In this paper we study the admissible rules of intermediate logics. We establish some general results on extensions of models and sets of formulas. These general results are then employed to provide a basis for the admissible rules of the Gabbay–de Jongh logics and to show that these logics have finitary unification type.

Goudsmit & Iemhoff () On unification and admissible rules in Gabbay–de Jongh logics. Annals of Pure and Applied Logic, 165(2):652-672 doi: 10.1016/j.apal.2013.09.003