October 2019

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

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

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

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 — 4:00 PM EDT

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.

Friday, October 4, 2019 — 1:00 PM EDT

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.

Friday, October 4, 2019 — 3:30 PM EDT

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.

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

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.

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

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 — 4:00 PM EDT

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.

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

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. 

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

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.

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