@article{10_4153_CJM_1971_070_x,
     author = {Karrass, A. and Solitar, D.},
     title = {Subgroups of {HNN} {Groups} and {Groups} with one {Defining} {Relation}},
     journal = {Canadian journal of mathematics},
     pages = {627--643},
     year = {1971},
     volume = {23},
     number = {4},
     doi = {10.4153/CJM-1971-070-x},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1971-070-x/}
}