## Seminars and Colloquia by Series

### Algebraic and Computational Aspects of Tensors

Series
Algebra Seminar
Time
Monday, March 27, 2017 - 16:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Ke YeUniversity of Chicago
Abstract: Tensors are direct generalizations of matrices. They appear in almost every branch of mathematics and engineering. Three of the most important problems about tensors are: 1) compute the rank of a tensor 2) decompose a tensor into a sum of rank one tensors 3) Comon’s conjecture for symmetric tensors. In this talk, I will try to convince the audience that algebra can be used to study tensors. Examples for this purpose include structured matrix decomposition problem, bilinear complexity problem, tensor networks states, Hankel tensors and tensor eigenvalue problems. In these examples, I will explain how algebraic tools are used to answer the three problems mentioned above.

### Sparse Multivariate Rational Function Model Discovery

Series
Algebra Seminar
Time
Friday, March 17, 2017 - 11:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Erich KaltofenNorth Carolina State University
Error-correcting decoding is generalized to multivariate sparse polynomial and rational function interpolation from evaluations that can be numerically inaccurate and where several evaluations can have severe errors (outliers''). Our multivariate polynomial and rational function interpolation algorithm combines Zippel's symbolic sparse polynomial interpolation technique [Ph.D. Thesis MIT 1979] with the numeric algorithm by Kaltofen, Yang, and Zhi [Proc. SNC 2007], and removes outliers (cleans up data'') by techniques from the Welch/Berlekamp decoder for Reed-Solomon codes. Our algorithms can build a sparse function model from a number of evaluations that is linear in the sparsity of the model, assuming that there are a constant number of ouliers and that the function probes can be randomly chosen.

### Low-rank approximations of binary forms

Series
Algebra Seminar
Time
Monday, March 13, 2017 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Hwangrae LeeAuburn University
For a given generic form, the problem of finding the nearest rank-one form with respect to the Bombieri norm is well-studied and completely solved for binary forms. Nonetheless, higher-rank approximation is quite mysterious except in the quadratic case. In this talk we will discuss such problems in the binary case.

### A Brill-Noether theorem for curves of a fixed gonality

Series
Algebra Seminar
Time
Monday, March 6, 2017 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Dhruv RaganathanIAS
The Brill-Noether varieties of a curve C parametrize embeddings of C of prescribed degree into a projective space of prescribed dimension. When C is general in moduli, these varieties are well understood: they are smooth, irreducible, and have the “expected” dimension. As one ventures deeper into the moduli space of curves, past the locus of general curves, these varieties exhibit intricate, even pathological, behaviour: they can be highly singular and their dimensions are unknown. A first measure of the failure of a curve to be general is its gonality. I will present a generalization of the Brill—Noether theorem, which determines the dimensions of the Brill—Noether varieties on a general curve of fixed gonality, i.e. “general inside a chosen special locus". The proof blends a study of Berkovich skeletons of maps from curves to toric varieties with tropical linear series theory. The deformation theory of logarithmic stable maps acts as the bridge between these ideas. This is joint work with Dave Jensen.

### Structured matrix computations via algebra

Series
Algebra Seminar
Time
Friday, March 3, 2017 - 11:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Lek-Heng LimUniversity of Chicago
We show that in many instances, at the heart of a problem in numerical computation sits a special 3-tensor, the structure tensor of the problem that uniquely determines its underlying algebraic structure. In matrix computations, a decomposition of the structure tensor into rank-1 terms gives an explicit algorithm for solving the problem, its tensor rank gives the speed of the fastest possible algorithm, and its nuclear norm gives the numerical stability of the stablest algorithm. We will determine the fastest algorithms for the basic operation underlying Krylov subspace methods --- the structured matrix-vector products for sparse, banded, triangular, symmetric, circulant, Toeplitz, Hankel, Toeplitz-plus-Hankel, BTTB matrices --- by analyzing their structure tensors. Our method is a vast generalization of the Cohn--Umans method, allowing for arbitrary bilinear operations in place of matrix-matrix product, and arbitrary algebras in place of group algebras. This talk contains joint work with Ke Ye and joint work Shmuel Friedland.

### Factorizations of PSD Matrix Polynomials and their Smith Normal Forms

Series
Algebra Seminar
Time
Monday, February 20, 2017 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Christoph HanselkaUniversity of Auckland
It is well-known, that any univariate polynomial matrix A over the complex numbers that takes only positive semidefinite values on the real line, can be factored as A=B^*B for a polynomial square matrix B. For real A, in general, one cannot choose B to be also a real square matrix. However, if A is of size nxn, then a factorization A=B^tB exists, where B is a real rectangular matrix of size (n+1)xn. We will see, how these correspond to the factorizations of the Smith normal form of A, an invariant not usually associated with symmetric matrices in their role as quadratic forms. A consequence is, that the factorizations canusually be easily counted, which in turn has an interesting application to minimal length sums of squares of linear forms on varieties of minimal degree.

### Galois action on homology of Fermat curves

Series
Algebra Seminar
Time
Monday, January 9, 2017 - 15:05 for 1 hour (actually 50 minutes)
Location
Sklles 005
Speaker
We prove a result about the Galois module structure of the Fermat curve using commutative algebra, number theory, and algebraic topology. Specifically, we extend work of Anderson about the action of the absolute Galois group of a cyclotomic field on a relative homology group of the Fermat curve. By finding explicit formulae for this action, we determine the maps between several Galois cohomology groups which arise in connection with obstructions for rational points on the generalized Jacobian. Heisenberg extensions play a key role in the result. This is joint work with R. Davis, V. Stojanoska, and K. Wickelgren.

### Conductors and minimal discriminants of hyperelliptic curves with rational Weierstrass points

Series
Algebra Seminar
Time
Monday, December 5, 2016 - 16:15 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Conductors and minimal discriminants are two measures of degeneracy of the singular fiber in a family of hyperelliptic curves. In the case of elliptic curves, the Ogg-Saito formula shows that (the negative of) the Artin conductor equals the minimal discriminant. In the case of genus two curves, equality no longer holds in general, but the two invariants are related by an inequality. We investigate the relation between these two invariants for hyperelliptic curves of arbitrary genus.

### Scheme theoretic tropicalization

Series
Algebra Seminar
Time
Monday, November 28, 2016 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Oliver LorscheidIMPA
Recent work of Jeff and Noah Giansiracusa exhibits a scheme theoretic structure for tropicalizations of classical varieties in terms of so-called semiring schemes. This works well in the framework of closed subvarieties of toric varieties, and Maclagan and Rincon recover the structure of a weighted polyhedral complex from the scheme theoretic tropicalization of a variety embedded into a torus.In this talk, I will review these ideas and show how these results can be extended by using blue schemes. This leads to an intrinsic notion of a tropicalization, independent from an embedding into an ambient space, and generalizes the above mentioned results to the broader context of log-schemes.

### Symmetry groupoids and weighted signatures of geometric objects

Series
Algebra Seminar
Time
Monday, November 14, 2016 - 15:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Peter OlverUniversity of Minnesota
In this talk, I will refine the concept of the symmetry group of a geometric object through its symmetry groupoid, which incorporates both global and local symmetries in a common framework. The symmetry groupoid is related to the weighted differential invariant signature of a submanifold, that is introduced to capture its fine grain equivalence and symmetry properties. The groupoid/signature approach will be connected to recent developments in signature-based recognition and symmetry detection of objects in digital images, including jigsaw puzzle assembly.