SALOMAA, A. On a homomorphic characterization of recursively enumerable languages. Annales Fennici Mathematici, [S. l.], n. 525, 1973. DOI: 10.5186/aasfm.1973.525. Disponível em: https://afm.journal.fi/article/view/134181. Acesso em: 7 apr. 2025.