On dot-depth two
F. Blanchet-Sadri · RAIRO - Theoretical Informatics and Applications · 1990
For positive integers m u , m k , congruences ~{m t m , } related to a version of the Ehrenfeuchi-Fraissé game are defined which correspond to level k of the Sraubtng hierarchy of star-free ïanguages Given any fîmte alphabet A, a necessary and sufficient condition is given for the monoids A*/~( rrti m% m ^ to be of dot-deàth exactly 2