URPONEN, T. On regular expressions over one letter and on commutative languages. Annales Fennici Mathematici, [S. l.], n. 517, 1972. DOI: 10.5186/aasfm.1972.517. Disponível em: https://afm.journal.fi/article/view/134173. Acesso em: 26 nov. 2024.