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@article{SF_1972__5_133920, author = {P.G. Trotter}, title = {A class of semigroups embeddable in groups.}, journal = {Semigroup forum}, pages = {1--13}, publisher = {mathdoc}, volume = {5}, year = {1972}, zbl = {0259.20055}, url = {http://geodesic.mathdoc.fr/item/SF_1972__5_133920/} }
P.G. Trotter. A class of semigroups embeddable in groups.. Semigroup forum, Tome 5 (1972), pp. 1-13. http://geodesic.mathdoc.fr/item/SF_1972__5_133920/