Forcing axioms via ground model interpretations
Philipp Schlicht, Christopher Turner · Bristol Research (University of Bristol) · 2021
We study principles of the form: if a name $σ$ is forced to have a certain property $φ$, then there is a ground model filter $g$ such that $σ^g$ satisfies $φ$. We prove a general correspondence connecting these name principles to forcing axioms. Special cases of the main theorem are: Any forcing axiom can be expressed as a name principle. For instance, $\mathsf{PFA}$ is equivalent to a principle for rank $1$ names (equivalently, nice names) for subsets of $ω_1$, and a principle for rank $2$ names for sets of reals. Moreover, $λ$-bounded forcing axioms are equivalent to name principles. Bagaria's characterisation of $\mathsf{BFA}$ via generic absoluteness is a corollary. We further systematically study name principles where $φ$ is a notion of largeness for subsets of $ω_1$ (such as being unbounded, stationary or in the club filter) and corresponding forcing axioms.