@article{10_21136_CMJ_1951_100031,
     author = {Schwarz, \v{S}tefan},
     title = {On semigroups having a kernel},
     journal = {Czechoslovak Mathematical Journal},
     pages = {229--264},
     year = {1951},
     volume = {1},
     number = {4},
     doi = {10.21136/CMJ.1951.100031},
     mrnumber = {0051221},
     zbl = {0048.25302},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.21136/CMJ.1951.100031/}
}