チューリング次数
In computability theory, a Turing degree ["X"] is low if the Turing jump ["X"′] is 0′. A set is low if it has low degree. Since every set is computable from its jump, any low set is computable in 0′, but the jump of sets computable in 0′ can bound any degree r.e. in 0′ (Schoenfield Jump Inversion). "X" being low says that its jump "X"′ has the least possible degree in terms of Turing reducibility for the jump of a set.