A generalization of Solovay's $\Sigma$-construction with application to intermediate models

Abstract

A $\Sigma$-construction of Solovay is extended to the case of intermediate sets which are not necessarily subsets of the ground model, with a more transparent description of the resulting forcing notion than in the classical paper of Grigorieff. As an application, we prove that, for a given name $t$ (not necessarily a name of a subset of the ground model), the set of all sets of the form $t[G]$ (the $G$-interpretation of $t$), $G$ being generic over the ground model, is Borel. This result was first established by Zapletal by a descriptive set theoretic argument.