@article{MM2_1975__17_154365,
     author = {H. Bresinsky},
     title = {Symmetric {Semigroups} of {Integers} {Generated} by 4 {Elements.}},
     journal = {Manuscripta mathematica},
     pages = {205--220},
     publisher = {mathdoc},
     volume = {17},
     year = {1975},
     zbl = {0317.10061},
     url = {http://geodesic.mathdoc.fr/item/MM2_1975__17_154365/}
}