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On modular mod $\ell$ Galois representations with exceptional images

Ian Kiming·Helena A. Verrill
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Abstract

We give a parametrization of the possible Serre invariants $(N,k,ν)$ of modular mod $\ell$ Galois representations of the exceptional types $A_4$, $S_4$, $A_5$, in terms of local data attached to the fields cut out by the associated projective representations. We show how this result combined with certain global considerations leads to an effective procedure that will determine for a given eigenform $f$ and prime $\ell$ whether a mod $\ell$ representation attached to $f$ is exceptional. We illustrate with numerical examples.

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Ian Kiming, Helena A. Verrill. 2003-11-06. On modular mod $\ell$ Galois representations with exceptional images. https://arxiv.org/abs/math/0311082

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