2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/33133Default logic can be regarded as a mechanism to represent families of belief sets of a reasoning agent. As such, it is inherently second-order. In this paper, we study the problem of representability of a family of theories as the set of extensions of a default theory. We give a complete solution to the representability by means of normal default theories. We obtain partial results on representability by arbitrary default theories. We construct examples of denumerable families of non-including theories that are not representable. We also study the concept of equivalence between default theories.Annals of Mathematics and Artificial Intelligence, 21 (1997), pp. 343-358Logic in Computer ScienceArtificial IntelligenceI.2.4, F.4.1, I.2.3text