@article{10_4153_CJM_1975_013_5,
     author = {McAdam, Stephen},
     title = {Going {Down} and {Open} {Extensions}},
     journal = {Canadian journal of mathematics},
     pages = {111--114},
     year = {1975},
     volume = {27},
     number = {1},
     doi = {10.4153/CJM-1975-013-5},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1975-013-5/}
}