@article{SF_1979__18_134362,
     author = {E.M. Norris},
     title = {The structure of an idempotent relation.},
     journal = {Semigroup forum},
     pages = {319--330},
     year = {1979},
     volume = {18},
     zbl = {0424.04001},
     url = {http://geodesic.mathdoc.fr/item/SF_1979__18_134362/}
}