@article{SF_1974__8_134051,
     author = {Thomas Nordahl},
     title = {Semigroups satisfying (xy)m = xmym .},
     journal = {Semigroup forum},
     pages = {332--346},
     year = {1974},
     volume = {8},
     zbl = {0298.20049},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__8_134051/}
}