@article{JRAM_1970__244_151043,
     author = {P.A. Grillet and Mario Petrich},
     title = {Extensions of an arbitrary semigroup.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {97--107},
     publisher = {mathdoc},
     volume = {244},
     year = {1970},
     zbl = {0221.20079},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1970__244_151043/}
}