University of Chicago

Mon 2 Apr 2012, 2:00pm
SPECIAL
Topology and related seminars
WMAX 110 (PIMS)

The cohomology groups of the pure string motion group are uniformly representation stable

WMAX 110 (PIMS)
Mon 2 Apr 2012, 2:00pm3:00pm
Abstract
In the past two years, Church, Farb and others have developed the concept of 'representation stability', an analogue of homological stability for a sequence of groups or spaces admitting group actions. In this talk, I will give an overview of this new theory, using the pure string motion group P\Sigma_n as a motivating example. The pure string motion group, which is closely related to the pure braid group, is not cohomologically stable in the classical sense  for each k>0, the dimension of the degree k rational cohomology of P\Sigma_n tends to infinity as n grows. The groups H^k(P\Sigma_n, \Q) are, however, representation stable with respect to a natural action of the hyperoctahedral group W_n  that is, in some precise sense, the description of the decomposition of these cohomology groups into irreducible W_nrepresentations stabilizes for n>>k. I will outline a proof of this result, verifying a conjecture by Church and Farb.
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University of Washington

Mon 2 Apr 2012, 3:10pm
Algebraic Geometry Seminar
WMAX 110

Vanishing theorems and their failure in positive characteristic

WMAX 110
Mon 2 Apr 2012, 3:10pm4:10pm
Abstract
The Kodaira vanishing theorem and its generalizations are extremely important tools in higher dimensional geometry and the failure of these theorems in positive characteristic causes great difficulties in extending the existing theories to that realm. In this talk I will discuss new results about cases where an appropriate vanishing theorem holds and cases where the expected one fails even in characteristic zero. These results are joint works (separately) with Christopher Hacon and with János Kollár.
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EOS, UBC

Tue 3 Apr 2012, 12:30pm
Scientific Computation and Applied & Industrial Mathematics
WMAX 110

Finite element based inversion for geoelectromagnetics

WMAX 110
Tue 3 Apr 2012, 12:30pm1:30pm
Abstract
High contrast in electrical conductivity motivates the investigation of electromagnetic methods in geophysics, for instance, for hydrocarbon and mineral exploration. A straightforward approach to modelling the spatial distribution of this parameter within the earth is the assumption of piecewise constant values, defined on a moderately fine tessellation of the volume under investigation by hexahedra or tetrahedra. We study here the solution of the 3D forward problem for timeharmonic electromagnetic fields using finite elements, based on the above mentioned tessellation. Furthermore, we seek to reconstruct the spatial distribution of conductivity of an overparameterized model by a regularised output least squares approach. Our model assumption, a piecewise constant coefficient, allows for simplifications of the forward solver which eventually lead to an overall faster imaging algorithm. The model assumption also requires special care when the regularisation operator is derived for unstructured meshes within the finite element framework.
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Institut de Mathématiques de Toulouse, Université Paul Sabatier

Tue 3 Apr 2012, 3:30pm
Diff. Geom, Math. Phys., PDE Seminar
WMAX 110

Excited MultiSolitons for a Nonlinear Schrödinger Equation

WMAX 110
Tue 3 Apr 2012, 3:30pm4:30pm
Abstract
We consider a nonlinear Schrödinger equation with a general nonlinearity. In space dimension 2 or higher, this equation admits solitons (standing/traveling waves) with a fixed profile which is not a ground state. These types of profiles are called excited states. Due to instability, excited solitons are singular objects for the dynamics of NLS. Nevertheless, we will show in this talk how to exhibit solutions of NLS behaving in large time like a sum of excited solitons with high relative speeds.
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University of Alberta

Tue 3 Apr 2012, 3:30pm
Algebraic Groups and Related Structures
MATX 1102

Essential dimension of Spinor and Clifford groups.

MATX 1102
Tue 3 Apr 2012, 3:30pm4:30pm
Abstract
We compute the exact value of essential dimension for split spinor groups and split even Clifford groups. I will also discuss some applications of these results to quadratic forms.
This talk is based on joint work with A. Merkurjev; see
www.math.unibielefeld.de/lag/man/455.html
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UBC

Tue 3 Apr 2012, 4:00pm
Discrete Math Seminar
MATH 126

Covering Maps and Sheaves on Graphs

MATH 126
Tue 3 Apr 2012, 4:00pm5:00pm
Abstract
The notions of covering maps, Galois theory and representations, Laplacians, L^2 Betti numbers, and sheaves are well known in many areas of mathematics. Despite the fact that these notions are easy to describe in the context of graph theory, these tools seem to be greatly underutilized there. Furthermore, such ideas can be used to prove theorems that have no obvious connection to sheaf theory, or even to graph theory.
We describe the above notions, and briefly describe the applications of sheaves and covering maps to (1) a solution of the Hanna Neumann Conjecture of the 1950's, and (2) an equivalence of two notions of "2independence," which are generalizations of ordinary linear independence (which has no obvious connection to graph theory). We also describe our original motivation to study sheaves on discrete structures that arose from complexity theory.
Application (2) seems to make essential use of a result regarding covering maps of graphs and "scaled Abelian limits" of Betti numbers, that may be described as a "baby version" of ladic cohomology.
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University of Toronto

Wed 4 Apr 2012, 3:00pm
SPECIAL
Department Colloquium
WMAX 110 (PIMS)

CRMFieldsPIMS Prize Lecture: Structure theory of Ramsey spaces and some of its applications

WMAX 110 (PIMS)
Wed 4 Apr 2012, 3:00pm4:00pm
Abstract
We give an overview of the most basic Ramsey theoretic principles such as the HalesJewett theorem and the HalpernLäuchli theorem and the corresponding Ramsey space theories that they lead to. The theory has natural counterparts both in the sense of dimensions and cardinalities of the structures. For example, we explain the close relationships between the finite and the infinitedimensional theory and we also explain the Ramsey theory of finite structures and its close relationship to the Ramsey theory of infinite structures. If time permits we will also explain some of the most recent advances in the density Ramsey theory with a particular emphasis on the new phenomena that show up in the context of infinite structures. We point out some of the successes in applying this theory to, for example, topological dynamics and functional analysis. Professor Todorcevic obtained his Ph.D. in Belgrade and currently holds a Canada Research Chair at the University of Toronto.
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University of Tokyo

Wed 4 Apr 2012, 4:00pm
SPECIAL
Topology and related seminars
WMAX 216

Selflinking number of transverse knots in general open books

WMAX 216
Wed 4 Apr 2012, 4:00pm5:00pm
Abstract
Every transverse knot in a contact 3manifold is represented as a closed braid in an open book. In this talk, based on a new technique called an open book foliation, we give a formula of selflinking number in terms of braids and open books. Surprisingly, our selflinking number formula essentially uses Johnson's homomorphism. This is a joint work with Keiko Kawamuro (Univ. Iowa).
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Université de Louvain

Thu 12 Apr 2012, 2:00pm
SPECIAL
Topology and related seminars
WMAX 216 (PIMS)

From the eversion of the sphere to spaces of knots

WMAX 216 (PIMS)
Thu 12 Apr 2012, 2:00pm3:00pm
Abstract
A famous result by Steven Smale states that we can turn the sphere insideout through immersions: this is called the eversion of the sphere. We will explain this result and the strategy of its proof which is a "cutandpaste" strategy quite standard in algebraic topology. This approach allows us to understand globally the space of all immersions of a given manifold in another one, like the space of all immersion of the sphere in R^3 in the case of Smale's eversion. This theory has been enhanced by Goodwillie in the 1990's to understand spaces of embeddings. We will explain how this can be applied to understand spaces of knots, that is the spaces of all embeddings of a circle into a fixed euclidean space.
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University of Vienna

Thu 12 Apr 2012, 3:15pm
WMAX 110 (PIMS)

Mathematics Seminar: Goedel in Vienna

WMAX 110 (PIMS)
Thu 12 Apr 2012, 3:15pm4:15pm
Details
A richly illustrated talk about Goedel's time in Vienna. These fifteen years cover his brilliant start at the University and his discovery of the incompleteness theorems, the confused period between Hitler's rise to power in Germany and the annexation of Austria, and Goedel's desperate struggle to leave nationalsocialist Vienna.
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UBC

Thu 12 Apr 2012, 4:00pm
Mathematical Education
MATH 126

Teaching Seminar: Infinity, limits and divisibility

MATH 126
Thu 12 Apr 2012, 4:00pm5:00pm
Abstract
This week Kyle Hambrook will conduct the discussion. We will study the section titled "Infinity, limits and divisibility". This section is composed of three papers:
 Developing notions of infinity (pdf)
 Layers of abstraction: theory and design for the instruction of limits concepts (pdf)
 Divisibility and transparency of number representations (pdf)
You will find access to the pdf version of these papers.
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UBC

Fri 13 Apr 2012, 1:00pm
Stochastic Dynamics Working Group
IAM Lounge

Modelling acquired drug resistance in HIV + individuals

IAM Lounge
Fri 13 Apr 2012, 1:00pm2:00pm
Abstract
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University of Vienna

Fri 13 Apr 2012, 3:00pm
Department Colloquium
MATH 100 (lecture hall)

PIMS/UBC distinguished colloquium: Sanctions on the Commons: Social Learning and the Social Contract

MATH 100 (lecture hall)
Fri 13 Apr 2012, 3:00pm4:00pm
Abstract
Evolutionary game theory helps to investigate the role of incentives in promoting cooperative behavior in joint enterprises. In particular, this lecture deals with the surprising effects of optional participation in collaborative enterprises. Coercion works better for voluntary rather than compulsory collaboration. A social contract need not be based on rational deliberation or the command of a higher authority. It can emerge spontaneously through social learning of individuals guided by no more than their myopic selfinterest.
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Fields Institute

Wed 18 Apr 2012, 3:00pm
Probability Seminar
WMAX 110

Density functional theory and optimal transport with Coulomb cost

WMAX 110
Wed 18 Apr 2012, 3:00pm4:00pm
Abstract
In this talk I explain a promising and previously unnoticed link between electronic structure of molecules and optimal transportation (OT), and I give some
first results. The `exact' mathematical model for electronic structure, the manyelectron Schroedinger equation, becomes computationally unfeasible for more than a dozen or so electrons. For larger systems, the standard model underlying a huge literature in computational physics/chemistry/materials science is density functional theory (DFT). In DFT, one only computes the singleparticle density instead of the full manyparticle wave function. In order to obtain a closed equation, one needs a closure assumption which expresses the pair density in terms of the singleparticle density rho.
We show that in the semiclassical HohenbergKohn limit, there holds an exact closure relation, namely the pair density is the solution to a optimal transport problem with Coulomb cost. We prove that for the case with $N=2$ electrons this problem has a unique solution given by an optimal map; moreover we derive an explicit formula for the optimal map in the case when $\rho$ is radially symmetric (note: atomic ground state densities are radially symmetric for many atoms such as He, Li, N, Ne, Na, Mg, Cu).
In my talk I focus on how to deal with its main mathematical novelties (cost decreases with distance; cost has a singularity on the diagonal). I also discus the derivation of the Coulombic OT problem from the manyelectron Schroedinger equation for the case with $N\ge 3$ electrons, and give some results and explicit solutions for the manymarginals OT problem.
Joint works with Gero Friesecke (TU Munich) and Claudia Klueppelberg (TU Munich).
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Thu 19 Apr 2012, 9:00am
SPECIAL
Graduate Student Center, Room 203

Doctoral Exam

Graduate Student Center, Room 203
Thu 19 Apr 2012, 9:00am12:00pm
Details
We provide results related to the study of prime points on level sets of homogeneous integral forms which are linear or quadratic. In the linear case we present an extension of the GreenTao Theorem, which finds affine copies of finite intervals in relatively dense subsets of the primes, to a higher dimensional setting in which one finds affine copies of suitably generic point configurations in relatively dense subsets of the Cartesian product of the primes. For general integral quadratic forms we present a result which is a BirchGoldbach type theorem for a single quadratic form with sufficient rank. This guarantees solubility among the primes on the level set of a quadratic form subject to local conditions. This is an extension of a well known result due to Hua.
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Thu 19 Apr 2012, 7:00pm
SPECIAL
PIMS Seminars and PDF Colloquiums
Asian Centre Auditorium, 1871 West Mall

The Modern Science of Origami

Asian Centre Auditorium, 1871 West Mall
Thu 19 Apr 2012, 7:00pm8:00pm
Abstract
From flapping birds to space telescopes: The modern science of Origami.
The last decade of this past century has been witness to a revolution in the development
and application of mathematical techniques to origami, the centuriesold Japanese art
of paperfolding. The techniques used in mathematical origami design range
from the abstruse to the highly approachable. In this talk, I will describe how geometric
concepts led to the solution of a broad class of origami folding problems  specifically,
the problem of efficiently folding a shape with an arbitrary number and arrangement of
flaps, and along the way, enabled origami designs of mindblowing complexity and
realism, some of which you'll see too. As often happens in mathematics, theory originally
developed for its own sake has led to some surprising practical applications. The
algorithms and theorems of origami design have shed light on longstanding
mathematical questions and have solved practical engineering problems. I will discuss
examples of how origami has enabled safer airbags, robdingnagian space telescopes,
and more.
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Oxford

Wed 25 Apr 2012, 3:00pm
Probability Seminar
WMAX 110

Optimal stopping under probability distortion

WMAX 110
Wed 25 Apr 2012, 3:00pm4:00pm
Abstract
We formulate an optimal stopping problem for a geometric Brownian motion where the probability scale is distorted by a general nonlinear function. The problem is inherently time inconsistent due to the Choquet integration involved. We develop a new approach, based on a reformulation of the problem where one optimally chooses the probability distribution or quantile function of the stopped state. An optimal stopping time can then be recovered from the obtained distribution/quantile function, either in a straightforward way for several important cases or in general via the Skorokhod embedding. This approach enables us to solve the problem in a fairly general manner with different shapes of the payoff and probability distortion functions. We also discuss economical interpretations of the results. In particular, we justify several liquidation strategies widely adopted in stock trading, including those of “buy and hold”, “cut loss or take profit”, “cut loss and let profit run”, and “sell on a percentage of historical high”.
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Dartmouth University

Thu 26 Apr 2012, 4:00pm
Number Theory Seminar
Room WMAX 216 (PIMS  UBC Campus)

Products of distinct cyclotomic polynomials

Room WMAX 216 (PIMS  UBC Campus)
Thu 26 Apr 2012, 4:00pm4:50pm
Abstract
A polynomial is a product of distinct cyclotomic polynomials if and only if it is a divisor over Z[x] of x^{n}–1 for some positive integer n. In this talk, we will examine two natural questions concerning the divisors of x^{n}–1: "For a given n, how large can the coefficients of divisors of x^{n}–1 be?" and "How often does x^{n}–1 have a divisor of every degree between 1 and n?" We will consider the latter question when x^{n}–1 is factored in both Z[x] and F_{p}[x], using sieve methods and other techniques from analytic number theory in order to obtain our results.
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UBC

Thu 26 Apr 2012, 5:10pm
Number Theory Seminar
Room WMAX 216 (PIMS  UBC Campus)

Escape of mass on Hilbert modular surfaces

Room WMAX 216 (PIMS  UBC Campus)
Thu 26 Apr 2012, 5:10pm6:00pm
Abstract
Quantum Unique Ergodicity has been a widely studied conjecture of Rudnick and Sarnak (1994), concerning the distribution of large frequency eigenstates on a negatively curved manifold. Arithmetic Quantum Unique Ergodicity (AQUE) restricts the problem to arithmetic manifolds, such as SL(2,Z) \ H, the classical modular surface. Work of Lindenstrauss (2006) combined with the elimination of escape of mass proved by Soundararajan (2010) confirmed AQUE for the classical modular surface. This talk is concerned with AQUE for Hilbert modular surfaces, and in particular, my thesis work involving the elimination of escape of mass in this case.
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Note for Attendees
Following PIMS Tea (2:45pm) the Lecture takes place from 34pm in WMAX 110.