## University COVID-19 update

### Questions about buildings and services? Visit the list of Modified Services.

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

# Events - 2020

Friday, December 11, 2020 — 3:30 PM EST

## Tutte Colloquium - Santanu Dey

Title: Sparse PSD approximation of the PSD cone

 Speaker: Santanu Dey Affiliation: H. Milton Stewart School of Industrial and Systems Engineering at Georgia Institute of Technology Zoom: Please email Emma Watson

Abstract:

While semidefinite programming (SDP) problems are polynomially solvable in theory, it is often difficult to solve large SDP instances in practice. One computational technique used to address this issue is to relax the global positive-semidefiniteness (PSD) constraint and only enforce PSD-ness on smaller k × k principal submatrices — we call this the sparse SDP relaxation.

Thursday, December 10, 2020 — 1:00 PM EST

## Algebraic Combinatorics Seminar - Laura Colmenarejo

Title: Chromatic symmetric functions of Dyck paths and $q$-rook theory

 Speaker: Laura Colmenarejo Affiliation: UMass Amherst Zoom: Contact Karen Yeats

Abstract:

Given a graph and a set of colors, a coloring of the graph is a function that associates each vertex in the graph with a color. In 1995, Stanley generalized this definition to symmetric functions by looking at the number of times each color is used and extending the set of colors to $\mathbb{Z}^+$. In 2012, Shareshian and Wachs introduced a refinement of the chromatic functions for ordered graphs as $q$-analogues.

Monday, December 7, 2020 — 11:30 AM EST

## Algebraic Graph Theory Seminar - Shahla Nasserasr

Title: Distinct Eignvalues and Sensitivity

 Speaker: Shahla Nasserasr Affiliation: Rochester Institute of Technology Zoom: Contact Soffia Arnadottir

Abstract:

For a graph $G$, the class of real-valued symmetric matrices whose zero-nonzero pattern of off-diagonal entries is described by the adjacencies in $G$ is denoted by $S(G)$. The inverse eigenvalue problem for the multiplicities of the eigenvalues of $G$ is to determine for which ordered list of positive integers $m_1\geq m_2\geq \cdots\geq m_k$ with $\sum_{i=1}^{k} m_i=|V(G)|$, there exists a matrix in $S(G)$ with distinct eigenvalues ${\lambda_1,\lambda_2,\cdots, \lambda_k}$ such that $\lambda_i$ has multiplicity $m_i$.

Friday, December 4, 2020 — 3:30 PM EST

## Tutte Colloquium - Oliver Pechenik

Title: Partial orders on the symmetric group

 Speaker: Oliver Pechenik Affiliation: University of Waterloo Zoom: Please email Emma Watson

Abstract:

The symmetric group of permutations is naturally a poset in at least 4 different ways, the (strong) Bruhat order and three flavors of weak order. Stanley showed in 1980 that the Bruhat order is Sperner, essentially meaning that the obvious large antichains are in fact the largest possible. The corresponding fact for weak orders was open until last year, when it was established by Gaetz and Gao.

Thursday, December 3, 2020 — 1:00 PM EST

## Algebraic Combinatorics Seminar - Loïc Foissy

Title: Twisted Hopf algebras

 Speaker: Loïc Foissy Affiliation: Université du Côte d'Opale Zoom: Contact Karen Yeats

Abstract:

A twisted Hopf algebra is a Hopf algebra in the category of linear species. The Fock functors allow to recover "classical" Hopf algebras from twisted ones. Numerous constructions and results can be lifted to the level of twisted bialgebras, such that cofreeness, shuffle and quasi-shuffles products, etc.

Monday, November 30, 2020 — 11:30 AM EST

## Algebraic Graph Theory Seminar - Krystal Guo

Title: Simple eigenvalues of graph

 Speaker: Krystal Guo Affiliation: University of Amsterdam Zoom: Contact Soffia Arnadottir

Abstract:

If v is an eigenvector for eigenvalue λ of a graph X and α is an automorphism of X, then α(v) is also an eigenvector for λ. Thus it is rather exceptional for an eigenvalue of a vertex-transitive graph to be simple. We study cubic vertex-transitive graphs with a non-trivial simple eigenvalue, and discover remarkable connections to arc-transitivity, regular maps and Chebyshev polynomials.

Monday, November 23, 2020 — 11:30 AM EST

## Algebraic Graph Theory Seminar - Nathan Lindzey

Title: Complexity Measures on the Symmetric Group and Beyond

 Speaker: Nathan Lindzey Affiliation: CU Boulder Zoom: Contact Soffia Arnadottir

Abstract:

A classical result in complexity theory states that a degree-d Boolean function on the hypercube can be computed using a decision tree of depth poly(d). Conversely, a Boolean function computed by a decision tree of depth d has degree at most d. Thus degree and decision tree complexity are polynomially related. Many other complexity measures of Boolean functions on the hypercube are polynomially related to the degree (e.g., approximate degree, certificate complexity, block sensitivity), and last year Huang famously added sensitivity to the list. Can we prove similar results for Boolean functions on other combinatorial domains?

Friday, November 20, 2020 — 3:30 PM EST

## Distinguished Tutte Lecture - Jordan Ellenberg

Title: Beyond rank

 Speaker: Jordan Ellenberg Affiliation: University of Wisconsin Zoom: Please email Emma Watson

Abstract:

The notion of the rank of a matrix is one of the most fundamental in linear algebra. The analogues of this notion in multilinear algebra — e.g., what is the “rank” of an m x n x p array of numbers? — is much more mysterious, but it also has proven to be useful in a wide array of contexts. I will talk about some questions and answers in “higher rank” coming from complexity theory, data science, geometric combinatorics, additive number theory, and commutative algebra.

Thursday, November 19, 2020 — 1:00 PM EST

## Algebraic Combinatorics Seminar - David Wagner

Title: Some new lemmas about polynomials with only real roots

 Speaker: David Wagner Affiliation: University of Waterloo Zoom: Contact Karen Yeats

Abstract:

Recent investigations in Ehrhart theory suggested some conjectures involving interlacing relations among polynomials with only real roots, and Veronese sections of them. Revisiting some old theorems, we find as corollaries some new lemmas which have been overlooked for a long time. One of these lemmas directly implies a strong form of the motivating conjecture.  Similar applications of the other lemmas are anticipated. This is ongoing joint work with Christos Athanasiadis (U. Athens).

Monday, November 16, 2020 — 11:30 AM EST

## Algebraic Graph Theory Seminar - Xiaohong Zhang

Title: Fractional revival on graphs

 Speaker: Xiaohong Zhang Affiliation: University of Waterloo Zoom: Contact Soffia Arnadottir

Abstract:

Let A be the adjacency matrix of a weighted graph, and let U(t)=exp(itA). If there is a time t such that U(t)e_a=\alpha e_a+\beta e_b, then we say there is fractional revival (FR) between a and b. For the special case when \alpha=0, we say there is perfect state transfer (PST) between vertices a and b. It is known that PST is monogamous (PST from a to b and PST from a to c implies b=c) and vertices a b are cospectral in this case. If \alpha\beta\neq 0, then there is proper fractional revival.

Friday, November 13, 2020 — 3:30 PM EST

## Tutte Colloquium - Nisheeth Vishnoi

Title: Sampling Under Symmetry

 Speaker: Nisheeth Vishnoi Affiliation: Yale University Zoom: Please email Emma Watson

Abstract:

Exponential densities on orbits of Lie groups such as the unitary group are endowed with surprisingly rich mathematical structure and. traditionally, arise in diverse areas of physics, random matrix theory, and statistics.

In this talk, we will discuss the computational properties of such distributions and also present new applications to quantum inference and differential privacy.

Thursday, November 12, 2020 — 1:00 PM EST

## Algebraic Combinatorics Seminar - Christos Athanasiadis

Title: Face enumeration and real-rootedness

 Speaker: Christos Athanasiadis Affiliation: University of Athens Zoom: Contact Karen Yeats

Abstract:

About fifteen years ago F. Brenti and V. Welker showed that the face enumerating polynomial of the barycentric subdivision of any Cohen-Macaulay simplicial complex has only real roots. It is natural to ask whether similar results hold when barycentric subdivision is replaced by more general types of triangulations, or when simplicial complexes are replaced by more general cell complexes.

Monday, November 9, 2020 — 8:00 PM EST

## Algebraic Graph Theory Seminar - Hajime Tanaka

Title: Scaling limits for the Gibbs states on distance-regular graphs with classical parameters

 Speaker: Hajime Tanaka Affiliation: Tohoku University Zoom: Contact Soffia Arnadottir

Abstract:

Limits of the normalized spectral distributions and other related probability distributions of families of graphs have been studied in the context of quantum probability theory as analogues of the central limit theorem. First I will review some of the previous work by Hora, Obata, and others, focusing on the case of distance-regular graphs, and emphasizing how the theory is related to the Terwilliger algebra.

Friday, November 6, 2020 — 3:30 PM EST

## Tutte Colloquium - Chloe Martindale

Title: Constructing broken SIDH parameters: a tale of De Feo, Jao, and Plut's serendipity

 Speaker: Chloe Martindale Affiliation: University of Bristol Zoom: Please email Emma Watson

Abstract:

This talk is motivated by analyzing the security of the cryptographic key exchange protocol SIDH (Supersingular Isogeny Diffie-Hellman), introduced by 2011 by De Feo, Jao, and Plut. We will first recall some mathematical background as well as the protocol itself. The 'keys' in this protocol are elliptic curves, which are typically described by equations in x and y of the form y^2 = x^3 + ax + b.

Thursday, November 5, 2020 — 1:00 PM EST

## Algebraic Combinatorics Seminar - Huda Ahmed and Yuanning Zhang

Title: Filtering Grassmannian cohomology via k-Schur functions

 Speakers: Huda Ahmed and Yuanning Zhang Affiliation: New York University and UC Berkeley Zoom: Contact Karen Yeats

Abstract:

This talk concerns the cohomology rings of complex Grassmannians. In 2003, Reiner and Tudose conjectured the form of the Hilbert series for certain subalgebras of these cohomology rings. We build on their work in two ways. First, we conjecture two natural bases for these subalgebras that would imply their conjecture using notions from the theory of k-Schur functions. Second we formulate an analogous conjecture for Lagrangian Grassmannians.

Joint work with Michael Feigen, Victor Reiner, and Ajmain Yamin.

Thursday, November 5, 2020 — 1:00 PM EST

## Algebraic Combinatorics Seminar - Jonathan Jedwab

Title: Packings of partial difference sets

 Speaker: Jonathan Jedwab Affiliation: Simon Fraser University Zoom: Contact Karen Yeats

Abstract:

Partial difference sets are highly structured group subsets that occur in various guises throughout design theory, finite geometry, coding theory, and graph theory. They admit only two possible nontrivial character sums and so are often studied using character theory.

Monday, November 2, 2020 — 11:30 AM EST

## Algebraic Graph Theory Seminar - Sabrina Lato & Christino Tamon

Title: Monogamy Violations in Perfect State Transfer

 Speakers: Sabrina Lato & Christino Tamon Affiliations: University of Waterloo & Clarkson Unversity Zoom: Contact Soffia Arnadottir

Abstract:

Continuous-time quantum walks on a graph are defined using a Hermitian matrix associated to a graph. For a quantum walk on a graph using either the adjacency matrix or the Laplacian, there can be perfect state transfer from a vertex to at most one other vertex in the graph.

Friday, October 30, 2020 — 3:30 PM EDT

## Tutte Colloquium - Logan Crew

Title: The Tutte Symmetric Function

 Speaker: Logan Crew Affiliation: University of Waterloo Zoom: Please email Emma Watson

Abstract:

The Tutte polynomial is one of the most celebrated and most well-studied graph functions, in part because it specializes to every graph polynomial with a linear deletion-contraction relation, such as the chromatic polynomial. In the 1990s, Stanley generalized the Tutte polynomial to a symmetric function, but at the cost of the deletion-contraction relation.

Thursday, October 29, 2020 — 1:00 PM EDT

## Algebraic Combinatorics Seminar - Florian Aigner

Title: qRSt: A probabilistic Robinson--Schensted correspondence for Macdonald polynomials

 Speaker: Florian Aigner Affiliation: Université du Québec à Montréal Zoom: Contact Karen Yeats

Abstract:

The Robinson--Schensted (RS) correspondence is a bijection between permutations and pairs of standard Young tableaux which plays a central role in the theory of Schur polynomials. In this talk, I will present a (q,t)-dependent probabilistic deformation of Robinson--Schensted which is related to the Cauchy identity for Macdonald polynomials.

Monday, October 26, 2020 — 11:30 AM EDT

## Algebraic Graph Theory Seminar - Ferdinand Ihringer

Title: Pseuodrandom Cliquefree Graphs, Finite Geometry, and Spectra

 Speaker: Ferdinand Ihringer Affiliation: Ghent University, Belgium Zoom: Contact Soffia Arnadottir

Abstract:

A regular graph is called optimally pseudorandom if its second largest eigenvalue in absolute value is, up to a constant factor, as small as possible. Determining the largest degree of an optimally pseudorandom graph without a clique of size s is a well-known open problem in extremal graph theory.

Friday, October 23, 2020 — 3:30 PM EDT

## Tutte Colloquium - David P. Williamson

Title: Semidefinite Programming Relaxations of the Traveling Salesman Problem

 Speaker: David P. Williamson Affiliation: Cornell University Zoom: Please email Emma Watson

Abstract:

Finding a polynomial-time solvable relaxation of the traveling salesman problem whose integrality gap better matches what is seen in practice has been an outstanding open problem in combinatorial optimization for some time.  We study several semidefinite programming relaxations of the traveling salesman problem proposed in the literature and show that all known relaxations have an unbounded integrality gap.

Thursday, October 22, 2020 — 1:00 PM EDT

## Algebraic Combinatorics Seminar - Reuven Hodges

Title: Coxeter combinatorics and spherical Schubert geometry

 Speaker: Reuven Hodges Affiliation: University of Illinois Zoom: Contact Karen Yeats

Abstract:

This talk will introduce spherical elements in a finite Coxeter system. These spherical elements are a generalization of Coxeter elements, that conjecturally, for Weyl groups, index Schubert varieties in the flag variety G/B that are spherical for the action of a Levi subgroup.

Wednesday, October 21, 2020 — 4:30 PM EDT

## Joint Colloquium PMath+CO - Nick Olson-Harris

Title: On the Theory of the Analytical Forms called Trees

 Speaker: Nick Olson-Harris Affiliation: University of Waterloo Zoom: Contact Maxwell Levit

Abstract:

Trees are among the most fundamental of combinatorial structures. Nowadays they appear all over mathematics and computer science, but this has not always been the case. Trees were first introduced, at least under that name, in an 1857 paper of Cayley by the same title as this talk.

Monday, October 19, 2020 — 3:00 PM EDT

## Graphs and Matroids Seminar - Erik Panzer

Title: The Hepp bound of a matroid: flags, volumes and integrals

 Speaker: Erik Panzer Affiliation: University of Oxford Zoom: Contact Rose McCarty

Abstract:

Invariants of combinatorial structures can be very useful tools that capture some specific characteristics, and repackage them in a meaningful way. For example, the famous Tutte polynomial of a matroid or graph tracks the rank statistics of its submatroids, which has many applications, and relations like contraction-deletion establish a very close connection between the algebraic structure of the invariant (e.g. Tutte polynomials) and the actual matroid itself.

Monday, October 19, 2020 — 11:30 AM EDT

## Algebraic Graph Theory Seminar - Christopher van Bommel

Title: Pretty Good State Transfer and Minimal Polynomials

 Speaker: Christopher van Bommel Affiliation: University of Manitoba Zoom: Contact Soffia Arnadottir

Abstract:

We examine conditions for a pair of strongly cospectral vertices to have pretty good quantum state transfer in terms of minimal polynomials, and provide cases where pretty good state transfer can be ruled out.

## Pages

### December 2020

S M T W T F S
29
30
1
2
5
6
8
9
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
1
2
1. 2021 (37)
1. April (6)
2. March (13)
3. February (8)
4. January (10)
2. 2020 (119)
1. December (5)
2. November (12)
3. October (12)
4. September (12)
5. August (11)
6. July (17)
7. June (11)
8. May (6)
9. March (11)
10. February (11)
11. January (11)
3. 2019 (167)
4. 2018 (136)
5. 2017 (103)
6. 2016 (137)
7. 2015 (136)
8. 2014 (88)
9. 2013 (48)
10. 2012 (39)
11. 2011 (36)
12. 2010 (40)
13. 2009 (40)
14. 2008 (39)
15. 2007 (15)