# Hyper-elliptic curve

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.

#### References

[1] | C. Chevalley, "Introduction to the theory of algebraic functions of one variable" , Amer. Math. Soc. (1951) |

[2] | G. Springer, "Introduction to Riemann surfaces" , Addison-Wesley (1957) pp. Chapt.10 |

[3] | I.R. Shafarevich, "Basic algebraic geometry" , Springer (1977) (Translated from Russian) |

#### Comments

The definition given in the main article (first sentence) is only valid in characteristic not equal to 2. In general, a hyper-elliptic curve can be defined as a double covering (cf. also Covering surface) of a rational curve.

#### References

[a1] | E. Arbarello, M. Cornalba, P.A. Griffiths, J.E. Harris, "Geometry of algebraic curves" , 1 , Springer (1985) |

**How to Cite This Entry:**

Hyper-elliptic curve.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Hyper-elliptic_curve&oldid=16119