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Congruence testing for odd subgroups of the modular group
Published online by Cambridge University Press: 01 May 2014
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 $\mathrm{PSL}_2(\mathbf{Z})$.
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- Research Article
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- © The Author(s) 2014