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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.

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 c_{2} 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 c_{2} 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.

Thursday, June 21, 2018 — 3:30 PM EDT

**Title**: The price of connectivity for domination

Speaker: | Paul Ouvrard |

Affiliation: | University of Bordeaux |

Room: | MC 5479 |

**Abstract**: The price of connectivity for dominating set in a graph G is the ratio between the minimum sizes of a connected dominating set and a dominating set of G.

Tuesday, June 19, 2018 — 3:00 PM EDT

**Title:** Minimization of convex compositions

Speaker: | Courtney Paquette |

Affiliation: | Lehigh University |

Room: | MC 6486 |

**Abstract:**

Numerous optimization tasks can be posed as minimization of a finite convex function composed with a smooth map. Phase retrieval and matrix factorization problems are common examples.

Friday, June 15, 2018 — 3:30 PM EDT

**Title:** Distributed coloring in planar graphs

Speaker: | Marthe Bonamy |

Affiliation: | University of Bordeaux |

Room: | MC 5501 |

**Abstract:**

We are concerned with efficiently coloring sparse graphs in the distributed setting with as few colors as possible. According to the celebrated Four Color Theorem, planar graphs can be colored with at most 4 colors, and the proof gives a (sequential) quadratic algorithm finding such a coloring.

Thursday, June 14, 2018 — 3:30 PM EDT

Title: Hamilton cycles in cubic planar graphs

Speaker: | Frantisek Kardos |

Affiliation: | University of Bordeaux |

Room: | MC 5479 |

**Abstract**: Tait conjectured in 1884 that each cubic planar graph contains a Hamilton cycle. Had the conjecture been true,

Tuesday, June 12, 2018 — 1:00 PM EDT

**Title**: On isogeny graphs of supersingular elliptic curves over finite fields- Part 2

Speaker: | Gora Adj |

Affiliation: | University of Waterloo |

Room: | MC 6486 |

**Abstract: **After having defined the supersingular isogeny graph, in this second talk

Friday, June 8, 2018 — 3:30 PM EDT

**Title:** Online Learning of Quantum States

Speaker: | Ashwin Nayak |

Affiliation: | University of Waterloo |

Room: | MC 5501 |

**Abstract:**

Suppose we have many copies of an unknown n-qubit state rho. We measure some copies of rho using a known two-outcome measurement E_1, then other copies using a measurement E_2, and so on.

Wednesday, June 6, 2018 — 3:30 PM EDT

**Title**: All Graphs are Beautiful

Speaker: | Alan Arroyo |

Affiliation: | University of Waterloo |

Room: | MC 5417 |

**Abstract**: In this talk I will show that all graphs are beautiful. The proof is by induction on g

Tuesday, June 5, 2018 — 1:00 PM EDT

**Title**: On isogeny graphs of supersingular elliptic curves over finite fields

Speaker: | Gora Adj |

Affiliation: | University of Waterloo |

Room: | MC 6486 |

**Abstract**: In this talk we will present some results on the isogeny graphs of supersingular elliptic curves

Friday, June 1, 2018 — 3:30 PM EDT

**Title:** Parameterized Approximation Algorithms for Steiner Network Problems

Speaker: | Andreas Feldmann |

Affiliation: | Charles University, Prague, Czech Republic |

Room: | MC 5501 |

**Abstract:**

Two standard approaches to handle NP-hard optimization problems are to develop approximation and parameterized algorithms.

Thursday, May 31, 2018 — 3:30 PM EDT

**Title**: Counting biased cliques

Speaker: | Peter Nelson |

Affiliation: | University of Waterloo |

Room: | MC 5479 |

**Abstract**: A biased graph is a pair consisting of an undirected graph G,

Thursday, May 31, 2018 — 11:30 AM EDT

**Title**: A single pass bijection between certain quarter plane lattice walks and certain Motzkin-like paths.

Speaker: | Karen Yeats |

Affiliation: | University of Waterloo |

Room: | MC 6486 |

**Abstract**: A p-tandem quarter plane walk is a walk starting at the origin and remaining in the first quadrant

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