@article{ARM_1981_17_1_a6,
     author = {Skula, Ladislav},
     title = {On $\delta n$-semigroups},
     journal = {Archivum mathematicum},
     pages = {43--52},
     year = {1981},
     volume = {17},
     number = {1},
     mrnumber = {672487},
     zbl = {0494.20036},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ARM_1981_17_1_a6/}
}