@article{ARM_1968_4_4_a5,
     author = {Zapletal, Josef},
     title = {Distinguishing subsets of semi-groups and groups},
     journal = {Archivum mathematicum},
     pages = {241--251},
     year = {1968},
     volume = {4},
     number = {4},
     mrnumber = {0263958},
     zbl = {0325.20059},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ARM_1968_4_4_a5/}
}