The index of a mod-$\ell$ Galois representation $\rho:\Gal_K\to G(\F_{\ell})$ is the subgroup index $[G(\F_{\ell}):\rho(\Gal_K)]$.
The index is $1$ if and only if the representation is surjective.
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- Last edited by John Cremona on 2023-03-24 11:22:41
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