Jake Bouvrie : Learning and Synchronization in Stochastic Neural Ensembles
- Graduate/Faculty Seminar ( 97 Views )We consider a biological learning model composed of coupled stochastic neural ensembles obeying a nonlinear gradient dynamics. The dynamics optimize a simple error criterion involving noisy observations provided by the environment, leading to a function that can be used to make decisions in the future. The uncertainty of the resulting decision function is characterized, and shown to be controlled in large part by trading off coupling strength (and/or network topology) against the ambient neuronal noise. Further connections with classical regularization notions in statistical learning theory will also be explored.
Hangjun Xu : Constant Mean Curvature Surfaces in Asymptotically Flat Manifolds
- Graduate/Faculty Seminar ( 107 Views )The study of surfaces with constant mean curvature (CMC) goes back to 1841 when Delaunay classified all CMC surfaces of revolution. There has been consistent work on finding CMC hypersurfaces in various ambient manifolds. In this talk, we will discuss some nice properties of CMC surfaces, and then the existence of CMC surfaces in the Schwarzschild, and in general, asymptotically flat manifolds.
Spencer Leslie : Intro to crystal graphs and their connections with number theory
- Graduate/Faculty Seminar ( 187 Views )I will review some basics of crystal bases for highest-weight representations for a semisimple Lie algebra. I will also point to some connections with number theory through Fourier coefficients of Eisenstein series, mostly in type A.
Dmitry Vagner : Higher Dimensional Algebra in Topology
- Graduate/Faculty Seminar ( 219 Views )In his letter, "Pursuing Stacks," Grothendieck advocated to Quillen for the use of "higher" categories to encode the higher homotopy of spaces. In particular, Grothendieck dreamt of realizing homotopy n-types as n-groupoids. This powerful idea both opened the field of higher dimensional algebra but also informed a paradigm in which the distinction between topology and algebra is blurred. Since then, work by Baez and Dolan among others further surveyed the landscape of higher categories and their relationship to topology. In this talk, we will explore this story, beginning with some definitions and examples of higher categories. We will then proceed to explain "the periodic table of higher categories" and the four central hypotheses of higher category theory. In particular, these give purely algebraic characterizations of homotopy types, manifolds, and generalized knots; and account for the general phenomena of stabilization in topology. No prerequisites beyond basic ideas in algebraic topology will be expected.
Dave Rose : Categorification and knot homology
- Graduate/Faculty Seminar ( 103 Views )Categorification can be viewed as the process of lifting scalar and polynomial invariants to homology theories having those invariants as (graded) Euler characteristics. In this talk, we will discuss categorification in general and as manifested in specific examples (ie Khovanov homology and knot Floer homology). Examples will be given showing how the categorified invariants are stronger and often more useful than the original invariants. I will motivate categorification using familiar constructions from (very basic) topology. It is my hope that this will make the discussion accessible to a wide audience. No prior knowledge of knot theory or category theory needed!
Gero Friesecke : Twisted X-rays and the determination of atomic structure
- Graduate/Faculty Seminar ( 116 Views )We find exact solutions of Maxwell's equations which yield discrete Bragg-peak-type diffraction patterns for helical structures, in the same way in which plane waves yield discrete diffraction patterns of crystals. We call these waves 'twisted X-rays', on account of its 'twisted' waveform. As in the crystal case, the atomic structure can be determined from the diffraction pattern. We demonstrate this by recovering the structure of the Pf1 virus (Protein Data Bank entry 1pfi) from its simulated diffraction data under twisted X-rays.
The twisted waves are found in a systematic way, by first answering a simpler question: could we derive plane waves from the goal that the diffraction pattern crystals is discrete? The answer is yes. Constructive interference at the intensity maxima trivially comes from the fact that the waves share the discrete translation symmetry of crystals. Destructive interference off the maxima is much more subtle, and - as I will explain in the talk - can be traced to the fact that the waves have a larger, continuous translation symmetry. Replacing the continuous translation group by the continuous helical group which extends the discrete symmetry of helical structures leads to twisted waves.
Once the waveforms are found, discreteness (or mathematically, extreme sparsity) of the diffraction pattern of helices under these waves can be proven by appealing to the generalisation of the Poisson summation formula to abelian groups which goes back to A. Weil, whose motivation came from number theory rather than structural biology.
Joint work with Dominik Juestel (TUM) and Richard James (University of Minnesota), SIAM J. Appl. Math. 76 (3), 2016, and Acta Cryst. A72, 190, 2016.
Dave Rose : The EilenbergMazur swindle
- Graduate/Faculty Seminar ( 137 Views )At some point in every mathematician's life they have seen the paradoxical 'proof' that 1=0 obtained by different groupings of the infinite sum 1-1+1-1+... As we learn, the issue is that this series does not converge. The Eilenberg-Mazur swindle is a twist on this argument which shows that A+B+A+B+... = 0 implies that A=0=B in certain situations where we can make sense of the infinite sum. In this talk, we will explore these swindles, touching on many interesting areas of mathematics along the way.
Margaret Regan : Using homotopy continuation to solve parametrized polynomial systems in applications
- Graduate/Faculty Seminar,Uploaded Videos ( 1694 Views )Many problems that arise in mathematics, science, and engineering can be formulated as solving a parameterized system of polynomial equations which must be solved for given instances of the parameters. One way to solve these systems is to use a common technique within numerical algebraic geometry called homotopy continuation. My talk will start with background on homotopy continuation and parametrized polynomial systems, followed by applications to problems in computer vision and kinematics. Of these, I will first present a new approach which uses locally adaptive methods and sparse matrix calculations to solve parameterized overdetermined systems in projective space. Examples will be provided in 2D image reconstruction to compare the new methods with traditional approaches in numerical algebraic geometry. Second, I will discuss a new definition of monodromy action over the real numbers which encodes tiered characteristics regarding real solutions. Examples will be given to show the benefits of this definition over a naive extension of the monodromy group (over the complex numbers). In addition, an application in kinematics will be discussed to highlight the computational method and impact on calibration.
Dave Rose : Cartans theorem on maximal tori
- Graduate/Faculty Seminar ( 121 Views )Cartan's theorem on maximal tori in compact Lie groups can be thought of as a generalization of the spectral theorem for unitary matrices. The goal of this talk will be to sketch the `topological' proof of this theorem, based on the Lefschetz fixed point theorem. Along the way, we'll encounter the flag variety, an interesting object whose geometry encodes the representation theory of the Lie group. Those who don't specialize in geometry or topology fear not, we will give examples showing that these concepts are very concrete objects familiar from linear algebra.
Demetre Kazaras:The geometry and topology of positive scalar curvature
- Graduate/Faculty Seminar,Uploaded Videos ( 1600 Views )I will give an informal overview of the history and status of my field. Local invariants of Riemannian metrics are called curvature, the weakest of which is known as "scalar curvature." The study of metrics with positive scalar curvature is very rich with >100 year old connections to General Relativity and smooth topology. Does this geometric condition have topological implications? The answer turns out to be "yes," but mathematicians continue to search for the true heart of the positive scalar curvature conditions.
Erin Beckman : A Look at Branching Processes
- Graduate/Faculty Seminar ( 124 Views )In 1873, a man named Francis Galton posed a question in Educational Times, calling for the mathematical study of the extinction of family surnames over time. Within a year, mathematician Henry Watson replied with a solution. But instead of ending there, this question opened up a new direction of mathematics: the study of branching processes. A branching process is a particle system in which the particles undergo splitting or branching events dictated by particular rules. This talk will introduce some examples of these systems (from the basic Galton-Watson model to more general branching-selection models), interesting questions people ask about branching processes, and some recent research done in this area.
Kash Balachandran : The Kakeya Conjecture
- Graduate/Faculty Seminar ( 134 Views )In 1917, Soichi Kakeya posed the question: What is the smallest amount of area required to continuously rotate a unit line segment in the plane by a full rotation? Inpsired by this, what is the smallest measure of a set in $\mathbb{R}^n$ that contains a unit line segment in every direction? Such sets are called Kakeya sets, and can be shown to have arbitrarily small measure w.r.t. n-dimensional Lebesgue measure [and in fact, measure zero]. The Kakeya conjecture asserts that the Hausdorff and Minkowski dimension of these sets in $\mathbb{R}^n$ is $n$. In this talk, I will introduce at a very elementary level the machinery necessary to understand what the Kakeya conjecture is asking, and how the Kakeya conjecture has consequences for fields diverse as multidimensional Fourier summation, wave equations, Dirichlet series in analytic number theory, and random number generation. I'll also touch on how tools from various mathematical disciplines from additive combinatorics and algebraic geometry to multiscale analysis and heat flow can be used to obtain partial results to this problem. The talk will be geared towards a general audience.
Hubert Bray : On Dark Matter, Galaxies, and the Large Scale Geometry of the Universe
- Graduate/Faculty Seminar ( 110 Views )More than 95% of the present day curvature of the universe is not a result of regular baryonic matter represented by the periodic table of elements. About 73% is well described by a geometrically natural cosmological constant, also referred to as dark energy, which results in a very small amount of curvature uniformly spread throughout the universe. We will explore the possibility that the remaining 23%, commonly referred to as dark mater, could also be explained very naturally from a geometric point of view.
Mark Iwen : Compressed Sensing for Manifold Data
- Graduate/Faculty Seminar ( 109 Views )We will discuss techniques for approximating a point in high-dimensional Euclidean space which is close to a known low-dimensional compact submanifold when only a compressed linear sketch of the point is available. More specifically, given a point, x, close to a known submanifold of R^D, we will consider linear measurement operators, M: R^D -> R^m, which have associated nonlinear inverses, A: R^m -> R^D, so that || x - A(Mx) || is small even when m << D. Both the design of good linear operators, M, and the design of stable nonlinear inverses, A, will be discussed. An algorithmic implementation of a particular nonlinear inverse will be presented, along with related stability bounds for the approximation of manifold data.
Christopher O'Neill : Mesoprimary Decomposition of Binomial Ideals using Monoid Congruences
- Graduate/Faculty Seminar ( 134 Views )Many algebraic objects are notorious for being easy to define, but hard to find explicitly. However, certain algebraic objects, when viewed with the "correct" combinatorial framework, become much easier to actually find. This allows us to compute much larger examples by hand, and often gives us insight into the object's underlying structure. In this talk, we will define irreducible decompositions of ideals, and explore their underlying combinatorial structure in the special case of monomial ideals in polynomial rings. As time permits, we will look at recent results in the case of binomial ideals. This talk will be accessible to anyone who has taken a course in Abstract Algebra.
Graham Cox : Unsolvable problems in geometry and topology
- Graduate/Faculty Seminar ( 113 Views )The resolution of Hilbert's tenth problem yields the following unsolvability result: there is no algorithm for determining whether or not a given polynomial equation p(x_1,...,x_n) = 0 with integer coefficients will admit an integer solution. After a few definitions and examples, I will discuss another well-known unsolvable problem: the word problem for finitely presented groups. It can be shown that there is no algorithm for determining when an arbitrary word in a finitely presented group is trivial. This has many remarkable topological consequences, including the result that there is no algorithm that will determine when two given manifolds are homeomorphic (provided the dimension is at least four). The unsolvability theorem also has significant geometric applications, allowing one to prove that certain manifolds admit an infinite number of contractible closed geodesics (regardless of the Riemannian structure).
Ioannis Sgouralis : Renal autoregulation in a dynamic nephrovascular model
- Graduate/Faculty Seminar ( 124 Views )Renal autoregulation stabilizes kidney functions and provides protection against blood pressure fluctuations. Autoregulation is mediated by two mechanisms: the *myogenic response,* where increased blood pressure elicits vascular constriction, and *tubuloglomerular feedback,* where salt excretion is balanced by adjustments of filtration rate. Coupling of the two mechanisms give rise to complex behaviours that are challenging to analyse. In the talk, I will describe a mathematical model of renal autoregulation, which represents both mechanisms and thus can be used to study the interactions developed among them. I will provide the necessary physiological background, and I will focus on the mathematical formulation of the involved processes. The talk will be accessible to everyone with basic understanding of differential equations.
Jeff Jauregui : Geometry and topology in low dimensions: an introduction togeometric flows
- Graduate/Faculty Seminar ( 126 Views )This talk will be geared toward first and second year grad students and/or anyone with limited geometry experience. We will discuss the idea of curvature for curves and surfaces and the notion of "best metrics." The classical Uniformization Theorem will be introduced from a modern angle: Ricci flow. This will motivate studying the Ricci flow in dimension 3 as a tool to understand topology in terms of geometry. Time permitting, we will finish by discussing the Geometrization Theorem.
Michael Abel : An introduction to Khovanov homology
- Graduate/Faculty Seminar ( 100 Views )Khovanov homology is a special case of a process known as categorification. The idea of categorification is to lift a known polynomial invariant of links to a homology theory whose isomorphism type is an invariant of links and whose ¡°Euler characteristic¡± is the original polynomial. In the case of Khovanov homology, this Euler characteristic is the famous Jones polynomial. After reviewing some basic knot theory and the construction of the Jones polynomial, we discuss the construction of Khovanov homology. Finally, we will discuss some topological applications of Khovanov homology.
Paul Bendich : Topological Data Analysis
- Graduate/Faculty Seminar ( 135 Views )TDA is now about fifteen years old, and is quickly becoming a widely applied tool in data analysis. In this talk, I'll describe how homology groups, a traditional algebraic invariant, can be turned into persistence diagrams, a robust statistical tool for dealing with high-dimensional data or embedded geometric objects. Time permitting, this talk should have some theory, some applications, and some algorithms, and perhaps even a proof.
Chung-Ru Lee : Introduction to Trace Formula
- Graduate/Faculty Seminar ( 194 Views )The Trace Formula can be understood roughly as an equation relating spectral data to geometric information. It is obtained via expansion of the trace of certain operators that are associated to the Representation Theory of an affine algebraic group, justifying its name. Therefore, the spectral side of the expansion by nature contains data of arithmetic interests. However, the spectral side is generally less accessible. Meanwhile, the geometric side consists of terms that can be written in a more explicit fashion. The computation of the geometric side, which is now referred to as the Orbital Integrals, thus come on the scene. In this talk, we plan to briefly introduce the general derivation of the (vaguely described) Trace Formula, and demonstrate a few concrete examples of it.
Anil Venkatesh : The Arithmetic of Modular Forms
- Graduate/Faculty Seminar ( 108 Views )In this talk, we investigate modular forms and their many connections to number theory. Modular forms are analytic functions on the upper half complex plane that satisfy certain functional equations. They arise in many contexts in number theory: from partitions of integers, to arithmetical divisor functions, to cutting edge research on special values of the Riemann zeta function. We discuss both classical and modern examples, with a view toward illustrating the profound connections between analysis, topology, and number theory.
Caitlin Leverson : Legendrian Knots, Augmentations, and Rulings
- Graduate/Faculty Seminar ( 110 Views )Given a contact structure (a plane field) on R^3, one can define a Legendrian knot to be an embedding of the circle such that the embedding is everywhere tangent to the plane field. Surgery along such a knot gives a way to construct new manifolds and so there is interest in classifying Legendrian knots. This turns out to be a finer classification than that of topological knots -- there are many different Legendrian unknots. Given a Legendrian knot, one can associate the Chekanov-Eliashberg differential graded algebra (DGA) generated by the crossings and then find augmentations of this DGA much like those in your standard algebraic topology course. This talk will give an overview of the relationships Joshua Sabloff and Dmitry Fuchs gave between such rulings and augmentations and how it relates to my current work.