@article{CMH_1986__61_140044,
     author = {Jonathan A. Hillman},
     title = {Abelian normal subgroups of two-knot groups.},
     journal = {Commentarii mathematici Helvetici},
     pages = {122--148},
     year = {1986},
     volume = {61},
     zbl = {0611.57015},
     url = {http://geodesic.mathdoc.fr/item/CMH_1986__61_140044/}
}