On reducible $\ell$-torsion representations of Picard curves

Source abstract

Let $C/\mathbb{Q}$ be a Picard curve and let $\ell\equiv1\pmod3$ be a prime such that the $λ$-torsion representation of the Jacobian of $C$ is reducible for a prime $λ$ of $\mathbb{Q}(ζ_3)$ above $\ell$. We describe the constraints imposed by complex conjugation on the $2$-dimensional subquotient $ρ$, in particular, showing that if it is absolutely irreducible, then up to twist it descends to $\mathbb{Q}$. We then give criteria for ruling out the irreducible case, using Dickson's classification of subgroups of $\operatorname{GL}_2(\mathbb{F}_{\ell})$, under the natural hypothesis that the characteristic polynomials of all elements in the image of $ρ$ split. We also develop algorithms to compute the set of $1$-dimensional Jordan--Hölder constitutents of the $\ell$-torsion representation over $\mathbb{Q}(ζ_3)$, and Frobenius characteristic polynomials of the $2$-dimensional subquotient $ρ$. We apply these methods to $381$ Picard curves with possibly reducible $7$-torsion representations, and to five corresponding $13$-torsion examples. Under the reducibility and splitting hypotheses, we rule out an irreducible $2$-dimensional subquotient in all of the examples considered, showing that the semisimplification of the $\ell$-torsion representation is a direct sum of $6$ characters.