Algebra/Number Theory/Combinatorics seminar

POSTPONED - Representations of Algebraic Groups

Category

Speaker

Eric Friedlander (University of Southern California)

Abstract

We consider affine algebraic groups G over a field k and rational (i.e., algebraic) actions of G on k-vector spaces V.  We seek to provide some understanding of such actions through examples, algebraic invariants, and geometric structures.  Our first example of an algebraic group is the general linear group GL_N, but we shall primarily focus on the example of the additive group G_a over k of characteristic p > 0. The representation theory of G_a is far too complicated to fully classify, but sufficiently accessible to reveal considerable structure.  Our efforts lead to finite subgroup schemes of G_a and finite dimensional sub-coalgebras of the algebraic functions k[G_a] = k[T].

 

Where

MDSL 126

Misc. Information

Information from Graphs in Voting Theory

Category

Speaker

Karl-Dieter Crisman (Gordon College)

Abstract

Those studying the mathematics of voting and choice have long used whatever tools are necessary to explain paradoxes - from dynamical systems to Voronoi diagrams. Recently, graph theory has proved to be quite useful in modeling situations where there are natural symmetry relationships between options in a given choice setup. We will see some recent results (including some due to HMC faculty) answering questions about voting for rank-orderings of candidates and committees. Then we will explore how graphs can be used to answer questions you didn't even know you had about ranking cyclic orders - such as how to seat people at a round table!

Where

MDSL 126

Misc. Information

Applications of differential algebra to algebraic independence of arithmetic functions

Category

Speaker

Wai Yan Pong (Cal State Dominguez Hills)

Abstract

We improve several results on algebriaic independence of arithmetic functions by a theorem of Ax's on differential Schanuel conjecture.

Where

Mudd Science Library 126, Pomona College

Misc. Information

On the Frobenius problem and its generalization

Category

Speaker

Lenny Fukshansky (CMC)

Abstract

Let N > 1 be an integer, and let 1 < a_1 < ... < a_N be relatively prime integers. Frobenius number of this N-tuple is defined to be the largest positive integer that cannot be represented as a linear combination of a_1,...,a_N with non-negative integer coefficients. More generally, the s-Frobenius number is defined to be the largest positive integer that has precisely s distinct representations like this, so that the classical Frobenius number can be thought of as the 0-Frobenius number. The condition that a_1,...,a_N are relatively prime implies that s-Frobenius numbers exist for every non-negative integer s. The general problem of determining the Frobenius number, given N and a_1,...,a_N, dates back to the 19-th century lectures of G. Frobenius and work of J. Sylvester, and has been studied extensively by many prominent mathematicians of the 20-th century, including P. Erdos. While this problem is now known to be NP-hard, there has been a number of successful efforts by various authors producing bounds and asymptotic estimates on the Frobenius number and its generalization. I will discuss some of these results, which are obtained by an application of techniques from Discrete Geometry.

Where

Mudd Science Library 126, Pomona College

Misc. Information

The neural ring: using algebraic geometry to analyze neural codes

Category

Speaker

Nora Youngs (HMC)

Abstract

Neurons in the brain represent external stimuli via neural codes. These codes often arise from stimulus-response maps, associating to each neuron a convex receptive field. An important problem confronted by the brain is to infer properties of a represented stimulus space without knowledge of the receptive fields, using only the intrinsic structure of the neural code. How does the brain do this? To address this question, it is important to determine what stimulus space features can - in principle - be extracted from neural codes. This motivates us to define the neural ring and a related neural ideal, algebraic objects that encode the full combinatorial data of a neural code. We find that these objects can be expressed in a "canonical form'' that directly translates to a minimal description of the receptive field structure intrinsic to the neural code. We analyze the algebraic properties of maps between these objects induced by natural maps between codes. We also find connections to Stanley-Reisner rings, and use ideas similar to those in the theory of monomial ideals to obtain an algorithm for computing the canonical form associated to any neural code, providing the groundwork for inferring stimulus space features from neural activity alone.

Where

Mudd Science Library 126, Pomona College

Misc. Information

Projective modules and A^1-homotopy theory

Category

Speaker

Aravind Asok (USC)

Abstract

I will discuss the theory of projective modules over smooth (affine) algebras from a modern point of view. In paticular, I will explain how ideas from homotopy theory can be used to answer classical questions like: when does a projective module of rank r split as the sum of a module of rank r-1 and a free module of rank 1?

Where

Mudd Science Library 126, Pomona College

Misc. Information

Representations of analytic functions as infinite products and some arithmetic applications

Category

Speaker

Bogdan Petrenko (Eastern Illinois University)

Abstract

See attached PDF.

Where

Mudd Science Library 126, Pomona College

Misc. Information

Cone theta functions and rational volumes of spherical polytopes

Category

Speaker

Sinai Robins (Nanyang Technological University - Singapore and Brown University)

Abstract

It is natural to ask when the spherical volume defined by the intersection of a sphere at the apex of an integer polyhedral cone is a rational number. This work sets up a dictionary between the combinatorial geometry of polyhedral cones with `rational volume' and the analytic behavior of certain associated cone theta functions, which we define from scratch. We use number theoretic methods to study a new class of polyhedral functions called cone theta functions, which are closely related to classical theta functions. One of our results shows that if K is a Weyl chamber for any finite crystallographic reflection group, then its cone theta function lies in a graded ring of classical theta functions (of different weights/dimensions) and in this sense is ‘almost’ modular. It is then natural to ask whether or not the conic theta functions are themselves modular, and we prove that in general they are not. In other words, we uncover relations between the class of integer polyhedral cones that have a rational solid angle at their apex, and the class of cone theta functions that are almost modular. This is joint work with Amanda Folsom and Winfried Kohnen.

Where

Mudd Science Library 126, Pomona College

Misc. Information

Finite Subgroups of the Quaternions

Category

Speaker

Will Murray (California State University Long Beach)

Abstract

Question:  Given a field, what interesting finite multiplicative groups does it contain? 

Answer:  None.  In algebra class we prove that any finite group in a field is cyclic. 

However, division rings (noncommutative fields) are much more interesting.  At the very least, we know that the quaternions contain the finite quaternion group {1,-1,i,-i,j,-j,k,-k}, which is not cyclic.  Lots of interesting geometry gives us a classification of all finite subgroups of the quaternions.  They correspond to subgroups of SU(2) and SO(3), which correspond in turn to rotation groups of the Platonic solids. If you know basic group theory, you'll be able to understand the talk.  Knowing the definitions of SU(2) and SO(3) would help, but we'll review those as we go along.

Where

MDSL 126

Misc. Information

Factoring Formal Power Series over Z

Category

Speaker

Jesse Elliott (California State University, Channel Islands)

Abstract

The polynomial (1+X)(2+X) is reducible as a polynomial over the integers but is irreducible as a formal power series over the integers.  On the other hand, the polynomial 6+X is irreducible as a polynomial over the integers but is reducible as a formal power series over the integers.  How does 6+X factor as a product of formal power series?  More generally, how does one represent a given polynomial over the integers as a product of irreducible formal power series?  We will provide a complete answer to this question.  The solution involves the ring of p-adic integers for primes p.

Where

MDSL 126

Misc. Information

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