Decision problems among the main subfamilies of rational relations

We consider the four families of recognizable, synchronous, deterministic rational and rational subsets of a direct product of free monoids. They form a strict hierarchy and we investigate the following decision problem: given a relation in one of the family, does it belong to a smaller family? We settle the problem entirely when all monoids have a unique generator and fill some gaps in the general case. In particular, adapting a proof of Stearns, we show that it is recursively decidable whether or not a deterministic subset of an arbitrary number of free monoids is recognizable. Also we exhibit a single exponential algorithm for determining if a synchronous relation is recognizable.

• Type: Journal
• Year: 2006
• Where: ITA
• Authors: Olivier Carton, Christian Choffrut, Serge Grigorieff
• Keywords: Communications, Free Monoids, ITA 2006, Rational Subsets, Strict Hierarchy