During 2016-17, the seminar takes place from 15:00-16:00 in Ross Building room N638. If you come by bus, the route 196A, 196B takes you to campus from Downsview subway station. 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 Laura Colmenarejo or Nantel Bergeron.
You may also be interested in the Algebraic Combinatorics Seminar at the Fields Institute.
Dates are listed in reverse-chronological order. Unless otherwise indicated, all talks will take place on Monday from 15:00-16:00 in N638 Ross Building (York University).
Date Speaker Title (click titles for abstract) 09 Jan. 2016 Steven Melczer
(U. of Waterloo)
12 Dec. 2016
(U. of Waterloo)
On some recent applications of combinatorics on words to noncommutative ring theoryIn this talk we will discuss some recent applications of combinatorics on words to noncommutative ring theory. We will use certain right-infinite words over finite alphabets to create interesting examples in graded ring theory. Namely, we will use right-infinite words obtained by iterating morphisms of the free monoid generated by an alphabet to produce a monomial algebra which is graded-nilpotent, semiprimitive, of Gelfand-Kirillov dimension 2, and finitely generated as a Lie algebra. This answers several questions of Greenfeld, Leroy, Smoktunowicz, and Ziembowski. Several open problems will be presented and discussed. This is joint work with Jason Bell. 5 Dec. 2016 Alex Fink
(Queen Mary U. of London)
The Tutte polynomial via Ehrhart theory and GKMTam\'as K\'alm\'an discovered a pair of univariate polynomials associated to a hypergraph which enumerate its spanning trees by internal and external activity. In the first part of the talk I'll discuss joint work with Amanda Cameron in which we extend this to a bivariate polynomial of polymatroids enumerating both activity statistics at once, using lattice point enumeration. On matroids our invariant agrees with the Tutte polynomial, though not in its most obvious basis. In the second part I'll introduce two other sources of formulae for the Tutte polynomial and similar invariants, and speculate on the relationships between them. One of these comes from joint work with David Speyer using K-theory of the Grassmannian, and the other is due to Kristin Shaw using Chern-Schwartz-Macpherson classes of hyperplane arrangement complements. 28 Nov. 2016 Li Ying
(Texas A&M U.)
Stability of the Heisenberg Product on Symmetric Functions.The Heisenberg product is an associative product defined on symmetric functions which interpolates between the usual product and Kronecker product. I will give the definition of this product and describe some properties of it. One well known thing about the Kronecker product of Schur functions is the stability phenomenon discovered by Murnaghan in 1938. I will give an analogous result for the Heisenberg product of Schur functions. 21 Nov. 2016 Farid Aliniaeifard
Normal supercharacter theory, Dyck paths, and Hopf structures.Normal supercharacter theory is a mechanism to substitute normal subgroups by conjugacy classes in such a way that this swelling simulate some features of irreducible charactes. We construct a normal supercharacter theory for the group of square matrices with entries in a finite field and ones on the diagonal and zeroes in all entries below the diagonal. The supercharacters in this supercharacter theory are indexed by Dyck paths. We show that this construction is identical with Scott Andrews' construction after gluing by torus group. Then we build up Hopf monoid structures base on these supercharacters. 14 Nov. 2016 (3:30pm) Mikhail Mazin
(Kansas State U.)
Rational Dyck Paths in the Non-Relatively Prime CaseIn the relatively prime case, the rational (n,m)-Dyck paths are in bijection with the (n,m)-invariant subsets of integers, considered up to shifts. This bijection brings a connection between rational Catalan combinatorics and the geometry of certain algebraic varieties. In particular, it allows one to reinterpret the dinv statistic as the dimension of the corresponding complex affine cell in a certain affine Springer fiber. The non-relatively prime case is more complicated. Although on the combinatorial side many things can be generalized, including the dinv statistic and even Shuffle conjecture, there is no known generalization of the geometric interpretation of the dinv statistic. In this talk, I will explain how one can extend the bijection between rational Dyck paths and the invariant subsets in Z to the non-relatively prime case. The natural obstacle is that the set of invariant subsets is not finite in the non-relatively prime case. One has to consider certain equivalence relation on the invariant subsets to make the bijection work. The hope is that this construction will lead to a geometric or representation theoretic interpretation of the dinv statistic in the non-relatively prime case. This is a joint project with Eugene Gorsky and Monica Vazirani. 14 Nov. 2016 (2:30pm) Alex Woo
(U. of Idaho)
Depth in classical Coxeter groupsMotivated by questions about sorting, Petersen and Tenner defined a notion of depth in an arbitrary Coxeter group and gave a combinatorial formula, with a constructive proof, for the depth of a permutation in terms of sums of sizes of exceedances. We do the same for the Coxeter groups of types B and D using the usual embedding into the symmetric group. We find that depth cannot always be realized with a minimal number of transpositions but can always be realized by a reduced product. We also characterize elements for which depth equals length and for which depth, length, and reflection length are all equal. This is joint work with Eli Bagno, Riccardo Biagioli, and Moti Novick. 07 Nov. 2016 Zach Teitler
(Boise State U.)
Waring ranks of homogeneous formsThe Waring rank of a homogeneous form is the number of terms needed to write it as a sum of powers of linear forms. It is related to secant varieties, provides a measure of the complexity of polynomials, and has applications in statistics, sciences, and engineering. I will discuss three topics related to Waring rank. (1) Waring ranks of general forms have been known for some time, but it is also of interest to determine Waring rank of particular forms such as the generic determinant and permanent. I will describe some recent results obtained via algebraic and geometric lower bounds for Waring rank; this is joint work with Jaroslaw Buczynski and with Harm Derksen. (2) A variation of a conjecture of Strassen asserts that the Waring rank of the sum of two forms in independent variables is the sum of the ranks of the summands. I will describe an elementary sufficient condition for a strong version of Strassen's conjecture. (3) It is an open question to determine the maximum Waring rank occurring among forms of a given degree, in a given number of variables. I will describe an upper bound; this is joint work with Gregoriy Blekherman. 31 Oct. 2016 Laura Colmenarejo
Stability in the combinatorics of symmetric functionsIn this talk, I will introduce the h-plethysm coefficients, related to the usual plethysm coefficients. I will present a combinatorial description of them. This description will be the tool to give a combinatorial proof of some well-known stability properties of the plethysm coefficients. I will also introduce some families of reduced kronecker coefficients. For these families, I will give their generating function as well as combinatorial descriptions in terms of plane partitions and quasipolynomials. These results are included in my thesis. 24 Oct. 2016 Dave Anderson
(Ohio State U.)
Diagrams and essential sets for signed permutationsThe essential set of a permutation, defined via its Rothe diagram by Fulton in 1992, gives a minimal list of rank conditions cutting out the corresponding Schubert variety in the flag manifold. I will describe an analogous notion for signed permutations, giving minimal conditions for Schubert varieties in flag varieties for other classical groups. This is related to a poset-theoretic construction of Reiner, Woo, and Yong, and thus gives a diagrammatic method for computing the latter. 17 Oct. 2016 Adeyemo Praise
The Three Presentations of T- Equivariant K- Theory of G/BLet G be a simple, simply connected, complex Lie group with a Borel subgroup B. G/B is the flag manifold associated to G. It is acted upon by maximal torus T contained in B by multiplication. We study the T- equivariant K-theory of G/B, the Grothendieck group of coherent sheaves on G/B. In this talk, I will give the three presentations of T-equivariant K-theory of this homogeneous space- Schubert presentation, Borel presentation and GKM ring. I will describe partially how to convert an element in one presentation to another. It represents the ongoing work with Shizuo Kaji. 3 Oct. 2016 Georg Merz
Newton Okounkov bodies - A bridge between algebraic and convex geometry.Inspired by work of A. Okounkov, Kaveh-Khovanskii and Lazarsfeld-Mustata independently introduced the notion of a Newton-Okounkov body which associates a convex body to a line bundle on a projective variety. This construction is a generalization of the toric case, and it allows to transport algebra geometric questions into such of convex geometry, and vice versa. In this talk I will give an outline of the construction, present some known results and give some generalizations of these in the setting of graded linear series. 26 Sep. 2016 John Maxwell Campbell
The evaluation of immaculate functions in terms of the ribbon basisThere is an elegant combinatorial formula for evaluating elements of the ribbon basis of NSym in terms of immaculate functions, but there is no known cancellation-free formula for expanding elements of the immaculate basis in terms of ribbon functions in NSym. However, recent research suggests that many different classes of immaculate functions may be evaluated in terms of ribbon functions using cancellation-free Jacobi-Trudi-like formulas. In this talk, we offer a sketch of a proof of a surprising Jacobi-Trudi-like formula for expanding immaculate-rectangles as linear combinations of ribbon functions. 12 Sep. 2016 Jake Levinson
(U. of Mich.)
(Real) Schubert Calculus from Marked Points on P^1I will describe a family of Schubert problems on the Grassmannian, defined using tangent flags to points of P^1 (or more generally, a stable curve) in its Veronese embedding. For Schubert problems having a finite set of solutions, the Shapiro-Shapiro Conjecture (later proven by Mukhin-Tarasov-Varchenko) proposed that, when the marked points are all real, the solutions would be as real as possible. More recently, Speyer gave a remarkable description of the real topology of the family in terms of Young tableaux and Schützenberger's jeu de taquin. I will go through this story, then give analogous results on the topology of one-dimensional Schubert problems (where the family consists of curves). In this case the combinatorics involves orbits of so-called "evacuation-shuffling", an algorithm related to tableau promotion and evacuation.
Below you will find links to the seminar webpages for previous years.
Year 2015-16 Year 2014-15 Year 2013-14 Year 2012-13 Year 2011-12 Year 2010-11 Year 2009-10 Fall 2008 & Winter 2009 Fall 2007 Winter 2006 Fall 2005 Winter 2005 Fall 2004 Winter 2004 Fall 2003 Winter 2003 Fall 2002 Winter 2002 Fall 2001 Winter 2001 Fall 2000 Winter 2000 Fall 1999 Winter 1999 Fall 1998 Winter 1998 Fall 1997