Hyper-elliptic curve

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A non-singular projective model of the affine curve , where is a polynomial without multiple roots of odd degree (the case of even degree may be reduced to that of odd degree ). The field of functions on a hyper-elliptic curve (a field of hyper-elliptic functions) is a quadratic extension of the field of rational functions; in this sense it is the simplest field of algebraic functions except for the field of rational functions. Hyper-elliptic curves are distinguished by the condition of the existence of a one-dimensional linear series of divisors of degree 2, defining a morphism of order 2 of the hyper-elliptic curve onto the projective straight line. The genus of a hyper-elliptic curve is , so that, for various odd , hyper-elliptic curves are birationally inequivalent. For one obtains the projective straight line; for an elliptic curve is obtained. Traditionally, curves of genus 0 and 1 are not called hyper-elliptic curves. The fractions of regular differential forms generate a subfield of genus 0 on a hyper-elliptic curve of genus ; this property is a complete characterization of hyper-elliptic curves.