On Recognizable Tree Languages Beyond the Borel Hierarchy

Abstract

We investigate the topological complexity of non Borel recognizable tree languages with regard to the difference hierarchy of analytic sets. We show that, for each integer $n\ge 1$, there is a $D_{{\omega }^n}({\mathbf{\Sigma }}_1^1)$-complete tree language L_n accepted by a (non deterministic) Muller tree automaton. On the other hand, we prove that a tree language accepted by an unambiguous Büchi tree automaton must be Borel. Then we consider the game tree languages $W_{(i,k)}$, for Mostowski-Rabin indices $(i,k)$. We prove that the $D_{{\omega }^n}({\mathbf{\Sigma }}_1^1)$-complete tree languages L_n are Wadge reducible to the game tree language $W_{(i,k)}$ for $k-i\ge 2$. In particular these languages $W_{(i,k)}$ are not in any class $D_{\alpha }({\mathbf{\Sigma }}_1^1)$ for $\alpha <{\omega }^{\omega }$.

To appear in Fundamenta Informaticae