チューリング次数
The Turing degree 0‘, which is the degree of the halting problem, is a PA degree. There are also PA degrees that are not above 0‘. For example, the low basis theorem implies that there is a low PA degree. On the other hand, Antonín Kučera has proved that there is a degree less than 0‘ that computes a DNR function but is not a PA degree (Jockusch 1989:197).