1. Some logical characterizations of the dot-depth hierarchy and applications;Blanchet-Sadri;Technical Report No. 88-03 of the Department of Mathematics and Statistics of McGill University,1988
2. Hierarchies de concatenation;Pin;RAIRO Inf. theor.,1984
3. A semigroup characterization of dot-depth one languages;Knast;RAIRO Inf. theor.,1983
4. On finite monoids having only trivial subgroups;Schützenberger;Inf. Control,1965
5. Counter-free Automata;McNaughton,1971