The **index** of an open subgroup $H$ of $\GL(2,\widehat\Z)$ is the positive integer $[\GL(2,\widehat\Z):H]$.

Similarly, the **index** of an open subgroup $H$ of $\GL(2,\Z_\ell)$ is the positive integer $[\GL(2,\Z_\ell):H]$.

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- Review status: beta
- Last edited by Andrew Sutherland on 2021-07-17 11:28:47

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