The Independence of Inherent Ambiguity From Complementedness Among Context-Free Languages
Thomas Nathaniel Hibbard, Joseph S. Ullian · Journal of the ACM · 1966
Call a (context-free) language unambiguous if it is not inherently ambiguous. In the absence of evidence to the contrary, the suspicion has arisen that the unambiguous languages might be precisely those languages with context-free complements. The two theorems presented in this paper lay the suspicion to rest by providing (1) an inherently ambiguous language with context-free complement and (2) an unambiguous language without context-free complement. This establishes the independence of inherent ambiguity from complementedness among the context-free languages.