## Hom Quandles

### Speaker

Alissa Crans (Loyola Marymount University)

### Abstract

Analogous to the case for groups, the collection of quandle homomorphisms, Hom(Q, X), has no natural quandle structure. However, if X is an abelian quandle, then the hom set does become a quandle with the obvious pointwise operation. We will consider examples and investigate properties of this hom quandle.

### Where

Millikan 2099, Pomona College

## Solving quadratic equations over Q-bar

### Speaker

Lenny Fukshansky (CMC)

### Abstract

Given a quadratic equation in N variables over a fixed number field K, there exists an algorithm to determine whether it has a non-trivial solution over K and to find such a solution. This matter becomes considerably more complicated for a system of quadratic equations: existence of a general such algorithm would contradict Matijasevich's negative answer to Hilbert's 10th problem. The problem however is more tractable if we allow searching in extensions of K. I will discuss an approach to this problem, which involves height functions, the common tools of Diophantine geometry. Our investigation extends previous results on small-height zeros of quadratic forms, including Cassels' theorem and its various generalizations and contributes to the literature of so-called "absolute" Diophantine results with respect to height.

### Where

Millikan 2099, Pomona College

### Misc. Information

Organizational meeting in the same room preceding the talk at 12:00 noon.

## Applications of combinatorial representation theory to machine learning

### Speaker

Lily Silverstein (CGU)

### Abstract

Some machine learning problems are naturally modeled by probability distributions (or other data) defined over a finite group. In this case the generalized Fourier transform, based on irreducible group representations, is a useful tool for designing efficient algorithms. In the case of the symmetric group, we can use combinatorial objects like Young diagrams to define an analogue to bandlimiting. Finally, I will talk about how certain probabilistic inferences can be performed directly in the Fourier domain, by considering the combinatorial decomposition of tensor products of representations. This talk is expository and based mainly on work done by Jonathan Huang and Risi Kondor.

### Where

Mudd Science Library 126, Pomona College

## Group Frames and Tests for Uniformity

### Speaker

Mike Orrison (HMC)

### Abstract

Group frames are special spanning sets of vectors in representations of finite groups that sometimes behave remarkably like orthogonal bases.  In this talk, I’ll explain why group frames are beginning to play a prominent role in the work I have been doing with Anna Bargagliotti on linear tests of uniformity for probability distributions defined on finite sets with symmetry.  Along the way, I’ll try to convince you that group frames should be a part of the next linear algebra course you teach!

### Where

Mudd Science Library 126, Pomona College

## Representations of Algebraic Groups

### 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

Mudd Science Library 126, Pomona College

## A local-global principle in the dynamics of polynomial maps

### Speaker

David Krumm (CMC)

### Abstract

Let $K$ be a number field and let $f \in K[x]$ be a polynomial. For any nonnegative integer $n$, let $f^n$ denote the $n$-fold composition of $f$ with itself. If $\tilde K$ is a field containing $K$, we say that an element $\alpha \in \tilde K$ is periodic for $f$ if there exists a positive integer $n$ such that $f^n(\alpha)=\alpha$. In that case, the least such $n$ is called the period of $\alpha$. It is clear that if $f$ has a point of period $n$ in $K$, then it has a point of period $n$ in any extension of $K$; in particular, for every finite place $v$ of $K$, $f$ has a point of period $n$ in the completion $K_v$. In this talk we will discuss whether the converse holds: if $f$ has a point of period $n$ in every nonarchimedean completion of $K$, must it then have a point of period $n$ in $K$?

### Where

Mudd Science Library 126, Pomona College

## The Fundamental Theorem of Perfect Simulation

Mark Huber (CMC)

### Abstract

In an induction, a problem is reduced recursively to a base case. In perfect simulation, there is no base case. Instead, a problem is randomly reduced to one of two problems, one of which is the original problem! The Fundamental Theorem of Perfect Simulation says that as long as the chance that the reduction does not require the original problem is greater than zero, then such a procedure terminates with probability 1 in finite time. In this talk, I'll explore and prove this fascinating theorem, and consider multiple ways in which it can be used to draw samples from one of the cornerstone models of statistical physics, the Ising model.

### Where

Mudd Science Library 126, Pomona College

## Ribbon biquandles and knotted surfaces

Sam Nelson (CMC)

### Abstract

Knotted surfaces can be represented with diagrams known as ch-diagrams where two diagrams represent ambient isotopic knotted surfaces iff they are related by a sequence of Yoshikawa moves. Recently Kauffman introduced a generalization of knotted surfaces by adding virtual crossings to ch-diagrams. In this talk we will define invariants of knotted and virtual knotted surfaces using algebraic objects known as ribbon biquandles.

### Where

Mudd Science Library 126, Pomona College

## Lattices from the Hermitian function field

### Speaker

Hiren Maharaj (CMC)

### Abstract

In recent joint work with Albrecht Boettcher, Lenny Fukshansky and Stephan Garcia on the Rosenbloom-Tsfasman function field lattices from the Hermitian function field we showed that these lattices are generated by their minimal vectors. We also have a lower bound on the total number of minimal vectors, and some properties of the automorphism groups of these lattices. I will talk about these results.

### Where

Mudd Science Library 126, Pomona College

## Very neat character sums: proof of the conjecture of Dobbertin, Helleseth, Kumar, and Martinsen

### Speaker

Daniel Katz (Cal State Northridge)

### Abstract

We consider Weil sums, which are obtained by summing the values of a finite field character with a polynomial argument. These are used to count points in varieties over finite fields, as well as to compute the correlation properties of sequences that arise in information theory and cryptography. Our Weil sums involve additive characters with binomial arguments. As we vary the coefficients of the binomial, we obtain a spectrum of values, which depends on the finite field used and the exponents that appear in the binomial. Some rare choices give very elegant spectra that attain only three distinct values as the coefficients are varied through the field (two values occur only in uninteresting degenerate cases). Ten infinite families of three-valued Weil sums have been found from 1966 to 2013. Numerical evidence was found by Dobbertin, Helleseth, Kumar, and Martinsen for one further inifnite family, which they conjectured (in 2001) to exist. This talk is about the proof of their conjecture. The methods employed are diverse: algebraic number theory, graph theory, trilinear forms, character sums, and counting points on curves. This is joint work with Philippe Langevin.

### Where

Mudd Science Library 126, Pomona College