Highly Undecidable Problems For Infinite Computations

Equipe de Logique Mathématique, CNRS et Université Paris 7, France; finkel@logique.jussieu.fr

Received: 23 January 2008
Accepted: 13 November 2008

Abstract

We show that many classical decision problems about 1-counter ω-languages, context free ω-languages, or infinitary rational relations, are Π½ -complete, hence located at the second level of the analytical hierarchy, and “highly undecidable”. In particular, the universality problem, the inclusion problem, the equivalence problem, the determinizability problem, the complementability problem, and the unambiguity problem are all Π½ -complete for context-free ω-languages or for infinitary rational relations. Topological and arithmetical properties of 1-counter ω-languages, context free ω-languages, or infinitary rational relations, are also highly undecidable. These very surprising results provide the first examples of highly undecidable problems about the behaviour of very simple finite machines like 1-counter automata or 2-tape automata.