DISCRETE CATS SEMINAR

Discrete CATS Seminar

Speaker:  Sophie Morel

                 Princeton University
 
Title:         Combinatorial proof of a character identity
 
Abstract:
 
The calculation of the (intersection) cohomology of a Siegel modular variety includes many difficult character identities (the fundamental lemma, for example). In this lecture, I want to concentrate on the character identity appearing at the infinite place, which involves, among other things, stable discrete series characters and appears to be related in some non-obvious way to an identity of Goresky, Kottwitz and MacPherson. Once we strip away all the Lie group complications, our identity becomes a very elementary statement and can proved directly using the geometry of the Coxeter complex of the symmetric group. The relation with the Goresky-Kottwitz-MacPherson identity also becomes clearer; in particular, neither identity follows from the other, but they should have a common generalization. This is joint work with Richard Ehrenborg and Margaret Readdy.
Date:
Location:
745 POT
Event Series:

Discrete CATS Seminar

Title: k-uniform displacement tableaux
 
Abstract: In this talk we will introduce a peculiar family of tableaux on rectangular partitions, known as k-uniform displacement tableaux. The primary curiosity of this family is the introduction of a rule that governs the distance between two boxes in a partition in which the same symbol occurs. Our main goals will be analyzing the ways of filling a partition using a minimal number of symbols, discussing an algorithm for constructing a new tableau with a minimal number of symbols from a given tableau, and the geometric implications of this work. If time permits we can also discuss generalizations of these concepts.
Date:
-
Location:
POT 745

Discrete CATS Seminar

Title: k-uniform displacement tableaux
 
Abstract: In this talk we will introduce a peculiar family of tableaux on rectangular partitions, known as k-uniform displacement tableaux. The primary curiosity of this family is the introduction of a rule that governs the distance between two boxes in a partition in which the same symbol occurs. Our main goals will be analyzing the ways of filling a partition using a minimal number of symbols, discussing an algorithm for constructing a new tableau with a minimal number of symbols from a given tableau, and the geometric implications of this work. If time permits we can also discuss generalizations of these concepts.
Date:
-
Location:
POT 745

Discrete CATS Seminar

Title: The Erdos-Lovasz Double-Critical Conjecture
 
Abstract: A simple, connected graph is said to be double-critical if removing any pair of adjacent vertices lowers the chromatic number of the graph by exactly two. In 1966, Paul Erdos and Laszlo Lovasz proposed the Double-Critical Conjecture which states that the complete graph is the only simple, connected graph that is a double-critical graph. This result has been proven when the chromatic number of a graph is less than six, but is still open for the other cases. In this talk, concepts and results related to this problem will be discussed.
Date:
-
Location:
POT 745

Discrete CATS Seminar

Title: The Erdos-Lovasz Double-Critical Conjecture
 
Abstract: A simple, connected graph is said to be double-critical if removing any pair of adjacent vertices lowers the chromatic number of the graph by exactly two. In 1966, Paul Erdos and Laszlo Lovasz proposed the Double-Critical Conjecture which states that the complete graph is the only simple, connected graph that is a double-critical graph. This result has been proven when the chromatic number of a graph is less than six, but is still open for the other cases. In this talk, concepts and results related to this problem will be discussed.
Date:
-
Location:
POT 745

Discrete CATS Seminar

Title: Algebraic Aspects of Lattice Simplices

Abstract: Given a lattice polytope P, there are open problems of interest related to the integer decomposition property, Ehrhart h*-unimodality, and Ehrhart positivity. In this talk, we will survey some recent results in this area, based on various joint works with Rob Davis, Morgan Lane, Fu Liu, and Liam Solus.

Date:
-
Location:
POT 745

Discrete CATS Seminar

Title: Algebraic Aspects of Lattice Simplices

Abstract: Given a lattice polytope P, there are open problems of interest related to the integer decomposition property, Ehrhart h*-unimodality, and Ehrhart positivity. In this talk, we will survey some recent results in this area, based on various joint works with Rob Davis, Morgan Lane, Fu Liu, and Liam Solus.

Date:
-
Location:
POT 745

Discrete CATS Seminar

Title: Mutation of friezes

 

Abstract: Frieze is an array of positive integers satisfying certain rules.  Friezes of type A were first studied by Conway and Coxeter in 1970's, but they gained fresh interest in the last decade in relation to cluster algebras.   Moreover, the categorification of cluster algebras developed in 2006 yields a new realization of friezes in terms of representation theory of Jacobian algebras.   In this talk, we will discuss friezes of types A and D and their mutations.

 

Date:
-
Location:
POT 745

Discrete CATS Seminar

Title: Mutation of friezes

 

Abstract: Frieze is an array of positive integers satisfying certain rules.  Friezes of type A were first studied by Conway and Coxeter in 1970's, but they gained fresh interest in the last decade in relation to cluster algebras.   Moreover, the categorification of cluster algebras developed in 2006 yields a new realization of friezes in terms of representation theory of Jacobian algebras.   In this talk, we will discuss friezes of types A and D and their mutations.

 

Date:
-
Location:
POT 745

Discrete CATS Seminar

Speaker:  Gabor Hetyei, UNC Charlotte

Title:  Alternation acyclic tournaments and the homogeneous Linial arrangement

Abstract:

We define a tournament to be alternation acyclic if it does not

contain a cycle in which descents and ascents alternate. We show that

these label the regions in a homogenized generalization of the Linial

arrangement. Using a result by Athanasiadis, we show that these are

counted by the median Genocchi numbers. By establishing a bijection

with objects defined by Dumont, we show that alternation acyclic

tournaments in which at least one ascent begins at each vertex, except

for the largest one, are counted by the Genocchi numbers of the first

kind. As an unexpected consequence, we obtain a simple model for the

normalized median Genocchi numbers.

Date:
Location:
108 Patterson Office Tower
Event Series: