@article{MAN_1981__256_163492,
     author = {Wilfried Hauenschild},
     title = {Subhypergroups and {Normal} {Subgroups.}},
     journal = {Mathematische Annalen},
     pages = {1--18},
     publisher = {mathdoc},
     volume = {256},
     year = {1981},
     zbl = {0465.22005},
     url = {http://geodesic.mathdoc.fr/item/MAN_1981__256_163492/}
}