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Upcoming talks and events:
Height and Happiness
Non-rigid designators play a major role in modal logics, when applied to aspects of natural language. Not only constants can be non-rigid, but also function symbols, and this is sometimes essential. I will discuss some examples, formalize them, and provide a semantics. If time permits, I will also present a nested sequent proof system to go with all this.
Introduction to remarkable cardinals
Ralf Schindler introduced remarkable cardinals because he discovered that they are precisely equiconsistent with the statement that the theory of $L(\mathbb R)$ is absolute for proper forcing. The statement that the theory of $L(\mathbb R)$ is absolute for all set forcing is closely related to whether $L(\mathbb R)\models {\rm AD}$. In contrast, remarkable cardinals sit relatively low in the large cardinal hierarchy; for instance, they are downward absolute to $L$. I will discuss the various equivalent characterizations of remarkable cardinals due to Schindler and show where the remarkable cardinals fit into the large cardinal hierarchy using results due to Schindler, Philip Welch and myself.
An extended abstract can be found here.
Structurability and countable Borel equivalence relations
The dynamical and descriptive set theoretic complexity of a countable Borel equivalence relation E can often be understood in terms of the kinds of countable first order structures which are compatible with E in a suitable sense. In this talk I will make this suitable sense precise by discussing the notion of Borel structurability. I will also discuss some recent joint work with Brandon Seward in which we show that the equivalence relation generated by the free part of the translation action of a countable group G on its powerset is structurably-universal among equivalence relations generated by free Borel actions of G.
The Consistency of Peano Arithmetic
In 1936, only a few years after the incompleteness theorems were proved, Gentzen proved the consistency of Peano arithmetic by using transfinite induction up to the ordinal epsilon_0. I will give a short proof of the result, based on on the simplification introduced by Schutte, and discuss some of the consequences.
New England Recursion & Definability Seminar, Spring 2014
The spring NERDS meeting this year will take place on Sunday, March 30, 2014, from 11 am until 4 pm, at Olin College in Needham, MA, fifteen miles west of Boston. Directions are available here. Further details will be posted when available, including speakers and titles of talks. NERDS is a one-day meeting on computability theory which takes place twice a year, at assorted sites throughout New England.
