The Applied Algebra Seminar
A Monday afternoon research seminar

The seminar is currently organized by Aram Dermenjian and Nantel Bergeron.

During 2020-21, the seminar is ONLINE at 15:00-16:00 EDT (GMT -4).
When we resume in person, we will continue to broadcast. The seminar room is RossBuilding room N638. If you come by public transportation, there is a York University subway station on the TTC Line 1 Yonge-Univerity route. If you come by car, you can find the available parking lots here.
The seminar has been running since 1997. The topics of talks have typically been any mixture of algebra with any other field: combinatorics, geometry, topology, physics, etc. Further down this page you will find links to the seminar webpages for previous years. The audience usually consists of 6–12 people, including several graduate students and post-docs. For this reason, speakers are encouraged to devote a portion of their talk to the suggestion of open problems and the directions for research in their area. If you are interested in speaking at the seminar, contact Aram Dermenjian or Nantel Bergeron.

You may also be interested in the Algebraic Combinatorics Seminar at the Fields Institute.

### Schedule

Dates are listed in reverse-chronological order.
All talks will be online with Zoom link, generaly at 15:00-16:00 (EDT [GMT-4]) on Monday
When we resume in person, we will continue to broadcast. Unless otherwise indicated, all in person talks will take place on Monday from 15:00-16:00 in N638 Ross Building (York University).

Date Speaker Title (click titles for abstract)
12 Apr. 2021
(Virtual seminar on Zoom)
Laura Colmenarejo
UMass Amherst
5 Apr. 2021
(Virtual seminar on Zoom)
TBA
29 Mar. 2021
(Virtual seminar on Zoom)
Maxime Bergeron
22 Mar. 2021
(Virtual seminar on Zoom)
Hugo Mlodecki
Université Paris-Saclay
15 Mar. 2021
(Virtual seminar on Zoom)
Rosa Orellana
Dartmouth College
8 Mar. 2021
(Virtual seminar on Zoom)
Daniel Tamayo
Université Paris-Saclay
1 Mar. 2021
(Virtual seminar on Zoom)
Foster Tom
University of California, Berkeley
22 Feb. 2021
(Virtual seminar on Zoom)
Anna Weigandt
Univeristy of Michigan
15 Feb. 2021
(Virtual seminar on Zoom)
No Seminar Family Day / Reading Week
8 Feb. 2021
(Virtual seminar on Zoom)
Matteo Mucciconi
Tokyo Institute of Technology
1 Feb. 2021
(Virtual seminar on Zoom)
Andrés Vindas Meléndez
University of Kentucky
25 Jan. 2021
(Virtual seminar on Zoom)
Olya Mandelshtam
Brown University
The multispecies TAZRP and modified Macdonald polynomials
Recently, a formula for the symmetric Macdonald polynomials $P_{\lambda}(X;q,t)$ was given in terms of objects called multiline queues, which also compute probabilities of a statistical mechanics model called the multispecies asymmetric simple exclusion process (ASEP) on a ring. It is natural to ask whether the modified Macdonald polynomials $\widetilde{H}_{\lambda}(X;q,t)$ can be obtained using a combinatorial gadget for some other statistical mechanics model. We answer this question in the affirmative. In this talk, we will give a new formula for $\widetilde{H}_{\lambda}(X;q,t)$ in terms of fillings of tableaux called polyqueue tableaux. We define a multispecies totally asymmetric zero range process (TAZRP) on a ring with parameter $t$, whose (unnormalized) stationary probabilities are computed by polyqueue tableaux, and whose partition function is equal to $\widetilde{H}_{\lambda}(X;1,t)$. This talk is based on joint work with Arvind Ayyer and James Martin.
18 Jan. 2021
(Virtual seminar on Zoom)
Sunita Chepuri
University of Michigan
Kazhdan-Lusztig Immanants for $k$-positive Matrices
Immanants are matrix functionals that generalize the determinant. One notable family of immanants are the Kazhdan-Lusztig immanants. These immanants are indexed by permutations and are defined as sums involving Kazhdan-Lusztig polynomials specialized at $q=1$. Kazhdan-Lusztig immanants have several interesting combinatorial properties, including that they are nonnegative on totally positive matrices. We give a condition on permutations that allows us to extend this theorem to the setting of $k$-positive matrices.
Winter Break
14 Dec. 2020
(Virtual seminar on Zoom)
Luis Serrano
Zapata Computing
Fundamentals and recent advances in machine learning and neural networks
In this talk we'll first go over the fundamental notions of machine learning such as linear regression and the perceptron algorithm. Then we'll delve into neural networks and some of their applications. If time permits, we'll get into some cutting edge material such as generative machine learning (the algorithms that draw faces as in www.thispersondoesnotexist.com) and some of their extensions.
7 Dec. 2020
(Virtual seminar on Zoom)
Emily Gunawan
Oklahoma University
Cambrian combinatorics on quiver representations
We will present a geometric model of the Auslander-Reiten quiver of a type A quiver together with a stability function for which all indecomposable modules are stable. We also introduce a new Catalan object which we call a maximal almost rigid representation. We define a partial order on the set of maximal almost rigid representations and use our new geometric model to show that this partial order is a Cambrian lattice. If there is time, we will discuss a connection to the biCambrian lattice of diagonal rectangulations (which are counted by the Baxter numbers). This is based on joint work with E. Barnard, E. Meehan, and R. Schiffler.
30 Nov. 2020
(Talk Video)
Christian Gaetz
MIT
Stable characters from permutation patterns
We study the expected value (and higher moments) of the number of occurrences of a fixed permutation pattern on conjugacy classes of the symmetric group S_n. We prove that this virtual character stabilizes as n grows, so that there is a single polynomial computing these moments on any conjugacy class of any symmetric group. Our proof appears to be the first application of partition algebras to the study of permutation patterns. This is joint work with Christopher Ryba.
23 Nov. 2020
(Talk Video)
Nicolle Gonzalez
UCLA
Affine Demazure crystals for nonsymmetric Macdonald polynomials
Macdonald polynomials have long been hailed as a breakthrough in algebraic combinatorics as they simultaneously generalize both Hall-Littlewood and Jack symmetric polynomials. The nonsymmetric Macdonald polynomials $E_a(X;q,t)$ are a further generalization which contain the symmetric versions as special cases. When specialized at $t =0$ the nonsymmetric Macdonald polynomials were shown by Bogdon and Sanderson to arise as characters of affine Demazure modules, which are certain truncations of highest weight modules. In this talk, I will describe a type A combinatorial crystal which realizes the affine Demazure module structure and recovers the results of Bogdon and Sanderson crystal-theoretically. The construction yields a filtration of these affine crystals by finite Demazure crystals via certain embedding operators that model those of Knop and Sahi for nonsymmetric Macdonald polynomials. Thus, we obtain an explicit combinatorial expansion of the specialized nonsymmetric Macdonald polynomials as graded sums of key polynomials. As a consequence, we derive a new combinatorial formula for the Kostka-Foulkes polynomials. This is joint work with Sami Assaf.
16 Nov. 2020
(Talk Video)
Aida Maraj
Max Planck Institute
Reciprocal ML-degree of Brownian Motion Tree Models
Brownian Motion Tree Models (BMTM) are multivariate Gaussian models that arise in phylogenetics when studying the evolution of species through time. They are realized by rooted directed trees. BMTM are wonderful as the space of their covariance matrices is a linear space of symmetric matrices, and the space of their concentration matrices is a toric variety. In applications, one is interested in computing the point in a model that is more probable for the observed data. The (reciprocal) Maximum Likelihood degree of the model gives an insight on the complexity of this problem. In BMTM the reciprocal ML-degree can be nicely computed from the structure of the tree. To prove this result we require help from toric geometry. This is based on joint work with T. Boege, J.I. Coons, C. Eur, and F. Röttger.
9 Nov. 2020
(Talk Video)
Galen Dorpalen-Barry
University of Minnesota
Cones of Hyperplane Arrangements through the Varchenko-Gea'lfand Ring
The coefficients of the characteristic polynomial of an arrangement in a real vector space have many interpretations. An interesting one is provided by the Varchenko-Gel'fand ring, which is the ring of functions from the chambers of the arrangement to the integers with pointwise multiplication. Varchenko and Gel'fand gave a simple presentation for this ring, along with a filtration whose associated graded ring has its Hilbert function given by the coefficients of the characteristic polynomial. We generalize these results to cones defined by intersections of halfspaces of some of the hyperplanes. Time permitting, we will discuss Varchenko-Gel'fand analogues of some well-known results in the Orlik-Solomon algebra regarding Koszulity and supersolvable arrangements.
2 Nov. 2020
(Talk Video)
Sophia Elia
FU Berlin
Congruence Normality for Simplicial Hyperplane Arrangements
Simplicial hyperplane arrangements still have much to reveal. In rank 3, it is not known whether the list of simplicial hyperplane arrangements is complete. We determine whether the associated posets of regions possess the combinatorial property of "congruence normality" for arrangements with up to 37 hyperplanes. We use methods from oriented matroids, which make the computations possible. This refines the structure of the list, breaking it into three separate combinatorial categories. In particular, we show that arrangements stemming from finite Weyl groupoids have congruence normal posets of regions. This is joint work with Jean-Philippe Labbé and Michael Cuntz.
26 Oct. 2020
(Talk Video)
Alex McDonough
Brown U.
A Higher-Dimensional Sandpile Map
Traditionally, the sandpile group is defined on a graph and the Matrix-Tree Theorem says that this group's size is equal to the number of spanning trees. An extension of the Matrix-Tree Theorem gives a relationship between the sandpile group and bases of a class of orientable arithmetic matroids. I provide a family of combinatorially meaningful maps between these two sets. This generalizes a bijection given by Backman, Baker, and Yuen and extends work by Duval, Klivans, and Martin. I will not assume any background beyond undergraduate linear algebra.
19 Oct. 2020
(Talk video)
Yibo Gao
MIT
The 1/3-2/3 Conjecture for Coxeter groups
The 1/3-2/3 Conjecture, originally formulated in 1968, is one of the best-known open problems in the theory of posets, stating that the balance constant of any non-total order is at least 1/3. By reinterpreting balance constants of posets in terms of convex subsets of the symmetric group, we extend the study of balance constants to convex subsets C of any Coxeter group. Remarkably, we conjecture that the lower bound of 1/3 still applies in any finite Coxeter group, with new and interesting equality cases appearing. We generalize several of the main results towards the 1/3-2/3 Conjecture to this new setting: we prove our conjecture when C is a weak order interval below a fully commutative element in any acyclic Coxeter group (a generalization of the case of width-two posets), we give a uniform lower bound for balance constants in all finite Weyl groups using a new generalization of order polytopes to this context, and we introduce generalized semiorders for which we resolve the conjecture. We hope this new perspective may shed light on the proper level of generality in which to consider the 1/3-2/3 Conjecture, and therefore on which methods are likely to be successful in resolving it. This is joint work with Christian Gaetz.
12 Oct. 2020 No Seminar Thanksgiving/Reading Week
5 Oct. 2020
(Talk video)
Yusra Naqvi
U. of Sydney
A gallery model for affine flag varieties
Galleries, which are special sequences of elements in a Coxeter group, provide a nice combinatorial way of studying flag varieties. In this talk, we will discuss what these objects are, how they relate to each other, and how this relationship gives us a convenient recursion for computing certain double coset intersections in affine flag varieties. This talk is based on joint work with Elizabeth Milicģevicģ, Petra Schwer and Anne Thomas.
28 Sep. 2020
(Talk video)
Wencin Poh
UC Davis
A crystal for stable Grothendieck polynomials
We construct a type A crystal, which we call the *-crystal, whose character is the stable Grothendieck polynomials for fully-commutative permutations. This crystal is a K-theoretic generalization of Morse-Schilling crystal on decreasing factorizations. Using the residue map, we showed that this crystal intertwines with the crystal on set-valued tableaux given by Monical, Pechenik and Scrimshaw. We also proved that this crystal is isomorphic to that of pairs of semistandard Young tableaux using a newly defined insertion called the *-insertion. The insertion offers a combinatorial interpretation to the Schur positivity of the stable Grothendieck polynomials for fully-commutative permutations. Furthermore, the *-insertion has interesting properties in relation to row Hecke insertion and the uncrowding algorithm. This is joint work with Jennifer Morse, Jianping Pan and Anne Schilling.
21 Sep. 2020 Logan Crew
U. of Waterloo
Edge Deletion-Contraction in the Chromatic and Tutte Symmetric Functions
"We consider symmetric function analogues of the chromatic and Tutte polynomials on graphs whose vertices have positive integer weights. We show that in this setting these functions admit edge deletion-contraction relations akin to those of the corresponding polynomials, and we use these relations to give enumerative and/or inductive proofs of properties of these functions. In particular we note that the Tutte symmetric function in this form is related to a family of vertex-weighted graph functions, from which we derive a recipe theorem and a spanning-tree expansion.
This is joint work with Sophie Spirkl."

### Archives

Below you will find links to the seminar webpages for previous years.