# Upcoming Seminars & Events

## Primary tabs

April 20, 2015
3:15pm - 4:30pm
##### Asymptotics of Chebyshev polynomials for general subsets of the real line
###### Analysis Seminar

Given a compact subset, E in R, the Chebyshev polynomials are the unique minmizers of the sup norm over E among all degree n monic polynomials. I will describe some recent results of Christensen, Simon and inchenko on this classical subject. We settle a 45 year old conjecture of Widom on the large n pointwise asymptotics for the case of finite gap sets. We also extend upper bounds on the norm that Totik and Widom obtained for the finite gap case to positive measure Cantor sets. Our proof in this case is new and even in the finite gap case is simpler and more explicit bounds than the earlier work.

Speaker: Barry Simon, Caltech
Location:
Fine Hall 314
April 20, 2015
4:30pm - 5:30pm
##### Efficient use of semidefinite programming for selection of rotamers in protein conformations and other applications
###### PACM/Applied Mathematics Colloquium

In this paper we study a semidefinite programming relaxation of the (NP-hard) side chain positioning problem. We show that the Slater constraint qualification (strict feasibility) fails for the SDP relaxation. We then show the advantages of using facial reduction to regularize the SDP. In fact, after applying facial reduction, we have a smaller problem that is more stable both in theory and in practice. We include a discussion of the background of SDP relaxations and exploiting pecial structure in various applications such as sensor network localization, SNL.  Work with: Forbes Burkowski (University of Waterloo) Yuen-Lam Cheung Voronin (University of Colorado, Boulder)

Speaker: Henry Wolkowicz, University of Waterloo
Location:
Fine Hall 214
April 21, 2015
4:30pm - 5:30pm
##### From molecular dynamics to kinetic theory and hydrodynamics
###### Department Colloquium

Please note that this colloquium talk is on Tuesday, not Wednesday.

Speaker: Laure Saint-Raymond, École Normale Supérieure
Location:
Fine Hall 314
April 21, 2015
4:30pm - 5:50pm
##### Singular Eigenvalue Perturbation Theory
###### Mathematical Physics Seminar

Eigenvalue Perturbation Theory is central to the theory of nonrelativistic quantum mechanics going back to Schrodinger's first papers. This lecture will review what is known about the eigenvalues in physical situations where one doesn't have simple convergence to a new isolated eigenvalue. Included are the anharmonic oscillator and Zeeman effect (divergent series and summability), autoionizing states in atoms (complex scaling and resonances), Stark effect (exponentially small resonances) and double wells (instantons).

Speaker: Barry Simon, Caltech
Location:
April 22, 2015
12:15pm - 5:30pm
##### Celebrating the work of Prof. Edward Nelson (1932-2014)
###### Department Colloquium, Special Events and Conferences

In place of the regular departmental colloquium and pre-colloquium presentations, we shall have three talks focused on different aspects of Prof. Edward Nelson's work:

12:15 - 1:15 (Fine Hall Common Room, 3rd fl.)
"Ed Nelson's Work in Analysis, Especially Related to Quantum Field Theory"
Barry Simon, California Institute of Technology

4:30 - 5:30 pm (Fine Hall 314)

"Questioning the Infinite"
Greg Lawler, University of Chicago
"Ed Nelson's stochatic mechanics"
Eric Carlen, Rutgers University
Presentations moderated by professor emeritus Simon Kochen.

Reminiscences will be held in the Common Room after the talks.
The family has arranged for a eucharist, with special intention for the memory of Edward Nelson, which will be celebrated on April 21 at 5:15 pm in the Marquand Transept of the University Chapel.  All are welcome to attend.

Speaker: ,
April 22, 2015
2:00pm - 3:00pm
##### Loop-Erased Random Walk
###### Probability Seminar

I will discuss recent work with Christian Benes and Fredrik Viklund on the convergence of  the Green's function of planar loop-erased random walk and the relationship to the convergence to the Schramm-Loewner evolution in the natural parametrization (Minkowski content). I will not assume that people know what the words in the last sentence mean --- I will describe the result and its importance.

Speaker: Greg Lawler, University of Chicago
Location:
Fine Hall 214
April 23, 2015
2:00pm - 3:30pm
##### Rigidity phenomena in random point sets and applications
###### Ergodic Theory & Statistical Mechanics

In several naturally occurring (infinite) point processes, the number the points inside a finite domain can be determined, almost surely, by the point configuration outside the domain. There are also other processes where such ''rigidity'' extends also to a number of moments of the mass distribution. The talk will focus on point processes with such curious "rigidity" phenomena, and their implications. We will also talk about applications to stochastic geometry and some questions in harmonic analysis.

Speaker: Subhro Ghosh, Princeton University
Location:
Fine Hall 601
April 23, 2015
3:00pm - 4:00pm
##### Incidences between points and lines and extremal configuration of lines in Euclidean spaces
###### Discrete Mathematics Seminar

In 1983, Szemer\'edi and Trotter proved a tight bound for the number of incidences between points and lines in the plane. Ever since then, incidence geometry has been a very active research area, bridging between computer science and mathematics, with many connections to diverse topics, from range searching algorithms to the Kakeya problem. Over 25 years later, Guth and Katz proved a tight incidence bound for points and lines in three dimensions. Their proof introduced methods from advanced algebra and especially from algebraic geometry which were not used in combinatorics before. This enabled Guth and Katz to (almost) settle the Erd\"os distinct distances problem - a problem which stubbornly stood open for over 60 years, despite very brave attempts to solve it. The work of Guth and Katz has given significant added momentum to incidence geometry, making many problems, deemed hopeless before the breakthrough, amenable to the new techniques. In this talk I will present the area of incidence geometry, before and after, highlighting the basics of the new algebraic'' approach, and will also present very recent results, joint with Micha Sharir, among which we studied incidences between points and lines in four dimensions, incidences between points and lines that lie on special surfaces and other related questions. We will also give a variety of interesting open related questions.

Speaker: Noam Solomon, Hebrew University
Location:
Fine Hall 224
April 23, 2015
4:30pm - 5:30pm
##### TBA - Jonathan Hanselman
###### Topology Seminar
Speaker: Jonathan Hanselman, Columbia University
Location:
Fine Hall 314
April 23, 2015
4:30pm - 5:30pm
##### Extensions of the Gross-Zagier formula
###### Princeton University/IAS Number Theory Seminar

I will first discuss the general conjectural picture relating algebraic cycles to L-functions and some extensions of the Gross-Zagier formula involving p-adic L-functions. This leads naturally to the question of constructing algebraic cycles corresponding to the vanishing of certain Rankin-Selberg L-functions at the center of symmetry. Finally, I will outline some new constructions of such cycles, based on work in progress with A. Ichino.

Speaker: Kartik Prasanna, University of Michigan
Location:
IAS Room S-101
April 24, 2015
1:30pm - 2:30pm
##### Periodic Symplectic Cohomologies
###### Symplectic Geometry Seminar

Periodic cyclic homology group associated to a mixed complex was introduced by Goodwillie. In this talk, I will explain how to apply this construction to the symplectic cochain complex of a Liouville domain and obtain two periodic symplectic cohomology theories, which are called periodic symplectic cohomology and finitely supported periodic symplectic cohomology, respectively. The main result is that there is a localization theorem for the finitely supported periodic symplectic cohomology.

Speaker: Jingyu Zhao, Columbia University
Location:
IAS Room S-101
April 24, 2015
3:00pm - 4:00pm
##### A priori estimates for semistable solutions of semilinear elliptic equations
###### Differential Geometry & Geometric Analysis Seminar

In this talk we will discuss semistable solutions of the boundary value problem $Lu+f(u)=0$ in $\Omega$ and $u=0$ on $\partial\Omega$, where $Lu:=\partial_i(a^{ij}u_j)$ is uniformly elliptic. By semistability we mean that the lowest Dirichlet eigenvalue of the linearized operator at u is nonnegative. The basic problem (which has a long history) is to obtain a priori $L^{\infty}$ bounds on a solution under minimal assumptions on $f(t)$.  A basic and standard assumption is that $u>0$ in $\Omega$  and $f\in C^2$ is positive, nondecreasing, and superlinear at infinity, i.e. $f(0)>0$, $f' \geq 0$ and $f(t)/t$ tends to infinity as $t$ tends to infinity. For radially symmetric solutions,  an $L^{\infty}$ bound for $u$ is known for $n\leq 9$. On the other hand there exists unbounded semistable solutions when $n\geq 10$ for $f(u)=e^u$. This problem, like many other semilinear elliptic problems studied in recent years, seems to be related to minimal surface stability but this still remains mysterious.

Speaker: Joel Spruck, Johns Hopkins University
Location:
Fine Hall 314
April 27, 2015
3:15pm - 4:30pm
##### TBA - Phil Gressman
###### Analysis Seminar
Speaker: Phil Gressman, UPenn
Location:
Fine Hall 314
April 27, 2015
4:30pm - 5:30pm
##### Fast algorithms for electronic structure analysis
###### PACM/Applied Mathematics Colloquium

Kohn-Sham density functional theory (KSDFT) is the most widely used electronic structure theory for molecules and condensed matter systems. For a system with N electrons, the standard method for solving KSDFT requires solving N eigenvectors for an O(N) * O(N) Kohn-Sham  Hamiltonian matrix.  The computational cost for such procedure is expensive and scales as O(N^3), and limits routine KSDFT calculations to hundreds of atoms.  In recent years, we have developed an alternative procedure called the  pole expansion and selected inversion (PEXSI) method [1-2].  The PEXSI method solves KSDFT without solving any eigenvalue and eigenvector, and directly evaluates physical quantities including electron density, energy, atomic force, density of states, and local density of states. The overall algorithm scales as at most O(N^2) for all materials including insulators, semiconductors and the difficult metallic systems.  The PEXSI method can be efficiently parallelized over 10,000 - 100,000 processors on high performance machines.  It has been integrated into standard electronic structure software packages such as SIESTA for ab initio materials simulation over 20,000 atoms [3].  Recently we have been able to use PEXSI to study electronic structure of large scale graphene nanoflakes [4] and  phosphorene nanoribbons [5] to unprecedented scale (more than 10,000 atoms).

[1] L. Lin, J. Lu, L. Ying, R. Car and W. E, Fast algorithm for extracting the diagonal of the inverse matrix with application to the electronic structure analysis of metallic systems, Commun. Math. Sci. 7, 755, 2009

[2] L. Lin, M. Chen, C. Yang and L. He, Accelerating atomic orbital-based electronic structure calculation via pole Expansion and selected inversion, J. Phys. Condens. Matter 25, 295501, 2013

[3] L. Lin, A. Garcia, G. Huhs and C. Yang, SIESTA-PEXSI: Massively parallel method for efficient and accurate ab initio materials simulation without matrix diagonalization, J. Phys. Condens. Matter 26, 305503, 2014

[4] W. Hu, L. Lin, C. Yang and J. Yang, Electronic structure of large-scale graphene nanoflakes, J. Chem. Phys. 141, 214704, 2014

[5] W. Hu, L. Lin and C. Yang, Edge reconstruction in armchair phosphorene nanoribbons revealed by discontinuous Galerkin density functional theory, submitted

Speaker: Lin Lin, UC-Berkeley
Location:
Fine Hall 214
April 28, 2015
4:30pm - 5:30pm
##### A class of gapped Hamiltonians on quantum spin chains and its classification
###### Mathematical Physics Seminar

The MPS (matrix product state) formalism gives a recipe to construct Hamiltonians in quantum spin chains from $n$-tuples of $k\times k$- matrices. This $n$-tuple defines a completely positive map and the existence of the uniform spectral gap of the Hamiltonian is related to the spectral property of the associated CP map. I would like to talk about a classification problem of this class of Hamiltonians. Through the relation between Hamiltonians and CP maps, the problem is reduced to the question of path connectedness of a class of CP maps.

Speaker: Yoshiko Ogata, University of Tokyo
Location:
April 29, 2015
1:30pm - 2:30pm
##### Another (q,t) world
###### Combinatorics

A well studied (q,t)-analogue of symmetric functions are the Macdonald polynomials. In this talk I will survey another (q,t)-analogue, where q is a prime power from a finite field and t is an indeterminate. Analogues of facts about the symmetric group S_n are given for GL_n(F_q), including (1) counting factorizations of certain elements into reflections, (2) combinatorial properties of appropriate (q,t)-binomial coefficients, (3) Hilbert series for invariants on polynomial rings. Some new conjectured explicit Hilbert series of rings of invariants over finite fields are given. This is joint work with Joel Lewis and Vic Reiner.

Speaker: Dennis Stanton , University of Minnesota
Location:
Fine Hall 214
April 29, 2015
4:30pm - 5:30pm
##### Zeta(3) in arithmetic and geometry
###### Department Colloquium

Euler proved in 1735 that zeta(2) = $\pi^2 /6$, and also computed the special values of zeta(n) at all positive even integers. Yet it took almost another 250 years for Apery proved that zeta(3) was irrational. In this talk, we shall talk about zeta(3) as well as its p-adic version, and the connection of these numbers to both arithmetic and geometry.

Speaker: Frank Calgary, Northwestern
Location:
Fine Hall 314
April 30, 2015
2:00pm - 3:30pm
##### Higher Rank Orbit Closures in Genus 3
###### Ergodic Theory & Statistical Mechanics

The moduli space of translation surfaces is stratified by the orders of the zeros of Abelian differentials. We classify GL^+(2,R) orbit closures in the strata of translation surfaces in genus 3 with at most two zeros, with the property that they have rank 2 (in the sense of Alex Wright).   This is joint work with Duc-Manh Nguyen and Alex Wright.

Speaker: David Aulicino, University of Chicago
Location:
Fine Hall 601
April 30, 2015
3:00pm - 4:00pm
##### Stability results in additive combinatorics and graph theory
###### Discrete Mathematics Seminar

We will talk about stability results in extremal combinatorics. Along the way, we solve a question of Erdos and Sarkozy on sumsets of integers avoiding perfect squares, and reprove the Posa-Seymour conjecture on hamiltonian cycles for large graphs. Joint work with A.Jamshed, A. Khalfalah, and E. Szemeredi.

Location:
Fine Hall 224
April 30, 2015
3:00pm - 4:00pm