If $\rho\colon\Gal_K\to G(\F_{\ell})$ is an mod-$\ell$ Galois representation with splitting field $L$, then a prime $\frak{p}\nmid\ell$ of $K$ is ramified if it is ramified in $L/K$.
Equivalently, a prime is ramified if the inertia subgroup of $\Gal_K$ for a prime above $\frak{p}$ is not contained in the kernel of $\rho$.
A prime which is not ramified is unramified.
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- Last edited by Andrew Sutherland on 2024-08-20 16:46:51
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- 2024-08-20 16:46:51 by Andrew Sutherland
- 2023-03-24 12:04:35 by John Cremona
- 2023-03-24 11:13:53 by John Cremona