An inverse Jacobian algorithm for Picard curves

Abstract

We study the inverse Jacobian problem for the case of Picard curves over $\mathbb{C}$. More precisely, we elaborate on an algorithm that, given a small period matrix $\Omega\in \mathbb{C}^{3\times 3}$ corresponding to a principally polarized abelian threefold equipped with an automorphism of order $3$, returns a Legendre-Rosenhain equation for a Picard curve with Jacobian isomorphic to the given abelian variety. Our method corrects a formula obtained by Koike-Weng in [Math. Comp., 74(249):499-518, 2005] which is based on a theorem of Siegel. As a result, we apply the algorithm to obtain (numerically) all the isomorphism classes of Picard curves with maximal complex multiplication attached to the sextic CM-fields with class number at most $4$. In particular, we obtain (conjecturally) the complete list of CM Picard curves defined over $\mathbb{Q}$. In the appendix, Vincent gives a correction to the generalization of Takase's formula for the inverse Jacobian problem for hyperelliptic curves given in [Balakrishnan-Ionica-Lauter-Vincent, LMS J. Comput. Math., 19(suppl. A):283-300, 2016].