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# Elliptic curves over a general field

This module defines the class `EllipticCurve_field`, based on `EllipticCurve_generic`, for elliptic curves over general fields.

class `sage.schemes.elliptic_curves.ell_field.``EllipticCurve_field`(K, ainvs)

  Bases: `sage.schemes.elliptic_curves.ell_generic.EllipticCurve_generic`

  Construct an elliptic curve from Weierstrass \(a\)-coefficients.

  INPUT:

  - `K` – a ring
  - `ainvs` – a list or tuple \([a_1, a_2, a_3, a_4, a_6]\) of Weierstrass coefficients.

  Note

  This class should not be called directly; use `sage.constructor.EllipticCurve` to construct elliptic curves.

  EXAMPLES:

      sage: E = EllipticCurve([1,2,3,4,5]); E
      Elliptic Curve defined by y^2 + x*y + 3*y = x^3 + 2*x^2 + 4*x + 5 over Rational Field
      sage: E = EllipticCurve(GF(7),[1,2,3,4,5]); E
      Elliptic Curve defined by y^2 + x*y + 3*y = x^3 + 2*x^2 + 4*x + 5 over Finite Field of size 7

  Constructor from \([a_4,a_6]\) sets \(a_1=a_2=a_3=0\):

      sage: EllipticCurve([4,5]).ainvs()
      (0, 0, 0, 4, 5)

  The base ring need not be a field:

      sage: EllipticCurve(IntegerModRing(91),[1,2,3,4,5])
      Elliptic Curve defined by y^2 + x*y + 3*y = x^3 + 2*x^2 + 4*x + 5 over Ring of integers modulo 91

  `base_field`()

    Returns the base ring of the elliptic curve.

    EXAMPLES:

        sage: E = EllipticCurve(GF(49, 'a'), [3,5])
        sage: E.base_ring()
        Finite Field in a of size 7^2

        sage: E = EllipticCurve([1,1])
        sage: E.base_ring()
        Rational Field

        sage: E = EllipticCurve(ZZ, [3,5])
        sage: E.base_ring()
        Integer Ring

  `descend_to`(K, f=None)

    Given an elliptic curve self defined over a field \(L\) and a subfield \(K\) of \(L\), return all elliptic curves over \(K\) which are isomorphic over \(L\) to self.

    INPUT:

    - \(K\) – a field which embeds into the base field \(L\) of self.
    - \(f\) (optional) – an embedding of \(K\) into \(L\). Ignored if \(K\) is \(\QQ\).

    OUTPUT:

    A list (possibly empty) of elliptic curves defined over \(K\) which are isomorphic to self over \(L\), up to isomorphism over \(K\).

    Note

    Currently only implemented over number fields. To extend to other fields of characteristic not 2 or 3, what is needed is a method giving the preimages in \(K^*/(K^*)^m\) of an element of the base field, for \(m=2,4,6\).

    EXAMPLES:

        sage: E = EllipticCurve([1,2,3,4,5])
        sage: E.descend_to(ZZ)
        Traceback (most recent call last):
        ...
        TypeError: Input must be a field.

        sage: F.<b> = QuadraticField(23)
        sage: G.<a> = F.extension(x^3+5)
        sage: E = EllipticCurve(j=1728*b).change_ring(G)
        sage: EF = E.descend_to(F); EF
        [Elliptic Curve defined by y^2 = x^3 + (27*b-621)*x + (-1296*b+2484) over Number Field in b with defining polynomial x^2 - 23]
        sage: all([Ei.change_ring(G).is_isomorphic(E) for Ei in EF])
        True

        sage: L.<a> = NumberField(x^4 - 7)
        sage: K.<b> = NumberField(x^2 - 7, embedding=a^2)
        sage: E = EllipticCurve([a^6,0])
        sage: EK = E.descend_to(K); EK
        [Elliptic Curve defined by y^2 = x^3 + b*x over Number Field in b with defining polynomial x^2 - 7,
        Elliptic Curve defined by y^2 = x^3 + 7*b*x over Number Field in b with defining polynomial x^2 - 7]
        sage: all([Ei.change_ring(L).is_isomorphic(E) for Ei in EK])
        True

        sage: K.<a> = QuadraticField(17)
        sage: E = EllipticCurve(j = 2*a)
        sage: E.descend_to(QQ)
        []

    TESTS:

    Check that trac ticket #16456 is fixed:

        sage: K.<a> = NumberField(x^3-2)
        sage: E = EllipticCurve('11a1').quadratic_twist(2)
        sage: EK = E.change_ring(K)
        sage: EK2 = EK.change_weierstrass_model((a,a,a,a+1))
        sage: EK2.descend_to(QQ)
        [Elliptic Curve defined by y^2 = x^3 + x^2 - 41*x - 199 over Rational Field]

        sage: k.<i> = QuadraticField(-1)
        sage: E = EllipticCurve(k,[0,0,0,1,0])
        sage: E.descend_to(QQ)
        [Elliptic Curve defined by y^2 = x^3 + x over Rational Field,
        Elliptic Curve defined by y^2 = x^3 - 4*x over Rational Field]

  `hasse_invariant`()

    Returns the Hasse invariant of this elliptic curve.

    OUTPUT:

    The Hasse invariant of this elliptic curve, as an element of the base field. This is only defined over fields of positive characteristic, and is an element of the field which is zero if and only if the curve is supersingular. Over a field of characteristic zero, where the Hasse invariant is undefined, a `ValueError` is returned.

    EXAMPLES:

        sage: E = EllipticCurve([Mod(1,2),Mod(1,2),0,0,Mod(1,2)])
        sage: E.hasse_invariant()
        1
        sage: E = EllipticCurve([0,0,Mod(1,3),Mod(1,3),Mod(1,3)])
        sage: E.hasse_invariant()
        0
        sage: E = EllipticCurve([0,0,Mod(1,5),0,Mod(2,5)])
        sage: E.hasse_invariant()
        0
        sage: E = EllipticCurve([0,0,Mod(1,5),Mod(1,5),Mod(2,5)])
        sage: E.hasse_invariant()
        2

    Some examples over larger fields:

        sage: EllipticCurve(GF(101),[0,0,0,0,1]).hasse_invariant()
        0
        sage: EllipticCurve(GF(101),[0,0,0,1,1]).hasse_invariant()
        98
        sage: EllipticCurve(GF(103),[0,0,0,0,1]).hasse_invariant()
        20
        sage: EllipticCurve(GF(103),[0,0,0,1,1]).hasse_invariant()
        17
        sage: F.<a> = GF(107^2)
        sage: EllipticCurve(F,[0,0,0,a,1]).hasse_invariant()
        62*a + 75
        sage: EllipticCurve(F,[0,0,0,0,a]).hasse_invariant()
        0

    Over fields of characteristic zero, the Hasse invariant is undefined:

        sage: E = EllipticCurve([0,0,0,0,1])
        sage: E.hasse_invariant()
        Traceback (most recent call last):
        ...
        ValueError: Hasse invariant only defined in positive characteristic

  `is_isogenous`(other, field=None)

    Returns whether or not self is isogenous to other.

    INPUT:

    - `other` – another elliptic curve.
    - `field` (default None) – Currently not implemented. A field containing the base fields of the two elliptic curves onto which the two curves may be extended to test if they are isogenous over this field. By default is_isogenous will not try to find this field unless one of the curves can be be extended into the base field of the other, in which case it will test over the larger base field.

    OUTPUT:

    (bool) True if there is an isogeny from curve `self` to curve `other` defined over `field`.

    METHOD:

    Over general fields this is only implemented in trivial cases.

    EXAMPLES:

        sage: E1 = EllipticCurve(CC, [1,18]); E1
        Elliptic Curve defined by y^2 = x^3 + 1.00000000000000*x + 18.0000000000000 over Complex Field with 53 bits of precision
        sage: E2 = EllipticCurve(CC, [2,7]); E2
        Elliptic Curve defined by y^2 = x^3 + 2.00000000000000*x + 7.00000000000000 over Complex Field with 53 bits of precision
        sage: E1.is_isogenous(E2)
        Traceback (most recent call last):
        ...
        NotImplementedError: Only implemented for isomorphic curves over general fields.

        sage: E1 = EllipticCurve(Frac(PolynomialRing(ZZ,'t')), [2,19]); E1
        Elliptic Curve defined by y^2 = x^3 + 2*x + 19 over Fraction Field of Univariate Polynomial Ring in t over Integer Ring
        sage: E2 = EllipticCurve(CC, [23,4]); E2
        Elliptic Curve defined by y^2 = x^3 + 23.0000000000000*x + 4.00000000000000 over Complex Field with 53 bits of precision
        sage: E1.is_isogenous(E2)
        Traceback (most recent call last):
        ...
        NotImplementedError: Only implemented for isomorphic curves over general fields.

  `is_quadratic_twist`(other)

    Determine whether this curve is a quadratic twist of another.

    INPUT:

    - `other` – an elliptic curves with the same base field as self.

    OUTPUT:

    Either 0, if the curves are not quadratic twists, or \(D\) if `other` is `self.quadratic_twist(D)` (up to isomorphism). If `self` and `other` are isomorphic, returns 1.

    If the curves are defined over \(\mathbb{Q}\), the output \(D\) is a squarefree integer.

    Note

    Not fully implemented in characteristic 2, or in characteristic 3 when both \(j\)-invariants are 0.

    EXAMPLES:

        sage: E = EllipticCurve('11a1')
        sage: Et = E.quadratic_twist(-24)
        sage: E.is_quadratic_twist(Et)
        -6

        sage: E1=EllipticCurve([0,0,1,0,0])
        sage: E1.j_invariant()
        0
        sage: E2=EllipticCurve([0,0,0,0,2])
        sage: E1.is_quadratic_twist(E2)
        2
        sage: E1.is_quadratic_twist(E1)
        1
        sage: type(E1.is_quadratic_twist(E1)) == type(E1.is_quadratic_twist(E2))   #trac 6574
        True

        sage: E1=EllipticCurve([0,0,0,1,0])
        sage: E1.j_invariant()
        1728
        sage: E2=EllipticCurve([0,0,0,2,0])
        sage: E1.is_quadratic_twist(E2)
        0
        sage: E2=EllipticCurve([0,0,0,25,0])
        sage: E1.is_quadratic_twist(E2)
        5

        sage: F = GF(101)
        sage: E1 = EllipticCurve(F,[4,7])
        sage: E2 = E1.quadratic_twist()
        sage: D = E1.is_quadratic_twist(E2); D!=0
        True
        sage: F = GF(101)
        sage: E1 = EllipticCurve(F,[4,7])
        sage: E2 = E1.quadratic_twist()
        sage: D = E1.is_quadratic_twist(E2)
        sage: E1.quadratic_twist(D).is_isomorphic(E2)
        True
        sage: E1.is_isomorphic(E2)
        False
        sage: F2 = GF(101^2,'a')
        sage: E1.change_ring(F2).is_isomorphic(E2.change_ring(F2))
        True

    A characteristic 3 example:

        sage: F = GF(3^5,'a')
        sage: E1 = EllipticCurve_from_j(F(1))
        sage: E2 = E1.quadratic_twist(-1)
        sage: D = E1.is_quadratic_twist(E2); D!=0
        True
        sage: E1.quadratic_twist(D).is_isomorphic(E2)
        True

        sage: E1 = EllipticCurve_from_j(F(0))
        sage: E2 = E1.quadratic_twist()
        sage: D = E1.is_quadratic_twist(E2); D
        1
        sage: E1.is_isomorphic(E2)
        True

  `is_quartic_twist`(other)

    Determine whether this curve is a quartic twist of another.

    INPUT:

    - `other` – an elliptic curves with the same base field as self.

    OUTPUT:

    Either 0, if the curves are not quartic twists, or \(D\) if `other` is `self.quartic_twist(D)` (up to isomorphism). If `self` and `other` are isomorphic, returns 1.

    Note

    Not fully implemented in characteristics 2 or 3.

    EXAMPLES:

        sage: E = EllipticCurve_from_j(GF(13)(1728))
        sage: E1 = E.quartic_twist(2)
        sage: D = E.is_quartic_twist(E1); D!=0
        True
        sage: E.quartic_twist(D).is_isomorphic(E1)
        True

        sage: E = EllipticCurve_from_j(1728)
        sage: E1 = E.quartic_twist(12345)
        sage: D = E.is_quartic_twist(E1); D
        15999120
        sage: (D/12345).is_perfect_power(4)
        True

  `is_sextic_twist`(other)

    Determine whether this curve is a sextic twist of another.

    INPUT:

    - `other` – an elliptic curves with the same base field as self.

    OUTPUT:

    Either 0, if the curves are not sextic twists, or \(D\) if `other` is `self.sextic_twist(D)` (up to isomorphism). If `self` and `other` are isomorphic, returns 1.

    Note

    Not fully implemented in characteristics 2 or 3.

    EXAMPLES:

        sage: E = EllipticCurve_from_j(GF(13)(0))
        sage: E1 = E.sextic_twist(2)
        sage: D = E.is_sextic_twist(E1); D!=0
        True
        sage: E.sextic_twist(D).is_isomorphic(E1)
        True

        sage: E = EllipticCurve_from_j(0)
        sage: E1 = E.sextic_twist(12345)
        sage: D = E.is_sextic_twist(E1); D
        575968320
        sage: (D/12345).is_perfect_power(6)
        True

  `isogenies_prime_degree`(l=None, max_l=31)

    Generic code, valid for all fields, for arbitrary prime \(l\) not equal to the characteristic.

    INPUT:

    - `l` – either None, a prime or a list of primes.
    - `max_l` – a bound on the primes to be tested (ignored unless \(l\) is None).

    OUTPUT:

    (list) All \(l\)-isogenies for the given \(l\) with domain self.

    METHOD:

    Calls the generic function `isogenies_prime_degree()`. This requires that certain operations have been implemented over the base field, such as root-finding for univariate polynomials.

    EXAMPLES:

        sage: F = QQbar
        sage: E = EllipticCurve(F, [1,18]); E
        Elliptic Curve defined by y^2 = x^3 + x + 18 over Algebraic Field
        sage: E.isogenies_prime_degree()
        Traceback (most recent call last):
        ...
        NotImplementedError: This code could be implemented for QQbar, but has not been yet.

        sage: F = CC
        sage: E = EllipticCurve(F, [1,18]); E
        Elliptic Curve defined by y^2 = x^3 + 1.00000000000000*x + 18.0000000000000 over Complex Field with 53 bits of precision
        sage: E.isogenies_prime_degree(11)
        Traceback (most recent call last):
        ...
        NotImplementedError: This code could be implemented for general complex fields, but has not been yet.

    Examples over finite fields:

        sage: E = EllipticCurve(GF(next_prime(1000000)), [7,8])
        sage: E.isogenies_prime_degree()
        [Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 970389*x + 794257 over Finite Field of size 1000003, Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 29783*x + 206196 over Finite Field of size 1000003, Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 999960*x + 78 over Finite Field of size 1000003, Isogeny of degree 13 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 878063*x + 845666 over Finite Field of size 1000003, Isogeny of degree 13 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 375648*x + 342776 over Finite Field of size 1000003, Isogeny of degree 17 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 347438*x + 594729 over Finite Field of size 1000003, Isogeny of degree 17 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 674846*x + 7392 over Finite Field of size 1000003, Isogeny of degree 23 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 390065*x + 605596 over Finite Field of size 1000003]
        sage: E.isogenies_prime_degree(2)
        [Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 970389*x + 794257 over Finite Field of size 1000003, Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 29783*x + 206196 over Finite Field of size 1000003, Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 999960*x + 78 over Finite Field of size 1000003]
        sage: E.isogenies_prime_degree(3)
        []
        sage: E.isogenies_prime_degree(5)
        []
        sage: E.isogenies_prime_degree(7)
        []
        sage: E.isogenies_prime_degree(13)
        [Isogeny of degree 13 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 878063*x + 845666 over Finite Field of size 1000003,
        Isogeny of degree 13 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 375648*x + 342776 over Finite Field of size 1000003]

        sage: E.isogenies_prime_degree([2, 3, 5, 7, 13])
        [Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 970389*x + 794257 over Finite Field of size 1000003, Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 29783*x + 206196 over Finite Field of size 1000003, Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 999960*x + 78 over Finite Field of size 1000003, Isogeny of degree 13 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 878063*x + 845666 over Finite Field of size 1000003, Isogeny of degree 13 from Elliptic Curve defined by y^2 = x^3 + 7*x + 8 over Finite Field of size 1000003 to Elliptic Curve defined by y^2 = x^3 + 375648*x + 342776 over Finite Field of size 1000003]
        sage: E.isogenies_prime_degree([2, 4])
        Traceback (most recent call last):
        ...
        ValueError: 4 is not prime.
        sage: E.isogenies_prime_degree(4)
        Traceback (most recent call last):
        ...
        ValueError: 4 is not prime.
        sage: E.isogenies_prime_degree(11)
        []
        sage: E = EllipticCurve(GF(17),[2,0])
        sage: E.isogenies_prime_degree(3)
        []
        sage: E.isogenies_prime_degree(2)
        [Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 2*x over Finite Field of size 17 to Elliptic Curve defined by y^2 = x^3 + 9*x over Finite Field of size 17, Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 2*x over Finite Field of size 17 to Elliptic Curve defined by y^2 = x^3 + 5*x + 9 over Finite Field of size 17, Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 2*x over Finite Field of size 17 to Elliptic Curve defined by y^2 = x^3 + 5*x + 8 over Finite Field of size 17]

        sage: E = EllipticCurve(GF(13^4, 'a'),[2,8])
        sage: E.isogenies_prime_degree(2)
        [Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 2*x + 8 over Finite Field in a of size 13^4 to Elliptic Curve defined by y^2 = x^3 + 7*x + 4 over Finite Field in a of size 13^4, Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 2*x + 8 over Finite Field in a of size 13^4 to Elliptic Curve defined by y^2 = x^3 + (8*a^3+2*a^2+7*a+5)*x + (12*a^3+3*a^2+4*a+4) over Finite Field in a of size 13^4, Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + 2*x + 8 over Finite Field in a of size 13^4 to Elliptic Curve defined by y^2 = x^3 + (5*a^3+11*a^2+6*a+11)*x + (a^3+10*a^2+9*a) over Finite Field in a of size 13^4]

        sage: E.isogenies_prime_degree(3)
        [Isogeny of degree 3 from Elliptic Curve defined by y^2 = x^3 + 2*x + 8 over Finite Field in a of size 13^4 to Elliptic Curve defined by y^2 = x^3 + 9*x + 11 over Finite Field in a of size 13^4]

    Example to show that separable isogenies of degree equal to the characteristic are now implemented:

        sage: E.isogenies_prime_degree(13)
        [Isogeny of degree 13 from Elliptic Curve defined by y^2 = x^3 + 2*x + 8 over Finite Field in a of size 13^4 to Elliptic Curve defined by y^2 = x^3 + 6*x + 5 over Finite Field in a of size 13^4]

    Examples over number fields (other than QQ):

        sage: QQroot2.<e> = NumberField(x^2-2)
        sage: E = EllipticCurve(QQroot2, j=8000)
        sage: E.isogenies_prime_degree()
        [Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + (-150528000)*x + (-629407744000) over Number Field in e with defining polynomial x^2 - 2 to Elliptic Curve defined by y^2 = x^3 + (-602112000)*x + 5035261952000 over Number Field in e with defining polynomial x^2 - 2,
        Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + (-150528000)*x + (-629407744000) over Number Field in e with defining polynomial x^2 - 2 to Elliptic Curve defined by y^2 = x^3 + (903168000*e-1053696000)*x + (14161674240000*e-23288086528000) over Number Field in e with defining polynomial x^2 - 2,
        Isogeny of degree 2 from Elliptic Curve defined by y^2 = x^3 + (-150528000)*x + (-629407744000) over Number Field in e with defining polynomial x^2 - 2 to Elliptic Curve defined by y^2 = x^3 + (-903168000*e-1053696000)*x + (-14161674240000*e-23288086528000) over Number Field in e with defining polynomial x^2 - 2]

        sage: E = EllipticCurve(QQroot2, [1,0,1,4, -6]); E
        Elliptic Curve defined by y^2 + x*y + y = x^3 + 4*x + (-6) over Number Field in e with defining polynomial x^2 - 2
        sage: E.isogenies_prime_degree(2)
        [Isogeny of degree 2 from Elliptic Curve defined by y^2 + x*y + y = x^3 + 4*x + (-6) over Number Field in e with defining polynomial x^2 - 2 to Elliptic Curve defined by y^2 + x*y + y = x^3 + (-36)*x + (-70) over Number Field in e with defining polynomial x^2 - 2]
        sage: E.isogenies_prime_degree(3)
        [Isogeny of degree 3 from Elliptic Curve defined by y^2 + x*y + y = x^3 + 4*x + (-6) over Number Field in e with defining polynomial x^2 - 2 to Elliptic Curve defined by y^2 + x*y + y = x^3 + (-128/3)*x + 5662/27 over Number Field in e with defining polynomial x^2 - 2, Isogeny of degree 3 from Elliptic Curve defined by y^2 + x*y + y = x^3 + 4*x + (-6) over Number Field in e with defining polynomial x^2 - 2 to Elliptic Curve defined by y^2 + x*y + y = x^3 + (-171)*x + (-874) over Number Field in e with defining polynomial x^2 - 2]

  `isogeny`(kernel, codomain=None, degree=None, model=None, check=True)

    Returns an elliptic curve isogeny from self.

    The isogeny can be determined in two ways, either by a polynomial or a set of torsion points. The methods used are:

    - Velu’s Formulas: Velu’s original formulas for computing isogenies. This algorithm is selected by giving as the `kernel` parameter a point or a list of points which generate a finite subgroup.
    - Kohel’s Formulas: Kohel’s original formulas for computing isogenies. This algorithm is selected by giving as the `kernel` parameter a polynomial (or a coefficient list (little endian)) which will define the kernel of the isogeny.

    INPUT:

    - `E` - an elliptic curve, the domain of the isogeny to

        initialize.

    - `kernel` - a kernel, either a point in `E`, a list of points

        in `E`, a univariate kernel polynomial or `None`. If initiating from a domain/codomain, this must be set to None. Validity of input is not fully checked.

    - `codomain` - an elliptic curve (default:None). If `kernel` is

        None, then this must be the codomain of a separable normalized isogeny, furthermore, `degree` must be the degree of the isogeny from `E` to `codomain`. If `kernel` is not None, then this must be isomorphic to the codomain of the normalized separable isogeny defined by `kernel`, in this case, the isogeny is post composed with an isomorphism so that this parameter is the codomain.

    - `degree` - an integer (default:None). If `kernel` is None,

        then this is the degree of the isogeny from `E` to `codomain`. If `kernel` is not None, then this is used to determine whether or not to skip a gcd of the kernel polynomial with the two torsion polynomial of `E`.

    - `model` - a string (default:None). Only supported variable is

        “minimal”, in which case if``E`` is a curve over the rationals, then the codomain is set to be the unique global minimum model.

    - `check` (default: True) does some partial checks that the

        input is valid (e.g., that the points defined by the kernel polynomial are torsion); however, invalid input can in some cases still pass, since that the points define a group is not checked.

    OUTPUT:

    An isogeny between elliptic curves. This is a morphism of curves.

    EXAMPLES:

        sage: F = GF(2^5, 'alpha'); alpha = F.gen()
        sage: E = EllipticCurve(F, [1,0,1,1,1])
        sage: R.<x> = F[]
        sage: phi = E.isogeny(x+1)
        sage: phi.rational_maps()
        ((x^2 + x + 1)/(x + 1), (x^2*y + x)/(x^2 + 1))

        sage: E = EllipticCurve('11a1')
        sage: P = E.torsion_points()[1]
        sage: E.isogeny(P)
        Isogeny of degree 5 from Elliptic Curve defined by y^2 + y = x^3 - x^2 - 10*x - 20 over Rational Field to Elliptic Curve defined by y^2 + y = x^3 - x^2 - 7820*x - 263580 over Rational Field

        sage: E = EllipticCurve(GF(19),[1,1])
        sage: P = E(15,3); Q = E(2,12);
        sage: (P.order(), Q.order())
        (7, 3)
        sage: phi = E.isogeny([P,Q]); phi
        Isogeny of degree 21 from Elliptic Curve defined by y^2 = x^3 + x + 1 over Finite Field of size 19 to Elliptic Curve defined by y^2 = x^3 + x + 1 over Finite Field of size 19
        sage: phi(E.random_point()) # all points defined over GF(19) are in the kernel
        (0 : 1 : 0)

        # not all polynomials define a finite subgroup trac #6384
        sage: E = EllipticCurve(GF(31),[1,0,0,1,2])
        sage: phi = E.isogeny([14,27,4,1])
        Traceback (most recent call last):
        ...
        ValueError: The polynomial does not define a finite subgroup of the elliptic curve.

    An example in which we construct an invalid morphism, which illustrates that the check for correctness of the input is not sufficient. (See trac 11578.):

        sage: R.<x> = QQ[]
        sage: K.<a> = NumberField(x^2-x-1)
        sage: E = EllipticCurve(K, [-13392, -1080432])
        sage: R.<x> = K[]
        sage: phi = E.isogeny( (x-564)*(x - 396/5*a + 348/5) )
        sage: phi.codomain().conductor().norm().factor()
        5^2 * 11^2 * 3271 * 15806939 * 4169267639351
        sage: phi.domain().conductor().norm().factor()
        11^2

  `isogeny_codomain`(kernel, degree=None)

    Returns the codomain of the isogeny from self with given kernel.

    INPUT:

    - `kernel` - Either a list of points in the kernel of the isogeny,

        or a kernel polynomial (specified as a either a univariate polynomial or a coefficient list.)

    - `degree` - an integer, (default:None) optionally specified degree

        of the kernel.

    OUTPUT:

    An elliptic curve, the codomain of the separable normalized isogeny from this kernel

    EXAMPLES:

        sage: E = EllipticCurve('17a1')
        sage: R.<x> = QQ[]
        sage: E2 = E.isogeny_codomain(x - 11/4); E2
        Elliptic Curve defined by y^2 + x*y + y = x^3 - x^2 - 1461/16*x - 19681/64 over Rational Field

  `quadratic_twist`(D=None)

    Return the quadratic twist of this curve by `D`.

    INPUT:

    - `D` (default None) the twisting parameter (see below).

    In characteristics other than 2, \(D\) must be nonzero, and the twist is isomorphic to self after adjoining \(\sqrt(D)\) to the base.

    In characteristic 2, \(D\) is arbitrary, and the twist is isomorphic to self after adjoining a root of \(x^2+x+D\) to the base.

    In characteristic 2 when \(j=0\), this is not implemented.

    If the base field \(F\) is finite, \(D\) need not be specified, and the curve returned is the unique curve (up to isomorphism) defined over \(F\) isomorphic to the original curve over the quadratic extension of \(F\) but not over \(F\) itself. Over infinite fields, an error is raised if \(D\) is not given.

    EXAMPLES:

        sage: E = EllipticCurve([GF(1103)(1), 0, 0, 107, 340]); E
        Elliptic Curve defined by y^2 + x*y  = x^3 + 107*x + 340 over Finite Field of size 1103
        sage: F=E.quadratic_twist(-1); F
        Elliptic Curve defined by y^2  = x^3 + 1102*x^2 + 609*x + 300 over Finite Field of size 1103
        sage: E.is_isomorphic(F)
        False
        sage: E.is_isomorphic(F,GF(1103^2,'a'))
        True

    A characteristic 2 example:

        sage: E=EllipticCurve(GF(2),[1,0,1,1,1])
        sage: E1=E.quadratic_twist(1)
        sage: E.is_isomorphic(E1)
        False
        sage: E.is_isomorphic(E1,GF(4,'a'))
        True

    Over finite fields, the twisting parameter may be omitted:

        sage: k.<a> = GF(2^10)
        sage: E = EllipticCurve(k,[a^2,a,1,a+1,1])
        sage: Et = E.quadratic_twist()
        sage: Et # random (only determined up to isomorphism)
        Elliptic Curve defined by y^2 + x*y  = x^3 + (a^7+a^4+a^3+a^2+a+1)*x^2 + (a^8+a^6+a^4+1) over Finite Field in a of size 2^10
        sage: E.is_isomorphic(Et)
        False
        sage: E.j_invariant()==Et.j_invariant()
        True

        sage: p=next_prime(10^10)
        sage: k = GF(p)
        sage: E = EllipticCurve(k,[1,2,3,4,5])
        sage: Et = E.quadratic_twist()
        sage: Et # random (only determined up to isomorphism)
        Elliptic Curve defined by y^2  = x^3 + 7860088097*x^2 + 9495240877*x + 3048660957 over Finite Field of size 10000000019
        sage: E.is_isomorphic(Et)
        False
        sage: k2 = GF(p^2,'a')
        sage: E.change_ring(k2).is_isomorphic(Et.change_ring(k2))
        True

  `quartic_twist`(D)

    Return the quartic twist of this curve by \(D\).

    INPUT:

    - `D` (must be nonzero) – the twisting parameter..

    Note

    The characteristic must not be 2 or 3, and the \(j\)-invariant must be 1728.

    EXAMPLES:

        sage: E=EllipticCurve_from_j(GF(13)(1728)); E
        Elliptic Curve defined by y^2  = x^3 + x over Finite Field of size 13
        sage: E1=E.quartic_twist(2); E1
        Elliptic Curve defined by y^2  = x^3 + 5*x over Finite Field of size 13
        sage: E.is_isomorphic(E1)
        False
        sage: E.is_isomorphic(E1,GF(13^2,'a'))
        False
        sage: E.is_isomorphic(E1,GF(13^4,'a'))
        True

  `sextic_twist`(D)

    Return the quartic twist of this curve by \(D\).

    INPUT:

    - `D` (must be nonzero) – the twisting parameter..

    Note

    The characteristic must not be 2 or 3, and the \(j\)-invariant must be 0.

    EXAMPLES:

        sage: E=EllipticCurve_from_j(GF(13)(0)); E
        Elliptic Curve defined by y^2 = x^3 + 1 over Finite Field of size 13
        sage: E1=E.sextic_twist(2); E1
        Elliptic Curve defined by y^2 = x^3 + 11 over Finite Field of size 13
        sage: E.is_isomorphic(E1)
        False
        sage: E.is_isomorphic(E1,GF(13^2,'a'))
        False
        sage: E.is_isomorphic(E1,GF(13^4,'a'))
        False
        sage: E.is_isomorphic(E1,GF(13^6,'a'))
        True

  `two_torsion_rank`()

    Return the dimension of the 2-torsion subgroup of \(E(K)\).

    This will be 0, 1 or 2.

    EXAMPLES:

        sage: E=EllipticCurve('11a1')
        sage: E.two_torsion_rank()
        0
        sage: K.<alpha>=QQ.extension(E.division_polynomial(2).monic())
        sage: E.base_extend(K).two_torsion_rank()
        1
        sage: E.reduction(53).two_torsion_rank()
        2

        sage: E = EllipticCurve('14a1')
        sage: E.two_torsion_rank()
        1
        sage: K.<alpha>=QQ.extension(E.division_polynomial(2).monic().factor()[1][0])
        sage: E.base_extend(K).two_torsion_rank()
        2

        sage: EllipticCurve('15a1').two_torsion_rank()
        2

  `weierstrass_p`(prec=20, algorithm=None)

    Computes the Weierstrass \(\wp\)-function of the elliptic curve.

    INPUT:

    - `mprec` - precision

    - `algorithm` - string (default:`None`) an algorithm identifier

        indicating using the `pari`, `fast` or `quadratic` algorithm. If the algorithm is `None`, then this function determines the best algorithm to use.

    OUTPUT:

    a Laurent series in one variable \(z\) with coefficients in the base field \(k\) of \(E\).

    EXAMPLES:

        sage: E = EllipticCurve('11a1')
        sage: E.weierstrass_p(prec=10)
        z^-2 + 31/15*z^2 + 2501/756*z^4 + 961/675*z^6 + 77531/41580*z^8 + O(z^10)
        sage: E.weierstrass_p(prec=8)
        z^-2 + 31/15*z^2 + 2501/756*z^4 + 961/675*z^6 + O(z^8)
        sage: Esh = E.short_weierstrass_model()
        sage: Esh.weierstrass_p(prec=8)
        z^-2 + 13392/5*z^2 + 1080432/7*z^4 + 59781888/25*z^6 + O(z^8)
        sage: E.weierstrass_p(prec=20, algorithm='fast')
        z^-2 + 31/15*z^2 + 2501/756*z^4 + 961/675*z^6 + 77531/41580*z^8 + 1202285717/928746000*z^10 + 2403461/2806650*z^12 + 30211462703/43418875500*z^14 + 3539374016033/7723451736000*z^16 + 413306031683977/1289540602350000*z^18 + O(z^20)
        sage: E.weierstrass_p(prec=20, algorithm='pari')
        z^-2 + 31/15*z^2 + 2501/756*z^4 + 961/675*z^6 + 77531/41580*z^8 + 1202285717/928746000*z^10 + 2403461/2806650*z^12 + 30211462703/43418875500*z^14 + 3539374016033/7723451736000*z^16 + 413306031683977/1289540602350000*z^18 + O(z^20)
        sage: E.weierstrass_p(prec=20, algorithm='quadratic')
        z^-2 + 31/15*z^2 + 2501/756*z^4 + 961/675*z^6 + 77531/41580*z^8 + 1202285717/928746000*z^10 + 2403461/2806650*z^12 + 30211462703/43418875500*z^14 + 3539374016033/7723451736000*z^16 + 413306031683977/1289540602350000*z^18 + O(z^20)

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