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 families, 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.
@article{ITA_2006__40_2_255_0, author = {Carton, Olivier and Choffrut, Christian and Grigorieff, Serge}, title = {Decision problems among the main subfamilies of rational relations}, journal = {RAIRO - Theoretical Informatics and Applications - Informatique Th\'eorique et Applications}, volume = {40}, year = {2006}, pages = {255-275}, doi = {10.1051/ita:2006005}, mrnumber = {2252638}, zbl = {1112.03008}, language = {en}, url = {http://dml.mathdoc.fr/item/ITA_2006__40_2_255_0} }
Carton, Olivier; Choffrut, Christian; Grigorieff, Serge. Decision problems among the main subfamilies of rational relations. RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications, Tome 40 (2006) pp. 255-275. doi : 10.1051/ita:2006005. http://gdmltest.u-ga.fr/item/ITA_2006__40_2_255_0/
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