Negation by Default

In this paper, we propose a new semantic framework for disjunctive logic programs by introducing the so called static expansions which extend to disjunctive programs the notions of stable, well-founded and stationary (or partial stable) models while at the same time generalizing the minimal model semantics of positive disjunctive programs. For every disjunctive program P; we de ne rst order extensions of P; called static expansions of P . Any static expansion of P provides the meaning or semantics for P by determining the set of true sentences about P . We show that among all static expansions of a disjunctive program there is always the least static expansion and we call it the static completion P of P . The static completion P is constructively de ned as a xed point of a natural minimal model operator and can be iteratively computed. The semantics de ned by the static completion P is called the static semantics of P . The static semantics always coincides with the set of sentences that are true in all static expansions of P . The class of static expansions represents a semantic framework which di ers signi cantly from the other semantics proposed recently for disjunctive programs and databases. It is also de ned for a much broader class of programs.