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@article{SF_1972__5_133949, author = {R.J. Warne}, title = {On the structure of semigroups which are unions of groups.}, journal = {Semigroup forum}, pages = {323--330}, publisher = {mathdoc}, volume = {5}, year = {1972}, zbl = {0275.20107}, url = {http://geodesic.mathdoc.fr/item/SF_1972__5_133949/} }
R.J. Warne. On the structure of semigroups which are unions of groups.. Semigroup forum, Tome 5 (1972), pp. 323-330. http://geodesic.mathdoc.fr/item/SF_1972__5_133949/