Published online by Cambridge University Press: 08 January 2021
For every countable wellordering $\alpha $ greater than
$\omega $, it is shown that clopen determinacy for games of length
$\alpha $ with moves in
$\mathbb {N}$ is equivalent to determinacy for a class of shorter games, but with more complicated payoff. In particular, it is shown that clopen determinacy for games of length
$\omega ^2$ is equivalent to
$\sigma $-projective determinacy for games of length
$\omega $ and that clopen determinacy for games of length
$\omega ^3$ is equivalent to determinacy for games of length
$\omega ^2$ in the smallest
$\sigma $-algebra on
$\mathbb {R}$ containing all open sets and closed under the real game quantifier.