Computer Science – Artificial Intelligence
Scientific paper
2011-09-21
Computer Science
Artificial Intelligence
Scientific paper
In knowledge bases expressed in default logic, outliers are sets of literals, or observations, that feature unexpected properties. This paper introduces the notion of strong outliers and studies the complexity problems related to outlier recognition in the fragment of acyclic normal unary theories and the related one of mixed unary theories. We show that recognizing strong outliers in acyclic normal unary theories can be done in polynomial time. Moreover, we show that this result is sharp, since switching to either general outliers, cyclic theories or acyclic mixed unary theories makes the problem intractable. This is the only fragment of default theories known so far for which the general outlier recognition problem is tractable. Based on these results, we have designed a polynomial time algorithm for enumerating all strong outliers of bounded size in an acyclic normal unary default theory. These tractability results rely on the Incremental Lemma, an interesting result on its own, that provides conditions under which a mixed unary default theory displays a monotonic reasoning behavior.
Angiulli Fabrizio
Ben-Eliyahu-Zohary R.
Palopoli Luigi
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