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Friday, September 28, 2018 — 3:30 PM EDT

Title: The Shapiro-Shapiro Conjecture

Speaker: Kevin Purbhoo
Affiliation: University of Waterloo
Room: MC 5501

Abstract:

Given four lines in 3-space, can you find a fifth line that intersects the other four? How many?

This is the smallest non-trivial example of a "Schubert problem". The answer, in this case, is not hard to compute: there are two such lines. Generalizations of this fact date back to 19th century work of Schubert.

Thursday, September 27, 2018 — 3:30 PM EDT

Title: A characterization of (p,q)-mixing when p/q < 4

Speaker: Ben Moore
Affiliation: University of Waterloo
Room: MC 5417

Abstract: Let Hom(G,H) be the graph whose vertex set is the set of H-colourings of G, and two H-colourings f and g are adjacent if f differs from g in at most one vertex.

Wednesday, September 26, 2018 — 4:00 PM EDT

Title: Classification, Regularization and Logistic Regression

Speaker: Haesol Im
Affiliation: University of Waterloo
Room: MC 5479

Abstract: For the first two seminar series in machine learning,

Friday, September 21, 2018 — 3:30 PM EDT

Title: Quantum advantage with shallow circuits

Speaker: David Gosset
Affiliation: University of Waterloo
Room: MC 5501

Abstract:

A constant-depth quantum circuit is a parallel quantum computation that proceeds for a constant number of time steps.  In this work we prove that constant-depth quantum circuits are more powerful than their classical counterparts.

Thursday, September 20, 2018 — 3:30 PM EDT

Title: Naji’s characterization of circle graphs

Speaker: Jim Geelen
Affiliation: University of Waterloo
Room: MC 5417

Abstract: A circle graph is the intersection graph of chords of a circle.

Friday, September 14, 2018 — 3:30 PM EDT

Title: Rapid mixing of Glauber dynamics for colorings below Vigoda’s 11/6 threshold 

Speaker: Michelle Delcourt
Affiliation: University of Waterloo
Room: MC 5501

Abstract:

A well-known conjecture in computer science and statistical physics is that Glauber dynamics on the set of k-colorings of a graph G on n vertices with maximum degree Δ is rapidly mixing for k ≥ Δ+2.

Friday, September 14, 2018 — 11:30 AM EDT

Title: Science of Security-- Could Such a Thing Exist?

Speaker: Paul van Oorschot
Affiliation: Carelton University
Room: MC 5501

Abstract: Recent years have seen increasing calls to make security research more "scientific". Who can argue with science being desirable?

Thursday, September 13, 2018 — 3:30 PM EDT

Title: Entropy and enumeration

Speaker: Jorn van der Pol
Affiliation: University of Waterloo
Room: MC 5417

Abstract: The information-theoretic concept of entropy is closely related to enumeration;

Friday, September 7, 2018 — 3:30 PM EDT

Title: The smallest eigenvalues of Hamming, Johnson and other graphs

Speaker: Sebastian Cioaba
Affiliation: University of Delaware
Room: MC 5501

Abstract:

The smallest eigenvalue of graphs is closely related to other graph parameters such as the independence number, the chromatic number or the max-cut.

Thursday, August 16, 2018 — 11:30 AM EDT

Title: Counting Partitions Inside a Rectangle

Speaker: Steve Melczer
Affiliation: University of Pennsylvania
Room: MC 6486

Abstract:

The study of integer partitions is a classic subject with applications ranging from number theory to representation theory and combinatorics.

Friday, August 3, 2018 — 3:30 PM EDT

Title: The geometry of matroids

Speaker:  Federico Ardila
Affiliation: San Francisco State University
Room: MC 5501

Abstract:

Matroid theory is a combinatorial theory of independence which has its origins in linear algebra and graph theory, and turns out to have deep connections with many other fields.

Friday, July 27, 2018 — 3:30 PM EDT

Title: Algorithms for Rank-1 Bimatrix Games

Speaker: Bernhard von Stengel
Affiliation: London School of Economics
Room: MC 5501

Abstract:

The rank of a bimatrix game is the matrix rank of the sum of the two payoff matrices.

Thursday, July 26, 2018 — 3:30 PM EDT

Title: Acyclic Colouring of Graphs on Surfaces

Speaker: Shayla Redlin
Affiliation: University of Waterloo
Room: MC 5479

Abstract: An acyclic k-colouring of a graph G is a proper k-colouring of G with no

Friday, July 20, 2018 — 3:30 PM EDT

Title: Claw-free matroids

Speaker: Peter Nelson
Affiliation: University of Waterloo
Room: MC 5501

Abstract: A simple binary matroid is claw-free if it has no independent rank-3 flat. I will discuss a structure theorem, obtained jointly with Kazuhiro Nomoto, that classifies these objects exactly.

Thursday, July 19, 2018 — 3:30 PM EDT

Title: Density and Structure of Homomorphism-Critical Graphs

Speaker: Evelyne Smith-Roberge
Affiliation: University of Waterloo
Room: MC 5417

Abstract: 

Let H be a graph. A graph G is H-critical if every proper subgraph of G admits a homomor-
phism to H, but G itself does not.

Tuesday, July 17, 2018 — 3:00 PM EDT

Title: Using Linear Algebra to do Matching Theory

Speaker: Justin Toth
Affiliation: University of Waterloo
Room: MC 5501

Abstract:

A matching in a graph is a set of edges with each vertex contained in at most one edge. A perfect matching is a matching in which each vertex is contained in some edge.

Thursday, July 12, 2018 — 3:30 PM EDT

Title: Gaps in the crossing numbers of drawings of the complete graph

Speaker: Bruce Richter
Affiliation: University of Waterloo
Room: MC 5479

Abstract: In the late 1990’s, a 5-author manuscript circulated proving the existence of intervals of integers that connot occur as the crossing number of a “good” drawing of the complete graph. 

Thursday, July 12, 2018 — 1:30 PM EDT

Title: Periodicity on Oriented Graphs by way of Transcendental Number Theory

Speaker: Sabrina Lato
Affiliation: University of Waterloo
Room: MC 6486

Abstract: Using the adjacency matrix of a graph, it is straightforward to show that perfect state transfer between two vertices at some time t implies that both vertices will be periodic at time 2t.

Friday, July 6, 2018 — 3:30 PM EDT

Title: The coloring problem for restricted graph classes

Speaker: Chinh T. Hoang
Affiliation: Wilfred Laurier University
Room: MC 5501

Abstract:

Let L be a set of graphs. Free(L) is the set of graphs that do not contain any graph in L as an induced subgraph.

Thursday, July 5, 2018 — 3:30 PM EDT

Title:Generalizing the problem of packing disjoint cycles

Speaker: Paul Wollan
Affiliation: University of Rome "La Sapienza"
Room: MC 5479

Abstract: A classic result of Erdos and Posa states that there exists a function f such that for all k,

Thursday, July 5, 2018 — 1:30 PM EDT

Title: Edge State Transfer

Speaker: Tina Chen
Affiliation: University of Waterloo
Room: MC 6486

Abstract: Most research about quantum state transfer on graphs use adjacency matrices as their Hamiltonians and investigate the transfers between single vertex states.

Friday, June 29, 2018 — 3:30 PM EDT

Title: The c2 invariant at p=2 by counting edge bipartitions

Speaker: Karen Yeats
Affiliation: University of Waterloo
Room: MC 5501

Abstract:

Since no one provided a visitor or volunteered for this week, I will explain how to prove a special case of a conjecture about the c2 invariant. 

Thursday, June 28, 2018 — 3:30 PM EDT

Title: Stretching drawings of graphs

Speaker: Alan Arroyo
Affiliation: University of Waterloo
Room: MC 5479

Abstract: Given a drawing D of a graph (where vertices are dots in the plane and edges are arbitrary curves connecting some pairs of dots),

Thursday, June 28, 2018 — 11:30 AM EDT

Title: The Many Faces of Circulation Algebras

Speaker: Nick Olson-Harris
Affiliation: University of Waterloo
Room: MC 6486

Abstract:  The circulation algebra is a commutative graded algebra associated to a graph, introduced by Wagner in 1998 to study flows.

Friday, June 22, 2018 — 3:30 PM EDT

Title: Two-level polytopes

Speaker: Kanstantsin Pashkovich
Affiliation: University of Waterloo
Room: MC 5501

Abstract:

This talk is dedicated to two-level polytopes. On one side, two-level polytopes form a well structured family of polytopes.

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