@article{SF_1977__14_186362,
     author = {J.A. Gerhard},
     title = {Semigroups with idempotent powers, {I.}},
     journal = {Semigroup forum},
     pages = {137--142},
     publisher = {mathdoc},
     volume = {14},
     year = {1977},
     zbl = {0364.20065},
     url = {http://geodesic.mathdoc.fr/item/SF_1977__14_186362/}
}