# Torsion subgroups of elliptic curves over number fields (including $$\QQ$$)¶

AUTHORS:

• Nick Alexander: original implementation over $$\QQ$$

• Chris Wuthrich: original implementation over number fields

• John Cremona: rewrote p-primary part to use division

polynomials, added some features, unified Number Field and $$\QQ$$ code.

class sage.schemes.elliptic_curves.ell_torsion.EllipticCurveTorsionSubgroup(E)

The torsion subgroup of an elliptic curve over a number field.

EXAMPLES:

Examples over $$\QQ$$:

sage: E = EllipticCurve([-4, 0]); E
Elliptic Curve defined by y^2  = x^3 - 4*x over Rational Field
sage: G = E.torsion_subgroup(); G
Torsion Subgroup isomorphic to Z/2 + Z/2 associated to the Elliptic Curve defined by y^2  = x^3 - 4*x over Rational Field
sage: G.order()
4
sage: G.gen(0)
(-2 : 0 : 1)
sage: G.gen(1)
(0 : 0 : 1)
sage: G.ngens()
2

sage: E = EllipticCurve([17, -120, -60, 0, 0]); E
Elliptic Curve defined by y^2 + 17*x*y - 60*y = x^3 - 120*x^2 over Rational Field
sage: G = E.torsion_subgroup(); G
Torsion Subgroup isomorphic to Trivial group associated to the Elliptic Curve defined by y^2 + 17*x*y - 60*y = x^3 - 120*x^2 over Rational Field
sage: G.gens()
()
sage: e = EllipticCurve([0, 33076156654533652066609946884,0,\
347897536144342179642120321790729023127716119338758604800,\
1141128154369274295519023032806804247788154621049857648870032370285851781352816640000])
sage: e.torsion_order()
16


Constructing points from the torsion subgroup:

sage: E = EllipticCurve('14a1')
sage: T = E.torsion_subgroup()
sage: [E(t) for t in T]
[(0 : 1 : 0),
(9 : 23 : 1),
(2 : 2 : 1),
(1 : -1 : 1),
(2 : -5 : 1),
(9 : -33 : 1)]


An example where the torsion subgroup is not cyclic:

sage: E = EllipticCurve([0,0,0,-49,0])
sage: T = E.torsion_subgroup()
sage: [E(t) for t in T]
[(0 : 1 : 0), (0 : 0 : 1), (-7 : 0 : 1), (7 : 0 : 1)]


An example where the torsion subgroup is trivial:

sage: E = EllipticCurve('37a1')
sage: T = E.torsion_subgroup()
sage: T
Torsion Subgroup isomorphic to Trivial group associated to the Elliptic Curve defined by y^2 + y = x^3 - x over Rational Field
sage: [E(t) for t in T]
[(0 : 1 : 0)]


Examples over other Number Fields:

sage: E = EllipticCurve('11a1')
sage: K.<i> = NumberField(x^2+1)
sage: EK = E.change_ring(K)
sage: from sage.schemes.elliptic_curves.ell_torsion import EllipticCurveTorsionSubgroup
sage: EllipticCurveTorsionSubgroup(EK)
Torsion Subgroup isomorphic to Z/5 associated to the Elliptic Curve defined by y^2 + y = x^3 + (-1)*x^2 + (-10)*x + (-20) over Number Field in i with defining polynomial x^2 + 1

sage: E = EllipticCurve('11a1')
sage: K.<i> = NumberField(x^2+1)
sage: EK = E.change_ring(K)
sage: T = EK.torsion_subgroup()
sage: T.ngens()
1
sage: T.gen(0)
(5 : -6 : 1)


Note: this class is normally constructed indirectly as follows:

sage: T = EK.torsion_subgroup(); T
Torsion Subgroup isomorphic to Z/5 associated to the Elliptic Curve defined by y^2 + y = x^3 + (-1)*x^2 + (-10)*x + (-20) over Number Field in i with defining polynomial x^2 + 1
sage: type(T)
<class 'sage.schemes.elliptic_curves.ell_torsion.EllipticCurveTorsionSubgroup_with_category'>


AUTHORS:

• Nick Alexander: initial implementation over $$\QQ$$.

• Chris Wuthrich: initial implementation over number fields.

• John Cremona: additional features and unification.

curve()

Return the curve of this torsion subgroup.

EXAMPLES:

sage: E = EllipticCurve('11a1')
sage: K.<i> = NumberField(x^2+1)
sage: EK = E.change_ring(K)
sage: T = EK.torsion_subgroup()
sage: T.curve() is EK
True

points()

Return a list of all the points in this torsion subgroup.

The list is cached.

EXAMPLES:

sage: K.<i> = NumberField(x^2 + 1)
sage: E = EllipticCurve(K,[0,0,0,1,0])
sage: tor = E.torsion_subgroup()
sage: tor.points()
[(0 : 1 : 0), (0 : 0 : 1), (-i : 0 : 1), (i : 0 : 1)]

sage.schemes.elliptic_curves.ell_torsion.torsion_bound(E, number_of_places=20)

Return an upper bound on the order of the torsion subgroup.

INPUT:

• E – an elliptic curve over $$\QQ$$ or a number field

• number_of_places (positive integer, default = 20) – the

number of places that will be used to find the bound

OUTPUT:

(integer) An upper bound on the torsion order.

ALGORITHM:

An upper bound on the order of the torsion group of the elliptic curve is obtained by counting points modulo several primes of good reduction. Note that the upper bound returned by this function is a multiple of the order of the torsion group, and in general will be greater than the order.

To avoid nontrivial arithmetic in the base field (in particular, to avoid having to compute the maximal order) we only use prime $$P$$ above rational primes $$p$$ which do not divide the discriminant of the equation order.

EXAMPLES:

sage: CDB = CremonaDatabase()
sage: from sage.schemes.elliptic_curves.ell_torsion import torsion_bound
sage: [torsion_bound(E) for E in CDB.iter()]
[6, 6, 6, 6, 6, 6]
sage: [E.torsion_order() for E in CDB.iter()]
[6, 6, 2, 6, 2, 6]


An example over a relative number field (see trac ticket #16011):

sage: R.<x> = QQ[]
sage: K.<b> = F.extension(x^2-3)
sage: E = EllipticCurve(K,[0,0,0,b,1])
sage: E.torsion_subgroup().order()
1


An example of a base-change curve from $$\QQ$$ to a degree 16 field:

sage: from sage.schemes.elliptic_curves.ell_torsion import torsion_bound
sage: f = PolynomialRing(QQ,'x')([5643417737593488384,0,
....:     -11114515801179776,0,-455989850911004,0,379781901872,
....:     0,14339154953,0,-1564048,0,-194542,0,-32,0,1])
sage: K = NumberField(f,'a')
sage: E = EllipticCurve(K, [1, -1, 1, 824579, 245512517])
sage: torsion_bound(E)
16
sage: E.torsion_subgroup().invariants()
(4, 4)