# Ramsey Quantifiers and PA($Q^2$)

Models of PAWednesday, September 21, 20166:30 pmGC 4214-03

# Ramsey Quantifiers and PA($Q^2$)

We can extend the language of first order logic to add in a new quantifier, $Q^2$, which binds two free variables. The intended interpretation of $Q^2 x,y\phi(x, y)$ is “There is an infinite (unbounded) set $X$ such that $\phi(x, y)$ holds for each $x \neq y \in X.$” The theory PA($Q^2$) is the theory of Peano Arithmetic in this augmented language (asserting that induction holds for all formulas, including with Ramsey quantifier) and can be thought of as a second order theory whose models are of the form $(M, \mathfrak{X})$ where $\mathfrak{X} \subseteq P(M)$. In this talk, I will present a few results due to Macintyre (1980) and Schmerl & Simpson (1982), namely that models of PA($Q^2$) correspond to models of the second order system $\Pi_1^1-CA_0$. If there is time, I will present Macintyre’s proof that so-called “strong” models of this theory correspond to $\kappa$-like models for some regular $\kappa$.