Thursday, March 30, 2023 — 1:00 PM EDT

Title: Arrangements of Pseudolines, Tropical Grassmannians, and Generalized Scattering Amplitudes

Speaker: Freddy Cachazo
Affiliation: Perimeter Institute
Room: MC 6029

Abstract: For each arrangement of (pseudo)lines on the projective plane, it is possible to construct a differential form that captures its combinatorial structure. The forms have simple poles whenever triangles shrink to a point in the arrangement, and share the same residue when two arrangements are connected via a "triangle flip".

Wednesday, March 29, 2023 — 2:30 PM EDT

Title: Distance-Regular and Distance-Biregular Graphs

Speaker: Sabrina Lato
Affiliation: University of Waterloo
Location: MC

Abstract: For a given diameter d and valency k, what is the maximum number of vertices a k-regular graph of diameter d can have, and what graphs meet that bound? Although there is a straightforward counting argument to bound the number of vertices using the structural information, the problem of characterizing the graphs that meet the bound turns out to be a problem in algebraic graph theory, and helps gives rise to the notion of distance-regular graphs.

Monday, March 27, 2023 — 8:00 PM EDT

Title: Inverse eigenvalue problem of a graph

Speaker: Jephian C.-H. Lin
Affiliation: National Sun Yat-sen University
Location: Please contact Sabrina Lato for Zoom link

Abstract:  We often encounter matrices whose pattern (zero-nonzero, or sign) is known while the precise value of each entry is not clear. Thus, a natural question is what we can say about the spectral property of matrices of a given pattern. When the matrix is real and symmetric, one may use a simple graph to describe its off-diagonal nonzero support.

Friday, March 24, 2023 — 3:30 PM EDT

Title: On the complexity of quantum partition functions

Speaker: David Gosset
Affiliation: University of Waterloo
Location: MC 5501 or contact Eva Lee for Zoom link

Abstract: Quantum complexity theory has been intertwined with the study of quantum many-body systems ever since Kitaev's insight that computing their ground energies is an intractable quantum constraint satisfaction problem that is complete for a quantum generalization of NP.

Friday, March 24, 2023 — 12:00 PM EDT

Title: Subgraph Polytopes and Independence Polytopes of Count Matroids

Speaker: David Aleman
Affiliation: University of Waterloo
Location: MC 6029

Abstract: Given a graph G=(V,E), the subgraph polytope of G is defined as the convex hull of the characteristic vector of the pairs (S,F) such that S is a non-empty subset of vertices and F is a set of edges contained in the induced subgraph G[S].

Thursday, March 23, 2023 — 1:00 PM EDT

Title: Quasisymmetric varieties, excedances, and bases for the Temperley--Lieb algebra

Speaker: Lucas Gagnon
Affiliation: York University
Location: MC 6029 please contact Olya Mandelshtam for Zoom link

Abstract:  This talk is about finding a quasisymmetric variety (QSV): a subset of permutations which (i) is a basis for the Temperley--Lieb algebra TL_n(2), and (ii) has a vanishing ideal (as points in n-space) that behaves similarly to the ideal generated by quasisymmetric polynomials.   While this problem is primarily motivated by classical (co-)invariant theory and generalizations thereof, the course of our investigation uncovered a number of remarkable combinatorial properties related to our QSV, and I will survey these as well. 

Monday, March 20, 2023 — 3:00 PM EDT

Title : A Closure Lemma for tough graphs and Hamiltonian degree conditions

Friday, March 17, 2023 — 3:30 PM EDT

Title: Steiner Cut Dominants

Speaker: Volker Kaibel
Affiliation: Otto von Guericke University Magdeburg
Location: MC 5501 or contact Eva Lee for Zoom link

Abstract: For a subset of terminals T of the nodes of a graph G a cut in G is called a T-Steiner cut if it subdivides T into two non-empty sets. The Steiner cut dominant of G is the Minkowski sum of the convex hull of the incidence vectors of T-Steiner cuts in G and the nonnegative orthant.

Friday, March 17, 2023 — 12:00 PM EDT

Title: Rectangle covers and bounding the extension complexity of the correlation polytope

Thursday, March 16, 2023 — 1:00 PM EDT

Title: Taking limits in Go-diagrams

Monday, March 13, 2023 — 11:30 AM EDT

Title: Edge domination in incidence graphs

Friday, March 10, 2023 — 3:30 PM EST

Title: Matroids without cliques

Speaker: Peter Nelson
Affiliation: University of Waterloo
Location: MC 5501 or contact Eva Lee for Zoom link

Abstract: The class of graphs that omit some fixed complete graph as a minor is very well-studied; in particular, the densest graphs in the class are known. The analogous question for matroids is just as well-motivated, but seems harder to answer. I will discuss some recent progress in this area, which reduces a bound from doubly exponential to singly exponential. This is joint work with Sergey Norin and Fernanda Rivera Omana.

Thursday, March 9, 2023 — 1:00 PM EST

Title: Bargain hunting in a Coxeter group

Tuesday, March 7, 2023 — 11:30 AM EST

Title: Distance-regular graphs with primitive automorphism groups

Monday, March 6, 2023 — 3:00 PM EST

Title: An f-coloring generalization of linear arboricity

Friday, March 3, 2023 — 3:30 PM EST

Title: Critical Points at Infinity for Hyperplanes of Directions

Speaker: Stephen Gillen
Affiliation: University of Waterloo
Location: MC 5501 or contact Eva Lee for Zoom link

Abstract: Analytic combinatorics in several variables (ACSV) analyzes the asymptotic growth of series coefficients of multivariate rational functions G/H in an exponent direction r. The poly-torus of integration T that arises from the multivariate Cauchy Integral Formula is deformed away from the origin into cycles around critical points of a “height function" h on V = V(H).

Friday, March 3, 2023 — 12:00 PM EST

Title: Conic lifts of convex sets

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