## Noga Alon : Gergen Lecture Seminar 3 Distance problems for Euclidean and other norms Lecture B: Coloring and ordering

- Gergen Lectures ( 0 Views )Distance problems in discrete geometry include fascinating examples of questions that are easy to state and hard to solve. Three of the best known problems of this type, raised in the 40s, are the Erd\H{o}s Unit Distance Problem, his Distinct Distances Problem, and the Hadwiger-Nelson Problem about the chromatic number of the unit distance graph in the plane. I will describe surprisingly tight recent solutions of the analogs of all three problems for typical norms, settling, in a strong form, questions and conjectures of Matou\v{s}ek, of Brass, of Brass, Moser and Pach, and of Chilakamarri. I will also discuss a related work about ordering points according to the sum of their distances from chosen vantage points. The proofs combine Combinatorial, Geometric and Probabilistic methods with tools from Linear Algebra, Topology, and Algebraic Geometry. Based on recent joint works with Matija Buci\'c and Lisa Sauermann, and with Colin Defant, Noah Kravitz and Daniel Zhu.

## Christopher Hacon : Birational geometry in characteristic $p>5$

- Gergen Lectures ( 285 Views )After the recent exciting progress in understanding the geometry of algebraic varieties over the complex numbers, it is natural to try to understand the geometry of varieties over an algebraically closed field of characteristic $p>0$. Many technical issues arise in this context. Nevertheless, there has been much recent progress. In particular, the MMP was established for 3-folds in characteristic $p>5$ by work of Birkar, Hacon, Xu and others. In this talk we will discuss some of the challenges and recent progress in this active area.

## Robert V. Kohn : A Variational Perspective on Wrinkling Patterns in Thin Elastic Sheets: What sets the patterns seen in geometry-driven wrinkling?

- Gergen Lectures ( 286 Views )The wrinkling of thin elastic sheets is very familiar: our skin
wrinkles, drapes have coarsening folds, and a sheet stretched
over a round surface must wrinkle or fold.

What kind of mathematics is relevant? The stable configurations of a
sheet are local minima of a variational problem with a rather special
structure, involving a nonconvex membrane term (which favors isometry)
and a higher-order bending term (which penalizes curvature). The bending
term is a singular perturbation; its small coefficient is the sheet
thickness squared. The patterns seen in thin sheets arise from energy
minimization -- but not in the same way that minimal surfaces arise
from area minimization. Rather, the analysis of wrinkling is an example
of "energy-driven pattern formation," in which our goal is to understand
the asymptotic character of the minimizers in a suitable limit (as the
nondimensionalized sheet thickness tends to zero).

What kind of understanding is feasible? It has been fruitful to focus
on how the minimum energy scales with sheet thickness, i.e. the "energy
scaling law." This approach entails proving upper bounds and
lower bounds that scale the same way. The upper bounds tend to be
easier, since nature gives us a hint. The lower bounds are more subtle,
since they must be ansatz-free; in many cases, the arguments used to
prove the lower bounds help explain "why" we see particular patterns.
A related but more ambitious goal is to identify the prefactor as well
as the scaling law; Ian Tobasco's striking recent work on geometry-driven
wrinkling has this character.

Lecture 1 will provide an overview of this topic (assuming no background
in elasticity, thin sheets, or the calculus of variations). Lecture 2 will
discuss some examples of tensile wrinkling, where identification of the
energy scaling law is intimately linked to understanding the local
length scale of the wrinkles. Lecture 3 will discuss our emerging
undertanding of geometry-driven wrinkling, where (as Tobasco has
shown) it is the prefactor not the scaling law that explains the
patterns seen experimentally.

## Robert V. Kohn : A Variational Perspective on Wrinkling Patterns in Thin Elastic Sheets: What sets the local length scale of tensile wrinkling?

- Gergen Lectures ( 282 Views )The wrinkling of thin elastic sheets is very familiar: our skin
wrinkles, drapes have coarsening folds, and a sheet stretched
over a round surface must wrinkle or fold.

What kind of mathematics is relevant? The stable configurations of a
sheet are local minima of a variational problem with a rather special
structure, involving a nonconvex membrane term (which favors isometry)
and a higher-order bending term (which penalizes curvature). The bending
term is a singular perturbation; its small coefficient is the sheet
thickness squared. The patterns seen in thin sheets arise from energy
minimization -- but not in the same way that minimal surfaces arise
from area minimization. Rather, the analysis of wrinkling is an example
of "energy-driven pattern formation," in which our goal is to understand
the asymptotic character of the minimizers in a suitable limit (as the
nondimensionalized sheet thickness tends to zero).

What kind of understanding is feasible? It has been fruitful to focus
on how the minimum energy scales with sheet thickness, i.e. the "energy
scaling law." This approach entails proving upper bounds and
lower bounds that scale the same way. The upper bounds tend to be
easier, since nature gives us a hint. The lower bounds are more subtle,
since they must be ansatz-free; in many cases, the arguments used to
prove the lower bounds help explain "why" we see particular patterns.
A related but more ambitious goal is to identify the prefactor as well
as the scaling law; Ian Tobasco's striking recent work on geometry-driven
wrinkling has this character.

Lecture 1 will provide an overview of this topic (assuming no background
in elasticity, thin sheets, or the calculus of variations). Lecture 2 will
discuss some examples of tensile wrinkling, where identification of the
energy scaling law is intimately linked to understanding the local
length scale of the wrinkles. Lecture 3 will discuss our emerging
undertanding of geometry-driven wrinkling, where (as Tobasco has
shown) it is the prefactor not the scaling law that explains the
patterns seen experimentally.

## Luis Caffarelli : Degenerate ellipticity and the porous media equation

- Gergen Lectures ( 234 Views )In the first lecture I will give a brief discussion of local and non local diffusion and degenerate ellipticity and different local and non local models for compressible flows in porous media.

In the second and third lectures I will discuss some properties of the
(infinitesimal) porous media equation, a non local in space model and
equations with memory.

## Luis Caffarelli : Degenerate ellipticity and the porous media equation

- Gergen Lectures ( 273 Views )In the first lecture I will give a brief discussion of local and non local diffusion and degenerate ellipticity and different local and non local models for compressible flows in porous media.

In the second and third lectures I will discuss some properties of the
(infinitesimal) porous media equation, a non local in space model and
equations with memory.

## Francis Brown : Periods, Galois theory and particle physics: Applications

- Gergen Lectures ( 286 Views )In the final lecture, I will propose how the Galois theory of periods should lead to a classification of periods by types. When applied to the set of Feynman integrals occurring in particle physics, experiments suggest the emergence of a `cosmic? Galois group of symmetries acting on the constants of high-energy physics.

## Francis Brown : Periods, Galois theory and particle physics: Galois theory and transcendence

- Gergen Lectures ( 302 Views )Classical Galois theory replaces the study of algebraic numbers with group theory. This idea is extremely powerful, and led to the proof of the insolubility of the general quintic equation. A deep idea, originating in the work of Grothendieck, is that Galois theory should extend to the theory of periods. I will describe a cheap way to set up such a theory and illustrate it in the case of multiple zeta values. It gives rise to a symmetry group which respects the algebraic identities satisfied by these numbers and explains their underlying structure.

## Francis Brown : Periods, Galois theory and particle physics: Amplitudes in high-energy physics

- Gergen Lectures ( 321 Views )In high-energy physics, interactions between fundamental particles can be represented by Feynman graphs. Almost all predictions for particle collider experiments are obtained by computing certain integrals associated to such graphs, called Feynman integrals, and a vast effort in the physics community worldwide is devoted to studying these quantities. Feynman integrals turn out to be periods, and surprisingly many are multiple zeta values. I will survey what is known and not known about these quantities.

## Francis Brown : Periods, Galois theory and particle physics: General introduction to periods

- Gergen Lectures ( 270 Views )A period is a certain kind of complex number which can be written as an integral of algebraic quantities. Kontsevich and Zagier conjectured that all identities between periods can be obtained from the elementary rules of calculus. After discussing several examples I will focus on the case of multiple zeta values which were first introduced in a special case by Euler, and now occur in numerous branches of mathematics. They satisfy many families of relations which are the subject of several open conjectures.

## Laszlo Lovasz : An informal conversation.

- Gergen Lectures ( 298 Views )For the third Gergen Lecture, Professor Laszlo will offer an informal conversation with interested students and faculty. This may include answering questions regarding prior lectures, such as details and proofs. He may also state open problems.

## Alice Guionnet : The spectrum of non-normal matrices, II: the Brown measure.

- Gergen Lectures ( 268 Views )In this talk, which is a continuation of Wednesday's lecture, we shall describe the natural candidate for the limit of the empirical measure of the eigenvalues of non-normal matrices, the so-called Brown measure. We will give some details about how to prove convergence towards such a limit, but also discuss the instability of such convergence.

## Jordan S. Ellenberg : Stability and Representations

- Gergen Lectures ( 278 Views )The notion of stability --speaking loosely, "sometimes an infinite sequence of vector spaces eventually starts being constant" -- appears in many branches of mathematics, perhaps most notably topology, where Harer's theorem about the stability of the homology of mapping class groups has driven decades of work. Some natural sequences of vector spaces are evidently NOT eventually constant: for instance, the space Q_n of quadratic polynomials in n variables has dimension (1/2)n(n-1), so gets larger and larger as n goes to infinity. On the other hand, Q_n carries an action of the symmetric group S_n by permutation of coordinates. We will discuss a new framework which allows us to speak meaningfully about what it means for a sequence of representations of S_n to be stable. It turns out that the structures we define are ubiquitous, appearing in topology (e.g. homology groups of configuration spaces and of moduli spaces of curves) algebraic combinatorics (e.g. the graded pieces of diagonal coinvariant algebras) and algebraic geometry (e.g. spaces of polynomials on discriminant and rank varieties.) We prove, for instance, that all these sequences of vector spaces have dimension which is eventually a polynomial in n.

## Jordan S. Ellenberg : Stability and Arithmetic Counting Problems

- Gergen Lectures ( 281 Views )A big theme in contemporary number theory is "arithmetic statistics": what does the class group of a random number field look like? What do the zeroes of a random L-function look like? What does a random rational point on a variety look iike? In this talk we will explain how arithmetic statistics problems over function fields are naturally tied to topological questions about stability for homology groups of certain moduli spaces; in particular, we will explain how a stability theorem for Hurwitz spaces (moduli spaces of finite branched covers of the line) can be used to prove a version of the Cohen-Lenstra conjectures over function fields. There will be some overlap with a talk I gave at Duke in December 2009, but many things which were speculations then are theorems now.

## Andrei Zelevinsky : Cluster algebras via quivers with potentials

- Gergen Lectures ( 283 Views )This lecture ties together the strands developed in the first two lectures. We discuss a recent proof (due to H. Derksen, J. Weyman, and the speaker) of a series of conjectures on cluster algebras by means of the machinery of quivers with potentials. An important ingredient of our argument is a categorification of cluster algebras using quiver Grassmannians, a family of projective algebraic varieties that are a far-reaching generalization of ordinary Grassmannians. Generalizing an idea due to P. Caldero, F. Chapoton and B. Keller, we show that the Euler characteristics of these varieties carry crucial information about the structure of cluster algebras.

## Andrei Zelevinsky : Quivers with potentials, their representation and mutations

- Gergen Lectures ( 272 Views )A quiver is a finite directed graph. A quiver representation assigns a finite-dimensional vector space to each vertex, and a linear map between the corresponding spaces to each arrow. A fundamental role in the theory of quiver representations is played by Bernstein-Gelfand-Ponomarev reflection functors associated to every source or sink of a quiver. In joint work with H. Derksen and J. Weyman (based on an earlier joint work with R. Marsh and M. Reineke) we extend these functors to arbitrary vertices. This construction is based on a framework of quivers with potentials; their representations are quiver representations satisfying relations of a special kind between the linear maps attached to arrows. The motivations for this work come from several sources: superpotentials in physics, Calabi-Yau algebras, and cluster algebras. However, no special knowledge will be assumed in any of these subjects, and the exposition aims to be accessible to graduate students.

## Felix Otto : Gergen Lecture - Speaker, Felix Otto

- Gergen Lectures ( 407 Views )In three specific examples, we shall demonstrate how the theory of partial differential equations (PDEs) relates to pattern formation in nature: Spinodal decomposition and the Cahn-Hilliard equation, Rayleigh-B\'enard convection and the Boussinesq approximation, rough crystal growth and the Kuramoto-Sivashinsky equation. These examples from different applications have in common that only a few physical mechanisms, which are modeled by simple-looking evolutionary PDEs, lead to complex patterns. These mechanisms will be explained, numerical simulation shall serve as a visual experiment. Numerical simulations also reveal that generic solutions of these deterministic equations have stationary or self-similar statistics that are independent of the system size and of the details of the initial data. We show how PDE methods, i. e. a priori estimates, can be used to understand some aspects of this universal behavior. In case of the Cahn-Hilliard equation, the method makes use of its gradient flow structure and a property of the energy landscape. In case of the Boussinesq equation, a ``driven gradient flow'', the background field method is used. In case of the Kuramoto-Sivashinsky equation, that mixes conservative and dissipative dynamics, the method relies on a new result on Burgers' equation.

## Richard Schoen : Positive scalar curvature and connections with relativity

- Gergen Lectures ( 275 Views )In this series of three lectures we will describe positivity conditions on Riemannian metrics including the classical conditions of positive sectional, Ricci, and scalar curvature. We will discuss open problems and recent progress including our recent proof of the differentiable sphere theorem (joint with Simon Brendle). That proof employs the Ricci flow, so we will spend some time explaining that technique. Finally we will discuss problems related to positive scalar curvature including some high dimensional issues which occur in that theory. If time allows we will describe recent progress on black hole topologies. These lectures, especially the first two, are intended for a general audience.

## Richard Schoen : Ricci flow and 1/4-pinching

- Gergen Lectures ( 258 Views )In this series of three lectures we will describe positivity conditions on Riemannian metrics including the classical conditions of positive sectional, Ricci, and scalar curvature. We will discuss open problems and recent progress including our recent proof of the differentiable sphere theorem (joint with Simon Brendle). That proof employs the Ricci flow, so we will spend some time explaining that technique. Finally we will discuss problems related to positive scalar curvature including some high dimensional issues which occur in that theory. If time allows we will describe recent progress on black hole topologies. These lectures, especially the first two, are intended for a general audience.

## Gang Tian : Geometry and Analysis of low-dimensional manifolds

- Gergen Lectures ( 35 Views )In this series of talks, I will focus on geometry and analysis of manifolds of dimension 2, 3 or 4. The first talk is a general introduction of this series. I will start the talk by reviewing some classical theories on Riemann surfaces and their recent variations in geometric analysis. Then we survey some recent progress on 3- and 4-manifolds. I hope that this talk will show some clues how geometric equations can be applied to studying geometry of underlying spaces. In the second talk, I will discuss recent works on the Ricci flow and its application to the geometrization of 3-manifolds, in particular, I will briefly discuss Perelman's work towards the Poincare conjecture. In last talk, I will discuss geometric equations in dimension 4 and how they can be applied to studying geoemtry of underlying 4-spaces. Some recent results will be discussed and some open problems will be given.

## Gang Tian : Geometry and Analysis of low-dimensional manifolds

- Gergen Lectures ( 37 Views )In this series of talks, I will focus on geometry and analysis of manifolds of dimension 2, 3 or 4. The first talk is a general introduction of this series. I will start the talk by reviewing some classical theories on Riemann surfaces and their recent variations in geometric analysis. Then we survey some recent progress on 3- and 4-manifolds. I hope that this talk will show some clues how geometric equations can be applied to studying geometry of underlying spaces. In the second talk, I will discuss recent works on the Ricci flow and its application to the geometrization of 3-manifolds, in particular, I will briefly discuss Perelman's work towards the Poincare conjecture. In last talk, I will discuss geometric equations in dimension 4 and how they can be applied to studying geoemtry of underlying 4-spaces. Some recent results will be discussed and some open problems will be given.

## Leo P. Kadanoff : Drips and Jets: Singularities, Topology Changes, and Scaling for Fluid Interfaces

- Gergen Lectures ( 38 Views )We investigate the behavior of the interface between two fluids. We are interested in the singularities which develop when the bridge connecting two pieces of fluid goes to zero thickness. One physical situation is Hele-Shaw flow: two fluids are trapped between parallel glass plates and feel frictional forces. Another situation is a cylindrically symmetrical stream, for example, in a dripping faucet. In a third case, a fluid in an electric or magnetic field which pulls off a piece of fluid ending in a sharp point. The last case has an interface between a light and a heavier fluid being sucked up as in a drinking straw. At a critical value of the sucking, a very thin bridge of the heavy fluid is formed. Analytical, numerical, and experimental methods are used to describe what happens right around the pinchoff of these bridges. Much of the resulting behavior can be understood via scaling arguments.