Permutations of periodic points of Weierstrass Prym eigenforms

Abstract

A Weierstrass Prym eigenform is an Abelian differential with a single zero on a Riemann surface possessing some special kinds of symmetries. Such surfaces come equipped with an involution, known as a Prym involution. They were originally discovered by McMullen and only arise in genus 2, 3 and 4. Moreover, they are classified by two invariants: discriminant and spin. We study how the fixed points for the Prym involution of Weierstrass Prym eigenforms are permuted. In previous work, the authors computed the permutation group induced by affine transformations in the case of genus 2, showing that they are dihedral groups depending only on the residue class modulo 8 of the discriminant $D$. In this work, we complete this classification by settling the case of genus 3, showing that the permutation group induced by the affine group on the set of its three (regular) fixed points is isomorphic to $\mathrm{Sym}_2$ when $D$ is even and a quadratic residue modulo 16, and to $\mathrm{Sym}_3$ otherwise. The case of genus 4 is trivial as the Pyrm involution fixes a single (regular) point. In both cases, these same groups arise when considering only parabolic elements of the affine group. By recent work of Freedman, when the Teichm\"uller curve induced by Weierstrass Prym eigenform is not arithmetic, the fixed points of the Prym involution coincide with the periodic points of the surface. Hence, in this case, our result also classifies how periodic points are permuted.