@article{SF_1974__7_134012,
     author = {Jane M. Day},
     title = {Semigroups with periodic properties.},
     journal = {Semigroup forum},
     pages = {292--309},
     year = {1974},
     volume = {7},
     zbl = {0275.22007},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__7_134012/}
}