Structural Logic and Abstract Elementary Classes with Intersection

Abstract

We give a syntactic characterization of abstract elementary classes (AECs) closed under intersections using a new logic with a quantifier for isomorphism types that we call structural logic: we prove that AECs with intersections correspond to classes of models of a universal theory in structural logic. This generalizes Tarski's syntactic characterization of universal classes. As a corollary, we obtain that any AEC with countable L\"owenheim-Skolem number is axiomatizable in $\mathbb{L}_{\infty, \omega} (Q)$, where $Q$ is the quantifier "there exists uncountably many".