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@article{PORMA_1961__20_4_114865, author = {Kelley, R.L. and Swingle, P.M.}, title = {Concerning minimal generating subsets of semigroups}, journal = {Portugaliae mathematica}, pages = {231--250}, publisher = {mathdoc}, volume = {20}, number = {4}, year = {1961}, mrnumber = {0144317}, zbl = {0102.02303}, language = {en}, url = {http://geodesic.mathdoc.fr/item/PORMA_1961__20_4_114865/} }
Kelley, R.L.; Swingle, P.M. Concerning minimal generating subsets of semigroups. Portugaliae mathematica, Tome 20 (1961) no. 4, pp. 231-250. http://geodesic.mathdoc.fr/item/PORMA_1961__20_4_114865/