#### Contact Info

Combinatorics & Optimization

University of Waterloo

Waterloo, Ontario

Canada N2L 3G1

Phone: 519-888-4567, ext 33038

PDF files require Adobe Acrobat Reader.

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

Combinatorics & Optimization

University of Waterloo

Waterloo, Ontario

Canada N2L 3G1

Phone: 519-888-4567, ext 33038

PDF files require Adobe Acrobat Reader.

University of Waterloo

University of Waterloo

43.471468

-80.544205

200 University Avenue West

Waterloo,
ON,
Canada
N2L 3G1

The University of Waterloo acknowledges that much of our work takes place on the traditional territory of the Neutral, Anishinaabeg and Haudenosaunee peoples. Our main campus is situated on the Haldimand Tract, the land granted to the Six Nations that includes six miles on each side of the Grand River. Our active work toward reconciliation takes place across our campuses through research, learning, teaching, and community building, and is centralized within our Office of Indigenous Relations.