**CUNY Graduate Center**

**Room 4214.03**

**Wednesdays 6:30pm-8pm**

**Organized by Roman Kossak**

**Calendar**

**February 27**

**Corey Switzer**,
CUNY

**Sets at arm's length**

**Abstract**

We will present the following striking theorem, due to Towsner: If $(M, \mathcal X) \models \mathsf{RCA}_0 + I\Sigma_n$ is countable, then for *any* (!!!) $W \subseteq M$ there is a countable expansion with the same first order part, $(M, \mathcal Y)$, where $\mathcal X \subseteq \mathcal Y$ so that $(M, \mathcal Y) \models \mathsf{RCA}_0 + I\Sigma_n$ and the set $W$ is $\Delta_{n+1}$ definable. Time permitting, we will then sketch some nice applications of this theorem to the study of conservative extensions of fragments of arithmetic.

**February 6**

**Whan Ki Lee**,
CUNY

**$\kappa$-like models**

**Abstract**

This is a continuation of a talk from last semester.

A model $(M, < ,\ldots)$ is said to be $\kappa$-like if $|M| = \kappa$ but for all $a \in M$, $|\{x \in M \mid x < a\}| < \kappa$. Based on the paper, the theory of $\kappa$-like models of arithmetic by R. Kaye, we will identify some axiom schemes true in such models of $I\Delta_0$ and investigate their interesting properties.

**Previous Semesters**