Congruence testing for odd subgroups of the modular group. Issue Volume 17:Issue A(2014) (May 2014) - Hamilton, Thomas
Abstract: We give a computationally effective criterion for determining whether a finite-index subgroup of $\mathrm{SL}_2(\mathbf{Z})$ is a congruence subgroup, extending earlier work of Hsu for subgroups of $\mathrm{PSL}_2(\mathbf{Z})$ .