Tree pushdown automata (Q1058866)

The paper presents a new type of automata called tree pushdown automata. This type of automata operates in the same manner as standard bottom-up tree automata except that there is an internal memory consisting of a finite sequence of trees (called tree stack), where there is exactly one tree stack for each read head in a bottom-up tree automaton. The main result of the paper is: The class of context-free tree languages is identical with the class of languages accepted by tree pushdown automata. The method shown resemble the methods used to show the equivalence between context-free string grammars and pushdown automata. Some directions of future research are presented at the end of the paper.