[bibtex key=HamkinsLinetskyMiller2007:ComplexityOfQuicklyDecidableORMSets]

The Ordinal Register Machine (ORM) is one of several different machine models for infinitary computability. We classify, by complexity, the sets that can be decided quickly by ORMs. In particular, we show that the arithmetical sets are exactly those sets that can be decided by ORMs in times uniformly less than ${\omega }^{\omega }$. Further, we show that the hyperarithmetical sets are exactly those sets that can be decided by an ORM in time uniformly less than ${\omega }_1^{CK}$.