@article{SF_1974__6_133958,
     author = {F. Knowles},
     title = {A characterization of a class of semigroups.},
     journal = {Semigroup forum},
     pages = {42--49},
     year = {1974},
     volume = {6},
     zbl = {0259.22004},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__6_133958/}
}