Department of Mathematics, University of California, Davis

Mon 3 Mar 2014, 3:00pm
SPECIAL
Institute of Applied Mathematics
LSK 460

Phase Retrieval, Random Matrices and Convex Optimization (IAMPIMS Distinguished Colloquium)

LSK 460
Mon 3 Mar 2014, 3:00pm4:00pm
Abstract
Phase retrieval is the centuryold problem of reconstructing a function, such as a signal or image, from intensity measurements, typically from the modulus of a diffracted wave. Phase retrieval problems  which arise in numerous areas including Xray crystallography, astronomy, diffraction imaging and quantum physics  are notoriously difficult to solve numerically. They also pervade many areas of mathematics, such as numerical analysis, harmonic analysis, algebraic geometry, combinatorics, and differential geometry. I will introduce a novel framework for phase retrieval, which comprises tools from optimization, random matrix theory, and compressive sensing. In particular, we will see that for certain types of random measurements a function, such as a signal or image, can be recovered exactly with high probability by solving a convenient semidefinite program without any assumption about the function whatsoever and under a mild condition on the number of measurements. Our method, known as PhaseLift, is also provably stable visavis noise. I will describe how this approach carries over to the classical phase retrieval setting using structured random illuminations. I conclude with some open problems.
hide

UBC

Mon 3 Mar 2014, 3:00pm
CRG Geometry and Physics Seminar
ESB 4127 (host: UBC)

Surjectivity and lifting the Weyl group action to the equivariant cohomology of a Springer fibre

ESB 4127 (host: UBC)
Mon 3 Mar 2014, 3:00pm4:00pm
Abstract
A famous result of Springer says that the Weyl group of a reductive algebraic group G (over C) acts on the cohomology of the subvariety X_u of the flag variety G/B consisting of the flags fixed by a unipotent u in G. This result was unexpected since W does not act on X_u itself. Recently, Kumar  Procesi and Goresky  MacPherson showed that Springer's action lifts to the equivariant cohomology of X_u with respect to the maximal torus in C_G(u) for so called parabolic unipotents u with the proviso that the cohomology morphism j*: H*(G/B) \to H*(X_u) is surjective. In this talk we will describe the parabolic unipotents for which j* is surjective and indicate a direct proof of lifting.
hide

UBC

Tue 4 Mar 2014, 2:00pm
Mathematical Education
Math 126

Comprehension of Arithmetic Word Problems: A Comparison of Successful and Unsuccessful Problem Solvers

Math 126
Tue 4 Mar 2014, 2:00pm3:00pm
hide

University of Victoria

Tue 4 Mar 2014, 3:30pm
Diff. Geom, Math. Phys., PDE Seminar
ESB2012 (PIMS)

Optimal Transportbased Model and Algorithms for Particle Image Velocimetry

ESB2012 (PIMS)
Tue 4 Mar 2014, 3:30pm4:30pm
Abstract
Particle Image Velocimetry (PIV) is a technique using successive laser images of particles immersed in a fluid to measure the velocity field of the fluid flow. Traditionally, crosscorrelation is employed to extract the field from each pair of recorded images. This talk will introduce a new approach based on Optimal Transport (OT) to approximate the velocity field. More specifically, we consider the solution of the L2 OT problem with initial and final densities given by successive images of tracers. We will first present a model for this situation and investigate the behaviour of the OT map with respect to the model's key parameters. Then, we will present some algorithms and numerical results applying this theory to synthetic and real examples. This is joint work with B.Khouider and M.Agueh.
hide

UAlberta

Wed 5 Mar 2014, 3:00pm
CRG Geometry and Physics Seminar
ESB 4127 (host: UAlberta)

An introduction to the GrossSiebert program (Part II)

ESB 4127 (host: UAlberta)
Wed 5 Mar 2014, 3:00pm4:00pm
Abstract
TBA
hide

UBC

Wed 5 Mar 2014, 3:00pm
Probability Seminar
ESB 2012

Hyperbolic random maps and unicellular maps

ESB 2012
Wed 5 Mar 2014, 3:00pm4:00pm
Abstract
Uniform infinite maps arise as local limits of uniformly chosen finite maps. Recently there has been keen interest in creating hyperbolic analogues of the uniform infinite planar maps. It is conjectured that uniformly distributed maps on surfaces with genus linear in the number of vertices should converge in the local topology to hyperbolic versions of uniform infinite maps. I will describe several models of such hyperbolic random maps which arise from several directions. Finally I will describe some recent results obtained for unicellular maps in high genus. Partly joint work with Omer Angel, Guillaume Chapuy and Nicolas Curien.
hide

University of Texas at Austin

Wed 5 Mar 2014, 3:15pm
Topology and related seminars
ESB 4133

Rightangled Artin subgroups of mapping class groups and Out(F)

ESB 4133
Wed 5 Mar 2014, 3:15pm4:15pm
Abstract
There are many analogies between the outer automorphism group of a free group Out(F) and the mapping class group of a surface Mod(S). I'll explain how each of these groups contains many rightangled Artin subgroups and how these subgroups can be used to understand the structure of both Mod(S) and Out(F). Interestingly, attempting to understand the properties of elements in rightangled Artin subgroups also reveals some major differences between Out(F) and Mod(S). I'll explain these differences and how they affect the study of Out(F).
hide

Mech. Eng. Dep. UBC

Wed 5 Mar 2014, 4:00pm
Fluids Lab Meeting
LSK 203

MacroSize Drop Encapsulation

LSK 203
Wed 5 Mar 2014, 4:00pm5:00pm
Abstract
Yield stress fluids have the property that they do not deform unless a given yield stress is exceeded. While in some flows this leads to unwanted features, this property can also be exploited in order to produce novel flow features. One example of such flows are viscoplastically lubricated (VPL) flows, in which a yield stress fluid is used to stabilize the interface in a multilayer flow, far beyond what might be expected for a typical viscousviscous interface. Here we extend this idea by considering the encapsulation of droplets within a viscoplastic fluid, for the purpose of transportation, e.g. in pipelines. The main advantage of this method, compared to others that involve capillary forces is that significantly larger droplets may be stably encapsulated, governed by the length scale of the flow and yield stress of the encapsulating fluid. We explore this setup both analytically and computationally. We show that sufficiently small droplets are held in the unyielded plug of the Poiseuille flow. As the length or radius of the droplets increase the carrier fluid eventually yields, potentially breaking the encapsulation. We study this process of breaking and give estimates for the limiting size of droplets that can be encapsulated.
Amir joined UBC for his master studies in the field of mechanical engineering in 2012. He is now working under supervision of Dr. Ian Frigaard in the complex fluids lab. He got his undergrad in 2011 from Sharif University of Technology.
hide

College of the Holy Cross

Thu 6 Mar 2014, 3:30pm
Number Theory Seminar
room ESB 4127 (PIMSUBC)

Weyl group multiple Dirichlet series of type C_n

room ESB 4127 (PIMSUBC)
Thu 6 Mar 2014, 3:30pm4:30pm
Abstract
We construct Weyl group multiple Dirichlet series associated to root systems of type C, through a combinatorial recipe involving GelfandTsetlin patterns. These Dirichlet series are associated with an nfold metaplectic cover of SO(2r+1) and we prove functional equations for them when n=1, via the CasselmanShalika formula. We also prove that our description matches the so called "stable case," as described for general root systems by Brubaker, Bump and Friedberg. This is joing work with Jennifer Beineke and Ben Brubaker.
hide

Daniel Valesin and Raimundo Briceno
UBC

Thu 6 Mar 2014, 4:00pm
Symbolic Dynamics and Ergodic Theory Seminar
Math Annex 1102

Phase Transitions and Computational Complexity (II)

Math Annex 1102
Thu 6 Mar 2014, 4:00pm5:30pm
hide

UBC

Fri 7 Mar 2014, 3:00pm
Graduate Student Seminar
MATX 1100

What is... the Fundamental Group?

MATX 1100
Fri 7 Mar 2014, 3:00pm4:30pm
Abstract
I will talk about one of the simplest and most important functors of algebraic topology, the fundamental group, which gives an algebraic object obtained from the loops on a space.
hide

UBC

Fri 7 Mar 2014, 3:00pm
Graduate Student Seminar
MATX 1100

The Wonderful World of   Disney  Modular Forms.

MATX 1100
Fri 7 Mar 2014, 3:00pm4:30pm
Abstract
In this talk, we will discuss modular forms, what they are and why they are amazing. Topics we will discuss include equalities of values of \sigma_{n} (sums of powers of divisors), the sum of four squares theorem, a discussion on Ramanujan's constant, an aside on polynomials with many prime terms, and perhaps even a bit of moonshine. Little will be completely proved but this talk will leave you in awe and wonder of this powerful concept.
hide

Department of Mathematics and Computer Science, Emory University

Mon 10 Mar 2014, 3:00pm
SPECIAL
Institute of Applied Mathematics
LSK 460

Complex Networks for Mathematicians (IAMPIMS Distinguished Colloquium)

LSK 460
Mon 10 Mar 2014, 3:00pm4:00pm
Abstract
Network Science is a rapidly growing interdisciplinary area at the intersection of mathematics, computer science, and a multitude of disciplines from the natural and life sciences to the social sciences and even the humanities. Network analysis methods are now widely used in proteomics, in the study of social networks (both human and animal), in finance, in ecology, in bibliometric studies, in archeology, and in a host of other fields.
In this talk I will introduce the audience to some of the mathematical and computational problems and methods of complex networks, with an emphasis on the basic notions of centrality and communicability. More specifically, I will describe some of the problems in largescale numerical linear algebra arising in this area, and how they differ from the corresponding problems encountered in more traditional applications of numerical analysis.
The talk will be accessible to students, requiring only a modest background in linear algebra and graph theory.
hide

Wisconsin

Mon 10 Mar 2014, 3:00pm
CRG Geometry and Physics Seminar
ESB 4127 (host: UBC)

The de Rham complex from the point of view of twisted derived intersections

ESB 4127 (host: UBC)
Mon 10 Mar 2014, 3:00pm4:00pm
Abstract
I shall present results of work with Arinkin and Hablicsek which allow us to understand the Frobenius pushforward of the de Rham complex as the structure sheaf of a twisted derived intersection. Similar considerations also apply for twisted de Rham complexes, yielding results which have applications in singularity theory and in the study of matrix factorizations. Using our theorems we recover and strengthen earlier results of DeligneIllusie, BarannikovKontsevich, and Sabbah. Our approach gives a new point of view on recent works of Joyce et al, generalizing results of Behrend, on understanding the holomorphic FukayaFloer homology.
hide

Montpellier / PIMSUBC

Tue 11 Mar 2014, 3:30pm
Diff. Geom, Math. Phys., PDE Seminar
ESB 2012

Spectral positivity on surfaces

ESB 2012
Tue 11 Mar 2014, 3:30pm4:30pm
Abstract
We shall see how the positivity of some Schr\"odinger operator on a surface gives information on its topology and its conformal type. The potent of the operators considered here involve the curvature of the surface and appear naturally in the study of minimal and constant mean curvature surfaces. It is a joint work with Pierre B\'erard.
hide

UAlberta

Wed 12 Mar 2014, 3:00pm
CRG Geometry and Physics Seminar
ESB 4127 (host: UAlberta)

A Variation of the BeilinsonHodge Conjecture

ESB 4127 (host: UAlberta)
Wed 12 Mar 2014, 3:00pm4:00pm
Abstract
Based on some recent joint work of J. Lewis, and others, we formulate a variation of the BeilinsonHodge conjecture pertaining to varieties defined over the complex numbers. In this talk, we explain the motivation for this conjecture, and some evidence in support of it.
hide

University of Warwick

Wed 12 Mar 2014, 3:00pm
Probability Seminar
ESB 2012

Large deviations and gradient flow

ESB 2012
Wed 12 Mar 2014, 3:00pm4:00pm
Abstract
We outline recent work uncovering intriguing connections between Otto's characterisation of diffusion as entropic gradient flow on one hand and largedeviation principles describing the microscopic picture (Brownian motion) on the other. Specifically, we connect macroscopic gradient flows with large deviation principles, and point out the potential of a bigger picture emerging: we indicate that in some nonequilibrium situations, entropies and thermodynamic free energies can be derived via large deviation principles. The approach advocated in the talk is different from the established hydrodynamic limit passage but extends a link that is well known in the equilibrium situation.
hide

Mech Eng Dept, University of Tehran

Wed 12 Mar 2014, 4:00pm
Fluids Lab Meeting
LSK 203

Discontinuous Galerkin Simulation of Compressible Flows on manycore GPUs

LSK 203
Wed 12 Mar 2014, 4:00pm5:00pm
Abstract
The Discontinuous Galerkin (DG) method is very suitable for studying flows with mesh and accuracy adaptation due to weakly imposition of interelement continuity. Moreover, DG provides sufficient stability for highorder finiteelement schemes by direct implementation of flux upwinding. Here a robust, highorder and fast DG solver was developed for compressible flows. For increasing the accuracy, Hermitian superparametric curved elements were designed for highorder boundary representation. For increasing the convergence rate, a NewtonKrylov algorithm was developed for pseudotime advancement of the semidiscrete system. Local timestep adjustment based on flow history was introduced to stabilize the solution especially in startup iterations. To maximize the computational efficiency, the developed code was parallelized on the stateoftheart Graphics Processing Unit (GPU) hardware with multiple layer of parallelism. According to compact nature of DG discretization, the fast blocktridiagonal solvers have been adapted recently both for structured and unstructured grids. A parallel Block Cyclic Reduction (BCR) linear solver was developed to work with DG on GPU. The developed solver has an improved speedup comparing with previous works due to optimizations in onchip memory bandwidth and floatingpoint arithmetic rate. The proposed GPUDG solver showed that it can be efficiently used in a wide range of highorder fluid flow simulations.
Dr. Baghapour finished his PhD at University of Tehran in the field of mechanical engineering in 2014. He worked under the supervision of Dr. Vahid Esfahanian in Vehicle, Fuel and Environment Research Institute (VFERI). He also got his MSc degree under the supervision of Dr. Mehdi Ashjaee in Laser Diagnostic laboratory of University of Tehran. He is now a research assistant at Simon Fraser University working on hydrodynamics.
hide

Université Lille 1

Thu 13 Mar 2014, 3:30pm
Number Theory Seminar
room MATH 126

Prime numbers: emergence and relevance of bilinear forms decomposition

room MATH 126
Thu 13 Mar 2014, 3:30pm4:30pm
Abstract
This talk will retrace the main steps of the modern theory of prime numbers and in particular how the combinatorial sieve combined with the Dirichlet series theory to give birth to the modern representation of the primes via a linear combination of terms, some of which being "linear", while the other ones are "bilinear". This will lead us to the recent developments of Green & Tao, Mauduit & Rivat, Tao, Helfgott, and Bourgain, Sarnak & Ziegler.
hide

UBC

Thu 13 Mar 2014, 4:00pm
Symbolic Dynamics and Ergodic Theory Seminar
Math Annex 1102

Phase Transitions and Computational Complexity (III)

Math Annex 1102
Thu 13 Mar 2014, 4:00pm5:30pm
hide

University of Southern California

Thu 13 Mar 2014, 4:00pm
Algebraic Groups and Related Structures
Math Annex (MATX) 1118

Strongly Dense Subgroups of Semisimple Algebraic Groups

Math Annex (MATX) 1118
Thu 13 Mar 2014, 4:00pm5:00pm
Abstract
Let G be a semisimple algebraic group over an algebraically closed field. A nonabelian free subgroup H is called strongly dense if every nonabelian subgroup of H is Zariski dense in G. We will discuss some recent results (with Breuillard, Green and Tao) regarding the existence of strongly dense subgroups and mention applications of these results to the BanachHausdorffTarski paradox, generation of finite simple groups of Lie type and the existence of families of expander graphs associated to finite groups of Lie type of fixed rank. If time permits, I will discuss some improvements with Breuillard and Larsen and the possibility of proving a generalizations of the Tits alternative.
hide

University of Southern California

Fri 14 Mar 2014, 3:00pm
Department Colloquium
ESB 2012 (PIMS)

Finite Simple Groups and Applications (PIMS/UBC Distinguished Colloquium)

ESB 2012 (PIMS)
Fri 14 Mar 2014, 3:00pm4:00pm
Abstract
The classification of finite simple groups is of fundamental importance in mathematics. It is also one of the longest and most complicated proofs in mathematics. We will very briefly discuss the result and a bit of history and then explain how it can and has been used to solve problems in many areas. We will end with mentioning some specific and perhaps surprising consequences in various fields.
hide

Stanford

Mon 17 Mar 2014, 3:00pm
CRG Geometry and Physics Seminar
ESB 4127 (host: UBC)

Categorified DonaldsonThomas invariants for sheaves

ESB 4127 (host: UBC)
Mon 17 Mar 2014, 3:00pm4:00pm
Abstract
We will present a joint work with YoungHoon Kiem on using family ChernSimons charts to construct perverse sheaves on moduli of sheaves that gives a categorification of DonaldsonThomas invariants.
hide

Mathematics Department, UBC

Mon 17 Mar 2014, 3:00pm
Institute of Applied Mathematics
LSK 460

Numerical simulations of proppant transport in hydraulic fractures induced by a slurry

LSK 460
Mon 17 Mar 2014, 3:00pm4:00pm
Abstract
Hydraulic fracturing (HF) is a process where the material, such as rock, is cracked by a pressurized fluid. Among many applications of HF, the most common use is the stimulation of production from oil and gas wells. To prevent fracture from closing after the pressure is reduced, the propping agents, such as sand, are pumped together with the fracturing fluid. The problems of fluiddriven fracture propagation and flow of the suspensions have been studied extensively, but not many works combine both and address the fracturing caused by a slurry. To fill the gap, the aim of this study is to develop a computational model for calculating the propagation of a fracture induced by the viscous fluid mixed with the spherical particles. First, the empirical constitutive law for the slurry is used to obtain the solution for the steady flow of the viscous fluid mixed with spherical particles in a channel. This solution is then used to formulate the conservation laws for the slurry and the particles, which govern the propagation of hydraulic fractures and the proppant transport inside them. The developments are applied to two fracture geometries  onedimensional KhristianovichZheltovGeertsma De Klerk (KGD) and pseudo3D (P3D). Numerical simulations show that the proposed method allows to capture the proppant plug (the region of compacted sand) formation and growth, as well as the gravitational settling for both geometries. Calculations with different proppant size demonstrate that bigger particles tend to settle faster, but, at the same time, promote fluid filtration through the plug, which supports further fracture propagation. Another problem that is addressed in this presentation is generation of proppant schedule. Certain proppant distribution inside the fracture is often desired at the end of pumping, however, only special proppant pumping schedule may lead to this. A new methodology of proppant schedule generation is introduced. It avoids solving an inverse problem and, at the same time, is more accurate than the commonly used procedure. One of the biggest advantages of the proposed approach is its compatibility with numerous HF simulators, making it a universal tool for generating a proppant schedule.
hide

Oregon State University

Tue 18 Mar 2014, 12:30pm
SPECIAL
Lunch Series on Teaching & Learning
Math 126

A dip in the shallow end is not enough: Developing a deeper approach to cultivate mathematics graduate students’ teaching practices

Math 126
Tue 18 Mar 2014, 12:30pm2:00pm
Abstract
The format of mathematics instruction in postsecondary contexts has remained problematic for undergraduate learners. Thus, the need to address the teaching practices of current and future teachers of postsecondary mathematics has become apparent. In preparing mathematics graduate students for their teaching responsibilities, many mathematics departments offer their new teaching assistants a onetime training session that addresses issues such as how to create quizzes, deal with disciplinary issues, and other practical matters. Beyond these shallow experiences, mathematics graduate students receive little support or mentorship for the development of their teaching practices. In this presentation, I will talk about three topics: my past research with mathematics graduate students, my current work on the creation of professional development experiences and support for mathematics graduate students, and my future work on the evolution of a deeper approach to cultivating mathematics graduate students’ teaching practices.
hide

GERAD and Ecole Polytechnique, Montreal

Tue 18 Mar 2014, 12:30pm
Scientific Computation and Applied & Industrial Mathematics
ESB 4133

The Projected GolubKahan Process for Constrained Linear LeastSquares Problems

ESB 4133
Tue 18 Mar 2014, 12:30pm2:00pm
Abstract
A preconditioned variant of the GolubKahan bidiagonalization process recently proposed by Arioli and Orban allows us to establish that SYMMLQ and MINRES applied to leastsquares problems in symmetric saddlepoint form perform redundant work and are combinations of methods such as LSQR and LSMR. A wellchosen preconditioner allows us to formulate a projected variant of the GolubKahan process that forms the basis of specialized numerical methods for linear leastsquares problems with linear equality constraints. As before, fullspace methods such as SYMMLQ and MINRES applied to the symmetric saddlepoint system defining the optimality conditions of such problems perform redundant work and are combinations of projected variants of LSQR and LSMR. We establish connections between numerical methods for leastsquares problems, fullspace methods and the projected and constraintpreconditioned Krylov methods of Gould, Orban and Rees.
hide

McGill University

Tue 18 Mar 2014, 4:00pm
Discrete Math Seminar
ESB 4133

Worstcase performance of online vector bin packing

ESB 4133
Tue 18 Mar 2014, 4:00pm5:00pm
Abstract
In the ddimensional bin packing problem (VBP), one is given vectors x1, x2, …, xn in Rd and the goal is to partition them into a minimum number of "feasible" sets. A set is feasible if the sum of its vectors does not have a component exceeding 1. Online VBP refers to the case where the vectors arrive sequentially and an algorithm must try to create these feasible sets on the fly. This problem has received renewed interest due to its relevance to placing virtual machines in a cloud platform.
The competitive ratio for an online algorithm is an upper bound on its worst case performance against an adversary which tries to choose a difficult sequence of incoming vectors. It had been outstanding for almost 20 years to clarify the gap between the best lower bound W(1) on the competitive ratio for online VBP versus the best upper bound of O(d). We settle this by describing a W (d / log d) lower bound. We also present several remaining open questions in the area.
hide

UBC

Wed 19 Mar 2014, 3:00pm
Probability Seminar
ESB 2012

Scaling limits and critical behaviour of the 4dimensional ncomponent phi^4 spin model

ESB 2012
Wed 19 Mar 2014, 3:00pm4:00pm
Abstract
The ncomponent phi^4 model is a ferromagnetic continuousspin model with interesting critical behaviour. In particular, the onecomponent model is predicted to be in the same universality class as the Ising model. We study the ncomponent model on the 4dimensional integer lattice, for all n greater than or equal to 1, with small coupling constant. We prove that the susceptibility has a logarithmic correction to mean field scaling, with exponent (n+2)/(n+8) for the logarithm.
We also analyse the asymptotic behaviour of the pressure as the critical point is approached, and prove that the specific heat hasfractional logarithmic scaling for n=1,2,3; double logarithmic scaling for n=4; and is bounded when n>4. In addition, for the model defined on the 4dimensional discrete torus, we prove that the scaling limit near the critical point is a multiple of the Gaussian free field on the continuum torus, whereas, in the subcritical regime, the scaling limit is Gaussian white noise with intensity equal to the susceptibility.
The proofs are based on a rigorous renormalisation group method.
This is joint work with Roland Bauerschmidt and David Brydges.
hide

UPenn

Wed 19 Mar 2014, 3:00pm
CRG Geometry and Physics Seminar
ESB 4127 (host: UAlberta)

Towards Unifying Toric Mirror Constructions

ESB 4127 (host: UAlberta)
Wed 19 Mar 2014, 3:00pm4:00pm
Abstract
I will discuss the recent work(inprogress) on unifying various mirror constructions of various authors, such as BatyrevBorisov and BerglundHübschKrawitz. This talk hopes to focus on questions, conjectures, and examples involved in this more generalized framework. This talk hopes not to focus on the difficulty of using a SmartBoard for seminars.
hide

University of Washington

Wed 19 Mar 2014, 3:15pm
Topology and related seminars
ESB 4133

Algebraic topology and algebraic torsors

ESB 4133
Wed 19 Mar 2014, 3:15pm4:15pm
Abstract
I will discuss some types of problems where techniques from algebraic topology have led to successful resolutions of open problems in algebraic geometry. Then, I will outline several future directions where better knowledge of the topology of classifying spaces of compact Lie groups could lead to more results in algebra.
hide

CS UBC

Thu 20 Mar 2014, 12:30pm
Graduate Student Seminar
Math 204

Application of twodimensional cell complexes to vector graphics

Math 204
Thu 20 Mar 2014, 12:30pm2:00pm
Abstract
Vector graphics software, such as Adobe Illustrator or Inkscape, typically use a datastructure made of paths independent from each other. This makes representing incidence relationships difficult.
With Rémi Ronfard and Michiel van de Panne, we developed a more topologyoriented datastructure, made of vertices, edges and faces, that I will first present. Then I will show how this combinatorial structure is in fact a presentation scheme whose geometric realization is (to the best of our computer scientist knowledge) a new kind of twodimensional "cell" complex, that we call PointCurveSurface complex (PCS complex). We are currently trying to prove that every twodimensional simplicial complex has a unique minimal PCS complex decomposition.
hide

Mathematics, UBC

Thu 20 Mar 2014, 12:30pm
Graduate Student Seminar
Math 204

Arithmetic Dynamics

Math 204
Thu 20 Mar 2014, 12:30pm2:00pm
Abstract
This short talk will focus on the interplay between number theory and dynamical systems by briefly describing some interesting yet simple to state questions.
hide

UCLA

Thu 20 Mar 2014, 3:30pm
Number Theory Seminar
room MATH 126

On the nontriviality of Heegner points modulo p

room MATH 126
Thu 20 Mar 2014, 3:30pm4:30pm
Abstract
Let l and p be distinct odd primes unramified in an imaginary quadratic extension K/Q. We outline the proof of the nontriviality of the padic formal group logarithm of Heegner points modulo p associated to the RankinSelberg convolution of an elliptic modular form of weight two and a theta series over the Z_{l}anticyclotomic extension of K. We also make remarks regarding the analogous nontriviality of generalised Heegner cycles.
hide

UBC

Thu 20 Mar 2014, 4:00pm
Symbolic Dynamics and Ergodic Theory Seminar
Math Annex 1102

Phase transitions and computational complexity (IV)

Math Annex 1102
Thu 20 Mar 2014, 4:00pm5:30pm
hide

University of Oregon

Fri 21 Mar 2014, 3:30pm
SPECIAL
Diff. Geom, Math. Phys., PDE Seminar
ESB 2012 (PIMS building) note the time and location change

Complex MongeAmpere equation on Kahler manifolds

ESB 2012 (PIMS building) note the time and location change
Fri 21 Mar 2014, 3:30pm4:30pm
Abstract
Complex MongeAmpere (CMA) equation is of fundamental importance in Kahler geometry. We will discuss regularity results for two versions of complex MongeAmpere equation which are extensively studied in Kahler geometry. The first is the classical CMA equation solved by S.T. Yau in 1970s to prove the Calabi conjecture. The second is a homogenous complex MongeAmpere, which is known as a geodesic equation of the space of Kahler metrics.
hide

Ohio State University

Mon 24 Mar 2014, 3:00pm
CRG Geometry and Physics Seminar
ESB 4127 (host: UBC)

Curves on Irreducible Holomorphic Symplectic Varieties

ESB 4127 (host: UBC)
Mon 24 Mar 2014, 3:00pm4:00pm
Abstract
The goal of the talk is to present derived category techniques to study holomorphic symplectic varieties. In particular, we study and answer the following questions:
(1) the HassettTschinkel Conjecture on the structure of the Mori cone of curves;
(2) the BogomolovTyurinHassettTschinkelHuybrechtsSawon Conjecture on the existence of Lagrangian fibrations;
(3) the KawamataMorrison Cone Conjecture.
Irreducible Holomorphic Symplectic varieties (IHS for short) are simply connected projective manifolds endowed with a unique (up to scalars) holomorphic symplectic form; K3 surfaces are the lowest dimensional example. In this talk we concentrate on IHS of K3^[n]type, namely IHS deformation equivalent to the punctual Hilbert scheme on a K3 surface. After giving a short introduction to the basics of IHS theory, we will present recent joint work with Arend Bayer on how to prove (1), (2), and (3) for moduli spaces of sheaves on K3 surfaces, by using derived categories and Bridgeland stability. If time permits, I will also sketch how to extend these results to all IHS of K3^[n]type, as recently proven by BayerHassettTschinkel, Mongardi, Matsushita, MarkmanYoshioka, and AmerikVerbitsky.
hide

Mathematics, UBC

Mon 24 Mar 2014, 3:00pm
Institute of Applied Mathematics
LSK 460

Logarithmic Expansions and the Stability of Periodic Patterns of Localize d Spots for ReactionDiffusion Systems in Two Dimensions

LSK 460
Mon 24 Mar 2014, 3:00pm4:00pm
Abstract
We determine the stability threshold for a periodic arrangment of localized spots for some singularly perturbed twocomponent reactiondiffusion systems including the GiererMeihardt, Schnakenburg, and GrayScott models, in R^2. In the semistrong interaction asymptotic limit where only one of the components has an asymptotically small diffusivity, the leading order stability threshold governing amplitude instabilities of the spots, as derived by WeiWinter (2001, 2003), is independent of the arrangement of the spots in the lattice. By combining a spectral approach based on FloquetBloch theory together with the method of matched asymptotic expansions and appropriate Fredholm solvability conditions, we calculate the next order term in the expansion of the stability threshold in terms of the regular parts of certain Green's functions. In this way, we derive an asymptotic result for the location of a realvalued band of continuous spectrum for the linearized operator when a stability parameter is close to its critical value. This result depends on an objective function defined in terms of both the Bloch wavevector and the particular lattice arrangment. From a numerical minmax optimization of this objective function it is shown that a regular hexagonal lattice of localized spots is the most stable. Joint work with Juncheng Wei (UBC), David Iron (Dalhousie) and John Rumsey (Dalhousie)
hide

Warwick

Tue 25 Mar 2014, 12:30pm
Scientific Computation and Applied & Industrial Mathematics
ESB 4133

Accelerating the Dimer Algorithm for Computing Saddle Points

ESB 4133
Tue 25 Mar 2014, 12:30pm2:00pm
Abstract
The dimer method is a simple hessianfree algorithm for computing index1 saddles. In this talk, I will review and analyze a few variants of this algorithm, focusing on some improvements to its efficiency, in particular adding preconditioning capabilities and linesearch based on a local merit function. I will demonstrate the efficiency of the new variant on a range of applications from academic toy problems, an atomistic problem and a PDE problem.
Despite these new improvements, we can currently give no global convergence guarantee. Indeed, we can construct counterexamples to global convergence. I will conclude my talk by explaining some of the difficulties we encountered.
hide

UBC

Tue 25 Mar 2014, 3:30pm
Diff. Geom, Math. Phys., PDE Seminar
ESB 2012

Complex analytic, onefrequency cocycles

ESB 2012
Tue 25 Mar 2014, 3:30pm4:30pm
Abstract
hide

University of Melbourne

Tue 25 Mar 2014, 4:00pm
Discrete Math Seminar
ESB 4133

Counting two interacting friendly walks near an attractive surface

ESB 4133
Tue 25 Mar 2014, 4:00pm5:00pm
Abstract
We will describe a class of two directed and friendly walks along the
square lattice that are restricted to the upper halfplane. One
motivation for studying this class is that it can be used to model an
idealised thermodynamic system of a DNA strand in a solvent near an
attractive surface. We will solve the combinatorial problem of finding
a closedform expression for the number of $N$ paired walks that
contain $L$ lowerwalkto$x$axis and $M$ lowerwalktoupperwalk
shared site contacts.
hide

UBC

Wed 26 Mar 2014, 3:15pm
Topology and related seminars
ESB 4133

A Classifying Space for Commutativity for Lie Groups

ESB 4133
Wed 26 Mar 2014, 3:15pm4:15pm
Abstract
In this talk we will define a space built out of all the commuting ntuples in a Lie group and discuss its role as a classifying space for commutativity. Applied to the unitary groups this gives rise to an infinite loop space and the notion of commutative Ktheory. We will also provide computations for the rational cohomology in terms of multsymmetric invariants. This is joint work with Jose Gomez.
hide

University of Tokyo

Thu 27 Mar 2014, 3:30pm
Number Theory Seminar
room MATH 126

Congruences of Hilbert modular forms over real quadratic fields and the special values of Lfunctions

room MATH 126
Thu 27 Mar 2014, 3:30pm4:30pm
Abstract
Let F be a totally real number field. We consider the problem to show how congruences between the Fourier coefficients of a Hilbert eigenform over F and a Hilbert Eisenstein series over F (of the same parallel weight (k,...,k)) give rise to corresponding congruences between the algebraic parts of the special values of the associated Lfunctions. In the case F = Q, the congruences of Lfunctions were obtained by Vatsal (k=2), HeumannVatsal (k≥2), and the speaker (k≥2). In this talk, we generalize Vatsal's work to the case F is a real quadratic field and k=2.
hide

UBC

Thu 27 Mar 2014, 4:00pm
Symbolic Dynamics and Ergodic Theory Seminar
Math Annex 1102

Phase transitions and computational complexity (V)

Math Annex 1102
Thu 27 Mar 2014, 4:00pm5:30pm
hide

UC Santa Barbara

Fri 28 Mar 2014, 3:00pm
Department Colloquium
ESB 2012 (PIMS)

The Emerging Roles and Computational Challenges of Stochasticity in Biological Systems (PIMS/UBC Distinguished Colloquium)

ESB 2012 (PIMS)
Fri 28 Mar 2014, 3:00pm4:00pm
Abstract
In recent years it has become increasingly clear that stochasticity plays an important role in many biological processes. Examples include bistable genetic switches, noise enhanced robustness of oscillations, and fluctuation enhanced sensitivity or “stochastic focusing".. Numerous cellular systems rely on spatial stochastic noise for robust performance. We examine the need for stochastic models, report on the state of the art of algorithms and software for modeling and simulation of stochastic biochemical systems, and identify some computational challenges.
hide

Note for Attendees
Refreshments start 15 minutes before the talk in the IAM Lounge, Room 306 of the LSK building.