On the Boolean Closure of Deterministic Top-Down Tree Automata

The class of Boolean combinations of tree languages recognized by deterministic top-down tree automata (also known as deterministic root-to-frontier automata) is studied. The problem of determining for a given regular tree language whether it belongs to this class is open. We provide some progress by two results: First, a characterization of this class by a natural extension of deterministic top-down tree automata is presented, and as an application we obtain a convenient method to show that certain regular tree languages are outside this class. In the second result, it is shown that, for fixed [Formula: see text], it is decidable whether a regular tree language is a Boolean combination of [Formula: see text] tree languages recognized by deterministic top-down tree automata.