Two-Way Automata with More than one Storage Medium

Abstract This paper is concerned with the computational power of two-way automata with more than one subrecursive storage medium. Two-way automata with a stack (a nonerasing stack or a pushdown store, respectively) and an arbitrary number of checking stacks are of special interest. They are able to accept exactly those sets which are elementary in the sense of Kalmar . If the number of checking stacks is fixed, then the computational power of the corresponding restricted classes of automata can also be characterized in terms of time and space complexity classes.