Relations enumerable from positive information

Abstract

We study countable structures from the viewpoint of enumeration reducibility. Since enumeration reducibility is based on only positive information, in this setting it is natural to consider structures given by their positive atomic diagram—the computable join of all relations of the structure. Fixing a structure A, a natural class of relations in this setting are the relations R such that R Aˆ is enumeration reducible to the positive atomic diagram of Aˆ for every Aˆ ∼= A – the relatively intrinsically positively enumerable (r.i.p.e.) relations. We show that the r.i.p.e. relations are exactly the relations that are definable by Σp1 formulas, a subclass of the infinitary Σ01 formulas. We then introduce a new natural notion of the jump of a structure and study its interaction with other notions of jumps.