $E_2$. MLDEs, of course, can be given in the form % $% \mathsf{L}(f) \,=\, D^n(f)+\sum_{i=1}^nQ_iD^i(f)\quad\text{where D=\frac{1}{2\pi\sqrt{-1}}\frac{d}{d\tau}.} %$ $\mathsf{L}(f) \,=\, D^n(f)+\sum_{i=1}^nQ_iD^i(f)$ where $D=\frac{1}{2\pi\sqrt{-1}}\frac{d}{d\tau}.$ Then it is not easy to know if the equation above is an MLDE except the fact that $Q_i$ are quasimodular forms. Very recently, Y.~Sakai and D.~Zagier (my collaborators) found formulas of $\mathsf{L}(f)$ by using the Rankin--Cohen products between $f$ and $g_i$. This is a modular form of weight $2i$, which is a linear function of the differential of~$Q_{j}$. Moreover, there are \textit{inversion formulas} which express $Q_i$ as a linear function of the derivatives of $g_{j}$. The most important fact is that the order $n$ and $n-1$ parts are equal to the so-called higher Serre derivative in the sense of Kaneko and Koike, where the group is $\Gamma_1$. (This holds for any Fuchsian group.) \\ Finally, the most important nature of my talk is that I will use a \textbf{blackboard} instead of \textbf{slides}ss.
Title: Iterative regularization methods for large-scale linear inverse problems
Seminar: Numerical Analysis and Scientific Computing
Speaker: Silvia Gazzola of University of Bath
Contact: James Nagy, jnagy@emory.edu
Date: 2019-08-27 at 2:00PM
Venue: MSC W301
Abstract:
Inverse problems are ubiquitous in many areas of Science and Engineering and, once discretized, they lead to ill-conditioned linear systems, often of huge dimensions: regularization consists in replacing the original system by a nearby problem with better numerical properties, in order to find a meaningful approximation of its solution. After briefly surveying some standard regularization methods, both iterative (such as many Krylov methods) and direct (such as Tikhonov method), this talk will introduce a recent class of methods that merge an iterative and a direct approach to regularization. In particular, strategies for choosing the regularization parameter and the regularization matrix will be emphasized, eventually leading to the computation of approximate solutions of Tikhonov problems involving a regularization term expressed in some p-norms.
Title: Learning from data through the lens of mathematical models: Bayesian Inverse Problems and Uncertainty Quantification
Seminar: Numerical Analysis and Scientific Computing
Speaker: Umberto Villa, Ph.D. of Washington University in St Louis
Contact: Alessandro Veneziani, ale@mathcs.emory.edu
Date: 2019-06-24 at 2:00PM
Venue: MSC W301
Abstract:
Recent years have seen rapid growth in the volume of observational and experimental data acquired from physical, biological or engineering systems. A fundamental question in several areas of science, engineering, medicine, and beyond is how to extract insight and knowledge from all of those available data. This process of learning from data is at its core a mathematical inverse problem. That is, given (possibly noisy) data and a (possibly uncertain) forward model describing the map from parameters to data, we seek to reconstruct or infer the parameters that characterize the model. Inverse problems are often ill-posed, i.e. their solution may not exist or may not be unique or may be unstable to perturbation in the data. Simply put, there may not be enough information in the data to fully determine the model parameters. In these cases, uncertainty is a fundamental feature of the inverse problem. The goal then is to both reconstruct the model parameters and quantify the uncertainty in such reconstruction. The ability to quantify these uncertainties is crucial to reliably predict the future behavior of the physical, biological or engineering systems, and to make informed decisions under uncertainty. This talk will illustrate the mathematical concepts and computational tools necessary for the solution of inverse problems in a deterministic and probabilistic (Bayesian) framework. Examples of inverse problems arising in imaging, geoscience, material engineering, and other fields of science will be presented. https://engineering.wustl.edu/Profiles/Pages/Umberto-Villa.aspx
Title: Discretize-Optimize Methods for Residual Neural Networks
Seminar: Numerical Analysis and Scientific Computing
Speaker: Derek Onken of Emory University
Contact: Lars Ruthotto, lruthotto@emory.edu
Date: 2019-04-26 at 2:00PM
Venue: MSC W301
Abstract:
Neural networks (discrete universal approximators) demonstrate impressive performance in myriad tasks. Specifically, Residual Neural Networks (ResNets) have won numerous image classification contests since they were introduced a few years ago. Deep learning centers around the addition of more and more layers (and thus parameters) to these networks in efforts to improve performance. In this talk, we interpret ResNets as a discretization of an ordinary differential equation (ODE). This viewpoint exposes the similarity between the learning problem and problems of optimal control of the ODE. We use a discretize-optimize approach for training the weights of the ResNet and study the impact of the particular discretization strategy on the network performance. Varying the discretization of the features and parameters allows us to determine if the improved accuracy from deeper architectures stems from the larger number of parameters or more layers.
Title: Using Mathematics to Aid in the Evaluation of Robotic Systems
Evans Hall Lecture: Mathematics/Robotic Systems
Speaker: Mili Shah, PhD of Associate Professor, Department of Mathematics, The Cooper Union
Contact: Kristen Schroeder, kfschro@emory.edu
Date: 2019-04-25 at 4:00PM
Venue: MSC: E208
Abstract:
Robotic systems use a variety of sensors to perform tasks that are assigned to them. In order to use data collected from these sensors, they must be registered with respect to a common coordinate frame. This talk will begin with an overview of the mathematical tools needed to tackle a registration problem and will conclude with current research and open problems related to the mathematics of evaluating robotic systems.