The space of solvable Pell-Abel equations

Abstract

Pell-Abel equation is a functional equation of the form P^{2}-DQ^{2} = 1, with a given polynomial D free of squares and unknown polynomials P and Q. We show that the space of Pell-Abel equations with the fixed degrees of D and of a primitive solution P is a complex manifold. We describe its connected components by an efficiently computable invariant. Moreover, we give various applications of this result, including torsion pairs on hyperelliptic curves, Hurwitz spaces and the description of the connected components of the space of primitive k-differentials with a unique zero on genus 2 Riemann surfaces.

An invariant describing the connected components of the space of solvable Pell-Abel equations is given. We relate this invariant with the parity invariant for k-differentials in genus 2. Two more applications, to extremal polynomials and Hurwitz spaces, are given