More projective geometry associated with unramified double covers of curves of genus four (Q1086311)

Let Y be a general curve of genus four over the complex field. With a 2- division point of \(Pic_{\theta}(Y)\) there is associated an unramified double cover of Y, a system Q of contact quadric to this cover, and a nonhyperelliptic curve X whose Jacobian is isomorphic to the associated Prym variety. - The paper under review is devoted to a study of Q and the construction of a bijection between the set of 28 bitangents to X (odd theta-characteristics of X) and the set of 28 degenerate quadrics of Q (pair of odd theta-characteristics of Y). The ultimate purpose of this work is to understand the geometry of the Schottky relation.