If $G\subseteq\Gamma$ is a subgroup of the modular group, then a closed set
$F\in\mathcal{H}\cup\mathbb{Q}\cup\{\infty\}$ is said
to be a **fundamental domain** for $G$ if:

- For any point $z\in\mathcal{H}$ there is a $g\in G$ such that $gz\in F$.
- If $z\not=z'\in F$ are equivalent with respect to the action of $G$, that is, if $z'=gz$ for some $g\in G$, then $z$ and $z'$ belong to $\partial F$, the boundary of F.

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- Review status: reviewed
- Last edited by David Farmer on 2019-04-11 22:47:15

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