Faculty of Mathematics, University of Waterloo

WebNotice Postings: 23-Feb-2015 through 01-Mar-2015

Postings for all Departments


Monday, 23 February 2015, 3:30AM -- DC 1304
Colloquium -- Applied Mathematics
Speaker: Niky Kamran, Department of Mathematics and Statistics | McGill University
Title: "Inverse scattering at fixed energy on asymptotically hyperbolic Liouville surfaces"
Remarks: Wine and Cheese Reception to follow in DC 1301
Abstract: We study an inverse scattering problem on asymptotically hyperbolic Liouville surfaces with two ends and prove that surfaces are determined uniquely (up to isometries) from the knowledge for a given fixed energy of the scattering operator for scalar waves. The main ingredients used to prove this result are the separability of the wave equation into a system of ODEs, the Complex Angular Momentum method and a reinterpretation of the partial scattering coefficients as generalized Weyl-Titchmarsh functions for a certain Sturm-Liouville equation having the complex angular momentum as spectral parameter. This is a joint work with Thierry Daude (Cergy-Pontoise) and François Nicoleau (Nantes).

Monday, 23 February 2015, 10:30AM -- DC 1304
Seminar -- Computer Science
Speaker: Dr. Vladimir Kim, Postdoctoral Scholar, Stanford University
Title: "Structure and Function in Large Collections of 3D Models"
Abstract: As large repositories of 3D shape collections grow, understanding the geometric data, especially encoding the inter-model similarity, their variations, semantics and functionality, is of central importance. My research addresses the challenge of deriving probabilistic models that capture common structure in large, unorganized, and diverse collections of 3D polygonal shapes. In my talk, I will present two such structural models and algorithms for inferring them, as well as their applications to exploration, organization, analysis, and synthesis of geometric data. First, I will describe part-based templates for encoding structural variations in collections of man-made shapes. In this work, we propose an automatic algorithm that starts with an initial template model and then jointly optimizes for part segmentation, point-to-point surface correspondence, and a compact deformation model for the templates to best explain the input shape collection. As output, the algorithm produces a set of probabilistic part-based templates that groups the original collection into clusters of objects capturing their styles and variations. The second structural model is motivated by the observation that a majority of man-made shapes are designed to be used by people. Thus, in order to fully understand their semantics, one needs to answer a fundamental question: “how do people interact with these objects?” This work demonstrates that variations in some man-made shapes can be encoded as variations in poses that a person would need to adopt in order to use an object. More specifically, we observe that when a person is interacting with an object, contact points usually share consistent local geometric features related to the anthropometric properties of corresponding parts and that human body is subject to kinematic constraints and priors. We learn these priors from example data and represent them with local region classifiers and global kinematic constraints. Given an input 3D shape, we use our structural model to predict a corresponding human pose, including contact points and kinematic parameters. Finally, I will demonstrate that part-based and human-centric models enable effective user interfaces for exploring geometric datasets and synthesizing novel data.

BIO

Vladimir G. Kim is a postdoctoral scholar at Stanford University working in Geometric Computation group. Vladimir obtained his Ph.D. from Princeton University in 2013. His main research interest is geometric analysis with applications in computer graphics and 3D vision. Vladimir's recent work focuses on data-driven algorithms that leverage large geometric repositories to analyze functional, structural, and semantic attributes of objects and environments. He also works on interactive tools to facilitate content creation, exploration, and manipulation of geometric and other visual data. Vladimir has served/will serve on program committees for SIGGRAPH 2015, SGP 2013, SGP 2014, SGP 2015, Eurographics (short papers) 2014, Eurographics (short papers) 2015, and CAD/Graphics 2015. Vladimir is a recipient of the Siebel Scholarship and the Alexander Graham Bell Canada Graduate Scholarship.

Monday, 23 February 2015, 11:00AM -- MC 6486
Graduate Student Seminar Seminar -- Combinatorics and Optimization
Speaker: Speaker #1: Miaolan Xie, Speaker #2: Fahimeh Rahimi, University of Waterloo
Title: "Speaker #1: Cone, Polytope and Ellipsoid, Speaker #2: Quantum Chromatic Number of a Graph""
Abstract: Speaker #1 Cone, polytope and ellipsoid are all objects of interest in optimization. In this talk, I will discuss some of their interesting relations and atheorem on minimum volume ellipsoid for sets in Euclidean space.

Speaker #2 The gaph coloring game is the game in which Alice and Bob try to convince the referee that they can color the vertices of a graph with \(c\) colors. The referee asks each of them about the color of a vertex and they have to answer the colors of those vertices. If he asks them about the color of the same vertex, they have to answer the same color, and if he asks them about the color of two adjacent vertices, then they have to answer different colors. The minimum number of colors such that Alice and Bob can win the game is the quantum chromatic number of the graph. This talk, describes the quantum chromatic number and presents some bounds for this number.

Monday, 23 February 2015, 2:30PM -- QNC 0101
Colloquium -- Institute for Quantum Computing
Speaker: Masahiro Hotta, Tohoku University
Title: "Quantum Energy Teleportation: Strong Local Passivity vs. LOCC"
Abstract: “Quantum Energy teleportation (QET) is a protocol that allows one to teleport energy making use of pre-existing entanglement of the ground state of quantum many-body systems or quantum fields. I will review the latest results on QET and I will explain its implications on information thermodynamics, such as quantum Maxwell demons and Black Hole thermodynamics. I will also comment on current experimental prospects for QET via the quantum Hall effect. “

Monday, 23 February 2015, 4:00PM -- M3-3103
Pure Math Colloquium Colloquium -- Pure Mathematics
Speaker: Ailana Fraser, University of British Columbia
Title: "“Minimal surfaces and eigenvalue problems”"
Remarks: Refreshments will be served in MC 5413 at 3:30 p.m. Everyone is welcome to attend.
Abstract: Finding sharp eigenvalue bounds and characterizing the extremals is a basic problem in geometric analysis. We will describe the structure of metrics which are obtained by maximizing the first eigen- value of the Dirichlet-to-Neumann map over all metrics on a surface with boundary. It turns out that the extremals are related to minimal surfaces in the ball with a natural boundary condition, and in some cases it is possible to use minimal surface theory to characterize the extremal metrics. This is joint work with R. Schoen.

Monday, 23 February 2015, 4:15PM -- MC 6486
Seminar -- Combinatorics and Optimization
Speaker: Sara Ahmadian, University of Waterloo
Title: "Achieving Anonymity via Clustering a result by Aggarwal, Feder, Kenthapadi, Khuller, Panigrahy, Thomas, Zhu."
Abstract: Publishing data for analysis from a table containing personal records, while maintaining individual privacy, is a problem of increasing importance today. In this paper, they propose a new method for anonymizing data records, where quasi-identifiers of data records are first clustered and then cluster centers are published. To ensure privacy of the data records, they impose the constraint that each cluster must contain no fewer than a pre-specified number of data records. Formally, we want to cluster given points such that each cluster contains at least a pre-specified number of points (each point is assigned to exactly one cluster) while minimizing the maximum radius of clusters which is defined as the maximum distance from a cluster center to a point in that cluster. This paper shows that it is NP-hard to approximate the problem in general metrics better than 2 and gives a 2-approximation algorithm for this problem. The presentation will be based on the following manuscript (http://theory.stanford.edu/~kngk/papers/anonymityViaClustering.pdf).

For more information about our reading group, please visit our webpage http://www.math.uwaterloo.ca/~k2georgi/reading.htm

Tuesday, 24 February 2015, 2:30PM -- MC 5479
Universal Algebra Seminar -- Pure Mathematics
Speaker: Ross Willard, Department of Pure Mathematics, University of Waterloo
Title: "“Barto’s NU Theorem (continued)”"
Abstract: This seminar is working through the paper ”Finitely related algebras in congruence dis- tributive varieties have near unanimity terms,” Canad. J. Math. 65 (2013), 3-21, by Libor Barto. In this lecture I will cover more preliminaries to the main argument.

Tuesday, 24 February 2015, 3:30PM -- MC 5413
Computability Learning Seminar -- Pure Mathematics
Speaker: Mohammad Mahmoud, Department of Pure Mathematics University of Waterloo
Title: "“Introduction to Reverse Mathematics”"
Abstract: Reverse mathematics is a program in mathematical logic that seeks to determine which axioms are required to prove theorems of mathematics (going back from the theorems to the axioms). Reverse mathematics makes use of subsystems of second order arithmetic ( a formal theory of the natural numbers and sets of natural numbers). These ”base” systems are too weak to prove most of the interesting theorems in mathematics, but powerful enough to state these theorems and maybe prove one from another. We describe five basic subsystems of second order arithmetic (Big Five) that occur frequently in reverse mathematics. In particular, we will focus on ACA0 and weaker systems (mathematics that can be done with sets from the arithmetic hierarchy) and compare their strength. Also we point out facts that explain the close connection between reverse mathematics and computability theory. Note: This is our first talk as we begin our new topic of Reverse Mathematics. We will be working through Hirschfeldt’s ”Slicing the Truth”.

Wednesday, 25 February 2015, 1:30PM -- MC 5417
Seminar -- Applied Mathematics
Speaker: Jody Klymak, School of Earth and Ocean Sciences | University of Victoria
Title: "Parameterizing turbulence in low Froude number stratified turbulent flows"
Abstract: Ocean mixing helps drive the global overturning circulation and set the distribution of ocean properties on both small and large scales. However, the mixing and turbulence that drives the mixing is often unresolved by numerical simulations, and therefore we need to parameterize it. Tides are one source of energy for this mixing, as stratified flow over rough topography creates internal waves that eventually break. For gentle topography this process is believed to be dominated by weakly non-linear wave-wave interactions. However, for abrupt topography substantial turbulence is generated directly at the topography through breaking non-linear lee waves. Here we demonstrate the phenomenology of these breaking waves, and discuss a simple a-priori method to parameterize the turbulence they produce.

Wednesday, 25 February 2015, 2:30PM -- MC 5403
Algebra Seminar -- Pure Mathematics
Speaker: Alex Berenbeim, Department of Pure Mathematics, University of Waterloo
Title: "“What Is Equivalence? Geometric realization, fibrations, and the ‘Logic’ of Space""
Abstract: Picking up where the previous talk left off, I will prove that the total singular complex functors and geometric realization functors form an adjoint pair. From here, fibred categories will be developed, using familiar examples from topology before establishing how fibred cat- egories correspond to logics pertaining to spaces. If time permits, I will make good on the original promise of showing the weak ω category is an algebra X for a contractible, normalized, globular operad P. If not, then you know there will be a thrilling concluding presentation!

Thursday, 26 February 2015, 12:00AM - ** Time Change ** - now at 12:00PM
** See updated entry for additional event information **
Speaker: Mike Zabrocki, York University
Title: "Macdonald Symmetric Functions and Parking Functions"

Thursday, 26 February 2015, 10:30AM -- DC 1304
Seminar -- Computer Science
Speaker: Dr. Simon Peter, Postdoctoral Research Associate, University of Washington
Title: "Building an Operating System for the Data Center"
Abstract: Data centers run a range of important applications with ever increasing performance demands, from cloud and server computing to Big Data and eScience. However, the scaling of CPU frequency has stalled in recent years, leading to hardware architectures that no longer transparently scale software performance. Two trends stand out: 1) Instead of frequency, hardware architectures increase the number of CPU cores, leaving complex memory system performance and CPU scheduling tradeoffs exposed to software. 2) Network and storage I/O performance continues to improve, but without matching improvements in CPU frequency. Software thus faces ever increasing I/O efficiency demands.

In my research, I address how operating systems (OSes) can handle these growing complexity and performance pressures to avoid becoming the limiting factor to performance. I first explain how current OS architecture is already inadequate to address these trends, limiting application performance. I then present how the OS can be redesigned to eliminate the performance limitations without compromising on existing features or application compatibility. I finish with an outlook on how these hardware trends affect software in the future and present ideas to address them.

BIO

Simon is a postdoctoral research associate at the University of Washington, where he leads research in operating systems and networks. His postdoctoral advisors are Tom Anderson and Arvind Krishnamurthy. Simon received a Ph.D. in Computer Science from ETH Zurich in 2012 and an MSc in Computer Science from the Carl-von-Ossietzky University Oldenburg, Germany in 2006.

Simon's research focus is on data center performance issues. For his work on the Arrakis high I/O performance operating system, he received the Jay Lepreau best paper award (2014) and the Madrona prize (2014). Previously, Simon has worked on the Barrelfish multicore operating system and conducted further award-winning systems research at various locations, including MSR Silicon Valley, MSR Cambridge, Intel Labs Germany, UC Riverside, and the NSF.

Thursday, 26 February 2015, 12:00PM -- MC 5479 true
Algebraic Combinatorics Seminar -- Combinatorics and Optimization
Speaker: Mike Zabrocki, York University
Title: "Macdonald Symmetric Functions and Parking Functions"
Abstract: A parking function can be thought of as a Dyck path of length n where the vertical edges are labeled with the integers 1 through n, increasing in the columns.

Haglund's "shuffle conjecture" from 2005 is a combinatorial formula for the symmetric function expression for $\nabla(e_n)$ with one term for each parking function. In 2008 Haglund, Morse and myself extended this conjecture to the action of nabla on a symmetric function indexed by a composition. The combinatorial formula has one term for each parking function which touches the diagonal according to the composition.

In the last few years Hicks, Garsia, Xin and myself were partially able to prove the compositional shuffle conjecture by showing algebraic recurrences on coefficients exist and agree with the combinatorics. In this talk I'll show algebraic recurrences which generalize those that were used to prove those results and draws a direct connection with Macdonald symmetric functions.

Thursday, 26 February 2015, 4:00PM -- M3 3127
Seminar -- Statistics & Actuarial Science
Speaker: Nokuthaba Sibanda,, Victoria University of Wellington, New Zealand
Title: "Monitoring outcomes in multistage healthcare procedures"
Abstract: Most statistical process control (SPC) programmes in healthcare focus on surveillance of outcomes at the final stage of a procedure, such as mortality or surgical failure rates. This approach ignores the multi-stage nature of such procedures, in which a patient progresses through several stages prior to the final stage. In this talk I will explore a novel approach to SPC programmes in healthcare. The proposed approach is based on multi-stage control charts that have been in use in industrial applications for decades. I will consider a number of univariate and multivariate test statistics for the control charts and explore their performance through simulations that are summarised using an empirical probability of true detection. The results show two main advantages of the multi-stage approach: (i) enables a closer study of all stages of a procedure and thus a clearer understanding of the potential sources of excess variation that lead to a shift in final stage outcome rates, (ii) changes in mid-stage outcomes rates can serve as a warning of pending changes in final stage outcomes rates if left unchecked.

Friday, 27 February 2015, 12:30PM -- PAS 2464
Artificial Intelligence Lab PhD Seminar -- Computer Science
Speaker: Trevor Bekolay, PhD candidate, David R. Cheriton School of Comp. Sci., Univ. Waterloo
Title: "Building a phenomenological model of the human ear"
Abstract: The human ear translates variations in air pressure to the sounds that we hear and interpret as music, speech, etc. It does this by separating the air pressure waves info frequency components. However, there are many differences between the output of the ear and a straightforward Fourier analysis. I will discuss these differences, and demonstrate them by showing how a phenomenological model of the human ear responds to different sounds.

Friday, 27 February 2015, 1:00PM -- MC 5413
Working Seminar on Approximate Groups Seminar -- Pure Mathematics
Speaker: Jason Bell, Department of Pure Mathematics University of Waterloo
Title: "“Approximate Groups: V”"
Abstract: We continue to follow van den Dries Seminaire Bourbaki article entitled Approximate Groups [after Hrushovski, and Breuillard, Green, Tao]. The subject involves the interaction of additive combinatorics and model theory.

Friday, 27 February 2015, 2:30PM -- MC 5413 true
Analysis Seminar Seminar -- Pure Mathematics
Speaker: Baris Ugurcan, Western University
Title: "“Potential theoretic analysis of a model from statistical mechanics: divisible sandpile and the discrete bi-Laplacian Gaussian field”"
Remarks: **Please note time change for this week**
Abstract: In the divisible sandpile model an initial mass distribution, composed of i.i.d. random variables on the vertices of an infinite vertex transitive graph, is disseminated by a local toppling rule so as to make the mass at each vertex ≤ 1. The whole process is governed by the equation s + ∆u = η, where ∆ is the discrete Laplacian operator and s, u, η respectively denote the initial mass distribution, the odometer function and the final mass configuration. Let m denote the mean mass per vertex. We show that for m < 1 the process stabilizes almost surely whereas in the case m > 1 it does not. Our main result is that, in the critical case namely when m = 1, the process does not stabilize a.s. The main interest in the divisible sandpile model is twofold: firstly it is a good starting place to study similar questions in the “abelian sandpile model” and secondly it leads to interesting questions in potential theory such as “under what conditions must a random harmonic function be an almost surely constant?”. We also give quantitative estimates for the divisible sandpile model on the discrete torus and relate the number of topplings to a discrete biLaplacian Gaussian field. This is joint work with Lionel Levine, Mathav Murugan and Yuval Peres.

Friday, 27 February 2015, 2:30PM -- DC 1331
Symbolic Computation Group Seminar -- Computer Science
Speaker: Andrew Arnold, PhD candidate, David R. Cheriton School of Comp. Sci., Univ. Waterloo
Title: "Output-Sensitive Algorithms for Sumset and Sparse Polynomial Multiplication"
Abstract: The most common computer representations of polynomials are the sparse and dense representations. A dense representation is a merely a list of coefficients. A sparse representation is a list of coefficient-exponent pairs representing the nonzero terms of a polynomial F. A sparse representation may be advantageous in terms of space when many of the coefficients of F are zero. An advantage of the dense representation is that it lends itself to fast FFT-based arithmetic, allowing for dense arithmetic in time softly linear in the combined size of the inputs and output. This is optimal up to poly-logarithmic factors. We present a probabilistic algorithm for multiplication of sparse polynomials with integer coefficients, sensitive to the size of the output. A challenge of this approach is that the size of the output varies for inputs of a fixed size. As a subroutine, we present an algorithm which computes the sumset (Minkowski sum) of two finite integer sets in time softly linear in the input and output size, assuming the integers are stored as words of a common size.

Friday, 27 February 2015, 3:30PM - ** Time Change ** - now at 2:30PM
** See updated entry for additional event information **
Speaker: Baris Ugurcan, Western University
Title: "“Potential theoretic analysis of a model from statistical mechanics: divisible sandpile and the discrete bi-Laplacian Gaussian field”"

Friday, 27 February 2015, 3:30PM -- MC 5479
Tutte Seminar Seminar -- Combinatorics and Optimization
Speaker: Peter Nelson, University of Waterloo
Title: "The structure of triangle-free graphs and geometries"
Abstract: Brandt and Thomass\'{e} proved that a triangle-free graph $G$ of minimum degree greater than $\frac{1}{3}|V(G)|$ has chromatic number at most $4$. Moreover, Luczak showed that for all $\epsilon > 0$, there exists $N = N(\epsilon)$ so that every triangle-free graph $G$ is obtained from a triangle-free graph on at most $N$ vertices by duplicating vertices. On the other hand, Hajnal proved that for all $\epsilon > 0$ and every integer $t$, there is a triangle-free graph $G$ with minimum degree at least $(\frac{1}{3}-\epsilon)|V(G)|$ and chromatic number at least $t$. Thus, the constant $\frac{1}{3}$ plays a very special role for the class of triangle-free graphs. I will discuss the above problems and recent work with Rutger Campbell and Jim Geelen where we obtain surprisingly similar results for triangle-free binary geometries.

Friday, 27 February 2015, 7:30PM -- Siegfried Hall, St. Jerome's University
Lecture -- Faculty of Mathematics
Speaker: Steven J. Banks and D. Marc Kilgour, New York University and Wilfrid Laurier University
Title: "Mathematics and Democracy"
Remarks: Free of Charge, Wheelchair Accessible, No Registration Required, Reception will follow the lecture. Parking is available at St Paul's free of charge for the lecture.
Abstract: A multitude of election systems have been proposed for choosing both single winners (for mayor, governor, or president) or multiple winners (to a council or committee). Those based on approval voting, which allows voters to vote for more than one candidate or party, are especially appealing. We look at the mathematics behind these systems, and how well they satisfy properties considered important in a democracy. We also analyze the usage of approval voting in electing, among other officials, Catholic popes and UN secretaries general. More recently, approval voting has been adopted by several major professional societies to elect their presidents and advisory councils. Based on this experience, we offer several recommendations for the use of approval voting in public elections.


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