1-Bounded TWA cannot be determinized

Abstract

Tree-walking automata are a natural sequential model for recognizing tree languages which is closely connected to XML. Two long standing conjectures say TWA cannot recognize all regular languages and that deterministic TWA are weaker than nondeterministic TWA. We consider a weaker model, 1-TWA, of tree walking automata that can make only one pass through a tree. We show that deterministic 1-TWA are weaker than 1-TWA. We also show a very simple language not recognized by 1-TWA. © Springer-Verlag Berlin Heidelberg 2003.