@article{JRAM_1971__248_151107,
     author = {J.W. Stepp and R.O. Fulp},
     title = {Semigroup {Extensions.}},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {28--41},
     publisher = {mathdoc},
     volume = {248},
     year = {1971},
     zbl = {0227.22009},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1971__248_151107/}
}