English

Elliptic mod \ell Galois representations which are not minimally elliptic

Number Theory 2016-09-07 v2

Abstract

In a recent preprint, F. Calegari has shown that for =2,3,5\ell = 2, 3, 5 and 7 there exist 2-dimensional surjective representations ρ\rho of \Gal(\Qˉ/\Q)\Gal(\bar{\Q}/\Q) with values in \F\F_\ell coming from the \ell-torsion points of an elliptic curve defined over \Q\Q, but not minimally, i.e., so that any elliptic curve giving rise to ρ\rho has prime-to-\ell conductor greater than the (prime-to-\ell) conductor of ρ\rho. In this brief note, we will show that the same is true for any prime >7\ell >7, concretely, we will show that for any such \ell the elliptic curve E:Y2=X(X3)(X31)E^\ell: \qquad Y^2 = X (X- 3^\ell ) (X - 3^\ell - 1) is semistable, has bad reduction at 3, the associated mod\mod \ell Galois representation ρ\rho is surjective, unramified at 3, and there is no elliptic curve with good reduction at 3 whose associated mod\mod \ell representation is isomorphic to ρ\rho.

Keywords

Cite

@article{arxiv.math/0409115,
  title  = {Elliptic mod \ell Galois representations which are not minimally elliptic},
  author = {Luis Dieulefait},
  journal= {arXiv preprint arXiv:math/0409115},
  year   = {2016}
}
R2 v1 2026-07-22T17:09:31.993Z