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Friday, December 13, 2019 — 3:30 PM EST

**Title:** Permanent Hardness from Linear Optics

Speaker: | Daniel Grier |

Affiliation: | University of Waterloo |

Room: | MC 5501 |

**Abstract:**

One of the great accomplishments in complexity theory was Valiant's 1979 proof that the permanent of a matrix is #P-hard to compute. Subsequent work simplified Valiant's ideas and even began to recast them as problems in quantum computing.

Thursday, December 12, 2019 — 3:00 PM EST

**Title:** OutFn, M_gn and renormalized topological field theory

Speaker: | Michael Borinsky |

Affiliation: | Nikhef |

Room: | MC 5417 |

**Abstract:**

I will report on recent joint work with Karen Vogtmann on the Euler characteristic of Out(F_n) and the moduli space of graphs.

Thursday, December 12, 2019 — 1:00 PM EST

**Title:** Graph covers with two new eigenvalues

Speaker: | Olha Silina |

Affiliation: | University of Waterloo |

Room: | MC 5479 |

**Abstract:**

I am going to talk about graph covers. If $Y$ is a cyclic cover of $X$, it turns out that $Y$ has the same spectrum as $X$ plus (possibly) some new eigenvalues.

Friday, December 6, 2019 — 3:03 PM EST

**Title:** Biometric security: Design, implementation, and challenges

Speaker: | Koray Karabina |

Affiliation: | Florida Atlantic University |

Room: | MC 5501 |

**Abstract:**

Biometric based cybersecurity technologies offer significant advantages in authentication, identification, and access control mechanisms.

Thursday, December 5, 2019 — 3:00 PM EST

**Title:** Inverses of Trees

Speaker: | Krystal Guo |

Affiliation: | Centre de Recherches Mathématiques & Université de Montréal |

Room: | MC 5417 |

**Abstract:**

A tree is invertible if and only if it has a perfect matching.

Godsil considers an invertible tree T and finds that the inverse of the adjacency matrix has entries in {0, ±1} and is the signed adjacency matrix of a graph which contains T.

Thursday, December 5, 2019 — 9:30 AM EST

**Title:** How can we optimize nonsmooth objectives globally?

Speaker: | Andreas Griewank |

Affiliation: | Humboldt University, Germany |

Room: | MC 5501 |

**Abstract:**

In machine learning objective functions that are only piecewise smooth and should be globally minimized abound. The standard method of dealing with them is to apply a stochastic gradient method disregarding the rare points of nonsmoothness and hoping for the best as far as global optimality of the computed solution is concerned.

Friday, November 29, 2019 — 3:30 PM EST

**Title: **Incompressibility of classical distributions

Speaker: | Debbie Leung |

Affiliation: | University of Waterloo |

Room: | MC 5501 |

**Abstract:**

We prove a general, robust, single-letter lower bound on the achievable rate for ensembles of classical states, which holds even when Alice and Bob share free entanglement and allow a constant local error.

Thursday, November 28, 2019 — 4:00 PM EST

**Title:** Resilience of the rank of random matrices

Speaker: | Jorn van der Pol |

Affiliation: | University of Waterloo |

Room: | MC 5501 |

**Abstract**:

I will discuss the preprint of this title by Ferber, Luh, and McKinley (arXiv:1910.03619).

Friday, November 22, 2019 — 3:30 PM EST

**Title:** Disjunctive Cuts through the V-Polyhedral Lens

Speaker: | Aleksandr Kazachkov |

Affiliation: | Polytechnique Montréal |

Room: | MC 5501 |

**Abstract:**

Cutting planes, or cuts, are a critical component of modern integer programming solvers, but existing cuts implemented in solvers are relatively simple compared to those in the literature. We discuss the primary reasons for this disparity, as well as our recently-proposed V-polyhedral framework for mitigating some of these difficulties encountered by prior "stronger" cuts.

Friday, November 22, 2019 — 1:00 PM EST

**Title:** Submodular function maximization via the multilinear relaxation and contention resolution schemes

Speaker: | Sharat Ibrahimpur |

Affiliation: | University of Waterloo |

Room: | MC 5417 |

**Abstract:**

I will present a general framework for maximizing a nonnegative submodular set function $f$ subject to a variety of packing type constraints including multiple matroid constraints, knapsack constraints, and their intersections.

Thursday, November 21, 2019 — 4:00 PM EST

**Title:** Kummer's Theorem on binomial coefficients, etc.

Speaker: | John Schanck |

Affiliation: | University of Waterloo |

Room: | MC 5501 |

**Abstract:**

Some of us like the primes and call ourselves pure mathematicians. Some of us like the Catalan numbers and call ourselves combinatorialists.

Friday, November 15, 2019 — 3:30 PM EST

**Title:** The combinatorics of parametric Feynman integrals

Speaker: | Marcel Golz |

Affiliation: | University of Waterloo |

Room: | MC 5501 |

**Abstract:**

Feynman integrals are used in perturbative quantum field theory to compute the probabilities of processes involving elementary particles. They can be represented as Feynman graphs and exhibit a rich combinatorial structure. The parametric representation of Feynman integrals is particularly suitable to be studied from a combinatorial perspective since it contains well known objects like the Kirchhoff polynomial.

Friday, November 15, 2019 — 1:00 PM EST

**Title:** A deterministic (1/2 + epsilon)-approximation for submodular maximizztion over a matroid

Speaker: | Ben Moore |

Affiliation: | University of Waterloo |

Room: | MC 5417 |

**Abstract:**

In 1978, it was shown that a natural greedy algorithm gives a 1/2 approximation to submodular maximization subject to a matroid constraint.

Thursday, November 14, 2019 — 4:00 PM EST

**Title:** Halfway to Rota's Basis Conjecture

Speaker: | Shayla Redlin |

Affiliation: | University of Waterloo |

Room: | MC 5501 |

**Abstract:**

Rota’s Basis Conjecture is that any rank-n matroid with n disjoint bases B_1, …, B_n has n disjoint transversal bases; a basis is transversal if it contains exactly one element from each B_i.

Thursday, November 14, 2019 — 3:00 PM EST

**Title:** From weakly separated collections to matroid subdivisions

Speaker: | Nick Early |

Affiliation: | Perimeter Institute for Theoretical Physics |

Room: | MC 5417 |

**Abstract:**

We study arrangements of slightly skewed tropical hyperplanes, called blades, on the vertices of a hypersimplex $\Delta_{k,n}$.

Friday, November 8, 2019 — 3:30 PM EST

**Title:** Circuit-hyperplane relaxation and matroid representation

Speaker: | Jim Geelen |

Affiliation: | University of Waterloo |

Room: | MC 5501 |

**Abstract:**

Relaxing a circuit-hyperplane in a representable matroid can destroy representability.

Thursday, November 7, 2019 — 4:00 PM EST

**Title:** Counting Pentagons in Triangle-free Binary Matroids

Speaker: | Adam Brown |

Affiliation: | University of Waterloo |

Room: | MC 5501 |

**Abstract:**

Every triangle-free graph with n vertices contains at most (n/5)^5 cycles of length five, and this value is attained by the balanced blowup of the 5-cycle.

Thursday, November 7, 2019 — 3:00 PM EST

**Title:** Two-colouring hypersurface complements in open Richardson varities

Speaker: | Kevin Purbhoo |

Affiliation: | University of Waterloo |

Room: | MC 5417 |

**Abstract: **

Given an algebraic hypersurface $H \subset \mathbb{R}^n$, we can always 2-colour the components of the complement $\mathbb{R}^n \setminus H$ such that adjacent components are of opposite colours.

Thursday, November 7, 2019 — 1:00 PM EST

**Title:** Cospectral and strongly cospectral vertices

Speaker: | Chris Godsil |

Affiliation: | University of Waterloo |

Room: | MC 5479 |

**Abstract:**

If $a$ is a vertex in a graph with adjacency matrix $A$, the \textsl{walk module} generated by $a$ is the $A$-invariant subspace spanned by the vectors $A^re_a$, for $r\ge0$.

Friday, November 1, 2019 — 3:30 PM EDT

**Title:** A Quantum Query Complexity Trichotomy for Regular Languages

Speaker: | Luke Schaeffer |

Affiliation: | University of Waterloo |

Room: | MC 5501 |

**Abstract:**

We consider the quantum query complexity of regular languages and discover a surprising trichotomy: each regular language has query complexity either Theta(1), ~Theta(sqrt(n)) or Theta(n).

Friday, November 1, 2019 — 1:00 PM EDT

**Title:** Maximizing non-monotone submodular functions

Speaker: | Zishen Qu |

Affiliation: | University of Waterloo |

Room: | MC 5417 |

**Abstract**:

Optimization of non-monotone submodular functions has applications in the maximum cut and maximum directed cut problems for graphs.

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

**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

**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

**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

**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.

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