#### Contact Info

Combinatorics & Optimization

University of Waterloo

Waterloo, Ontario

Canada N2L 3G1

Phone: 519-888-4567, ext 33038

PDF files require Adobe Acrobat Reader.

Monday, January 31, 2022 — 11:30 AM EST

**Title: **Hadamard diagonalizable graphs of small order

Speaker: | Steve Butler |

Affiliation: | Iowa State University |

Zoom: | Contact Sabrina Lato |

**Abstract:**

A graph whose Laplacian matrix has a full set of eigenvectors with entries in {1,-1} is said to be Hadamard diagonalizable (i.e. there exists a Hadamard matrix which diagonalizes the Laplacian matrix). We demonstrate that the only diagonalizable graphs on n=8k+4 vertices are K_n and K_{n/2,n/2}.

Friday, January 28, 2022 — 3:30 PM EST

**Title:** Traveling Salesman Problems: Approximation Algorithms and Black-Box Reductions

Speaker: | Jens Vygen |

Affiliation: | University of Bonn |

Zoom: | Please email Emma Watson |

**Abstract:**

We survey the recent progress on approximation algorithms and integrality ratios for variants of the traveling salesman problem, with a focus on black-box reductions from one problem to another. In particular, we explain recent results for the Path TSP and the Capacitated Vehicle Routing Problem, which are joint works with Vera Traub and Rico Zenklusen and with Jannis Blauth and Vera Traub.

Thursday, January 27, 2022 — 1:00 PM EST

**Title:** Springer fibers and the Delta Conjecture at t=0

Speaker: | Sean Griffin |

Affiliation: | UC Davis |

Zoom: | Please email Olya Mandelshtam |

**Abstract:**

Springer fibers are a family of varieties that have remarkable connections to combinatorics and representation theory. Springer used them to geometrically construct all of the irreducible representations of the symmetric group (Specht modules). Moreover, they give a geometric meaning to Hall-Littlewood symmetric functions.

Thursday, January 27, 2022 — 11:30 AM EST

**Title:** Random Self-reducibility of Ideal-SVP via Arakelov Random Walks

Speaker: | Pravek Sharma |

Affiliation: | University of Waterloo |

Zoom: | Please email Jesse Elliott |

**Abstract:**

Fixing a number field, the space of all ideal lattices, up to isometry, is naturally an Abelian group, called the *Arakelov class group*. This fact, well known to number theorists, has so far not been explicitly used in the literature on lattice-based cryptography. Remarkably, the Arakelov class group is a combination of two groups that have already led to significant cryptanalytic advances: the class group and the unit torus.

Tuesday, January 25, 2022 — 3:00 PM EST

**Title:** A Matching Theoretic Flat Wall Theorem

Speaker: | Archontia Giannopoulou |

Affiliation: | University of Athens |

Zoom: | http://matroidunion.org/?page_id=2477 or please email Shayla Redlin |

**Abstract:**

One of the key theorems in Graph Minors is the Flat Wall Theorem which asserts the existence of a large wall under certain conditions. In this talk, we discuss about graphs with perfect matchings and their relationship with digraphs. Our main focus is on a matching theoretic analogue of the Flat Wall Theorem for bipartite graphs excluding a fixed matching minor.

Monday, January 24, 2022 — 11:30 AM EST

**Title:** Minimal relations for an algebra inspired by algebraic graph theory

Speaker: | Erika Pirnes |

Affiliation: | University of Wisconsin-Madison |

Zoom: | Contact Sabrina Lato |

**Abstract:**

The balanced algebra has two generators, R and L, and its defining relations are that any pair of balanced words commutes. For example, RL and LR are balanced (contain the same number of both generators), so in the balanced algebra, (RL)(LR)=(LR)(RL).

Friday, January 21, 2022 — 3:30 PM EST

**Title:** Guessing with little data

Speaker: | Manuel Kauers |

Affiliation: | Johannes Kepler University |

Zoom: | Please email Emma Watson |

**Abstract:**

A popular and powerful technique in experimental mathematics takes as input the first few terms of an infinite sequence and returns plausible candidates for recurrence equations that the sequence may satisfy. In a way, the search for such candidates is a generalization of polynomial interpolation. For polynomial interpolation, it is well known and easy to see that d+1 sample points are needed in order to recover a polynomial of degree d. Similarly, it turns out that (r+1)*(d+2) consecutive terms of a sequence are needed in order to detect a linear recurrence of order r with polynomial coefficients of degree at most d.

Tuesday, January 18, 2022 — 3:00 PM EST

**Title:** Enumerating Matroids and Linear Spaces

Speaker: | Mehtaab Sawhney |

Affiliation: | MIT |

Zoom: | Please email Shayla Redlin |

**Abstract:**

We show that the number of linear spaces on a set of $n$ points and the number of rank-3 matroids on a ground set of size $n$ are both of the form $(cn+o(n))^{n^2/6}$, where $c=e^{\sqrt 3/2-3}(1+\sqrt 3)/2$. This is the final piece of the puzzle for enumerating fixed-rank matroids at this level of accuracy: there are exact formulas for enumeration of rank-1 and rank-2 matroids, and it was recently proved by van der Hofstad, Pendavingh, and van der Pol that for constant $r\ge 4$ there are $(e^{1-r}n+o(n))^{n^{r-1}/r!}$ rank-$r$ matroids on a ground set of size $n$.

Monday, January 17, 2022 — 11:30 AM EST

**Title:** Laplacian States

Speaker: | Chris Godsil |

Affiliation: | University of Waterloo |

Zoom: | Conatact Sabrina Lato |

**Abstract:**

It is customary to assume that the initial state of a continuous quantum walk on a graph $X$ is a vertex. However the Laplacian matrix of a graph with vertex set $V(X)$ is positive semidefinite, and can be scaled to produce a density matrix, and so provides an initial state for a walk on $X$.

Thursday, January 13, 2022 — 1:00 PM EST

**Title:** Crystal invariant theory and geometric RSK

Speaker: | Gabriel Frieden |

Affiliation: |
Université du Québec à Montréal (UQAM) |

Zoom: | Contact Olya Mandelshtam |

**Abstract:**

The original problem of classical invariant theory was to describe the invariants of SL_m acting on a polynomial ring in an m \times n matrix of variables. One way to solve this problem is to consider the polynomial ring as a GL_m \times GL_n representation, and decompose this representation into its irreducible components.

Thursday, January 13, 2022 — 11:30 AM EST

**Title:** A New Adaptive Attack on SIDH

Speaker: | Valerie Gilchrist |

Affiliation: | University of Waterloo |

Zoom: | Please email Jesse Elliott |

**Abstract:**

The SIDH key exchange is the main building block of SIKE, the only isogeny based scheme involved in the NIST standardization process. In 2016, Galbraith et al. presented an adaptive attack on SIDH. In this attack, a malicious party manipulates the torsion points in his public key in order to recover an honest party's static secret key, when having access to a key exchange oracle.

Tuesday, January 11, 2022 — 3:00 PM EST

**Title:** Edge-Disjoint Linkage in Infinite Graphs

Speaker: | Amena Assem |

Affiliation: | University of Waterloo |

Zoom: | http://matroidunion.org/?page_id=2477 or contact Shayla Redlin |

**Abstract:**

In 1980 Thomassen conjectured that, for odd k, an edge-connectivity of k is enough for a graph to be weakly k-linked, meaning any k pairs of terminals can be linked by k edge-disjoint paths. The best known result to date for finite graphs is from 1991, by Andreas Huck, and assumes an edge-connectivity of k+1 for odd k.

Monday, January 10, 2022 — 11:30 AM EST

**Title:** Quantum walks do not like bridges

Speaker: | Emanuel Juliano |

Affiliation: | Universidade Federal de Minas Gerais |

Zoom: | Contact Sabrina Lato |

**Abstract:**

In this talk we consider graphs with two cut vertices joined by a bridge, and prove that there can be no quantum perfect state transfer between these vertices, unless the graph has no other vertex.

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.