November 2019

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.

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). 

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 $r\ge0$.

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 — 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.

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 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 $\Delta_{k,n}$.

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.

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.

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. 

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 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.

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.

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