Counting in trees for free

It is known that MSO logic for ordered unranked trees is undecidable if Presburger constraints are allowed at children of nodes. We show here that a decidable logic is obtained if we use a modal fixpoint logic instead. We present a characteriza-tion of this logic by means of deterministic Presburger tree automata and show how it can be used to express numerical document queries. Surprisingly, the complexity of satisfiability for the extended logic is asymptotically the same as for the original fixpoint logic. The non-emptiness for Presburger tree automata (PTA) is PSPACE-complete, which is moderate given that it is already PSPACE-hard to test whether the complement of a regular expression is non-empty. We also identify a subclass of PTAs with a tractable non-emptiness problem. Further, to decide whether a tree t satisfies a formula ' is polynomial in the size of ' and linear in the size of t. A technical construction of independent interest is a linear time construction of a Presburger formula for the Parikh image of a regular language