The **$\PSL_2$-index** of a modular curve $H$ is the index of $\Gamma_H := \pm H\cap \SL_2(\Z)$ in $\PSL_2(\Z)$; this index is also the degree of the map $j : \Gamma_H \backslash \overline{\mathfrak{h}} \to X(1)$, where $\overline{\mathfrak{h}}$ is the completed upper half-plane.

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- Review status: beta
- Last edited by John Cremona on 2022-12-12 06:57:34

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**History:**(expand/hide all)

- 2022-12-12 06:57:34 by John Cremona
- 2022-03-25 09:09:30 by John Voight
- 2022-03-25 08:27:14 by John Voight
- 2022-03-25 08:26:59 by John Voight
- 2022-03-24 22:52:39 by Andrew Sutherland

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