On Undecidability of Non-monotonic Logic
Marek A. Suchenek · 2006
Abstract. The degree of undecidability of nonmonotonic logic is investigated. A proof is provided that arithmetical but not recursively enumerable sets of sentences definable by nonmonotonic default logic are elements of Δn+1 but not Σn nor Πn for some n ≥ 1 in Kleene-Mostowski hierarchy of arithmetical sets.