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 Events
Gabriel Samson
INSA Rennes, France
Wed 4 Sep 2013, 4:00pm SPECIAL
One Time Event
LSK 203
Behavior of a static bubble in a yield stress fluid
LSK 203
Wed 4 Sep 2013, 4:00pm-5:00pm

Details

Special Event: Fluid Lab Meetings
Abstract:
Non-bearing materials currently used in building applications (walls or bulkheads) are generally formed by panels or blocks. Such materials require good thermal performances and minimal mechanical strength. Mineral foams are relevant solutions. We focus on calcium sulfate foams containing a large amount of air trapped in bubbles. The foam production starts with fluid suspension obtained by quick mixing of water, mineral binder and surfactants. Foaming is applied before the quick setting of the mineral matrix. 

To study the interaction between foaming and rheological properties evolution, we use a transparent fluid model (dispersion of carbopol 676 in water). The air bubbles stability in yield stress polymer gel is analyzed by picture processing. Carbopol gel is placed in plexiglas container. Bubbles are inserted with a variable volume using a syringe for injection controlling gas pressure. Several phenomena are studied. The impact of the yield stress on a single bubble stability, growth, sphericity is first investigated. The contact conditions between is then studied. Experiments reveal surfactant role in foam stability. Results leads to a better understand of the foams stability at fresh state. It gives us tools to optimize the formulation regarding mechanical and thermal properties targets.


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Vitali Vougalter
University of Cape Town
Thu 5 Sep 2013, 1:00pm SPECIAL
Diff. Geom, Math. Phys., PDE Seminar
MATX 1102
Existence and nonlinear stability of stationary states for the semi-relativistic Schroedinger-Poisson system
MATX 1102
Thu 5 Sep 2013, 1:00pm-2:00pm

Abstract

We study the stationary states of the semi-relativistic Schroedinger-Poisson system in the repulsive (plasma physics) Coulomb case. In particular, we establish the existence and the nonlinear stability of a wide class of stationary states by means of the energy-Casimir method. Moreover, we establish global well-posedness results for the semi-relativistic Schroedinger-Poisson system in appropriate functional spaces.
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Microsoft Research
Thu 5 Sep 2013, 3:30pm
Probability Seminar
MATH 126 (previously listed as at 3 pm and in Henry Angus 254)
Random walks on groups and the Kaimanovich-Vershik 1983 conjecture for lamplighter groups
MATH 126 (previously listed as at 3 pm and in Henry Angus 254)
Thu 5 Sep 2013, 3:30pm-4:30pm

Abstract

Let G be an infinite group with a finite symmetric generating set S. The corresponding Cayley graph on G has an edge between x,y in G if their ratio xy^{-1} is in S. Kaimanovich-Vershik (1983), building on fundamental results of Furstenberg, Derrienic and Avez,  showed that  G admits non-constant bounded harmonic functions iff the entropy of simple random walk on G grows linearly in time;  Varopoulos (1985) showed that this is equivalent to the random walk escaping with a positive asymptotic speed. Kaimanovich and Vershik (1983)  also described the lamplighter groups (groups of exponential growth consisting of finite lattice configurations) where (in dimension at least 3) the simple random walk has positive speed, yet the probability of returning to the starting point does not decay exponentially. They conjectured a complete description of the bounded harmonic functions on these groups; In dimension 5 and above, their conjecture was proved by Anna Erschler (2011). In the talk, I will discuss the background and present a proof of the Kaimanovich-Vershik conjecture for all dimensions, obtained in joint work with Russ Lyons; the case of dimension 3 is the most delicate. No prior knowledge of group theory will be assumed.
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University of Augsburg
Thu 5 Sep 2013, 4:00pm
Topology and related seminars
ESB 2012
Positive scalar curvature: Existence and Classification
ESB 2012
Thu 5 Sep 2013, 4:00pm-5:00pm

Abstract

Scalar curvature measures the asymptotic volume growth of small balls in Riemannian manifolds. In the case of positive scalar curvature the growth rate is smaller than in the flat, euclidean case. Typical examples are round spheres of dimension at least two.

We will discuss the interplay of analytic, geometric and topological methods for the investigation of manifolds of positive scalar curvature.
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Microsoft Research
Fri 6 Sep 2013, 3:00pm
Department Colloquium
ESB 2012 (PIMS)
Search games and Optimal Kakeya Sets (PIMS/UBC Distinguished Colloquium)
ESB 2012 (PIMS)
Fri 6 Sep 2013, 3:00pm-4:00pm

Abstract

 A planar set that contains a unit segment in every direction is called a Kakeya set. These sets have been studied intensively in geometric measure theory and harmonic analysis since the work of Besicovich (1928); we find a new connection to game theory and probability. A hunter and a rabbit move on an n-vertex cycle without seeing each other until they meet. At each step, the hunter moves to a neighboring vertex or stays in place, while the rabbit is free to jump to any node. Thus they are engaged in a zero sum game, where the payoff is the capture time.   We show that every rabbit strategy yields a Kakeya set; the optimal rabbit strategy is based on a discretized Cauchy random walk, and it yields a Kakeya set K consisting of 4n triangles, that has minimal area among such Kakeya sets. Passing to the scaling limit yields a simple construction of a random Kakeya set with zero area from two Brownian motions.  (Talk based on joint work withY. Babichenko, R. Peretz, P. Sousi and P. Winkler).

Note for Attendees

 Coffee, tea and cookies served at 2:30pm in the PIMS Lounge.
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Mon 9 Sep 2013, 3:00pm SPECIAL
Institute of Applied Mathematics
LSK 306
IAM Welcome Reception
LSK 306
Mon 9 Sep 2013, 3:00pm-5:00pm

Abstract

Institute of Applied Mathematics (IAM) Students, Postdoctoral Fellows, Faculty and Staff gather to introduce and re-introduce themselves in an informal setting over refreshments. Even if you are not a member but interested in the IAM and its activities, you are welcome to join us. 
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Ohio State University
Mon 9 Sep 2013, 3:10pm
CRG Geometry and Physics Seminar
ESB 4127 (via video from U Alberta)
Counting disks in toric varieties
ESB 4127 (via video from U Alberta)
Mon 9 Sep 2013, 3:10pm-4:10pm

Abstract

CRG Geometry and Physics seminar.
This talk is hosted by U Alberta.

Abstract: For a toric manifold X and a Lagrangian torus fiber L in X, Fukaya-Oh-Ohta-Ono defined open Gromov-Witten invariants which are virtual enumerations of holomorphic disks in X with boundary conditions in L. Qualitative and quantitative properties of these open Gromov-Witten invariants play important roles in the symplectic geometry and mirror symmetry of X. Open Gromov-Witten invariants are difficult to compute because standard methods in Gromov-Witten theory (such as virtual localization) are not applicable. In this talk we explain a geometric method that leads to a complete calculation of these open Gromov-Witten invariants for compact semi-Fano toric manifolds. This is joint work with K. Chan, S.-C. Lau, N. C. Leung.
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Technion, Israel
Tue 10 Sep 2013, 12:30pm
Scientific Computation and Applied & Industrial Mathematics
ESB 4133
A Multilevel Algorithm for L_1 Minimization with Application to Sparse Representation of Signals
ESB 4133
Tue 10 Sep 2013, 12:30pm-2:00pm

Abstract

The area of sparse representation of signals is drawing tremendous attention in recent years in diverse fields of science and engineering. The idea behind the model is that a signal can be approximated as a linear combination of a few "atoms'' from a pre-specified and over-complete "dictionary'' (typically represented by columns from a matrix with more columns than rows). The sparse representation of a signal is often achieved by minimizing an L1 penalized least squares functional.
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University of Augsburg
Wed 11 Sep 2013, 3:00pm SPECIAL
Topology and related seminars
ESB 4133
The stable free rank of symmetry of products of spheres
ESB 4133
Wed 11 Sep 2013, 3:00pm-4:00pm

Abstract

A well known conjecture in the theory of transformation groups states that if p is a prime and (Z/p)^r acts freely on a product of k spheres, then r ≤ k. We prove this assertion if p is large compared to the dimension of the product of spheres. The argument builds on tame homotopy theory for non-simply connected spaces.
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Applied and Computational Mathematics, Rice
Thu 12 Sep 2013, 12:30pm SPECIAL
Scientific Computation and Applied & Industrial Mathematics
ESB 4133
Many-core Algorithms for High-order Finite Element Methods: When Time to Solution Matters
ESB 4133
Thu 12 Sep 2013, 12:30pm-2:00pm

Abstract

The ultimate success of many modeling applications depends on time to solution. I will illustrate the critical nature of time to solution by describing a joint project between my group at Rice University and Dr David Fuentes at the MD Anderson Cancer Center. The project goal is to evaluate the role and viability of using finite element modeling as part of the treatment planning process for MR Guided Laser Induced Thermal Therapy.
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Centro de Algebra da Universidade de Lisboa
Fri 13 Sep 2013, 11:00am SPECIAL
Discrete Math Seminar
ESB 4133
The Commuting Graph of the Symmetric Inverse Semigroup
ESB 4133
Fri 13 Sep 2013, 11:00am-12:00pm

Abstract

The commuting graph of a finite non-commutative semigroup S, denoted G(S), is a simple graph whose vertices are the non-central elements of S and two distinct vertices x,y are adjacent if xy=yx. This definition generalizes the corresponding concept of the commuting graph of a non-Abelian group.

Our work looks at the commuting graph of the symmetric inverse semigroup I(X). For a finite set X, let I(X) be the semigroup of all partial injective transformations on X under composition. The semigroup is universal for the class of inverse semigroups in the sense that every inverse semigroup can be embedded in I(X) for some finite set X, analog to the situation of the symmetric groups Sym(X) in group theory.

In 1989, Burns and Goldsmith classified the maximum order abelian subgroups of Sym(X). We extend this result to the semigroup I(X). As a consequence, we obtain a formula for the clique number of the commuting graph of  I(X).We also calculate the diameter of I(X) when |X| is prime or even, and obtain tight bounds on it in the remaining cases.

This is joint work with Joao Araujo (CAUL) and  Janusz Konieczny (Mary Washington).

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UBC
Fri 13 Sep 2013, 3:00pm
Department Colloquium
MATX 1100
Big data and big models: On Inverse Problems that are Too Large to be Solved
MATX 1100
Fri 13 Sep 2013, 3:00pm-4:00pm

Abstract

In recent years large data sets have been collected and analyzed, typically, by using some machine learning algorithms. However, many types of data demand a much more in-depth analysis that requires simulation, that is, solving partial differential equations, and optimization to estimate parameters.

In this talk we discuss examples for such data sets in earth science. We describe the setting in which vast amounts of geophysical data is collected from the air. We present the large scale modeling that is required to simulate such a data set and the inverse problems that arise from these types of problems. We show that by using traditional techniques these problems cannot be solved in reasonable time on reasonable hardware.

We then discuss a new set of algorithms that enable us to solve such problems. These algorithms are based on a concept we call domain of interest computation for the forward coupled with stochastic programming for the inverse. We show that by using this combination we are able to solve very large scale inverse problems, using a rather modest hardware in reasonable time.
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University of Washington
Mon 16 Sep 2013, 3:00pm
PIMS Seminars and PDF Colloquiums
ESB 2012
Combinatorics of Matrix Varieties
ESB 2012
Mon 16 Sep 2013, 3:00pm-4:00pm

Abstract

Many interesting and familiar varieties are collections of matrices. Two of the most familiar are the determinantal varieties and the matrix Schubert varieties. Often, these varieties are stable under the row operations of linear algebra. This talk will be about such varieties, specifically the equivariant K-theory classes of such varieties. Familiar combinatorial objects show up here, such as Schur polynomials and matroids. After giving a lengthy overview, I will move towards discussing the "shape" of the K-class of a variety. No background material will be assumed.
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Mathematics, UBC
Mon 16 Sep 2013, 3:00pm
Institute of Applied Mathematics
LSK 460
Successes and Challenges of Industrial Mathematics
LSK 460
Mon 16 Sep 2013, 3:00pm-4:00pm

Abstract

Our world is becoming increasingly complex: nanoscale processes need to be understood for the next generation of materials; global financial markets need to be understood to be able to optimize and stabilize the world's economy; long time environmental forecasts need to be done to understand the impact of global warming. Many of the underlying questions in these fields involve abstract models and require accurate computational approximation. The models may not involve new, research mathematics, but are often beyond the expertise of application experts. Mathematicians have the skills to contribute to these questions. Several examples of successful collaboration of mathematicians in industrial problems will be outlined, including the speaker's work in a larger group working with Ballard Power Systems on simulation tools for Hydrogen fuel cells. Some of the challenges to this collaboration will be discussed, including resistance of industrial engineers to working with mathematicians, but also resistance by our own community to recognizing the value of this activity. Some additional remarks will be made on the employment opportunities for mathematicians working in the Industry.
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Emil Wiedemann
UBC
Tue 17 Sep 2013, 3:30pm
Diff. Geom, Math. Phys., PDE Seminar
ESB 2012
Relaxation of Variational Problems for Orientation-Preserving Maps
ESB 2012
Tue 17 Sep 2013, 3:30pm-4:30pm

Abstract

It is well-known that variational problems may fail to have a classical minimiser if the integrand is not convex. In the 1930s, L. C. Young suggested a relaxation of such problems, where the minimising map is allowed to be measure-valued. In physical applications (e.g. elasticity theory), one often looks at variational problems for gradients of vector fields. A crucial problem in the context of relaxation is to characterise those measure-valued maps that arise as limits of a sequence of gradients. While this was achieved by D. Kinderlehrer and P. Pedregal about 20 years ago, the question remained open whether a similar characterisation could be found under the additional constraint that the gradients have positive determinant, i.e. the underlying maps be orientation-preserving. I will present such a characterisation, recently obtained in joint work with K. Koumatos (Oxford) and F. Rindler (Warwick).  
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University of Washington
Tue 17 Sep 2013, 4:00pm
Discrete Math Seminar
ESB 4133
Extending the parking spaces
ESB 4133
Tue 17 Sep 2013, 4:00pm-5:00pm

Abstract

A length n parking function is a sequence of integers whose non-decreasing rearrangement (q_1,q_2,\dots,q_n) satisfies q_i \leq i. Any rearrangement of a parking function thus remains a parking function and one can consider the permutation representation P_n of the symmetric groups S_n generated by such sequences. In this talk I will describe a representation of S_{n+1} whose restriction to S_n is P_n. This will be viewed in the context of a result of Stanley which extends the regular representation of S_n to a representation of S_{n+1}. I'll include a review of the combinatorics of symmetric group representations. This is joint work with Brendon Rhoades.
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UBC
Wed 18 Sep 2013, 3:00pm
Probability Seminar
ESB 2012
Limit stochastic differential equations (SDEs) for products of random matrices
ESB 2012
Wed 18 Sep 2013, 3:00pm-4:00pm

Abstract

We consider the Markov process given by products of i.i.d. random matrices that are perturbations of a fixed non-random matrix and the randomness is coupled with some small coupling constant. Such random products occur in terms of transfer matrices for random quasi-one dimensional Schrodinger operators with i.i.d. matrix potential. Letting the number of factors going to infi nity and the random disorder going to zero in a critical scaling we obtain a limit process for a certain Schur complement of the random products. This limit is described by an SDE. This allows us to obtain a limit SDE for the Markov processes given by the action of the random products on Grassmann and flag manifolds. Applied to random quasi-one dimensional Schrodinger operators we can describe the limiting eigenvalue process in a critical scaling by the zero process of a determinant of a matrix-valued function described by an SDE.
Joint work with B. Virag.
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Univeristy of Oxford
Wed 18 Sep 2013, 3:15pm SPECIAL
Topology and related seminars
ESB 4133
Configurations spaces revisited and homology stability for some diffeomorphism groups
ESB 4133
Wed 18 Sep 2013, 3:15pm-4:15pm

Abstract

Homology stability for families of discrete groups such as the symmetric groups, linear groups, braid groups and mapping class groups are well-known. Extensions to diffeomorphism groups of manifolds more generally have only been proved recently in a few special cases. We will revisit some classical results on configuration spaces, extend them to the equivariant setting, and prove homology stability for so-called symmetric diffeomorphism groups for arbitrary manifolds.
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Colorado State University
Thu 19 Sep 2013, 3:30pm
Number Theory Seminar
room MATH 126
Arithmetic occult periods
room MATH 126
Thu 19 Sep 2013, 3:30pm-4:30pm

Abstract

It occasionally happens that, for a certain type of complete intersection, the associated complex moduli space is actually (open in) an arithmetic quotient of a complex ball.  I will describe recent work, particularly involving the case of cubic surfaces, which suggests that this unexpected structure is the complex realization of a morphism of integral moduli spaces.  A modest payoff of the general theory is the calculation, in characteristic at least five, of the Galois group of the 27 lines on a sufficiently general cubic surface.
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University of Oxford
Fri 20 Sep 2013, 3:00pm SPECIAL
Department Colloquium
ESB 2012 (PIMS)
Cobordisms: old and new (PIMS/UBC Distinguished Colloquium Lecture)
ESB 2012 (PIMS)
Fri 20 Sep 2013, 3:00pm-4:00pm

Abstract


Cobordisms have played an important role in the classification of manifolds through work of René Thom from the 1950s. In a different way, they are fundamental to the mathematical formulation for topological quantum field theory of Atiyah, Segal and Witten. For a long time these two theories did not seem to interact. We will explain how more recent results have unified the two.

Note for Attendees

 Coffee, tea and cookies served at 2:30pm in the PIMS Lounge.
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Mathematics, SFU
Mon 23 Sep 2013, 3:00pm
Institute of Applied Mathematics
LSK 460
An informal introduction to rigorous computing
LSK 460
Mon 23 Sep 2013, 3:00pm-4:00pm

Abstract

This talk will present the under utilized ideas of rigorous numerics as a means of solving problems in PDE analysis. The core concepts such as interval arithmetic, automatic differentiation and radii polynomial will be discussed and recent extensions using mesh adaptivity will be presented. A fully automated curve following code for a class of elliptic problems will be explained.
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UBC
Mon 23 Sep 2013, 3:10pm
CRG Geometry and Physics Seminar
ESB 4127 (host: UBC)
Categorical Heisenberg actions on Hilbert schemes of points
ESB 4127 (host: UBC)
Mon 23 Sep 2013, 3:10pm-4:10pm

Abstract

 

We define actions of certain Heisenberg algebras on the Hilbert schemes of
points on ALE spaces. This lifts constructions of Nakajima and Grojnowski
from cohomology to K-theory and derived categories of coherent sheaves.
This action can be used to define Lie algebra actions (using categorical
vertex operators) and subsequently braid group actions and knot
invariants.
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Centro de Algebra da Universidade de Lisboa
Mon 23 Sep 2013, 4:00pm SPECIAL
Department Colloquium
MATX 1100; note unusual day
Square Packings and Optimality Proofs
MATX 1100; note unusual day
Mon 23 Sep 2013, 4:00pm-5:00pm

Abstract

Suppose you have a fixed number of boxes with a square base. These are transported in containers that also have a square base. In order to save space one wants to make containers small while still being able to hold all boxes. How large would the smallest such container be, and how would one need to pack the boxes into such a container? Questions such as these are called packing problems.

In mathematical language, this particular question would be considered as square packing, an area that traces its origin to a paper by Erdös and Graham. For small numbers of packed squares, tight (and conjectured optimal) packings have been obtained by a variety of methods, including computerized search. In contrast, only a surprisingly small number of non-trivial cases have actually been proved to be optimal, all of which involving relatively simple packings.

In this talk, which is accessible to undergraduate students, we give an overview on the subject and demonstrate the techniques used in proving square packings optimal.

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Roger Donaldson
Midvale Applied Mathematics, Inc.
Tue 24 Sep 2013, 12:30pm
Scientific Computation and Applied & Industrial Mathematics
ESB 4133
Statistics in the face of heavy-tailed data: a case-study in analyzing website traffic
ESB 4133
Tue 24 Sep 2013, 12:30pm-2:00pm

Abstract

A common experiment performed by web companies is the so-called A/B test, which compares two software designs assigning different interfaces or functionalities to two groups of randomly-selected users.  Although it is usual to design these experiments as hypothesis tests, standard tests (such as t-tests) overestimate confidence, since web traffic is heavy-tailed.  It is typical for a small number of individuals to radically affect the statistics of their test group.

In this talk, I discuss my work as a consultant for deviantART, an online art community, as we developed reliable models for their traffic data, as well as subsequent hypothesis tests.

This talk is both a review of properties and methods for handling heavy-tailed data, and an advertisement for the utility of probabilistic modeling in industry.

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UBC
Tue 24 Sep 2013, 3:30pm
Diff. Geom, Math. Phys., PDE Seminar
ESB 2012
Symmetric Monge-Kantorovich problems and polar decompositions of vector fields
ESB 2012
Tue 24 Sep 2013, 3:30pm-4:30am

Abstract

For any given integer N larger than 2, we show that every bounded measurable vector field is N-cyclically monotone up to a measure preserving N-involution. The proof involves the solution of a multidimensional symmetric Monge-Kantorovich problem, which we first study in the case of a general cost function on a product domain. The proof exploits a remarkable duality between measure preserving transformations that are N-involutions and those Hamiltonians that are N-cyclically antisymmetric.
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CNRS / U. Bordeaux
Tue 24 Sep 2013, 4:00pm
Discrete Math Seminar
ESB 4133
On the sandpile model on complete or bipartite complete graphs
ESB 4133
Tue 24 Sep 2013, 4:00pm-5:00pm

Abstract

The sandpile model was introduced by physicists Bak, Tang and Wiesenfeld in 1988. Then it was identified as a prototype of self organized criticality. This discrete model of diffusion is generally defined on any connected graph, and a configuration/state corresponds to a distribution of grains on vertices.  Here are some of its general properties. It induces a Markov chain with uniform stationary distribution on some recurrent configurations in bijection with spanning trees. The distribution of external activity of Tutte polynomial corresponds to the distribution of the number of grains. Addition of recurrent configurations leads to a finite abelian group for each graph, sometimes called in other contexts the critical group or the Picard's group.

During this talk, I will focus on the case of complete and bipartite complete graphs. The symmetries of these graphs simplify a lot the analysis but leads, according to me, to not so completely trivial results. A study related to the evolution of the Markov chain to the recurrent configurations simply leads to an apparently not so well-known extension, due to Chottin (1975), of a classical cyclic lemma used to count Dyck words. The analysis of Dhar's criterion, testing recurrence of a given configuration, simply leads to bistatistics related to the q,t-Catalan numbers studied by Bergeron, Garsia, Haglund and Haiman in their study of the space of diagonal harmonics. On the complete graphs, we obtain an algorithm of linear arithmetic complexity for the rank parameter introduced by Baker and Norine in 2007 for their analogue of Riemann-Roch theorem for graphs.

Based on joint works with Mark Dukes, Michele D'adderio, Jean-Christophe Aval, Angela Hicks and Robert Cori.
[arXiv:1208.0024,arXiv:1301.4803, arXiv:1307.7740, arXiv:1308.5325]
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Tel Aviv University
Wed 25 Sep 2013, 3:00pm
Probability Seminar
ESB 2012
New results on zeroes of stationary Gaussian functions
ESB 2012
Wed 25 Sep 2013, 3:00pm-4:00pm

Abstract

 We consider (complex) Gaussian analytic functions on a horizontal strip, whose distribution is invariant with respect to horizontal shifts (i.e., "stationary"). Let N(T) be the number of zeroes in [0,T] x [a,b]. First, we present an extension of a result by Wiener, concerning the existence and characterization of the limit N(T)/T as T approaches infinity. Secondly, we characterize the growth of the variance of N(T). For the last part, we consider real stationary Gaussian functions on the real axis and discuss the "gap probability" (i.e., the probability that the function has no zeroes in [0,T]). This part is a joint work with Ohad Feldheim.
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UAlberta
Wed 25 Sep 2013, 3:10pm
CRG Geometry and Physics Seminar
ESB 4127 (host: UAlberta)
Homological Projective Duality via Variation of Geometric Invariant Theory Quotients
ESB 4127 (host: UAlberta)
Wed 25 Sep 2013, 3:10pm-4:10pm

Abstract

I will briefly Homological Projective Duality, an analog of projective duality in the setting of derived categories due to Kuznetsov. I will explain the relationship between this duality, GLSMs, and variation of geometric invariant theory quotients as part of joint work with M. Ballard, D. Deliu, U. Isik, and L. Katzarkov.
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Tel Aviv University
Wed 25 Sep 2013, 4:00pm
Probability Seminar
ESB 2012
Rigidity of 3-colorings of the d-dimensional discrete torus
ESB 2012
Wed 25 Sep 2013, 4:00pm-5:00pm

Abstract

 We prove that a uniformly chosen proper coloring of Z_{2n}^d with 3 colors has a very rigid structure when the dimension d is sufficiently high. The coloring almost surely takes one color on almost all of either the even or the odd sub-lattice. In particular, one color appears on nearly half of the lattice sites. This model is the zero temperature case of the 3-states anti-ferromagnetic Potts model, which has been studied extensively in statistical mechanics. The proof involves results about graph homomorphisms and various combinatorial methods, and follows a topological intuition. Joint work with Ron Peled.
 
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Thibaut Divoux
Research Scientist at CNRS, France
Wed 25 Sep 2013, 4:00pm
One Time Event
LSK 203
Soft solids: how do they yield
LSK 203
Wed 25 Sep 2013, 4:00pm-5:00pm

Details

Abstract:
Yield stress fluids encompass a large class of materials, from granular media and foams to dense assemblies of colloidal hard spheres and glassy suspensions of soft particles such as emulsions, microgels, etc. These soft glassy systems share the following feature: they display a solid-like behavior at rest, while they flow as viscous liquids when stressed above their yield stress. Such solid-fluid or yielding transition can be seen as a stress-induced unjamming scenario. The question of whether this shear-induced fluidization displays universal features, in a way similar to jamming driven by temperature or by volume fraction, has triggered much research effort in the recent years. Experimentally, difficulties arise from the need to measure deformations and flows close to yielding at vanishingly small shear rates with sufficient spatial and temporal resolutions. 

In this talk, I will briefly review the current state of research on the steady state reached by a soft glassy system above yielding. I will then concentrate on the spatiotemporal fluidization dynamics of a "simple" yield stress material, namely a carbopol microgel, that presents negligible aging and thixotropy. Through long experiments combining standard rheology and ultrasonic velocimetry under imposed strain or stress, I will show that the material undergoes a transient regime characterized by (i) a short-time creep regime reminiscent of the primary, or Andrade, creep in solid materials followed by (ii) a long-lasting shear banding regime that progressively gives way to homogeneous flow. As a key result, the duration of the shear-banding regime decreases as power laws of the applied shear rate and of the applied viscous stress. These power laws nicely combine to recover the Herschel-Bulkley law characteristic of the steady-state rheology of our microgel, thus providing an interesting interpretation of this widely used phenomenological law.
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University of Newcastle, Australia
Thu 26 Sep 2013, 3:30pm
Number Theory Seminar
room ASB 10908 (IRMACS - SFU) *
Mahler's Method, digital expansions, and algebraic numbers (or not)
room ASB 10908 (IRMACS - SFU) *
Thu 26 Sep 2013, 3:30pm-4:30pm

Abstract

In this talk, we survey past, present, and possible future results concerning the arithmetic nature of low complexity sequences. For example, what properties can be exhibited by numbers whose base expansion can be determined by a finite automaton? In the current context, this line of questioning was unknowingly initiated by Mahler, and later championed by Loxton and van der Poorten following the work of Cobham and Mendes France. In addition to describing some historical work, this talk will describe some of the the current advancements and generalisations concerning Mahler's method.

Note for Attendees

* People can attend the talk via videoconferencing in room MATH 126 at UBC.
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UBC
Fri 27 Sep 2013, 3:00pm
Department Colloquium
MATX 1100
Harmonic analysis on p-adic groups: how much does it depend on p?
MATX 1100
Fri 27 Sep 2013, 3:00pm-4:00pm

Abstract

This talk is about an application of methods originating in model theory to harmonic analysis on p-adic groups. It will start with the review of the definition of the p-adic numbers, and
then focus on a way to do integration over the field of p-adic numbers in a way that does not depend on p (this is where the model theory comes in).  Then I will describe the application of these ideas to harmonic analysis, in particular, a way to get a uniform in p estimate for  orbital integrals (which will be defined in the talk).

Note for Attendees

Please join your colleagues for refreshments in MATH 125 at 2:45 p.m. prior to today's colloquium.
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Wes Maciejewski
UBC
Mon 30 Sep 2013, 2:30pm
Math Education Research Reading
MATH 126
Purposes and Methods of Research in Mathematics Education (Alan H. Schoenfeld)
MATH 126
Mon 30 Sep 2013, 2:30pm-3:20pm

Abstract

The Mathematics Education Research Reading Group will meet monthly to review and discuss current papers about research in mathematics education.  Anyone is welcome to participate.  For this first meeting's paper we have chosen a broad overview of the purposes and methods in mathematics education research.  Future meetings will focus on peer-reviewed studies.

The paper can be obtained at the following link: http://www.ams.org/notices/200006/fea-schoenfeld.pdf
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UAlberta
Mon 30 Sep 2013, 3:10pm
CRG Geometry and Physics Seminar
ESB 4127 (host: UAlberta)
Homological Projective Duality via Variation of Geometric Invariant Theory Quotients (Part 2)
ESB 4127 (host: UAlberta)
Mon 30 Sep 2013, 3:10pm-4:10pm

Abstract

 Continuation of talk on Sep. 25.
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