@article{AFM_1972_501-520_a3,
     author = {Topi Urponen},
     title = {On regular expressions over one letter and on commutative languages},
     journal = {Annales Fennici Mathematici},
     publisher = {mathdoc},
     volume = {501-520},
     year = {1972},
     doi = {10.5186/aasfm.1972.517},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.5186/aasfm.1972.517/}
}