The classical modular group as a subgroup of GL(2, ℤ)

Abstract

The title is somewhat misleading, since the classical modular group Г= PSL(2, ℤ) is certainly not a subgroup of GL(2, ℤ). What is meant of course are the faithful representations of F as a subgroup of GL(2, ℤ), where Г is to be thought of as the free product of a cyclic group of order 2 and a cyclic group of order 3. No such representation is possible as a subgroup of SL(2, ℤ); it is necessary to have matrices of determinant −1 as well.