@article{10_4153_CJM_1974_021_1,
     author = {Karrass, A. and Pietrowski, A. and Solitar, D.},
     title = {An {Improved} {Subgroup} {Theorem} for {HNN} {Groups} with {Some} {Applications}},
     journal = {Canadian journal of mathematics},
     pages = {214--224},
     year = {1974},
     volume = {26},
     number = {1},
     doi = {10.4153/CJM-1974-021-1},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1974-021-1/}
}