TY  - JOUR
AU  - Mason, A.W.
TI  - Non-Standard, Normal Subgroups and Non-Normal, Standard Subgroups of the Modular Group
JO  - Canadian mathematical bulletin
PY  - 1989
SP  - 109
EP  - 113
VL  - 32
IS  - 1
UR  - http://geodesic.mathdoc.fr/articles/10.4153/CMB-1989-016-5/
DO  - 10.4153/CMB-1989-016-5
ID  - 10_4153_CMB_1989_016_5
ER  -