Löwenheim–Skolem theorems for non-classical first-order algebraizable logics

This article is a contribution to the model theory of non-classical first-order predicate logics. In a wide framework of first-order systems based on algebraizable logics, we study several notions of homomorphisms between models and find suitable definitions of elementary homomorphism, elementary substructure and elementary equivalence. Then we obtain (downward and upward) Lowenheim-Skolem theorems for these non-classical logics, by direct proofs and by describing their models as classical two-sorted models.