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Thursday, April 11, 2024 2:00 pm - 3:00 pm EDT (GMT -04:00)

Algebraic and Enumerative Combinatorics - Alex Kroitor

Title: Lattice Paths Through ACSV

Speaker: Alex Kroitor
Affiliation: University of Waterloo
Location: MC 5479

There will be a pre-seminar presenting relevant background at the beginning graduate level starting at 1pm.

Abstract: Analytic combinatorics in several variables uses complex analytic results to find coefficients in the series expansions of meromorphic functions. Typically this is used to find asymptotics of sequences by examining their associated generating functions. In the 2000's Bousquet-Melou (and others) used the kernel method, introduced in the late 60s and 70s, to find generating function expressions (in terms of certain multivariate rational functions) for certain kinds of walks in restricted regions. In particular Melczer and Mishna found asymptotics for these restricted walks when the step sets are symmetric in every axis.

Friday, April 12, 2024 3:30 pm - 4:30 pm EDT (GMT -04:00)

Tutte Colloquium - Bertrand Guenin

Title: Bridging the gap between Linear and Integer Programming

Speaker: Bertrand Guenin
Affiliation: University of Waterloo
Location: MC 5501

Abstract: Informally, an Integer Program is obtained from a Linear Program by adding the condition that some variables be restricted to be integer. What kind of other restrictions lead to interesting classes of optimization problems? One such class of problems is obtained by restricting the variables in a Linear Program to be dyadic rationals, i.e. rationals where the denominator is a power of two. These problems borrow features from both Linear Programming and Integer Programming. Notably, they can be solved in polynomial time, but optimal solutions may have large support.

Monday, April 15, 2024 11:30 am - 12:30 pm EDT (GMT -04:00)

Algebraic Graph Theory - John Jasper

Title: New strongly regular graphs from optimal line packings

Speaker: John Jasper
Affiliation: Air Force Institute of Technology
Location: Please contact Sabrina Lato for Zoom link.

Abstract: In this talk, we'll explore several new constructions of strongly regular graphs. Specifically, we will show how some known optimal line packings can be coerced into generating new families of SRGs. We will also introduce a new construction of optimal line packings, yielding additional infinite families of SRGs.

Thursday, April 18, 2024 2:00 pm - 3:00 pm EDT (GMT -04:00)

Algebraic and Enumerative Combinatorics - Jeremy Chizewer

Title: Analytic Methods and Combinatorial Plants

Speaker: Jeremy Chizewer
Affiliation: University of Waterloo
Location: MC 5479

There will be a pre-seminar presenting relevant background at the beginning graduate level starting at 1pm

Abstract: In this talk, I will present the results of my masters thesis. I examine three applications of analytic methods to problems in combinatorics. By coincidence, each problem involves a combinatorial structure named for a plant--AVL trees, cactus graphs, and sunflowers--which we refer to collectively as combinatorial plants.

Monday, April 22, 2024 8:00 pm - 9:00 pm EDT (GMT -04:00)

Algebraic Graph Theory - Akihiro Munemasa

Title: Abelian covers of association schemes with applications to SIC-POVM

Speaker: Akihiro Munemasa
Affiliation: Tohoku University
Location: Please contact Sabrina Lato for Zoom link.

Abstract: Godsil and Hensel (1992) developed a theory of abelian covers of complete graphs to construct antipodal distance-regular graphs of diameter 3. More recently, Coutinho, Godsil, Shirazi and Zhan (2016) showed that equiangular tight frames can be constructed from covers of complete graphs in terms of cyclic groups of prime order. In this talk, we introduce covers of (not necessarily symmetric) association scheme of d classes in terms of an (not necessarily cyclic) abelian group G of order d.

Tuesday, April 23, 2024 3:00 pm - 4:00 pm EDT (GMT -04:00)

Graphs and Matroids - Jonathan Leake

Title: Lorentzian polynomials

Speaker: Jonathan Leake
Affiliation: University of Waterloo
Location: MC 5417

Abstract: Lorentzian (aka completely log-concave) polynomials were recently developed by Brändén-Huh and Anari-Liu-Oveis Gharan-Vinzant to settle Mason's conjectures on the log-concavity of the number of size-k independent sets of a matroid. In this talk, we will define these polynomials and sketch the proof of these conjectures. Along the way we will also state other results which will demonstrate the very strong connection between matroids and Lorentzian polynomials.

Monday, April 29, 2024 11:30 am - 12:30 pm EDT (GMT -04:00)

Algebraic Graph Theory - Dmitriy Panasenko

Title: Deza graphs and vertex connectivity

Speaker: Dmitriy Panasenko
Affiliation: Umeå University
Location: Please contact Sabrina Lato for Zoom link.

Abstract: A k-regular graph on v vertices is called a Deza graph with parameters (v, k, b, a), b ≥ a if the number of common neighbors of any two distinct vertices takes two values: a or b. A Deza graph is called a strictly Deza graph if it has diameter 2 and is not strongly regular.

In this talk we will discuss the enumeration of strictly Deza graphs and the enumeration of special subclass of strictly Deza graphs called divisible design graphs. We will also describe the constructions of divisible design graphs found during the enumeration.

Thursday, May 16, 2024 2:00 pm - 3:00 pm EDT (GMT -04:00)

Algebraic & Enumerative Combinatorics - Sarah Brauner

Title: Configuration spaces and combinatorial algebras

Speaker: Sarah Brauner
Affiliation: UQAM
Location: MC 6029

There will be a pre-seminar presenting relevant background at the beginning graduate level starting at 1pm.

Abstract:  In this talk, I will discuss connections between configuration spaces, an important class of topological space, and combinatorial algebras arising from the theory of reflection groups. In particular, I will present work relating the cohomology rings of some classical configuration spaces—such as the space of n ordered points in Euclidean space—with Solomon’s descent algebra and the peak algebra. The talk will be centered around two questions.

First, how are these objects related?

Second, how can studying one inform the other? This is partially joint work with Marcelo Aguiar and Vic Reiner.

Friday, May 17, 2024 3:30 pm - 4:30 pm EDT (GMT -04:00)

Distinguished Tutte Lecture - Katya Scheinberg

Title: Stochastic  Oracles and Where to Find Them

Speaker: Katya Scheinberg
Affiliation: Cornell University
Location: MC 5501

Abstract: Continuous optimization is a mature field, which has recently undergone major expansion and change. One of the key new directions is the development of methods that do not require exact information about the objective function. Nevertheless, the majority of these methods, from stochastic gradient descent to "zero-th order" methods use some kind of approximate first order information. We will overview different methods of obtaining this information, including simple stochastic gradient via sampling, robust gradient estimation in adversarial settings, traditional and randomized finite difference methods and more.

We will discuss what key properties of these inexact, stochastic first order oracles are useful for convergence analysis of optimization methods that use them. 

Tuesday, May 21, 2024 1:30 pm - 2:30 pm EDT (GMT -04:00)

URA Seminar - Stephen Melczer

Title: Adventures in Enumeration

Speaker: Stephen Melczer
Affiliation: University of Waterloo
Location: MC 5479

Abstract: We make the argument that by combining pure mathematical tools with computational insights and applications from a vast array of disciplines, combinatorics is the perfect area to see all the wonders of math on display. Applications discussed include the analysis of classical algorithms, restricted permutations, models predicting the shape of biomembranes, queuing theory, random walks, ratchet models for gene expression, maximum likelihood degree in algebraic statistics, transcendence of zeta values, sampling algorithms for perfect matchings in bipartite graphs, and parallel synthesis for DNA storage.