Model-theoretic properties of the m-c.e. degrees

Abstract

In this article, we investigate model-theoretic properties of various Turing degree structures in the hierarchy of Δ 0 2-sets which is well known in the literature as Ershov Hierarchy. In particular, questions of definability of m-c.e. degrees in the structure of n-c.e. degrees, where m<n, elementary equivalences, extendability of partial order extensions are considered. © 2010 The Author. Published by Oxford University Press. All rights reserved.