Constraints on Extensions of a Default Theory

In this paper, the representability of a family of theories as the setof extensions of a default theory is studied. First, a new necessarycondition is given for the representability by means of general defaulttheories; then a sufficient one is presented. The families of theoriesrepresented by default theories are also fully characterized. Finally,the paper gives an example of denumerable families of mutuallyinconsistent theories that are represented by a default theory butnot by normal ones.