no. 4
A semigroup characterization of dot-depth one languages
RAIRO. Informatique théorique, Tome 17 (1983) no. 4, pp. 321-330 
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     author = {Knast, Robert},
     title = {A semigroup characterization of dot-depth one languages},
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     pages = {321--330},
     year = {1983},
     publisher = {EDP Sciences},
     volume = {17},
     number = {4},
     mrnumber = {743892},
     zbl = {0522.68063},
     language = {en},
     url = {https://www.numdam.org/item/ITA_1983__17_4_321_0/}
}
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Knast, Robert. A semigroup characterization of dot-depth one languages. RAIRO. Informatique théorique, Tome 17 (1983) no. 4, pp. 321-330. https://www.numdam.org/item/ITA_1983__17_4_321_0/

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