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Events for June – October 2008

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Sep. 02, 2008 2pm (MATH 350)	Lie Theory Z. Strider McGregor-Dorsey (CU) X A full heap is an isomorphism class of partially ordered sets with labeling taken from some underlying labeled graph, satisfying certain conditions. A Dynkin diagram is graph which encodes, via a set of rules, a generalized Cartan matrix. In this talk, we classify all the full heaps over Dynkin diagrams of affine type A, defining and giving examples of these structures along the way. There will be lots of pictures. Full Heaps over Dynkin Diagrams of Affine Type A
Sep. 11, 2008 2pm (MATH 350)	Lie Theory Robert Guralnick (USC) X A derangement is a fixed point free permutation. By an old elementary result of Jordan, every transitive finite permutation group contains a derangement. We will discuss some recent refinements of this result on the proportion of derangements together with applications to Brauer groups and rational maps between curves. In particular, we will discuss the Fulman-Guralnick solution of the Boston-Shalev conjecture on a lower bound for the proportion of derangements for finite simple groups. Derangements in Finite Permutation Groups
Sep. 16, 2008 1pm (Math 350)	PDE/Analysis Stephen Preston X I will describe how the Brouwer fixed point theorem generalizes to the Schauder fixed point theorem in infinite dimensions, then show how this theorem can be used to solve nonlinear elliptic partial differential equations. In addition I will describe how it can be used to prove global existence for two-dimensional hydrodynamics. This will be a two-part talk. Fixed point theorems for PDEs, Part 1
Sep. 16, 2008 2pm (MATH 350)	Lie Theory Nat Thiem (CU) X Supercharacter theory is an exciting new development in representation theory that replaces precision with nice combinatorics. In particular, by studying nearly irreducible representations (so-called super-representations) rather than irreducible ones, one can often study previously intractable representation theories without sacrificing too much information. This talk gives an introduction to supercharacters and describes some of the many open problems this point of view suggests. Some open problems in supercharacter theory
Sep. 23, 2008 1pm (Math 350)	PDE/Analysis Stephen Preston X I will describe how the Brouwer fixed point theorem generalizes to the Schauder fixed point theorem in infinite dimensions, then show how this theorem can be used to solve nonlinear elliptic partial differential equations. In addition I will describe how it can be used to prove global existence for two-dimensional hydrodynamics. This will be a two-part talk. Fixed point theorems for PDEs, Part 2
Sep. 23, 2008 2pm (MATH 350)	Lie Theory Nat Thiem (CU) X The symmetric group has a well-known and beautiful combinatorial structure inextricably linked with the ring of symmetric functions. In the first of two talks, we will review the combinatorial structure of the character theory of the symmetric group and its relation to the ring of symmetric functions. In the second, we will explore an analogue obtained via the supercharacter theory of the finite groups of unipotent upper-triangular matrices. A p-group analogue to symmetric group combinatorics I
Sep. 30, 2008 1pm (Math 350)	PDE/Analysis Anca Radulescu X We attempt to model the neurodegenerative process in schizophrenia, building upon neural vulnerability and dysregulation hypotheses. We attribute the mental disorder to a hereditary predisposition that reduces the individual’s psychological threshold towards stimuli, to the point where even minor daily stresses will directly trigger psychotic experiences. The behavior of our model depends on an set of physiological parameters (synaptic strengths, blood cortisol, dopamine regulation, autoimmunity). Interpretation of this dependence of parameters of the system’s dynamics offers an analytical explanation for the "normality/disease" dichotomy. The concept of "bifurcation" could be the key to understanding the threshold between these two states. Dynamical systems and schizophrenia
Sep. 30, 2008 2pm (MATH 350)	Lie Theory Nat Thiem (CU) X The symmetric group has a well-known and beautiful combinatorial structure inextricably linked with the ring of symmetric functions. In the first of two talks, we will review the combinatorial structure of the character theory of the symmetric group and its relation to the ring of symmetric functions. In the second, we will explore an analogue obtained via the supercharacter theory of the finite groups of unipotent upper-triangular matrices. A p-group analogue to symmetric group combinatorics II
Oct. 07, 2008 1pm (Math 350)	PDE/Analysis Arlan Ramsay X There are several sets of axioms for both Euclidean and hyperbolic plane geometry. If the axioms in each such set are combined, we could say that we have definitions of Euclidean or hyperbolic planes. My goal is to describe one formulation and how the uniqueness theorems can be proved in that context. In this formulation, the real numbers are taken as a starting point, rather than deriving the real numbers from other properties of planes. The two uniqueness theorems have a number of steps in common, consisting of what are called neutral theorems, i.e., ones that do not depend on any assumption about the number of lines parallel to a given line through a given point. Indeed one of the theorems is that there is always at least one such parallel line. I will only mention some of the theorems more central to the development. Axiomatic geometry, Euclidean and otherwise, part 1
Oct. 07, 2008 2pm (MATH 350)	Lie Theory Richard Green (CU) X Many finite Coxeter groups occur naturally as groups of automorphisms of polytopes (higher dimensional polyhedra). Some of my recent work considers subcomplexes of these polytopes, that is, spaces obtained by deleting interiors of certain faces. If these subcomplexes are chosen carefully, their homology groups (don't worry, I'll explain this) afford representations of the Coxeter group. I will discuss how one might find the characters of such representations, concentrating mostly on the case of type D. This will be a good excuse to discuss the representation theory of finite Coxeter groups of classical type (A, B, D) over the complex numbers. Homology representations of Coxeter groups I
Oct. 14, 2008 1pm (Math 350)	PDE/Analysis Arlan Ramsay X There are several sets of axioms for both Euclidean and hyperbolic plane geometry. If the axioms in each such set are combined, we could say that we have definitions of Euclidean or hyperbolic planes. My goal is to describe one formulation and how the uniqueness theorems can be proved in that context. In this formulation, the real numbers are taken as a starting point, rather than deriving the real numbers from other properties of planes. The two uniqueness theorems have a number of steps in common, consisting of what are called neutral theorems, i.e., ones that do not depend on any assumption about the number of lines parallel to a given line through a given point. Indeed one of the theorems is that there is always at least one such parallel line. I will only mention some of the theorems more central to the development. Axiomatic geometry, Euclidean and otherwise, part 2
Oct. 14, 2008 2pm (MATH 350)	Lie Theory Richard Green (CU) X Many finite Coxeter groups occur naturally as groups of automorphisms of polytopes (higher dimensional polyhedra). Some of my recent work considers subcomplexes of these polytopes, that is, spaces obtained by deleting interiors of certain faces. If these subcomplexes are chosen carefully, their homology groups (don't worry, I'll explain this) afford representations of the Coxeter group. I will discuss how one might find the characters of such representations, concentrating mostly on the case of type D. This will be a good excuse to discuss the representation theory of finite Coxeter groups of classical type (A, B, D) over the complex numbers. Homology representations of Coxeter groups II
Oct. 28, 2008 2pm (MATH 350)	Lie Theory Hugh Denoncourt (CU) X Stanley's formula for the number of reduced expressions of a permutation regarded as a Coxeter group element raises the question of how to enumerate the reduced expressions of an arbitrary Coxeter group element. We provide a framework for answering this question by constructing combinatorial objects that represent the inversion set and the reduced expressions for an arbitrary Coxeter group element. It will be shown that this construction generalizes the balanced labellings of Fomin et al. The framework also provides a formula for the length of an element formed by deleting a generator from a Coxeter group element. Some combinatorial models for reduced expressions
Oct. 29, 2008 1pm (Math 350)	PDE/Analysis Lawrence Baggett X Let L be a linear differential operator, and let f be a smooth function with compact support K. Is there a solution to the equation Lu = f? The answer is "no" in general, but the answer to a related question is often "yes." The related question is, does there exist a function g, for which g restricted to K equals f, and for which there is a solution to the equation Lu = g? In this talk I will give some beginning hints about how noncommutative harmonic analysis (representation theory) can be used to show that the answer to the second question is "yes" for certain left-invariant differential operators on Lie groups. Local solvability of certain differential and difference operators

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