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# Construct elliptic curves as Jacobians

An elliptic curve is a genus one curve with a designated point. The Jacobian of a genus-one curve can be defined as the set of line bundles on the curve, and is isomorphic to the original genus-one curve. It is also an elliptic curve with the trivial line bundle as designated point. The utility of this construction is that we can construct elliptic curves without having to specify which point we take as the origin.

EXAMPLES:

    sage: R.<u,v,w> = QQ[]
    sage: Jacobian(u^3+v^3+w^3)
    Elliptic Curve defined by y^2 = x^3 - 27/4 over Rational Field
    sage: Jacobian(u^4+v^4+w^2)
    Elliptic Curve defined by y^2 = x^3 - 4*x over Rational Field

    sage: C = Curve(u^3+v^3+w^3)
    sage: Jacobian(C)
    Elliptic Curve defined by y^2 = x^3 - 27/4 over Rational Field

    sage: P2.<u,v,w> = ProjectiveSpace(2, QQ)
    sage: C = P2.subscheme(u^3+v^3+w^3)
    sage: Jacobian(C)
    Elliptic Curve defined by y^2 = x^3 - 27/4 over Rational Field

One can also define Jacobians of varieties that are not genus-one curves. These are not implemented in this module, but we call the relevant functionality:

    sage: R.<x> = PolynomialRing(QQ)
    sage: f = x**5 + 1184*x**3 + 1846*x**2 + 956*x + 560
    sage: C = HyperellipticCurve(f)
    sage: Jacobian(C)
    Jacobian of Hyperelliptic Curve over Rational Field defined
    by y^2 = x^5 + 1184*x^3 + 1846*x^2 + 956*x + 560

REFERENCES:

[WpJacobianVariety] | http://en.wikipedia.org/wiki/Jacobian_variety

`sage.schemes.elliptic_curves.jacobian.``Jacobian`(X, **kwds)

  Return the Jacobian.

  INPUT:

  - `X` – polynomial, algebraic variety, or anything else that has a Jacobian elliptic curve.
  - `kwds` – optional keyword arguments.

  The input `X` can be one of the following:

  - A polynomial, see `Jacobian_of_equation()` for details.
  - A curve, see `Jacobian_of_curve()` for details.

  EXAMPLES:

      sage: R.<u,v,w> = QQ[]
      sage: Jacobian(u^3+v^3+w^3)
      Elliptic Curve defined by y^2 = x^3 - 27/4 over Rational Field

      sage: C = Curve(u^3+v^3+w^3)
      sage: Jacobian(C)
      Elliptic Curve defined by y^2 = x^3 - 27/4 over Rational Field

      sage: P2.<u,v,w> = ProjectiveSpace(2, QQ)
      sage: C = P2.subscheme(u^3+v^3+w^3)
      sage: Jacobian(C)
      Elliptic Curve defined by y^2 = x^3 - 27/4 over Rational Field

      sage: Jacobian(C, morphism=True)
      Scheme morphism:
        From: Closed subscheme of Projective Space of dimension 2 over Rational Field defined by:
        u^3 + v^3 + w^3
        To:   Elliptic Curve defined by y^2 = x^3 - 27/4 over Rational Field
        Defn: Defined on coordinates by sending (u : v : w) to
              (u*v^7*w + u*v^4*w^4 + u*v*w^7 :
               v^9 + 3/2*v^6*w^3 - 3/2*v^3*w^6 - w^9 :
               -v^6*w^3 - v^3*w^6)

`sage.schemes.elliptic_curves.jacobian.``Jacobian_of_curve`(curve, morphism=False)

  Return the Jacobian of a genus-one curve

  INPUT:

  - `curve` – a one-dimensional algebraic variety of genus one.

  OUTPUT:

  Its Jacobian elliptic curve.

  EXAMPLES:

      sage: R.<u,v,w> = QQ[]
      sage: C = Curve(u^3+v^3+w^3)
      sage: Jacobian(C)
      Elliptic Curve defined by y^2 = x^3 - 27/4 over Rational Field

`sage.schemes.elliptic_curves.jacobian.``Jacobian_of_equation`(polynomial, variables=None, curve=None)

  Construct the Jacobian of a genus-one curve given by a polynomial.

  INPUT:

  - `F` – a polynomial defining a plane curve of genus one. May be homogeneous or inhomogeneous.
  - `variables` – list of two or three variables or `None` (default). The inhomogeneous or homogeneous coordinates. By default, all variables in the polynomial are used.
  - `curve` – the genus-one curve defined by `polynomial` or # `None` (default). If specified, suitable morphism from the jacobian elliptic curve to the curve is returned.

  OUTPUT:

  An elliptic curve in short Weierstrass form isomorphic to the curve `polynomial=0`. If the optional argument `curve` is specified, a rational multicover from the Jacobian elliptic curve to the genus-one curve is returned.

  EXAMPLES:

      sage: R.<a,b,c> = QQ[]
      sage: f = a^3+b^3+60*c^3
      sage: Jacobian(f)
      Elliptic Curve defined by y^2 = x^3 - 24300 over Rational Field
      sage: Jacobian(f.subs(c=1))
      Elliptic Curve defined by y^2 = x^3 - 24300 over Rational Field

  If we specify the domain curve the birational covering is returned:

      sage: h = Jacobian(f, curve=Curve(f));  h
      Scheme morphism:
        From: Projective Curve over Rational Field defined by a^3 + b^3 + 60*c^3
        To:   Elliptic Curve defined by y^2 = x^3 - 24300 over Rational Field
        Defn: Defined on coordinates by sending (a : b : c) to
              (216000*a*b^7*c + 12960000*a*b^4*c^4 + 777600000*a*b*c^7 :
               216000*b^9 + 19440000*b^6*c^3 - 1166400000*b^3*c^6 - 46656000000*c^9 :
               -216000*b^6*c^3 - 12960000*b^3*c^6)
      sage: h([1,-1,0])
      (0 : 1 : 0)

  Plugging in the polynomials defining \(h\) allows us to verify that it is indeed a rational morphism to the elliptic curve:

      sage: E = h.codomain()
      sage: E.defining_polynomial()(h.defining_polynomials()).factor()
      (-10077696000000000) * c^3 * b^3 * (a^3 + b^3 + 60*c^3) * (b^6 + 60*b^3*c^3 + 3600*c^6)^3

  By specifying the variables, we can also construct an elliptic curve over a polynomial ring:

      sage: R.<u,v,t> = QQ[]
      sage: Jacobian(u^3+v^3+t, variables=[u,v])
      Elliptic Curve defined by y^2 = x^3 + (-27/4*t^2) over
      Multivariate Polynomial Ring in u, v, t over Rational Field

  TESTS:

      sage: from sage.schemes.elliptic_curves.jacobian import Jacobian_of_equation
      sage: Jacobian_of_equation(f, variables=[a,b,c])
      Elliptic Curve defined by y^2 = x^3 - 24300 over Rational Field

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