[full paper] |

**Equivalent Sets of Formulas for Circumscriptions**

*Yves Moinard, Raymond Rolland*Circumscription is a way of using classical logic in order to modelize rules with exceptions and implicit knowledge. Formula circumscription is easier to use in order to modelize a given situation. We describe when two sets of formulas give the same result when circumscribed, introducing two kinds of equivalence. For ordinary equivalence, the two sets give the same circumscription, and for the strong equivalence, when completed by any arbitrary set, the two sets give the same circumscription. The strong equivalence corresponds simply to having the same closure for logical ``and'' and ``or''. For the ordinary equivalence, there exists also always a greatest set. Our answer to these two equivalence problems for the case of propositional formula circumscription is exhaustive. This gives rise to various notions of formulas positive with respect to a given set of formulas. When starting from ordinary propositional circumscription, things remain simple enough, and we provide a syntactical description of all these equivalent sets, even in the infinite case.

*Keywords:*Nonmonotonic reasoning, Knowledge representation, Automatic reasoning

*Citation:* Yves Moinard, Raymond Rolland: Equivalent Sets of Formulas for Circumscriptions. In W.Horn (ed.): ECAI2000, Proceedings of the 14th European Conference on Artificial Intelligence, IOS Press, Amsterdam, 2000, pp.479-483.

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