Free Quotients of Congruence Subgroups of SL2 Over a Coordinate Ring. A.W. Mason — 1988 Mathematische Zeitschrift
On subgroups of GL (n, A) which are generated by commutators. II. A.W. Mason — 1981 Journal für die reine und angewandte Mathematik
On Subgroups of GL(n, A) which Are Generated by Commutators. A.W. Mason; W.W. Stothers — 1974 Inventiones mathematicae