## Seminar Categories

- Applied and Computational Math Colloquium (1)
- Climate Seminar (78)
- Colloquium (3)
- MCFAM Seminar (7)
- PDE Seminar (1)

## Current Series

Thu Sep 13 |
## Colloquium3:35pm - Vincent Hall 16Singularities of the Ricci flow and Ricci solitons Huai-Dong Cao, Lehigh University, Ordway Visitor Understanding formation of singularities has been an important subject in the study of the Ricci flow and other geometric flows. It turns out generic singularities in the Ricci flow are modeled on shrinking Ricci solitons. In this talk, I will discuss some of the recent progress on classifications of shrinking Ricci solitons and their stability/instability with respect to Perelman's $\nu$-entropy. |

Tue Sep 25 |
## Colloquium3:35pm - Vincent Hall 16Microlocal codimension-three conjecture Kari Vilonen, University of Melbourne, Ordway Visitor Special functions (or distributions) can be understood and analyzed in terms of the systems of differential equations they satisfy. To this end, a general theory of systems of linear (micro) differential equations was developed by the Sato school in Kyoto. This point of view, in its various incarnations, is now ubiquitous in many parts of mathematics. For example, in the geometric Langlands program and representation theory it allows us to replace functions and group representations by geometric objects, perverse sheaves or D-modules. It has been well-known for a long time that the description of these objects gains more symmetry when one passes to the cotangent bundle. We explain the shape of the general microlocal structure of these objects and discuss, in particular, the role played by the codimension-three conjecture which was proved by Masaki Kashiwara and the speaker a few years ago. |

Thu Oct 11 |
## Colloquium3:35pm - Vincent Hall 16Propagation of bistable fronts through a perforated wall Hiroshi Matano, Meiji University, Tokyo, Ordway Visitor We consider a bistable reaction-diffusion equation on ${\bf R}^N$ in the presence of an obstacle $K$, which is a wall of infinite span with periodically arrayed holes. More precisely, $K$ is a closed subset of ${\bf R}^N$ with smooth boundary such that its projection onto the $x_1$-axis is bounded, while it is periodic in the rest of variables $(x_2,\ldots, x_N)$. We assume that ${\bf R}^N \setminus K$ is connected. Our goal is to study what happens when a planar traveling front coming from $x_1 = +\infty$ meets the wall $K$. We first show that there is clear dichotomy between `propagation' and `blocking'. In other words, the traveling front either completely penetrates through the wall or is totally blocked, and that there is no intermediate behavior. This dichotomy result will be proved by what we call a De Giorgi type lemma for an elliptic equation on ${\bf R}^N$. Then we will discuss sufficient conditions for blocking, and those for propagation. This is joint work with Henri Berestycki and Francois Hamel. |

Thu Nov 15 |
## Colloquium3:35pm - Vincent Hall 16(Log)-Epiperimetric Inequality and the Regularity of Variational Problems Luca Spolaor, MIT In this talk I will present a new method for studying the regularity of minimizers to variational problems. I will start by introducing the notion of blow-up, using as a model case the so-called Obstacle problem. Then I will state the (Log)-epiperimetric inequality and explain how it is used to prove uniqueness of the blow-up and regularity results for the solution near its singular set. I will then show the flexibility of this method by describing how it can be applied to other free-boundary problems and to (almost)-area minimizing currents. Finally I will describe some future applications of this method both in regularity theory and in other settings. |

Tue Nov 27 |
## Colloquium3:35pm - Vincent Hall 16Surface bundles, monodromy, and arithmetic groups Bena Tshishiku, Harvard University Fiber bundles with fiber a surface arise in many areas including hyperbolic geometry, symplectic geometry, and algebraic geometry. Up to isomorphism, a surface bundle is completely determined by its monodromy representation, which is a homomorphism to a mapping class group. This allows one to use algebra to study the topology of surface bundles. Unfortunately, the monodromy representation is typically difficult to "compute" (e.g. determine its image). In this talk, I will discuss some recent work toward computing monodromy groups for holomorphic surface bundles, including certain examples of Atiyah and Kodaira. This can be applied to the problem of counting the number of ways that certain 4-manifolds fiber over a surface. This is joint work with Nick Salter. |

Thu Nov 29 |
## Colloquium10:00am - Vincent Hall 570The role of Energy in Regularity Max Engelstein, MIT The calculus of variations asks us to minimize some energy and then describe the shape/properties of the minimizers. It is perhaps a surprising fact that minimizers to ``nice" energies are more regular than one, a priori, assumes. A useful tool for understanding this phenomenon is the Euler-Lagrange equation, which is a partial differential equation satisfied by the critical points of the energy. However, as we teach our calculus students, not every critical point is a minimizer. In this talk we will discuss some techniques to distinguish the behavior of general critical points from that of minimizers. We will then outline how these techniques may be used to solve some central open problems in the field. We will then turn the tables, and examine PDEs which look like they should be an Euler-Lagrange equation but for which there is no underlying energy. For some of these PDEs the solutions will regularize (as if there were an underlying energy) for others, pathological behavior can occur |

Thu Nov 29 |
## Colloquium3:35pm - Vincent Hall 16Interpolative decomposition and its applications Lexing Ying, Stanford, Ordway Visitor Interpolative decomposition is a simple and yet powerful tool for approximating low-rank matrices. After discussing the theory and algorithm, I will present a few new applications of interpolative decomposition in numerical partial differential equations, quantum chemistry, and machine learning. |

Thu Dec 06 |
## Colloquium3:35pm - Vincent Hall 16Number theory over function fields and geometry Will Sawin, Columbia University The function field model involves taking problems in |

Tue Jan 22 |
## Colloquium2:30pm - Vincent Hall 16Multiscale Problems in Cell Biology Chuan Xue, Ohio State University Complex biological systems involve multiple space and time scales. To get an integrated understanding of these systems involves multiscale modeling, computation and analysis. In this talk, I will discuss two such examples in cell biology and illustrate how to use multiscale methods to explain experimental data. The first example is on chemotaxis of bacterial populations. I will present recent progress on embedding information of single cell dynamics into models of cell population dynamics. I will clarify the scope of validity of the well-known Patlak-Keller-Segel chemotaxis equation and discuss alternative models when it breaks down. The second example is on the axonal cytoskeleton dynamics in health and disease. I will present a stochastic multiscale model that gave the first mechanistic explanation for the cytoskeleton segregation phenomena observed in many neurodegenerative diseases. |

Thu Jan 31 |
## Colloquium3:35pm - Vincent Hall 16From infinite random matrices over finite fields to square ice Leonid Petrov, University of Virginia Asymptotic representation theory of symmetric groups is a rich and beautiful subject with deep connections with probability, mathematical physics, and algebraic combinatorics. A one-parameter deformation of this theory related to infinite random matrices over a finite field leads to randomization of the classical Robinson-Schensted correspondence between words and Young tableaux. Exploring such randomizations we find unexpected applications to six vertex (square ice) type models and traffic systems on a 1-dimensional lattice. |

Tue Feb 05 |
## Colloquium3:35pm - Vincent Hall 16A Multiscale/Multiphysics Coupling Framework for Heart Valve Damage Yue Yu, Lehigh University Bioprosthetic heart valves (BHVs) are the most popular artificial replacements for diseased valves that mimic the structure of native valves. However, the life span of BHVs remains limited to 10-15 years, and the mechanisms that underlie BHVs failure remain poorly understood. Therefore, developing a unifying mathematical framework which captures material damage phenomena in the fluid-structure interaction environment would be extremely valuable for studying BHVs failure. Specifically, in this framework the computational domain is composed of three subregions: the fluid (blood) , the fracture structure (damaged BHVs) modeled by the recently developed nonlocal (peridynamics) theory, and the undamaged thin structure (undamaged BHVs). These three subregions are numerically coupled to each other with proper interface boundary conditions. In this talk, I will introduce two sub-problems and the corresponding numerical methods we have developed for this multiscale/multiphysics framework. In the first problem the coupling strategy for fluid and thin structure is investigated. This problem presents unique challenge due to the large deformation of BHV leaflets, which causes dramatic changes in the fluid subdomain geometry and difficulties on the traditional conforming coupling methods. To overcome the challenge, the immersogeometric method was developed where the fluid and thin structure are discretized separately and coupled through penalty forces. To ensure the capability of the developed method in modeling BHVs, we have verified and validated this method. The second problem focuses on the nonlocal Neumann-type interface boundary condition which plays a critical role in the fluidperidynamics coupling framework. In the nonlocal models the Neumann-type boundary conditions should be defined in a nonlocal way, namely, on a region with non-zero volume outside the surface, while in fluidstructure interfaces the hydrodynamic loadings from the fluid side are typically provided on a sharp co-dimensional one surface. To overcome this challenge, we have proposed a new nonlocal Neumann-type boundary condition which provides an approximation of physical boundary conditions on a sharp surface, with an optimal asymptotic convergence rate to the local counter part. Based on this new boundary condition, we have developed a fluidperidynamics coupling framework without overlapping regions. |

Tue Mar 05 |
## Colloquium3:35pm - Vincent Hall 16Cohomology of Shimura varieties Sug Woo Shin, U.C. Berkeley Shimura varieties are a certain class of algebraic varieties over number fields with lots of symmetries, introduced by Shimura and Deligne nearly half a century ago. They have been playing a central role in number theory and other areas. Langlands proposed a program to compute the L-functions and cohomology of Shimura varieites in 1970s; this was refined by Langlands-Rapoport and Kottwitz in 1980s. I will review some old and recent results in this direction. |

Thu Mar 14 |
## Colloquium3:35pm - Vincent Hall 16Short-Time Asymptotic Methods In Financial Mathematics Jose Figueroa-Lopez, Washington University In this talk, we will be concerned with the average values of certain random functionals of the path of a stochastic process during a given time period. High-order asymptotic characterizations of such values when the time period shrinks to 0 have a wide range of applications. In statistics, they are instrumental in establishing infill asymptotic properties of high-frequency based statistical methods of stochastic processes. In finance, they have been used as model selection and calibration tools based on near expiration option prices. In some Engineering problems, they also show up as a method to solve a problem in continuous time by looking at the analogous problem in discrete time and shrinking the time step to 0. These short-time asymptotic methods are especially useful in the study of complex models with jumps and stochastic volatility due to the lack of tractable formulas and efficient statistical and numerical procedures. In this talk, I will discuss some recent advances in the area and illustrate their broad relevance in several contexts. Bio: Dr. Figueroa-López is a Professor in the Department of Mathematics and Statistics at Washington University in St. Louis. He currently serves as part of the executive committee and the chair of the statistics committee of the Department. Formerly he was Associate Professor of Statistics at Purdue University, where he served as Associate Director of the Computational Finance Program and as a member of the University Senate. Professor Figueroas ongoing research includes short-time asymptotics of jump-diffusion models, diffusion limits of Limit Order Book models, optimal limit order placement problems, market making via reinforced learning, and optimal tuning of high-frequency based econometric methods. He was awarded the NSF career award in 2012 and currently has two active NSF grants on the interplay of finance, statistics, and probability. He is an Associate Editor of the SIAM Journal on Financial Mathematics (SIFIN) and a former Associate Editor of Electronic Journal of Statistics. |

Tue Apr 16 |
## Colloquium3:35pm - Vincent Hall 16What's known about the Birch and Swinnerton-Dyer Conjecture Ordway Visitor Christopher Skinner, Princeton University The celebrated Birch and Swinnerton-Dyer (BSD) Conjecture connects the structure of the rational points on an elliptic curve defined over the rationals to the analytic properties of its associated Hasse-Weil L-function. This talk will recall the BSD conjecture (and its various parts) and related conjectures and survey some of the known results toward them, especially recent work. This talk is intended for a general mathematical audience: no prior acquaintance with elliptic curves or even non-elementary number theory will be assumed. |

Thu Apr 18 |
## Colloquium3:35pm - Vincent Hall 16Teichmueller flow and complex geometry of Moduli Spaces Vlad Markovic - Ordway Visitor, Caltech I will explain why in general the Caratheodory and Teichmueller metrics do not agree on Teichmueller spaces and why this yields a proof of the convexity conjecture of Siu. Moreover, I will illustrate how deep theorems in Teichmueller dynamics play an important role in classifying Teichmueller discs where the two metrics agree. |

Thu Apr 25 |
## Colloquium3:35pm - Vincent Hall 16Recent progress on the Gan-Gross-Prasad and Ichino-Ikeda conjectures for unitary groups Raphael Beuzart-Plessis, CNRS and Columbia University A celebrated result of Waldspurger from the eighties express the central value of certain base-change L-functions for $GL_2$ as toric periods of modular forms or generalizations thereof. In the mid 2000s Gan, Gross and Prasad have formulated conjectural generalizations to higher rank classical groups relating the non-vanishing of central values of certain automorphic L-functions to the non-vanishing of certain explicit integrals of automorphic forms that are called 'automorphic periods'. These predictions have been subsequently refined by Ichino-Ikeda and N.Harris into precise identities relating the two invariants. These conjectures also have local counterparts which concern certain branching laws in the representation theory of real or p-adic groups. Most of these conjectures have now been established for unitary groups. This talk aims to give an introduction to this circle of ideas and to review recent results on the subject. |

Tue Apr 30 |
## Colloquium3:35pm - Vincent Hall 16The NTRU Class of cryptosystems, lattices, and post quantum cryptography Jeffrey Hoffstein, Brown University I'll discuss the nature and origins of public key cryptography, and the dangers that the development of quantum computers pose to the security of the internet and virtually all cryptosystems presently in widespread use. All public key cryptographic systems are based on a hard mathematical problem, and I'll explain how, while originally despised, hard problems based on lattices have evolved to become the most promising direction for the development of quantum resistant cryptography. I'll focus on NTRU, the earliest effective and efficient public key cryptosystem, which was developed in 1996 by myself, Jill Pipher and Joe Silverman. I will also discuss the recent history of public key cryptography. The talk will require no previous knowledge of lattices or cryptography, and will be aimed at a wide audience. |

Thu May 02 |
## Colloquium3:35pm - Vincent Hall 16Singularity Formation in 3D Euler Equations and Related Models Thomas Hou, Ordway Vsitor, Caltech Whether the 3D incompressible Euler equations can develop a finite time singularity from smooth initial data is a long-standing open question in mathematical fluid dynamics. Recent computations have provided strong numerical evidence that the 3D Euler equations develop a finite time singularity from smooth initial data. I will report some recent progress in providing a rigorous justification of the singularity formation in the 3D Euler equations and related models. |

Thu Sep 05 |
## Colloquium3:35pm - Vincent Hall 16Colloquium Alina Chertock, NSCU |

Thu Sep 19 |
## Colloquium3:35pm - Vincent Hall 16Colloquium Solomon Friedberg, Boston College |

Thu Oct 17 |
## Colloquium3:35pm - Vincent Hall 16Colloquium Marcelo Aguiar, Cornell University |