@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/}
}