Published online by Cambridge University Press: 21 July 2020
We characterize the determinacy of $F_\sigma $ games of length
$\omega ^2$ in terms of determinacy assertions for short games. Specifically, we show that
$F_\sigma $ games of length
$\omega ^2$ are determined if, and only if, there is a transitive model of
${\mathsf {KP}}+{\mathsf {AD}}$ containing
$\mathbb {R}$ and reflecting
$\Pi _1$ facts about the next admissible set.
As a consequence, one obtains that, over the base theory ${\mathsf {KP}} + {\mathsf {DC}} + ``\mathbb {R}$ exists,” determinacy for
$F_\sigma $ games of length
$\omega ^2$ is stronger than
${\mathsf {AD}}$, but weaker than
${\mathsf {AD}} + \Sigma _1$-separation.