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.