@article{10_4153_CJM_1972_112_0,
     author = {Karrass, A. and Solitar, D.},
     title = {On a {Theorem} of {Cohen} and {Lyndon} {About} {Free} {Bases} for {Normal} {Subgroups}},
     journal = {Canadian journal of mathematics},
     pages = {1086--1091},
     year = {1972},
     volume = {24},
     number = {6},
     doi = {10.4153/CJM-1972-112-0},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1972-112-0/}
}