Double Coverings Of Hyperelliptic Real Algebraic Curves

Abstract

We consider double and (possibly) branched coverings : X ! X0 between real algebraic curves where X is hyperelliptic. We are interested in the topology of such coverings and also in describing them in terms of algebraic equations. In this article we completely solve these two problems. We first analyse the topological features and ramification data of such coverings. Second, for each isomorphism class of these coverings we then describe a representative, with defining polynomial equations for X and for X0, a formula for the involution that generates the coveri transformation group, and a rational formula forng the covering projection : X ! X0.