The $\PSL_2$-index of a modular curve $X_H$ 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. This is equal to the index of the modular curve when $-1 \in H$, and equals half the index if $-1 \notin H$.
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- Review status: beta
- Last edited by Jun Bo Lau on 2024-12-10 15:02:37
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- 2024-12-10 15:02:37 by Jun Bo Lau
- 2024-03-24 05:56:07 by Andrew Sutherland
- 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