Rus, Teodor. “Some Observations Concerning the Algebraic Treating of the Formal Languages”.
Anal. Numér. Théor. Approx.
, vol. 4, no. 2, Aug. 1975, pp. 179-00,
https://ictp.acad.ro/jnaat/journal/article/view/1975-vol4-no2-art9
.