title: Post's program and incomplete recursively enumerable sets.
abstract: A set A of nonnegative integers is recursively enumerable (r.e.) if A can be computably listed. It is shown that there is a first-order property, Q(X), definable in E, the lattice of r.e. sets under inclusion, such that (i) if A is any r.e. set satisfying Q(A) then A is nonrecursive and Turing incomplete and (ii) there exists an r.e. set A satisfying Q(A). This resolves a long open question stemming from Post's program of 1944, and it sheds light on the fundamental problem of the relationship between the algebraic structure of an r.e. set A and the (Turing) degree of information that A encodes.
authors.affiliation: Department of Mathematics, University of California, Berkeley, CA 94720, USA.
authors.forename: L
authors.initials: L
authors.lastname: Harrington
authors.forename: R I
authors.initials: RI
authors.lastname: Soare