FINITELY EXPANDABLE DEEP PDAs

As special cases of deep pushdown automata, we discuss finitely expandable deep pushdown automata that always contain a bounded number of non-input symbols in their pushdown stores. Based on these automata, it establishes an infinite hierarchy of language families that coincides with the hierarchy resulting from matrix grammars of finite index. Thus finitely expandable deep pushdown automata can are an automaton counterpart to these grammars. It also follows that deleting transitions do not add more power to finitely expandable deep pushdown automata. In a final section, we suggest some open problems.