@article{SF_1974__8_134049,
     author = {Diana Lindsey},
     title = {Relations {Invariant} under semigroup actions.},
     journal = {Semigroup forum},
     pages = {298--311},
     publisher = {mathdoc},
     volume = {8},
     year = {1974},
     zbl = {0292.20063},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__8_134049/}
}