@article{10_4153_CMB_1989_016_5,
     author = {Mason, A.W.},
     title = {Non-Standard, {Normal} {Subgroups} and {Non-Normal,} {Standard} {Subgroups} of the {Modular} {Group}},
     journal = {Canadian mathematical bulletin},
     pages = {109--113},
     year = {1989},
     volume = {32},
     number = {1},
     doi = {10.4153/CMB-1989-016-5},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CMB-1989-016-5/}
}