# Yet more forcing in arithmetic: life in a second-order world

Models of PAWednesday, September 14, 20166:30 pmGC 4214-03

# Yet more forcing in arithmetic: life in a second-order world

### The CUNY Graduate Center

In previous semesters of this seminar, I have talked about how the technique of forcing, originally developed by Cohen for building models of set theory, can be used to produce models of arithmetic with various properties. In this talk, I will present a forcing proof of Harrington’s theorem on the conservativity of $\mathsf{WKL}_0$ over $\mathsf{RCA}_0$. More formally, any countable model of $\mathsf{RCA}_0$ can be extended to a model of $\mathsf{WKL}_0$ with the same first-order part. As an immediate corollary, we get that any $\Pi^1_1$ sentence provable by $\mathsf{WKL}_0$ is already provable by $\mathsf{RCA}_0$.

Kameryn Williams is a graduate student in mathematics at the CUNY Graduate
Center, specializing in set theory and mathematical logic. He received a
bachelor’s degree in mathematics from Boise State University in 2012.

Posted by on August 29th, 2016