The Algebraic Combinatorics Seminar
Scheduled for Fridays at 3:00 PM

#### Schedule

In reverse-chronological order.

Date Speaker Title (click titles for abstract)
15 Apr. 2016
Nantel Bergeron Toward a cancelation free formula for the antipode of Hypergraphs (Part IV: Orientations)
8 Apr. 2016
Nantel Bergeron Toward a cancelation free formula for the antipode of Hypergraphs (Part III: Hopf Monoid structure)
1 Apr. 2016
Iain Moffat (University of London) The Tutte Polynomial via Hopf algebras
The Tutte polynomial is one of the most important, and best studied, graph polynomials. It is important not only because it encodes a large amount of combinatorial information about a graph, but also because of its applications to areas such as statistical physics and knot theory. Because of its importance the Tutte polynomial has been extended to various classes of combinatorial object. For some objects there is more than one definition of a "Tutte polynomial". For example, there are three different definitions for the Tutte polynomial of graphs in surfaces: M. Las Vergnasâ€™ 1978 polynomial, B. BollobÃ¡s and O. Riordanâ€™s 2002 ribbon graph polynomial, and V. Kruskalâ€™s polynomial from 2011. On the other hand, for some objects, such as digraphs, there is no wholly satisfactory definition of a Tutte polynomial. Why is this? Why are there three different Tutte polynomials of graphs in surfaces? Which can claim to be the Tutte polynomial of a graph in a surface? More generally, what does it mean to be the Tutte polynomial of a class of combinatorial objects? In this talk I will describe how Hopf algebras can be used to canonically construct Tutte polynomials of combinatorial objects, and, using this framework, will offer answers to these questions. This is joint work with Thomas Krajewski and Adrian Tanasa.
11 Mar. 2016
Mike Zabrocki Character evaluated at root of unity and Kronecker product of symmetric functions(Hopf Structure)
4 Mar. 2016
Shu Xiao Li The quotient k[x,y]/, part II
We continue work on the quotient k[x,y]/. This time, we focus on the case of infinitely many variables. I will attempt to construct explicitly its hilbert basis and give a proof. Hopefully we could also discuss about the case of finitely many variables.
26 Feb. 2016
Nantel Bergeron Toward a cancelation free formula for the antipode of Hypergraphs (Part II: and total order)
12 Feb. 2016
Nantel Bergeron Toward a cancelation free formula for the antipode of Hypergraphs (Part I: the Graph case)
5 Feb. 2016
Mike Zabrocki Character evaluated at root of unity and Kronecker product of symmetric functions( The h-tilde basis)
29 Jan. 2016
Mike Zabrocki Character evaluated at root of unity and Kronecker product of symmetric functions
22 Jan. 2016
SPECIAL Talk
Yuly Billig
(Carleton University)
Sturm-Liouville problem in Fock space
We compute the eigenvalues of the differential operator \Sum_{a+b=c+d} x_a x_b d/dx_c d/dx_d acting on the Fock space C[x_1, x_2, ...]. It turns out that this operator is diagonalizable with integer non-negative eigenvalues. Not surprisingly, the eigenvectors and the eigenvalues are indexed by Young diagrams.
15 Jan. 2016
Shu Xiao Li We discuss the quotient of polynomials in two sets of variables by the diagonaly-quasysymmetric function and show some partial results
27 Nov. 2015
Sirous Homayouni Fomin-Kirillov algebra
Buchberger's algorithm generates a Grobner basis ($GB$) for $I$, the ideal generated by the relations among generators $x_{ij}=-x_{ji}$ of Fomin-Kirillov algebra $FK(n)$. We show that elements of any degree $d$, of a $GB$ (with a polynomial ordering) generated by Buchberger's algorithm for the ideal $I$, are \textit{z-star} polynomials, i.e., polynomials with all variables of the form $x_{\alpha z}$, for a fix $z$, where \$1\leq\alpha
20 Nov. 2015
Nantel Bergeron Quotient involving Combinatorial Hopf Algebra
I will show that as we pass from n to n+1, The basis of the quotient for n is included in the quotient for n+1. In fact I show several potential inclusion that has consequences on the structure of the Hilbert series of the quotient. I will also recall that when we have free Hopf algebra we can determine the infinite Hilbert series. I will do so for quotient of r-QSym/s-QSym. That last part was inspired by talks during A. Lauve's visit.
13 Nov. 2015
Shu Xiao Li discussion on the dimension of quotients involving r-quasisymmetric functions
6 Nov. 2015
Farid Aliniaeifard The dimension of the quotient of polynomials by ideal generated by non-constant generalized quasi-symmetric functions
We consider a certain action of the symmetric group on polynomials called local action. Then we try to find the dimension of the quotient of polynomials by the ideal generated by the orbits of the local action.
30 Oct. 2015
Mike Zabrocki Symmetric group and Gl_n characters
I will review the construction of the Gl_n and S_n characters in the tensor algebra and then present a simple to state open problem: how do we express the S_n characters as a monomial symmetric function expansion in the eigenvalues of the permutation matrix?
23 Oct. 2015
Room 230
Shu Xiao Li Unimodality and Fibonomials
16 Oct. 2015
Nantel Bergeron Hopf monoid of Matroid and Positroid
I describe the Hopf monoid of matroid (with linear order) and the Hopf submonoid of Positroid.
9 Oct. 2015
Room 230
Yannic Vargas The product of monomial basis in the Hopf algebra of Malvenuto-Reutenauer
We will work out some examples to understand two combinatorial interpretations for the coefficients of the product between the monomial basis in the Hopf algebra of Malvenuto-Reutenauer, based on planar posets and permutation patterns.
2 Oct. 2015
NO SEMINAR
25 Sep. 2015 Nantel Bergeron and Carolina Benedetti Hopf algebras on matroids
We will work out some examples of the several Hopf algebras on Matroids.
18 Sep. 2015 Jean-Baptiste Priez Non-commutative Frobenius characteristic of generalized parking functions
11 Sep. 2015 Nantel Bergeron What to do this year
27 Aug. 2015 Tom Denton (Google Inc) How to Win a Data Science Competition With Algebraic Combinatorics
As an early foray into the intersection of combinatorics and machine learning, I tried my hand at a Kaggle competition on identifying circles of friends in the Facebook graph. I'll give an overview of my solution, including a deep dive on spectral clustering, a graph clustering algorithm with roots in algebraic graph theory.
21 Aug. 2015
SERIES of TALKS
Carolina Benedetti Matroid, Positroid and Combinatorial Hopf Algebras.
14 Aug. 2015
SERIES of TALKS
Carolina Benedetti Matroid, Positroid and Combinatorial Hopf Algebras.
7 Aug. 2015
SERIES of TALKS
Carolina Benedetti Matroid, Positroid and Combinatorial Hopf Algebras.

Notes.

• Unless otherwise indicated, the seminar meets on Fridays at 3:15 pm in Room 210 of the Fields Institute.
• If you are interested in speaking at the seminar, contact Nantel Bergeron.
• You may also be interested in the Applied Algebra Seminar (Monday afternoons at York University).

About the seminar. Every year we pick a new topics to explore.

Year Topic
2014-2015 Fiboland, Symmetric and non symmetric functions
2013-2014 Fiboland, a world of Catalan and Fibonacci numbers
2012-2013 NSym and the Immaculate Basis
2011-2012 k-Schur functions and affine permutations
2010-2011 Littlewood Richardson rule k-Schur functions.
2009-2010 Idempotents and weakly ordered semigroups. (q,t) Catalan Numbers.
2008-2009 Littlewood-Richardson Rule, Shifted Tableaux and P-Schur functions
2007-2008 Open problems around k-Schur functions and non-commutative symmetric functions
2006-2007 Open problems
2005-2006 Cluster Algebras and Quivers
Spring 2005 Formal languages and analytic classes of functions
Fall 2004 (Quasi-) Symmetric functions in noncommutative variables and applications
Winter 2003 Crystal Bases and Representation Theory, Super-algebras, etc.
Fall 2003 Quasi-Symmetric functions and applications
Fall 2002 Crystal Bases and Representation Theory