# Events - October 2019

Thursday, October 31, 2019 — 4:00 PM EDT

## Graphs and Matroids Seminar - James Davies

Title: Edge-maximal graphs on surfaces

 Speaker: James Davies Affiliation: University of Waterloo Room: MC 5501

Abstract:

It is straightforward to show that, with the exception of small complete graphs, every edge-maximal planar graph triangulates the plane.

Thursday, October 31, 2019 — 3:00 PM EDT

## Algebraic Combinatorics Seminar - Alexandru Nica

Title: Interpolated versions of the Central Limit Theorem, and crossings of pair-partitions

 Speaker: Alexandru Nica Affiliation: University of Waterloo Room: MC 5417

Abstract:

Thursday, October 31, 2019 — 1:00 PM EDT

## Algebraic Graph Theory Seminar - Soffia Arnadottir

Title: Perfect state transfer on Cayley graphs

 Speaker: Soffia Arnadottir Affiliation: University of Waterloo Room: MC 5479

Abstract:

Perfect state transfer on Cayley graphs

Wednesday, October 30, 2019 — 3:30 PM EDT

## Joint C&O/PureMath Colloquium - Hayley Reid

Title: Cutting a square into triangles of equal area

 Speaker: Hayley Reid Affiliation: University of Waterloo Room: MC 5501

Abstract:

Suppose you are given a square and asked to cut it into n triangles of equal area. If n is even the problem is almost trivial, but when n is odd the problem becomes much harder.

Thursday, October 24, 2019 — 4:00 PM EDT

## Graphs and Matroids Seminar - Rose McCarty

Title: Sublinear separators in intersection graphs of convex shapes

 Speaker: Rose McCarty Affiliation: University of Waterloo Room: MC 5501

Abstract:

A balanced separator of an n-vertex graph is set of vertices whose deletion leaves only components of size at most 2n/3.

Thursday, October 24, 2019 — 3:00 PM EDT

## Algebraic Combinatorics Seminar - John Machacek

Title: Boundary measurement and sign variation in real projective space

 Speaker: John Machacek Affiliation: York University Room: MC 5417

Abstract:

Thursday, October 24, 2019 — 1:00 PM EDT

## Algebraic Graph Theory Seminar - Xiaohong Zhang

Title: Perfect state transfer on Hadamard diagonalizable graphs

 Speaker: Xiaohong Zhang Affiliation: University of Waterloo Room: MC 5479

Abstract:

A (weighted) graph whose Laplacian matrix is diagonalizable by a Hadamard matrix is said to be Hadamard diagonalizable.

Thursday, October 17, 2019 — 1:00 PM EDT

## Algebraic Graph Theory Seminar - Harmony Zhan

Title: Discrete quantum walks, embeddings and designs

 Speaker: Harmony Zhan Affiliation: York University Room: MC 5479

Abstract:

We construct a quantum walk from an embedding of a graph.

Friday, October 11, 2019 — 3:30 PM EDT

## Tutte Colloquium - Laurent Poirrier

Title: On the depth of cutting planesOn the depth of cutting planes

 Speaker: Laurent Poirrier Affiliation: University of Waterloo Room: MC 5501

Abstract:

We tackle one of the most important open problems in computational integer programming: cut selection.

For four decades, cutting planes were believed to be useful only for structured combinatorial problems. This changed in 1995 when Balas, Ceria and Cornuéjols showed that Gomory cuts could helpfully strengthen the formulation of general integer programming problems. Since then, many other cut generation techniques have been developed, but their practical success has been moderate at best.

Friday, October 11, 2019 — 1:00 PM EDT

## Combinatorial Optimization Reading Group - Justin Toth

Title: Maximizing a Monotone Submodular Function subject to a Matroid Constraint

 Speaker: Justin Toth Affiliation: University of Waterloo Room: MC 5417

Abstract:

Based on the paper by Calinescu, Chekuri, Pál, and Vondrák of the same title. We will study a randomized $(1-\frac{1}{e})$-approximation algorithm for the titular problem.

Thursday, October 10, 2019 — 4:00 PM EDT

## Graphs and Matroids Seminar - Logan Grout

Title: A Pseudoforest Analogue of the Strong Nine Dragon Tree Conjecture

 Speaker: Logan Grout Affiliation: University of Waterloo Room: MC 5501

Abstract:

In 2016, Jiang and Yang proved the Nine Dragon Tree Conjecture, a strengthening of the classical arboricity result of Nash-Williams (1964). On the way to developing this proof, Fan, Lim Song, and Yang proved an analogous result for decomposing graphs into pseudoforests, which is a strengthening of Hakimi’s Theorem.

Thursday, October 10, 2019 — 3:00 PM EDT

## Algebraic Combinatorics Seminar - Hugh Thomas

Title: Scattering amplitudes and associahedra

 Speaker: Hugh Thomas Affiliation: UQAM Room: MC 5417

Abstract:

The classic approach to scattering amplitudes sums a contribution from a (potentially very large) number of Feynman diagrams.

Thursday, October 10, 2019 — 1:00 PM EDT

## Algebraic Graph Theory Seminar - Chris Godsil

Title: Quantum Colouring and Derangements

 Speaker: Chris Godsil Affiliation: University of Waterloo Room: MC 5479

Abstract:

Work in quantum information has lead to the introduction of quantum colourings.

Friday, October 4, 2019 — 3:30 PM EDT

## Tutte Colloquium - Bruce Richter

Title: Extending drawings of K(n) to pseudolines and pseudocircles

 Speaker: Bruce Richter Affiliation: University of Waterloo Room: MC 5501

Abstract:

In the early part of the 21st century, it was shown that the number of crossings in a straight-line drawing of K(n) is at least the number H(n), which is conjectured to be the crossing number of K(n). In fact, it is now known that, for n at least 10, the inequality is strict.

Friday, October 4, 2019 — 1:00 PM EDT

## Combinatorial Optimization Reading Group - Ishan Bansal

Title: Greedy Heuristic for Maximizing Submodular Set Functions

 Speaker: Ishan Bansal Affiliation: University of Waterloo Room: MC 5417

Abstract:

Several hard combinatorial optimization problems can be posed in the following framework: maximize a submodular function over its domain subject to a cardinality constraint.

Thursday, October 3, 2019 — 4:00 PM EDT

## Graphs and Matroids Seminar - Lise Turner

Title:

 Speaker: Lise Turner Affiliation: University of Waterloo Room: MC 5501

Abstract:

There are several different notions of what it means for a graph to converge. One popular notion for sparse graphs is Benjamini-Schramm convergence which focuses on local properties of the graphs.

Thursday, October 3, 2019 — 3:00 PM EDT

## Algebraic Combinatorics Seminar - David Wagner

Title: Discrete diffusion on graphs and real hyperplane arrangements

 Speaker: David Wagner Affiliation: University of Waterloo Room: MC 6483

Abstract:

In 2016, Duffy et al. introduced the following process on a graph.  Initially, each vertex has some integer number of chips'' placed there (possibly negative).

Thursday, October 3, 2019 — 1:00 PM EDT

## Algebraic Graph Theory Seminar - Ada Chan

Title: Type II Matrices

 Speaker: Ada Chan Affiliation: York University Room: MC 5479

Abstract:

In 1867,  Sylvester defined an {\sl inverse orthogonal matrix} as an $n\times n$ complex matrix $W$ satisfying

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