Abstract

We prove the following results: every recursively enumerable set approximated by finite sets of some set M of recursively enumerable sets with index set in π 2 is an element of M , provided that the finite sets in M are canonically enumerable. If both the finite sets in M and in M̄ are canonically enumerable, then the index set of M is in σ 2 ∩ π 2 if and only if M consists exactly of the sets approximated by finite sets of M and the complement M̄ consists exactly of the sets approximated by finite sets of M̄ . Under the same condition M or M̄ has a non-empty subset with recursively enumerable index set, if the index set of M is in σ 2 ∩ π 2 . If the finite sets in M are canonically enumerable, then the following three statements are equivalent: the index set of M is in σ 2 \ π 2 , the index set of M is σ 2 -complete, the index set of M is in σ 2 and some sequence of finite sets in M approximate a set in M̄ . Finally, for every n ⩾ 2, an index set in σ n \ π n is presented which is not σ n -complete