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Can one determine whether a given Eisenstein series ( for GL_{2}(Q)) is overconvergent, just by looking at the associated Galois representation?

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Can you say a bit more about what you mean by "Eisenstein series" in this context? Something you get by $p$-adically interpolating classical Eisenstein series as the weight varies? Something more highfalutin? – Ramsey Aug 22 '11 at 15:53
Sorry, about being imprecise. I meant the one obtained by p-adic interpolation. Basically, given a p-adic modular forms, how can we characterise whether it is overconvergent by looking at the Galois representation? Recent theorems of Emerton, Kisin answer the question in certain situations. – jkl Aug 23 '11 at 6:57
up vote 3 down vote accepted

In the paper "Lissite de la Courbe de Hecke aux points Eisenstein critiques", Bellaiche and Chenevier completely classify all reducible Galois representations coming from overconvergent eigenforms of tame level 1. They are precisely those coming from either the ordinary family of Eisenstein series, or one of the "critical" Eisenstein series

$E^{crit}_w(q) = q + \sum a_nq^n, \quad a_p=p^{k-1}, \quad a_\ell=\epsilon(\ell) + \ell^{k-1}$.

Here $w:\mathbb{Z}_p^*\rightarrow \overline{\mathbb{Q}_p^*}$ is the character $x\mapsto \epsilon(x)x^k$ where $k\geq 2$, $\epsilon$ is finite order, and $(\epsilon,k)\neq(\mathbf{1},2)$.

The classification is Proposition 4 of Section 4.

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