TY  - JOUR
AU  - A.W. Mason
TI  - Free Quotients of Congruence Subgroups of SL2 Over a Coordinate Ring.
JO  - Mathematische Zeitschrift
PY  - 1988
SP  - 39
EP  - 52
VL  - 198
IS  - 3
PB  - mathdoc
UR  - http://geodesic.mathdoc.fr/item/MZ_1988__198_3_173908/
ID  - MZ_1988__198_3_173908
ER  -