The Existential Fragment of Third Order Logic and Third Order Relational Machines

Abstract

We introduce a new sub logic of third order logic (TO), the logic TOϖ, as a semantic restriction of TO. We focus on the existential fragment of TOϖ which we denote Σ²ϖ , and we study its relational complexity by introducing a variation of the non deterministic relational machine, which we denote 3-NRM, where we allow third order relations in the relational store of the machine.We then prove that Σ²ϖ characterizes exactly NEXPTIME3,r