Contact Info
Combinatorics & Optimization
University of Waterloo
Waterloo, Ontario
Canada N2L 3G1
Phone: 519-888-4567, ext 33038
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Friday, June 26, 2020 3:30 PM EDT
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
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
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
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
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
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
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
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
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
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
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.