A Uniformization Theorem for Nested Word to Word Transductions

We study the class of relations implemented by nested word to word transducers (also known as visibly pushdown transducers). We show that any such relation can be uniformized by a functional relation from the same class, implemented by an unambiguous transducer. We give an exponential upper bound on the state complexity of the uniformization, improving a previous doubly exponential upper bound. Our construction generalizes a classical construction by Schützenberger for the disambiguation of nondeterministic finite-state automata, using determinization and summarization constructions on nested word automata. Besides theoretical interest, our procedure can be the basis for synthesis procedures for nested word to word transductions.