@article{MOMA_1974__78_186323,
     author = {Elbert M. Pirtle},
     title = {A {Generalization} of the {Class} {Group.}},
     journal = {Monatshefte f\"ur Mathematik},
     pages = {237--245},
     year = {1974},
     volume = {78},
     zbl = {0289.13008},
     url = {http://geodesic.mathdoc.fr/item/MOMA_1974__78_186323/}
}