Abstract. We show that many classical decision problems about 1-counter ω-languages, context free ω-languages, or infinitary rational relations, are Π 1 2 -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 Π 1 2 -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.
Some completeness results for low levels of the analytical hierarchy for decision problems on infinitary rational relations were shown in REF .
Highly Undecidable Problems For Infinite Computations
RAIRO - Theoretical Informatics and Applications 43, 2 (2009) 339-364
Mathematics Computer Science