A Necessary Condition for the Rationality of the Zeta Function of a Regular Language

Abstract We show that if the zeta function of a regular language L is rational, then there exist cyclic languages L 1 and L 2 such that the generating function of L is the difference of the generating functions of L 1 and L 2 . We show also that it is decidable whether or not the zeta function of a given regular language is rational. If it is rational, it can be computed effectively.