On the Hierarchy of ¢ 0 -Real Numbers
Xizhong Zheng, Theoretische Informatik · 2004
A real number x is called ∆0 2 if its binary expansion corresponds to a ∆02-set of natural numbers. Such reals are just the limits of computable sequences of rational numbers and hence also called computably approximable. Depending on how fast the sequences converge, ∆0 2-reals have different levels of effectiveness. This leads to various hierarchies of ∆0 2 reals. In this paper we summarize several recent developments related to such kind of hierarchies.