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The cusps of a subgroup $\Gamma$ of the modular group are equivalence classes of points in $\mathbb{Q}\cup\infty$ under the action of $\Gamma$ by linear fractional transformation, where for $\gamma=\left(\begin{array}{ll}a&b\\c&d \end{array}\right)\in\Gamma,$ we define $\gamma\infty = \frac{a}{c}$ when $c\neq 0$, and $\gamma\infty = \infty$ when $c=0$.

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• Review status: reviewed
• Last edited by David Farmer on 2019-05-01 14:10:53
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