Coding along trees and remarkable cardinals

Speaker: 

Zach Norwood

Institution: 

UCLA

Time: 

Monday, February 12, 2018 - 4:00pm to 5:30pm

Host: 

Location: 

RH 440R

A major project in set theory aims to explore the connection between large cardinals and so-called generic absoluteness principles, which assert that forcing notions from a certain class cannot change the truth value of (projective, for instance) statements about the real numbers. For example, in the 80s Kunen showed that absoluteness to ccc forcing extensions is equiconsistent with a weakly compact cardinal. More recently, Schindler showed that absoluteness to proper forcing extensions is equiconsistent with a remarkable cardinal. (Remarkable cardinals will be defined in the talk.) Schindler's proof does not resemble Kunen's, however, using almost-disjoint coding instead of Kunen's innovative method of coding along branchless trees. We show how to reconcile these two proofs, giving a new proof of Schindler's theorem that generalizes Kunen's methods and suggests further investigation of non-thin trees.

Adventures in Binary Quadratic Forms

Speaker: 

Vadim Ponomarenko

Institution: 

San Diego State University

Time: 

Thursday, May 24, 2018 - 3:00pm to 4:00pm

Location: 

RH 306

Since Fermat characterized (without proof) those integers represented by the quadratic form x^2+y^2, number theorists have been extending these results.  Recently a paper appeared in Journal of Number Theory answering the question for x^2 ± xy ± y^2.  It turns out that this was not news (although JNT refuses to correct or retract).  Naively, today's speaker extended these results, through elementary means.  This talk will outline these methods, and contrast them with more traditional techniques.

Choosing distinct points on cubic curves

Speaker: 

Weiyan Chen

Institution: 

University of Minnesota

Time: 

Tuesday, April 17, 2018 - 3:00pm to 4:00pm

Host: 

Location: 

RH 340P

It is a classical topic dating back to Maclaurin (1698–1746) to study certain special points on smooth cubic plane curves, such as the 9 inflection points (Maclaurin and Hesse), the 27 sextatic points (Cayley), and the 72 points "of type 9" (Gattazzo). Motivated by these algebro-geometric constructions, we ask the following topological question: is it possible to choose n distinct points on a smooth cubic plane curve as the curve varies continuously in family, for any integer n other than 9, 27 and 72? We will present both constructions and obstructions to such continuous choices of points, state a classification theorem for them, and discuss conjectures and open questions.

Braids, Polynomials and Hilbert's 13th Problem

Speaker: 

Jesse Wolfson

Institution: 

UC Irvine

Time: 

Thursday, March 1, 2018 - 4:00pm to 5:00pm

Location: 

RH 306

There are still completely open fundamental questions about polynomials in one variable. One example is Hilbert's 13th Problem, a conjecture going back long before Hilbert.  Indeed, the invention of algebraic topology grew out of an effort to understand how the roots of a polynomial depend on the coefficients. The goal of this talk is to explain part of the circle of ideas surrounding these questions. 

Along the way, we will encounter some beautiful classical objects - the space of monic, degree d square-free polynomials, algebraic functions, lines on cubic surfaces, level structures on Jacobians, braid groups, Galois groups, and configuration spaces - all intimately related to each other, all with mysteries still to reveal.

FALL 2017/2018 GRADUATE STUDENT MENTOR PROGRAM

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Learning seminar: An introduction to large cardinals and L

Speaker: 

Ryan Sullivant

Institution: 

UCI

Time: 

Monday, February 5, 2018 - 4:00pm to 5:30pm

Host: 

Location: 

RH 440R

In this talk, we will cover the basics of measurable cardinals and their relationship to non-trivial elementary embeddings.  We proceed with basic facts about the constructible universe, L.  After laying this groundwork, we show L cannot have a measurable cardinal.  Time permitting, we will discuss the dichotomy introduced by Jensen's covering lemma: either L is a good approximation to V, or there is a non-trivial elementary embedding from L to L.

 

Quasiflats in hierarchically hyperbolic spaces

Speaker: 

Jason Behrstock

Institution: 

CUNY

Time: 

Monday, April 2, 2018 - 4:00pm to 5:00pm

Host: 

Location: 

RH 340P

Hierarchically hyperbolic spaces provide a uniform framework for working with many important examples, including mapping class groups, right angled Artin groups, Teichmuller space, and others. In this talk I'll provide an introduction to studying groups and spaces from this point of view. This discussion will center around recent work in which we classify quasiflats in these spaces, thereby resolving a number of well-known questions and conjectures. This is joint work with Mark Hagen and Alessandro Sisto.

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