Post's theorem
Post's theorem characterizes the arithmetical hierarchy using Turing jumps: a set is at level [0mΣ⁰ₙ exactly when it is computably enumerable relative to the (n−1)st jump of the empty set, and at level Π⁰ₙ when its complement is.
Post's theorem characterizes the arithmetical hierarchy using Turing jumps: a set is at level [0mΣ⁰ₙ exactly when it is computably enumerable relative to the (n−1)st jump of the empty set, and at level Π⁰ₙ when its complement is.