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Please note: The University of Waterloo is closed for all events until further notice.

# Events - June 2020

Friday, June 26, 2020 — 3:30 PM EDT

## Tutte Colloquium - David Wagner

Title: Discrete diffusion on graphs and real hyperplane arrangements

 Speaker: David Wagner Affiliation: University of Waterloo Zoom: Please email Emma Watson To view the slides: Click here

Abstract:

In 2016, Duffy, Lidbetter, Messinger, and Nowakowski introduced the following variation of a chip-firing model on a graph. At time zero, there is an integer number of "chips" at each vertex. Time proceeds in discrete steps.  At each step, each edge is examined (in parallel) -- one chip is moved from the greater end to the lesser end if the ends are not equal.

Thursday, June 25, 2020 — 2:30 PM EDT

## Algebraic Combinatorics Seminar - Ali Mahmoud

Title: 2-Connected Chord Diagrams and Applications in QFT

 Speaker: Ali Mahmoud Affiliation: University of Waterloo Zoom: Contact Karen Yeats

Abstract:

A functional equation for 2-connected chord diagrams is derived, then is used to calculate asymptotic information for the number of 2-connected chord diagrams by means of alien derivatives applied to factorially divergent power series.

Friday, June 19, 2020 — 3:30 PM EDT

## Tutte Colloquium - Nicolas Trotignon

Title: Widths in even-hole-free graphs

 Speaker: Nicolas Trotignon Affiliation: CNRS - LIP - École Normale Supérieure de Lyon Zoom: Please email Emma Watson

Abstract:

Historically, the study of even-hole-free graphs is motivated by the analogy with perfect graphs. The decomposition theorems that are known for even-hole-free graphs are seemingly more powerful than the ones for perfect graphs: the basic classes and the decompositions are in some respect more restricted. But strangely, in an algorithmic perpective, much more is known for perfect graphs.

Thursday, June 18, 2020 — 2:30 PM EDT

## Algebraic Combinatorics Seminar - Victor Reiner

Title: Sandpiles and representation theory

 Speaker: Victor Reiner Affiliation: University of Minnesota Zoom: Contact Karen Yeats

Abstract:

For an undirected graph, its sandpile group is an interesting isomorphism invariant-- it is a finite abelian group that describes the integer cokernel of the graph's Laplacian matrix.

Thursday, June 18, 2020 — 1:03 PM EDT

## Combinatorial Optimization Reading Group - Julián Romero

Title: Graph coloring of graphs with large girth is hard for the Nullstellensatz

 Speaker: Julián Romero Affiliation: University of Waterloo Zoom: Contact Sharat Ibrahimpur

Abstract:

In this talk we will discuss a method to solve combinatorial problems using hierarchies of systems of linear equations using Hilbert's Nullstellensatz. In particular, we will study the behaviour of these hierarchies for deciding the non-$k$-colorabilty of graphs.

Friday, June 12, 2020 — 3:30 PM EDT

## Tutte Colloquium - Courtney Paquette

Title: Halting Time is Predictable for Large Models: A Universality Property and Average-case Analysis

 Speaker: Courtney Paquette Affiliation: University of Waterloo Zoom: Please email Emma Watson

Abstract:

Average-case analysis computes the complexity of an algorithm averaged over all possible inputs. Compared to worst-case analysis, it is more representative of the typical behavior of an algorithm, but remains largely unexplored in optimization. One difficulty is that the analysis can depend on the probability distribution of the inputs to the model.

Friday, June 12, 2020 — 1:30 PM EDT

## Combinatorial Optimization Reading Group - Vishnu V. Narayan

Title: One Dollar Each Eliminates Envy

 Speaker: Vishnu V. Narayan Affiliation: McGill University Zoom: Contact Sharat Ibrahimpur

Abstract:

We study the fair division of a collection of $m$ indivisible goods amongst a set of $n$ agents. Whilst envy-free allocations typically do not exist in the indivisible goods setting, envy-freeness can be achieved if some amount of a divisible good (money) is introduced.

Thursday, June 11, 2020 — 2:30 PM EDT

## Algebraic Combinatorics Seminar - Steve Melczer

Title: An Upper Bound on Graphical Partitions

 Speaker: Steve Melczer Affiliation: UQAM/CRM Zoom Contact Karen Yeats

Abstract:

An integer partition is called graphical if it can be realized as the size-ordered degree sequence of a simple graph (with no loops or multiple edges).  In his 1736 paper on the Königsberg bridge problem, arguably the origin of graph theory, Euler gave a necessary condition for a partition to be graphical: its sum must be even.

Friday, June 5, 2020 — 3:30 PM EDT

## Distinguished Tutte Lecture - Lauren K. Williams

Title: Matroids, tropical geometry, and positivity

 Speaker: Lauren K. Williams Affiliation: Harvard University & Radcliffe Institute Zoom: Please email Emma Watson.

Abstract:

The theory of matroids -- a class of combinatorial objects which simultaneously generalize graphs as well as vectors in a vector space -- was pioneered by William Tutte in his 1948 PhD thesis. Matroids are also closely connected to the Grassmannian and the tropical Grassmannian.  In recent years, mathematicians and physicists have been exploring positive notions of all of these objects, finding applications to scattering amplitudes and shallow water waves.  In my talk I will give an introduction to matroids, tropical geometry, and positivity, and survey some of the beautiful results and applications.

Friday, June 5, 2020 — 1:00 PM EDT

## Combinatorial Optimization Reading Group - Cedric Koh

Title: A Strongly Polynomial Label-Correcting Algorithm for Linear Systems with Two Variables per Inequality

 Speaker: Cedric Koh Affiliation: London School of Economics and Political Science Zoom: Contact Sharat Ibrahimpur

Abstract:

In this talk, I will present a strongly polynomial label-correcting algorithm for solving the feasibility of linear systems with two variables per inequality. The algorithm is based on the Newton–Dinkelbach method for fractional combinatorial optimization, and extends previous work of Madani (2002).

Thursday, June 4, 2020 — 2:30 PM EDT

## Algebraic Combinatorics Seminar - Lukas Nabergall

Title: Weighted generating functions for weighted chord diagrams

 Speaker: Lukas Nabergall Affiliation: University of Waterloo Zoom: Contact Karen Yeats

Abstract:

Motivated by the universal property of the Connes-Kreimer Hopf algebra of rooted trees and Hopf subalgebras arising from so-called combinatorial Dyson-Schwinger equations, we introduce a class of two-variable recursive functional equations involving Hochschild 1-cocycle operators.

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