Paper detail

Three Applications to Rational Relations of the High Undecidability of the Infinite Post Correspondence Problem in a Regular omega-Language

It was noticed by Harel in [Har86] that "one can define $Σ_1^1$-complete versions of the well-known Post Correspondence Problem". We first give a complete proof of this result, showing that the infinite Post Correspondence Problem in a regular $ω$-language is $Σ_1^1$-complete, hence located beyond the arithmetical hierarchy and highly undecidable. We infer from this result that it is $Π_1^1$-complete to determine whether two given infinitary rational relations are disjoint. Then we prove that there is an amazing gap between two decision problems about $ω$-rational functions realized by finite state Büchi transducers. Indeed Prieur proved in [Pri01, Pri02] that it is decidable whether a given $ω$-rational function is continuous, while we show here that it is $Σ_1^1$-complete to determine whether a given $ω$-rational function has at least one point of continuity. Next we prove that it is $Π_1^1$-complete to determine whether the continuity set of a given $ω$-rational function is $ω$-regular. This gives the exact complexity of two problems which were shown to be undecidable in [CFS08].