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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- 2019-05-01 14:10:53 by David Farmer (Reviewed)
- 2019-04-13 15:39:33 by David Farmer
- 2019-04-13 15:39:15 by David Farmer
- 2019-04-13 15:32:10 by David Farmer
- 2016-03-24 12:00:27 by Andreea Mocanu

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