G-automata, counter languages and the Chomsky hierarchy

Abstract

We consider how the languages of $G$-automata compare with other formal language classes. We prove that if the word problem of a group $G$ is accepted by a machine in the class $\mathcal M$ then the language of any $G$-automaton is in the class $\mathcal M$. It follows that the so called {\emph counter languages} (languages of $\mathbb Z^n$-automata) are context-sensitive, and further that counter languages are indexed if and only if the word problem for $\mathbb Z^n$ is indexed.