University of Aveiro

Thu 8 Oct 2015, 3:00pm
Symbolic Dynamics and Ergodic Theory Seminar
Math Annex 1102

Multidimensional Convolutional Codes

Math Annex 1102
Thu 8 Oct 2015, 3:00pm4:00pm
Abstract
An introductory talk on one dimensional convolutional codes will be given on Tuesday, October 6.
Multidimensional (nD) convolutional codes generalize one dimensional (1D) convolutional codes and correspond to multidimensional systems widely studied in the systems theory literature. These codes have a practical potential in applications as they are very suitable to encode data recorded in n dimensions, e.g., pictures, videos, storage media, wireless applications, etc. However, in comparison to 1D convolutional codes, little is known in the area of nD convolutional codes and much more needs to be done to make it attractive for practical applications. From a mathematical point of view, these codes can be viewed as F[x_1,...,x_n]submodules of F[x_1,...,x_n]^n, where F is a finite field and F[x_1,...,x_n] is the ring of polynomials in several variables over F. In this talk we present a construction of an nD convolutional code based on superregular matrices with excellent errorcorrecting capabilities.
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University of Auckland

Fri 9 Oct 2015, 3:00pm
SPECIAL
Institute of Applied Mathematics
C680 HSBC Hall, Robson Square Campus (downtown)

The dynamics of aircraft as ground vehicles

C680 HSBC Hall, Robson Square Campus (downtown)
Fri 9 Oct 2015, 3:00pm4:00pm
Abstract
Aircraft are designed to fly but also need to operate efficiently and safely as vehicles on the ground. The tricycle configuration of commercial aircraft presents challenges for manoeuvres, such as highspeed turns off a runway. The talk will present results of a collaboration with Airbus into the stability of ground manoeuvres, whose central idea is to employ tools from bifurcation analysis to relevant industryvalidated aircraft models. Compared to standard extensive numerical simulations, this approach has been demonstrated to have potential efficiency benefits during the design stage. In particular, it allows for detailed studies of the nature of instabilities that need to be avoided in practice. As an example, we show that the sudden loss of lateral stability of a midsize passenger aircraft turning on the ground is due to a canard phenomenon that arises due to a nonobvious slowfast splitting.
This is joint work with Etienne Coetzee (Airbus), James Rankin (INRIA France), Mathieu Desroches (INRIA France) and Mark Lowenberg (University of Bristol).
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University of Auckland

Fri 9 Oct 2015, 4:00pm
SPECIAL
Institute of Applied Mathematics
C680 HSBC Hall, Robson Square Campus (downtown)

Shaken but not stirred: using mathematics in earthquakes

C680 HSBC Hall, Robson Square Campus (downtown)
Fri 9 Oct 2015, 4:00pm5:00pm
Abstract
Predicting the behaviour of a structure when subjected to an earthquake is an important problem from Civil Engineering. Here, we consider a planar posttensioned frame, which can be modelled as a twodegreeoffreedom system that is equivalent to the analytical model of a tied rocking block on an elastic foundation. The frame remains structurally sound as long as the tilt angle of the frame does not exceed a certain maximal angle. A standard approach to studying the dynamics would be to run simulations, where it is assumed that the earthquake is a pure sine wave with varying frequency and amplitude. Such a bruteforce approach establishes a region in the frequencyamplitude plane for which the structural stability of the frame eventually fails. We propose a novel approach that calculates the failure region in a much more efficient way by determining the failure boundary directly. Our method is based on continuation of a suitable twopoint boundary value problem. Our computations demonstrate that the failure boundary is only piecewise smooth and the results highlight further interesting details of how the dynamics is organised in the frequencyamplitude plane.
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Politechic University of Catalonia

Tue 13 Oct 2015, 3:30pm
Diff. Geom, Math. Phys., PDE Seminar
ESB 2012

On singular solutions for the fractional Yamabe problem

ESB 2012
Tue 13 Oct 2015, 3:30pm4:30pm
Abstract
Abstract: We construct some ODE solutions for the fractional Yamabe problem in conformal geometry. The fractional curvature, a generalization of the usual scalar curvature, is defined from the conformal fractional Laplacian, which is a nonlocal operator constructed on the conformal infinity of a conformally compact Einstein manifold.
These ODE solutions are a generalization of the usual Delaunay and, in particular, solve the fractional Yamabe problem
$$ (\Delta)^\gamma u= c_{n, {\gamma}}u^{\frac{n+2\gamma}{n2\gamma}}, u>0 \ \mbox{in} \ \r^n \backslash \{0\},$$
with an isolated singularity at the origin.
This is a fractional order ODE for which new tools need to be developed. The key of the proof is the computation of the fractional Laplacian in polar coordinates.
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University of Kentucky

Tue 13 Oct 2015, 4:00pm
Topology and related seminars
ESB 4133

The etalocal motivic sphere

ESB 4133
Tue 13 Oct 2015, 4:00pm5:00pm
Abstract
The Hopf map eta is nilpotent in the stable homotopy groups of spheres. This is not so for the motivic Hopf map, considered as an element of the motivic stable homotopy groups of spheres. This suggests that the etalocal part of motivic stable homotopy theory is an interesting object of study. We will describe this for the base fields C and R.
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Université Paris Diderot

Wed 14 Oct 2015, 3:00pm
Probability Seminar
ESB 2012

Cutoff for nonbacktracking random walks on sparse random graphs

ESB 2012
Wed 14 Oct 2015, 3:00pm4:00pm
Abstract
A finite ergodic Markov chain exhibits cutoff if its distance to stationarity remains close to 1 over a certain number of iterations and then abruptly drops to near 0 on a much shorter time scale. Here we consider nonbacktracking random walks on random graphs with a given degree sequence. Under a general sparsity condition, we establish the cutoff phenomenon, determine its precise window, and prove that the cutoff profile approaches a remarkably simple, universal shape. This is a joint work with Justin Salez (ParisDiderot).
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Columbia University

Fri 16 Oct 2015, 2:00pm
Number Theory Seminar
room MATH 126

TBA (note different seminar day and time)

room MATH 126
Fri 16 Oct 2015, 2:00pm3:00pm
Abstract
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UBC Math

Fri 16 Oct 2015, 3:00pm
Department Colloquium

TBA

Fri 16 Oct 2015, 3:00pm4:00pm
Abstract
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University of Fribourg

Mon 19 Oct 2015, 3:00pm
Algebraic Geometry Seminar
MATH 126

Motivic classes of classifying stacks and their invariants

MATH 126
Mon 19 Oct 2015, 3:00pm4:00pm
Abstract
After introducing the class of the classifying stack of a (finite) group, BG, in the Grothendieck ring of algebraic stacks, I will present certain cohomological invariants for a group  the Ekedahl invariants.
I am going to show that the class of BG is trivial if G is a finite subgroup of GL_3(k) or if G is a finite linear (or projective) reflection group. (k is a algebraically closed field of characteristic zero.) I will also show that the Ekedahl invariants of the discrete 5Heisenberg group are trivial.
These results relate naturally to Noether's Problem and to its obstruction, the Bogomolov multiplier.
At the end of the talk, I will link these results to the study of the motivic classes of the quotient varieties V/G by showing that such classes and the classes of BG exhibit the same combinatorial structure. Therefore, despite the title and technical terminology I will aim at making the talk enjoyable also by the combinatorial community.
(Partial joint work with Emanuele Delucchi.)
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University of Toronto

Mon 19 Oct 2015, 4:15pm
Algebraic Geometry Seminar
MATH 126

Theta lifts and currents on Shimura varieties

MATH 126
Mon 19 Oct 2015, 4:15pm5:15pm
Abstract
The Shimura varieties X attached to orthogonal and unitary groups come equipped with a large collection of socalled special cycles. Examples include Heegner divisors on modular curves and HirzebruchZagier cycles on Hilbert modular surfaces. We will review work of Borcherds and Bruinier using regularised theta lifts for the pair (SL_2,O(V)) to construct Green currents for special divisors. Then we will explain how to construct other interesting currents on X using the dual pair (Sp_4,O(V)). We will show that one obtains currents in the image of the regulator map of a certain motivic complex of X. Finally, we will describe how an argument using the SiegelWeil formula allows to relate the values of these currents to the product of a special value of an Lfunction and a period on a certain subgroup of Sp_4.
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Pacific Institute for the Mathematical Sciences and Université de Tours

Wed 21 Oct 2015, 3:00pm
Probability Seminar
ESB 2012

Discrete harmonic functions in the quadrant

ESB 2012
Wed 21 Oct 2015, 3:00pm4:00pm
Abstract
In this talk we shall be interested in discrete harmonic functions in cones (in particular, in the quarter plane). The generating function of these harmonic functions satisfies a functional equation (closed to a wellknown functional equation that appears in the context of enumeration of confined walks in combinatorics). We shall show the link between these harmonic functions and a oneparameter family of conformal mappings. One of the motivations to that study is to condition (in the sense of Doob) random walks never to leave cones.
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Reed College

Wed 21 Oct 2015, 3:15pm
Topology and related seminars
ESB 4133

Tensor triangular geometry of the stable motivic homotopy category

ESB 4133
Wed 21 Oct 2015, 3:15pm4:15pm
Abstract
In Balmer's framework of tensor triangular geometry, the prime thick tensor ideals in a tensor triangulated category C form a space which admits a continuous map to the Zariski spectrum Spec^h(End_u(1)) of homogeneous prime ideals in the graded endomorphism ring of the unit object. (Here the grading is induced by an element u of the Picard group of C.) If C is the stable motivic homotopy category and u is the punctured affine line, then this endomorphism ring is the MilnorWitt Ktheory ring of the base field. I will describe work by my student, Riley Thornton, which completely determines the homogeneous Zariski spectrum of MilnorWitt Ktheory in terms of the orderings on the base field. I will then comment on work in progress which uses the structure of this spectrum to study the thick subcategories of the stable motivic homotopy category.
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SFU

Thu 22 Oct 2015, 3:30pm
Number Theory Seminar
room MATH 126

An arithmetic graph invariant with applications in quantum field theory

room MATH 126
Thu 22 Oct 2015, 3:30pm4:30pm
Abstract
I will give an overview of things we know about c_{2} invariant of a graph. This is an invariant investigated principally by Brown and Schnetz which comes from counting points on the hypersurface defined by the Kirchhoff polynomial of a graph. This invariant predicts many properties of the Feynman integral of the graph. It connects with deep things like modular forms and motives. Many computations involving it come down to playing around with polynomials defined from the graph and so its also combinatorial. The fun and power of it come from the interplay of all three of these things.
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University of Tennessee, Knoxville

Tue 27 Oct 2015, 3:30pm
Diff. Geom, Math. Phys., PDE Seminar
ESB 2012

TBA

ESB 2012
Tue 27 Oct 2015, 3:30pm4:30am
Abstract
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Cornell University

Wed 28 Oct 2015, 3:00pm
Probability Seminar


Wed 28 Oct 2015, 3:00pm4:00pm
Abstract
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Carleton University

Thu 29 Oct 2015, 3:30pm
Number Theory Seminar
room MATH 126

TBA

room MATH 126
Thu 29 Oct 2015, 3:30pm4:30pm
Abstract
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Fri 30 Oct 2015, 3:00pm
Department Colloquium

reserved

Fri 30 Oct 2015, 3:00pm4:00pm
Abstract
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University of Washington

Tue 3 Nov 2015, 4:00pm
Discrete Math Seminar
ESB 4127

TBA

ESB 4127
Tue 3 Nov 2015, 4:00pm5:00pm
Abstract
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University of Washington

Wed 4 Nov 2015, 3:00pm
Probability Seminar
ESB 2012

Finitely dependent graph homomorphisms

ESB 2012
Wed 4 Nov 2015, 3:00pm4:00pm
Abstract
When a child randomly paints a coloring book, adjacent regions receive distinct colors whereas distant regions remain independent. It took mathematicians until 2014 to replicate this effect, when Holroyd and Liggett discovered the first stationary kdependent qcolorings. In this talk, I will discuss an extension of Holroyd and Liggett's construction which associates a canonical insertion procedure to every finite graph. The known colorings turn out to be diamonds in the rough: apart from multipartite analogues, they are the only kdependent processes which arise from finite graphs in this manner. Time permitting, I will present extensions of these results to weighted graphs and shifts of finite type. Joint work with Alexander Holroyd.
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UBC

Thu 5 Nov 2015, 3:30pm
Number Theory Seminar
room MATH 126

TBA

room MATH 126
Thu 5 Nov 2015, 3:30pm4:30pm
Abstract
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UBC

Thu 12 Nov 2015, 3:30pm
Number Theory Seminar
room MATH 126

TBA

room MATH 126
Thu 12 Nov 2015, 3:30pm4:30pm
Abstract
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Fri 13 Nov 2015, 3:00pm
Department Colloquium

reserved

Fri 13 Nov 2015, 3:00pm4:00pm
Abstract
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University of Texas at Austin

Mon 16 Nov 2015, 3:00pm
Algebraic Geometry Seminar
MATH 126

TBA

MATH 126
Mon 16 Nov 2015, 3:00pm4:00pm
Abstract
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Thu 19 Nov 2015, 3:30pm
Number Theory Seminar
room MATH 126


room MATH 126
Thu 19 Nov 2015, 3:30pm4:30pm
Abstract
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Thu 26 Nov 2015, 3:30pm
Number Theory Seminar
room MATH 126


room MATH 126
Thu 26 Nov 2015, 3:30pm4:30pm
Abstract
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Thu 3 Dec 2015, 3:30pm
Number Theory Seminar
room MATX 1101 (note different room)


room MATX 1101 (note different room)
Thu 3 Dec 2015, 3:30pm4:30pm
Abstract
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Mainz

Fri 4 Dec 2015, 3:00pm
Department Colloquium
MATX 1100

TBA

MATX 1100
Fri 4 Dec 2015, 3:00pm4:00pm
Abstract
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University of Toronto

Tue 5 Jan 2016, 3:30pm
Diff. Geom, Math. Phys., PDE Seminar
ESB2012

Vortex filaments in the Euler equation

ESB2012
Tue 5 Jan 2016, 3:30pm4:30pm
Abstract
Abstract: Classical fluid dynamics arguments suggest that in certain
limits, the evolution of thin vortex filaments in an ideal incompressible
fluid should roughly be governed by an equation called the binormal
curvature flow. However, these classical arguments rely on assumptions
that are so unrealistic that it would be hard even to extract from them a
precise conjecture that admits any realistic possibility of a proof. We
present a different approach to this question that yields a reasonable
formulation of a conjecture and strong supporting evidence, and that
clarifies the very substantial obstacles to a full proof. Parts of the
talk are based on joint work with Didier Smets and with Christian Seis
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Princeton University

Fri 26 Feb 2016, 3:00pm
SPECIAL
Department Colloquium / PIMS Seminars and PDF Colloquiums
ESB 2012

TBAPIMS/UBC Distinguished Colloquium

ESB 2012
Fri 26 Feb 2016, 3:00pm4:00pm
Abstract
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UC Berkeley

Fri 4 Mar 2016, 3:00pm
SPECIAL
Department Colloquium
ESB 2012

PIMS Hugh Morris Lecture

ESB 2012
Fri 4 Mar 2016, 3:00pm4:00pm
Abstract
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Note for Attendees
Note this is the first of two talks at Robson Square campus. The second talk will be followed by a reception at 5:00.