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Thursday, November 7, 2019 1:00 pm - 1:00 pm EST (GMT -05:00)

Algebraic Graph Theory Seminar - Chris Godsil

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 - 3:00 pm EST (GMT -05:00)

Algebraic Combinatorics Seminar - Kevin Purbhoo

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 - 4:00 pm EST (GMT -05:00)

Graphs and Matroids Seminar - Adam Brown

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 14, 2019 3:00 pm - 3:00 pm EST (GMT -05:00)

Algebraic Combinatorics Seminar - Nick Early

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 - 4:00 pm EST (GMT -05:00)

Graphs and Matroids Seminar - Shayla Redlin

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 - 1:00 pm EST (GMT -05:00)

Combinatorial Optimization Reading Group - Benjamin Moore

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 - 3:30 pm EST (GMT -05:00)

Tutte Colloquium - Marcel Golz

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 22, 2019 1:00 pm - 1:00 pm EST (GMT -05:00)

Combinatorial Optimization Reading Group - Sharat Ibrahimpur

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.