@article{SF_1974__9_134060,
     author = {Chester, jr. John},
     title = {A theorem on free envelopes.},
     journal = {Semigroup forum},
     pages = {74--75},
     publisher = {mathdoc},
     volume = {9},
     year = {1974},
     zbl = {0295.20001},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__9_134060/}
}