# Elliptic curves over number fields¶

An elliptic curve $$E$$ over a number field $$K$$ can be given by a Weierstrass equation whose coefficients lie in $$K$$ or by using base_extend on an elliptic curve defined over a subfield.

One major difference to elliptic curves over $$\QQ$$ is that there might not exist a global minimal equation over $$K$$, when $$K$$ does not have class number one. Another difference is the lack of understanding of modularity for general elliptic curves over general number fields.

Currently Sage can obtain local information about $$E/K_v$$ for finite places $$v$$, it has an interface to Denis Simon’s script for 2-descent, it can compute the torsion subgroup of the Mordell-Weil group $$E(K)$$, and it can work with isogenies defined over $$K$$.

EXAMPLE:

```sage: K.<i> = NumberField(x^2+1)
sage: E = EllipticCurve([0,4+i])
sage: E.discriminant()
-3456*i - 6480
sage: P= E([i,2])
sage: P+P
(-2*i + 9/16 : -9/4*i - 101/64 : 1)
```
```sage: E.has_good_reduction(2+i)
True
sage: E.local_data(4+i)
Local data at Fractional ideal (i + 4):
Reduction type: bad additive
Local minimal model: Elliptic Curve defined by y^2 = x^3 + (i+4) over Number Field in i with defining polynomial x^2 + 1
Minimal discriminant valuation: 2
Conductor exponent: 2
Kodaira Symbol: II
Tamagawa Number: 1
sage: E.tamagawa_product_bsd()
1
```
```sage: E.simon_two_descent()
(1, 1, [(i : 2 : 1)])
```
```sage: E.torsion_order()
1
```
```sage: E.isogenies_prime_degree(3)
[Isogeny of degree 3 from Elliptic Curve defined by y^2 = x^3 + (i+4) over Number Field in i with defining polynomial x^2 + 1 to Elliptic Curve defined by y^2 = x^3 + (-27*i-108) over Number Field in i with defining polynomial x^2 + 1]
```

AUTHORS:

• Robert Bradshaw 2007
• John Cremona
• Chris Wuthrich

REFERENCE:

• [Sil] Silverman, Joseph H. The arithmetic of elliptic curves. Second edition. Graduate Texts in Mathematics, 106. Springer, 2009.
• [Sil2] Silverman, Joseph H. Advanced topics in the arithmetic of elliptic curves. Graduate Texts in Mathematics, 151. Springer, 1994.
class sage.schemes.elliptic_curves.ell_number_field.EllipticCurve_number_field(x, y=None)¶

Bases: sage.schemes.elliptic_curves.ell_field.EllipticCurve_field

Elliptic curve over a number field.

EXAMPLES:

```sage: K.<i>=NumberField(x^2+1)
sage: EllipticCurve([i, i - 1, i + 1, 24*i + 15, 14*i + 35])
Elliptic Curve defined by y^2 + i*x*y + (i+1)*y = x^3 + (i-1)*x^2 + (24*i+15)*x + (14*i+35) over Number Field in i with defining polynomial x^2 + 1
```
conductor()¶

Returns the conductor of this elliptic curve as a fractional ideal of the base field.

OUTPUT:

(fractional ideal) The conductor of the curve.

EXAMPLES:

```sage: K.<i>=NumberField(x^2+1)
sage: EllipticCurve([i, i - 1, i + 1, 24*i + 15, 14*i + 35]).conductor()
Fractional ideal (21*i - 3)
sage: K.<a>=NumberField(x^2-x+3)
sage: EllipticCurve([1 + a , -1 + a , 1 + a , -11 + a , 5 -9*a  ]).conductor()
Fractional ideal (6*a)
```

A not so well known curve with everywhere good reduction:

```sage: K.<a>=NumberField(x^2-38)
sage: E=EllipticCurve([0,0,0, 21796814856932765568243810*a - 134364590724198567128296995, 121774567239345229314269094644186997594*a - 750668847495706904791115375024037711300])
sage: E.conductor()
Fractional ideal (1)
```

An example which used to fail (see trac #5307):

```sage: K.<w>=NumberField(x^2+x+6)
sage: E=EllipticCurve([w,-1,0,-w-6,0])
sage: E.conductor()
Fractional ideal (86304, w + 5898)
```

An example raised in #11346:

```sage: K.<g> = NumberField(x^2 - x - 1)
sage: E1 = EllipticCurve(K,[0,0,0,-1/48,-161/864])
sage: [(p.smallest_integer(),e) for p,e in E1.conductor().factor()]
[(2, 4), (3, 1), (5, 1)]
```
gens(verbose=0, lim1=5, lim3=50, limtriv=10, maxprob=20, limbigprime=30)¶

Returns some generators of this elliptic curve. Check rank() or rank_bounds() to verify the number of generators.

Note

The optional parameters control the Simon two descent algorithm; see the documentation of simon_two_descent() for more details.

INPUT:

• verbose – 0, 1, 2, or 3 (default: 0), the verbosity level
• lim1 – (default: 5) limit on trivial points on quartics
• lim3 – (default: 50) limit on points on ELS quartics
• limtriv – (default: 10) limit on trivial points on elliptic curve
• maxprob – (default: 20)
• limbigprime – (default: 30) to distinguish between small and large prime numbers. Use probabilistic tests for large primes. If 0, don’t use probabilistic tests.

OUTPUT:

The linearly independent elements given by the Simon two-descent.

Note

For non-quadratic number fields, this code does return, but it takes a long time.

EXAMPLES:

```sage: K.<a> = NumberField(x^2 + 23, 'a')
sage: E = EllipticCurve(K, '37')
sage: E == loads(dumps(E))
True
sage: E.gens()
[(-1 : 0 : 1), (1/2*a - 5/2 : -1/2*a - 13/2 : 1)]
```

Here is a curve with two-torsion, so here the algorithm does not uniquely determine the rank:

```sage: Qrt5.<rt5>=NumberField(x^2-5)
sage: E=EllipticCurve([0,5-rt5,0,rt5,0])
sage: E.gens()
[(3/2*rt5 + 5/2 : -9/2*rt5 - 15/2 : 1), (-1/2*rt5 + 3/2 : 3/2*rt5 - 9/2 : 1), (0 : 0 : 1)]
```

IMPLEMENTATION:

Uses Denis Simon’s PARI/GP scripts from http://www.math.unicaen.fr/~simon/.

global_integral_model()¶

Return a model of self which is integral at all primes.

EXAMPLES:

```sage: K.<i> = NumberField(x^2+1)
sage: E = EllipticCurve([i/5,i/5,i/5,i/5,i/5])
sage: P1,P2 = K.primes_above(5)
sage: E.global_integral_model()
Elliptic Curve defined by y^2 + (-i)*x*y + (-25*i)*y = x^3 + 5*i*x^2 + 125*i*x + 3125*i over Number Field in i with defining polynomial x^2 + 1
```

trac #7935:

```sage: K.<a> = NumberField(x^2-38)
sage: E = EllipticCurve([a,1/2])
sage: E.global_integral_model()
Elliptic Curve defined by y^2 = x^3 + 1444*a*x + 27436 over Number Field in a with defining polynomial x^2 - 38
```

trac #9266:

```sage: K.<s> = NumberField(x^2-5)
sage: w = (1+s)/2
sage: E = EllipticCurve(K,[2,w])
sage: E.global_integral_model()
Elliptic Curve defined by y^2 = x^3 + 2*x + (1/2*s+1/2) over Number Field in s with defining polynomial x^2 - 5
```

trac #12151:

```sage: K.<v> = NumberField(x^2 + 161*x - 150)
sage: E = EllipticCurve([25105/216*v - 3839/36, 634768555/7776*v - 98002625/1296, 634768555/7776*v - 98002625/1296, 0, 0])
sage: E.global_integral_model()
Elliptic Curve defined by y^2 + (33872485050625*v-31078224284250)*x*y + (2020602604156076340058146664245468750000*v-1871778534673615560803175189398437500000)*y = x^3 + (6933305282258321342920781250*v-6422644400723486559914062500)*x^2 over Number Field in v with defining polynomial x^2 + 161*x - 150
```
global_minimal_model(proof=None)¶

Returns a model of self that is integral, minimal at all primes.

Note

This is only implemented for class number 1. In general, such a model may or may not exist.

INPUT:

• proof – whether to only use provably correct methods (default controlled by global proof module). Note that the proof module is number_field, not elliptic_curves, since the functions that actually need the flag are in number fields.

OUTPUT:

A global integral and minimal model.

EXAMPLES:

```sage: K.<a> = NumberField(x^2-38)
sage: E = EllipticCurve([0,0,0, 21796814856932765568243810*a - 134364590724198567128296995, 121774567239345229314269094644186997594*a - 750668847495706904791115375024037711300])

sage: E2 = E.global_minimal_model()
sage: E2 # random (the global minimal model is not unique)
Elliptic Curve defined by y^2 + a*x*y + (a+1)*y = x^3 + (a+1)*x^2 + (368258520200522046806318444*a-2270097978636731786720859345)*x + (8456608930173478039472018047583706316424*a-52130038506793883217874390501829588391299) over Number Field in a with defining polynomial x^2 - 38

sage: E2.local_data()
[]
```

See trac #11347:

```sage: K.<g> = NumberField(x^2 - x - 1)
sage: E = EllipticCurve(K,[0,0,0,-1/48,161/864]).integral_model().global_minimal_model(); E
Elliptic Curve defined by y^2 + x*y + y = x^3 + x^2 over Number Field in g with defining polynomial x^2 - x - 1
sage: [(p.norm(), e) for p, e in E.conductor().factor()]
[(9, 1), (5, 1)]
sage: [(p.norm(), e) for p, e in E.discriminant().factor()]
[(9, 1), (-5, 2)]
```
has_additive_reduction(P)¶

Return True if this elliptic curve has (bad) additive reduction at the prime $$P$$.

INPUT:

• P – a prime ideal of the base field of self, or a field element generating such an ideal.

OUTPUT:

(bool) True if the curve has additive reduction at $$P$$, else False.

EXAMPLES:

```sage: E=EllipticCurve('27a1')
sage: [(p,E.has_additive_reduction(p)) for p in prime_range(15)]
[(2, False), (3, True), (5, False), (7, False), (11, False), (13, False)]

sage: K.<a>=NumberField(x^3-2)
sage: P17a, P17b = [P for P,e in K.factor(17)]
sage: E = EllipticCurve([0,0,0,0,2*a+1])
sage: [(p,E.has_additive_reduction(p)) for p in [P17a,P17b]]
[(Fractional ideal (4*a^2 - 2*a + 1), False),
(Fractional ideal (2*a + 1), True)]
```
has_bad_reduction(P)¶

Return True if this elliptic curve has bad reduction at the prime $$P$$.

INPUT:

• P – a prime ideal of the base field of self, or a field element generating such an ideal.

OUTPUT:

(bool) True if the curve has bad reduction at $$P$$, else False.

Note

This requires determining a local integral minimal model; we do not just check that the discriminant of the current model has valuation zero.

EXAMPLES:

```sage: E=EllipticCurve('14a1')
sage: [(p,E.has_bad_reduction(p)) for p in prime_range(15)]
[(2, True), (3, False), (5, False), (7, True), (11, False), (13, False)]

sage: K.<a>=NumberField(x^3-2)
sage: P17a, P17b = [P for P,e in K.factor(17)]
sage: E = EllipticCurve([0,0,0,0,2*a+1])
sage: [(p,E.has_bad_reduction(p)) for p in [P17a,P17b]]
[(Fractional ideal (4*a^2 - 2*a + 1), False),
(Fractional ideal (2*a + 1), True)]
```
has_good_reduction(P)¶

Return True if this elliptic curve has good reduction at the prime $$P$$.

INPUT:

• P – a prime ideal of the base field of self, or a field element generating such an ideal.

OUTPUT:

(bool) – True if the curve has good reduction at $$P$$, else False.

Note

This requires determining a local integral minimal model; we do not just check that the discriminant of the current model has valuation zero.

EXAMPLES:

```sage: E=EllipticCurve('14a1')
sage: [(p,E.has_good_reduction(p)) for p in prime_range(15)]
[(2, False), (3, True), (5, True), (7, False), (11, True), (13, True)]

sage: K.<a>=NumberField(x^3-2)
sage: P17a, P17b = [P for P,e in K.factor(17)]
sage: E = EllipticCurve([0,0,0,0,2*a+1])
sage: [(p,E.has_good_reduction(p)) for p in [P17a,P17b]]
[(Fractional ideal (4*a^2 - 2*a + 1), True),
(Fractional ideal (2*a + 1), False)]
```
has_multiplicative_reduction(P)¶

Return True if this elliptic curve has (bad) multiplicative reduction at the prime $$P$$.

Note

See also has_split_multiplicative_reduction() and has_nonsplit_multiplicative_reduction().

INPUT:

• P – a prime ideal of the base field of self, or a field

element generating such an ideal.

OUTPUT:

(bool) True if the curve has multiplicative reduction at $$P$$, else False.

EXAMPLES:

```sage: E=EllipticCurve('14a1')
sage: [(p,E.has_multiplicative_reduction(p)) for p in prime_range(15)]
[(2, True), (3, False), (5, False), (7, True), (11, False), (13, False)]

sage: K.<a>=NumberField(x^3-2)
sage: P17a, P17b = [P for P,e in K.factor(17)]
sage: E = EllipticCurve([0,0,0,0,2*a+1])
sage: [(p,E.has_multiplicative_reduction(p)) for p in [P17a,P17b]]
[(Fractional ideal (4*a^2 - 2*a + 1), False), (Fractional ideal (2*a + 1), False)]
```
has_nonsplit_multiplicative_reduction(P)¶

Return True if this elliptic curve has (bad) non-split multiplicative reduction at the prime $$P$$.

INPUT:

• P – a prime ideal of the base field of self, or a field element generating such an ideal.

OUTPUT:

(bool) True if the curve has non-split multiplicative reduction at $$P$$, else False.

EXAMPLES:

```sage: E=EllipticCurve('14a1')
sage: [(p,E.has_nonsplit_multiplicative_reduction(p)) for p in prime_range(15)]
[(2, True), (3, False), (5, False), (7, False), (11, False), (13, False)]

sage: K.<a>=NumberField(x^3-2)
sage: P17a, P17b = [P for P,e in K.factor(17)]
sage: E = EllipticCurve([0,0,0,0,2*a+1])
sage: [(p,E.has_nonsplit_multiplicative_reduction(p)) for p in [P17a,P17b]]
[(Fractional ideal (4*a^2 - 2*a + 1), False), (Fractional ideal (2*a + 1), False)]
```
has_split_multiplicative_reduction(P)¶

Return True if this elliptic curve has (bad) split multiplicative reduction at the prime $$P$$.

INPUT:

• P – a prime ideal of the base field of self, or a field element generating such an ideal.

OUTPUT:

(bool) True if the curve has split multiplicative reduction at $$P$$, else False.

EXAMPLES:

```sage: E=EllipticCurve('14a1')
sage: [(p,E.has_split_multiplicative_reduction(p)) for p in prime_range(15)]
[(2, False), (3, False), (5, False), (7, True), (11, False), (13, False)]

sage: K.<a>=NumberField(x^3-2)
sage: P17a, P17b = [P for P,e in K.factor(17)]
sage: E = EllipticCurve([0,0,0,0,2*a+1])
sage: [(p,E.has_split_multiplicative_reduction(p)) for p in [P17a,P17b]]
[(Fractional ideal (4*a^2 - 2*a + 1), False), (Fractional ideal (2*a + 1), False)]
```
height_pairing_matrix(points=None, precision=None)¶

Returns the height pairing matrix of the given points.

INPUT:

• points - either a list of points, which must be on this curve, or (default) None, in which case self.gens() will be used.
• precision - number of bits of precision of result (default: None, for default RealField precision)

EXAMPLES:

```sage: E = EllipticCurve([0, 0, 1, -1, 0])
sage: E.height_pairing_matrix()
[0.0511114082399688]
```

For rank 0 curves, the result is a valid 0x0 matrix:

```sage: EllipticCurve('11a').height_pairing_matrix()
[]
sage: E=EllipticCurve('5077a1')
sage: E.height_pairing_matrix([E.lift_x(x) for x in [-2,-7/4,1]], precision=100)
[  1.3685725053539301120518194471  -1.3095767070865761992624519454 -0.63486715783715592064475542573]
[ -1.3095767070865761992624519454   2.7173593928122930896610589220   1.0998184305667292139777571432]
[-0.63486715783715592064475542573   1.0998184305667292139777571432  0.66820516565192793503314205089]

sage: E = EllipticCurve('389a1')
sage: E = EllipticCurve('389a1')
sage: P,Q = E.point([-1,1,1]),E.point([0,-1,1])
sage: E.height_pairing_matrix([P,Q])
[0.686667083305587 0.268478098806726]
[0.268478098806726 0.327000773651605]
```

Over a number field:

```sage: x = polygen(QQ)
sage: K.<t> = NumberField(x^2+47)
sage: EK = E.base_extend(K)
sage: EK.height_pairing_matrix([EK(P),EK(Q)])
[0.686667083305586 0.268478098806726]
[0.268478098806726 0.327000773651605]
```
```sage: K.<i> = QuadraticField(-1)
sage: E = EllipticCurve([0,0,0,i,i])
sage: P = E(-9+4*i,-18-25*i)
sage: Q = E(i,-i)
sage: E.height_pairing_matrix([P,Q])
[  2.16941934493768 -0.870059380421505]
[-0.870059380421505  0.424585837470709]
sage: E.regulator_of_points([P,Q])
0.164101403936070
```
integral_model()¶

Return a model of self which is integral at all primes.

EXAMPLES:

```sage: K.<i> = NumberField(x^2+1)
sage: E = EllipticCurve([i/5,i/5,i/5,i/5,i/5])
sage: P1,P2 = K.primes_above(5)
sage: E.global_integral_model()
Elliptic Curve defined by y^2 + (-i)*x*y + (-25*i)*y = x^3 + 5*i*x^2 + 125*i*x + 3125*i over Number Field in i with defining polynomial x^2 + 1
```

trac #7935:

```sage: K.<a> = NumberField(x^2-38)
sage: E = EllipticCurve([a,1/2])
sage: E.global_integral_model()
Elliptic Curve defined by y^2 = x^3 + 1444*a*x + 27436 over Number Field in a with defining polynomial x^2 - 38
```

trac #9266:

```sage: K.<s> = NumberField(x^2-5)
sage: w = (1+s)/2
sage: E = EllipticCurve(K,[2,w])
sage: E.global_integral_model()
Elliptic Curve defined by y^2 = x^3 + 2*x + (1/2*s+1/2) over Number Field in s with defining polynomial x^2 - 5
```

trac #12151:

```sage: K.<v> = NumberField(x^2 + 161*x - 150)
sage: E = EllipticCurve([25105/216*v - 3839/36, 634768555/7776*v - 98002625/1296, 634768555/7776*v - 98002625/1296, 0, 0])
sage: E.global_integral_model()
Elliptic Curve defined by y^2 + (33872485050625*v-31078224284250)*x*y + (2020602604156076340058146664245468750000*v-1871778534673615560803175189398437500000)*y = x^3 + (6933305282258321342920781250*v-6422644400723486559914062500)*x^2 over Number Field in v with defining polynomial x^2 + 161*x - 150
```
is_global_integral_model()¶

Return true iff self is integral at all primes.

EXAMPLES:

```sage: K.<i> = NumberField(x^2+1)
sage: E = EllipticCurve([i/5,i/5,i/5,i/5,i/5])
sage: P1,P2 = K.primes_above(5)
sage: Emin = E.global_integral_model()
sage: Emin.is_global_integral_model()
True
```
is_isogenous(other, proof=True, maxnorm=100)¶

Returns whether or not self is isogenous to other.

INPUT:

• other – another elliptic curve.
• proof (default True) – If False, the function will return True whenever the two curves have the same conductor and are isogenous modulo $$p$$ for all primes $$p$$ of norm up to maxp. If True, the function returns False when the previous condition does not hold, and if it does hold we attempt to see if the curves are indeed isogenous. However, this has not been fully implemented (see examples below), so we may not be able to determine whether or not the curves are isogenous..
• maxnorm (integer, default 100) – The maximum norm of primes $$p$$ for which isogeny modulo $$p$$ will be checked.

OUTPUT:

(bool) True if there is an isogeny from curve self to curve other.

EXAMPLES:

```sage: x = polygen(QQ, 'x')
sage: F = NumberField(x^2 -2, 's'); F
Number Field in s with defining polynomial x^2 - 2
sage: E1 = EllipticCurve(F, [7,8])
sage: E2 = EllipticCurve(F, [0,5,0,1,0])
sage: E3 = EllipticCurve(F, [0,-10,0,21,0])
sage: E1.is_isogenous(E2)
False
sage: E1.is_isogenous(E1)
True
sage: E2.is_isogenous(E2)
True
sage: E2.is_isogenous(E1)
False
sage: E2.is_isogenous(E3)
True
```
```sage: x = polygen(QQ, 'x')
sage: F = NumberField(x^2 -2, 's'); F
Number Field in s with defining polynomial x^2 - 2
sage: E = EllipticCurve('14a1')
sage: EE = EllipticCurve('14a2')
sage: E1 = E.change_ring(F)
sage: E2 = EE.change_ring(F)
sage: E1.is_isogenous(E2)
True
```
```sage: x = polygen(QQ, 'x')
sage: F = NumberField(x^2 -2, 's'); F
Number Field in s with defining polynomial x^2 - 2
sage: k.<a> = NumberField(x^3+7)
sage: E = EllipticCurve(F, [7,8])
sage: EE = EllipticCurve(k, [2, 2])
sage: E.is_isogenous(EE)
Traceback (most recent call last):
...
ValueError: Second argument must be defined over the same number field.
```

Some examples from Cremona’s 1981 tables:

```sage: K.<i> = QuadraticField(-1)
sage: E1 = EllipticCurve([i + 1, 0, 1, -240*i - 400, -2869*i - 2627])
sage: E1.conductor()
Fractional ideal (4*i + 7)
sage: E2 = EllipticCurve([1+i,0,1,0,0])
sage: E2.conductor()
Fractional ideal (4*i + 7)
sage: E1.is_isogenous(E2)
Traceback (most recent call last):
...
NotImplementedError: Curves appear to be isogenous (same conductor, isogenous modulo all primes of norm up to 1000), but no isogeny has been constructed.
sage: E1.is_isogenous(E2, proof=False)
True
```

In this case E1 and E2 are in fact 9-isogenous, as may be deduced from the following:

```sage: E3 = EllipticCurve([i + 1, 0, 1, -5*i - 5, -2*i - 5])
sage: E3.is_isogenous(E1)
True
sage: E3.is_isogenous(E2)
True
sage: E1.isogeny_degree(E2)
9
```
is_local_integral_model(*P)¶

Tests if self is integral at the prime ideal $$P$$, or at all the primes if $$P$$ is a list or tuple.

INPUT:

• *P – a prime ideal, or a list or tuple of primes.

EXAMPLES:

```sage: K.<i> = NumberField(x^2+1)
sage: P1,P2 = K.primes_above(5)
sage: E = EllipticCurve([i/5,i/5,i/5,i/5,i/5])
sage: E.is_local_integral_model(P1,P2)
False
sage: Emin = E.local_integral_model(P1,P2)
sage: Emin.is_local_integral_model(P1,P2)
True
```
isogeny_degree(other)¶

Returns the minimal degree of an isogeny between self and other, or 0 if no isogeny exists.

INPUT:

• other – another elliptic curve.

OUTPUT:

(int) The degree of an isogeny from self to other, or 0.

Warning

Not all isogenies over number fields are yet implemented. Currently the code only works if there is a chain of isogenies from self to other of degrees 2, 3, 5, 7 and 13.

EXAMPLES:

```sage: x = QQ['x'].0
sage: F = NumberField(x^2 -2, 's'); F
Number Field in s with defining polynomial x^2 - 2
sage: E = EllipticCurve('14a1')
sage: EE = EllipticCurve('14a2')
sage: E1 = E.change_ring(F)
sage: E2 = EE.change_ring(F)
sage: E1.isogeny_degree(E2)
2
sage: E2.isogeny_degree(E2)
1
sage: E5 = EllipticCurve('14a5').change_ring(F)
sage: E1.isogeny_degree(E5)
6
```
kodaira_symbol(P, proof=None)¶

Returns the Kodaira Symbol of this elliptic curve at the prime $$P$$.

INPUT:

• P – either None or a prime ideal of the base field of self.
• proof – whether to only use provably correct methods (default controlled by global proof module). Note that the proof module is number_field, not elliptic_curves, since the functions that actually need the flag are in number fields.

OUTPUT:

The Kodaira Symbol of the curve at P, represented as a string.

EXAMPLES:

```sage: K.<a>=NumberField(x^2-5)
sage: E=EllipticCurve([20, 225, 750, 625*a + 6875, 31250*a + 46875])
sage: bad_primes = E.discriminant().support(); bad_primes
[Fractional ideal (-a), Fractional ideal (7/2*a - 81/2), Fractional ideal (-a - 52), Fractional ideal (2)]
sage: [E.kodaira_symbol(P) for P in bad_primes]
[I0, I1, I1, II]
sage: K.<a> = QuadraticField(-11)
sage: E = EllipticCurve('11a1').change_ring(K)
sage: [E.kodaira_symbol(P) for P in K(11).support()]
[I10]
```
local_data(P=None, proof=None, algorithm='pari')¶

Local data for this elliptic curve at the prime $$P$$.

INPUT:

• P – either None or a prime ideal of the base field of self.
• proof – whether to only use provably correct methods (default controlled by global proof module). Note that the proof module is number_field, not elliptic_curves, since the functions that actually need the flag are in number fields.
• algorithm (string, default: “pari”) – Ignored unless the base field is $$\QQ$$. If “pari”, use the PARI C-library ellglobalred implementation of Tate’s algorithm over $$\QQ$$. If “generic”, use the general number field implementation.

OUTPUT:

If $$P$$ is specified, returns the EllipticCurveLocalData object associated to the prime $$P$$ for this curve. Otherwise, returns a list of such objects, one for each prime $$P$$ in the support of the discriminant of this model.

Note

The model is not required to be integral on input.

For principal $$P$$, a generator is used as a uniformizer, and integrality or minimality at other primes is not affected. For non-principal $$P$$, the minimal model returned will preserve integrality at other primes, but not minimality.

EXAMPLES:

```sage: K.<i> = NumberField(x^2+1)
sage: E = EllipticCurve([1 + i, 0, 1, 0, 0])
sage: E.local_data()
[Local data at Fractional ideal (i - 2):
Reduction type: bad non-split multiplicative
Local minimal model: Elliptic Curve defined by y^2 + (i+1)*x*y + y = x^3 over Number Field in i with defining polynomial x^2 + 1
Minimal discriminant valuation: 1
Conductor exponent: 1
Kodaira Symbol: I1
Tamagawa Number: 1,
Local data at Fractional ideal (-3*i - 2):
Reduction type: bad split multiplicative
Local minimal model: Elliptic Curve defined by y^2 + (i+1)*x*y + y = x^3 over Number Field in i with defining polynomial x^2 + 1
Minimal discriminant valuation: 2
Conductor exponent: 1
Kodaira Symbol: I2
Tamagawa Number: 2]
sage: E.local_data(K.ideal(3))
Local data at Fractional ideal (3):
Reduction type: good
Local minimal model: Elliptic Curve defined by y^2 + (i+1)*x*y + y = x^3 over Number Field in i with defining polynomial x^2 + 1
Minimal discriminant valuation: 0
Conductor exponent: 0
Kodaira Symbol: I0
Tamagawa Number: 1
```

An example raised in #3897:

```sage: E = EllipticCurve([1,1])
sage: E.local_data(3)
Local data at Principal ideal (3) of Integer Ring:
Reduction type: good
Local minimal model: Elliptic Curve defined by y^2 = x^3 + x + 1 over Rational Field
Minimal discriminant valuation: 0
Conductor exponent: 0
Kodaira Symbol: I0
Tamagawa Number: 1
```
local_information(P=None, proof=None)¶

code{local_information} has been renamed code{local_data} and is being deprecated.

local_integral_model(*P)¶

Return a model of self which is integral at the prime ideal $$P$$.

Note

The integrality at other primes is not affected, even if $$P$$ is non-principal.

INPUT:

• *P – a prime ideal, or a list or tuple of primes.

EXAMPLES:

```sage: K.<i> = NumberField(x^2+1)
sage: P1,P2 = K.primes_above(5)
sage: E = EllipticCurve([i/5,i/5,i/5,i/5,i/5])
sage: E.local_integral_model((P1,P2))
Elliptic Curve defined by y^2 + (-i)*x*y + (-25*i)*y = x^3 + 5*i*x^2 + 125*i*x + 3125*i over Number Field in i with defining polynomial x^2 + 1
```
local_minimal_model(P, proof=None, algorithm='pari')¶

Returns a model which is integral at all primes and minimal at $$P$$.

INPUT:

• P – either None or a prime ideal of the base field of self.
• proof – whether to only use provably correct methods (default controlled by global proof module). Note that the proof module is number_field, not elliptic_curves, since the functions that actually need the flag are in number fields.
• algorithm (string, default: “pari”) – Ignored unless the base field is $$\QQ$$. If “pari”, use the PARI C-library ellglobalred implementation of Tate’s algorithm over $$\QQ$$. If “generic”, use the general number field implementation.

OUTPUT:

A model of the curve which is minimal (and integral) at $$P$$.

Note

The model is not required to be integral on input.

For principal $$P$$, a generator is used as a uniformizer, and integrality or minimality at other primes is not affected. For non-principal $$P$$, the minimal model returned will preserve integrality at other primes, but not minimality.

EXAMPLES:

```sage: K.<a>=NumberField(x^2-5)
sage: E=EllipticCurve([20, 225, 750, 625*a + 6875, 31250*a + 46875])
sage: P=K.ideal(a)
sage: E.local_minimal_model(P).ainvs()
(0, 1, 0, a - 33, -2*a + 64)
```
period_lattice(embedding)¶

Returns the period lattice of the elliptic curve for the given embedding of its base field with respect to the differential $$dx/(2y + a_1x + a_3)$$.

INPUT:

• embedding - an embedding of the base number field into $$\RR$$ or $$\CC$$.

Note

The precision of the embedding is ignored: we only use the given embedding to determine which embedding into QQbar to use. Once the lattice has been initialized, periods can be computed to arbitrary precision.

EXAMPLES:

First define a field with two real embeddings:

```sage: K.<a> = NumberField(x^3-2)
sage: E=EllipticCurve([0,0,0,a,2])
sage: embs=K.embeddings(CC); len(embs)
3
```

For each embedding we have a different period lattice:

```sage: E.period_lattice(embs[0])
Period lattice associated to Elliptic Curve defined by y^2 = x^3 + a*x + 2 over Number Field in a with defining polynomial x^3 - 2 with respect to the embedding Ring morphism:
From: Number Field in a with defining polynomial x^3 - 2
To:   Algebraic Field
Defn: a |--> -0.6299605249474365? - 1.091123635971722?*I

sage: E.period_lattice(embs[1])
Period lattice associated to Elliptic Curve defined by y^2 = x^3 + a*x + 2 over Number Field in a with defining polynomial x^3 - 2 with respect to the embedding Ring morphism:
From: Number Field in a with defining polynomial x^3 - 2
To:   Algebraic Field
Defn: a |--> -0.6299605249474365? + 1.091123635971722?*I

sage: E.period_lattice(embs[2])
Period lattice associated to Elliptic Curve defined by y^2 = x^3 + a*x + 2 over Number Field in a with defining polynomial x^3 - 2 with respect to the embedding Ring morphism:
From: Number Field in a with defining polynomial x^3 - 2
To:   Algebraic Field
Defn: a |--> 1.259921049894873?
```

Although the original embeddings have only the default precision, we can obtain the basis with higher precision later:

```sage: L=E.period_lattice(embs[0])
sage: L.basis()
(1.86405007647981 - 0.903761485143226*I, -0.149344633143919 - 2.06619546272945*I)

sage: L.basis(prec=100)
(1.8640500764798108425920506200 - 0.90376148514322594749786960975*I, -0.14934463314391922099120107422 - 2.0661954627294548995621225062*I)
```
rank(verbose=0, lim1=5, lim3=50, limtriv=10, maxprob=20, limbigprime=30)¶

Return the rank of this elliptic curve, if it can be determined.

Note

The optional parameters control the Simon two descent algorithm; see the documentation of simon_two_descent() for more details.

INPUT:

• verbose – 0, 1, 2, or 3 (default: 0), the verbosity level
• lim1 – (default: 5) limit on trivial points on quartics
• lim3 – (default: 50) limit on points on ELS quartics
• limtriv – (default: 10) limit on trivial points on elliptic curve
• maxprob – (default: 20)
• limbigprime – (default: 30) to distinguish between small and large prime numbers. Use probabilistic tests for large primes. If 0, don’t use probabilistic tests.

OUTPUT:

If the upper and lower bounds given by Simon two-descent are the same, then the rank has been uniquely identified and we return this. Otherwise, we raise a ValueError with an error message specifying the upper and lower bounds.

Note

For non-quadratic number fields, this code does return, but it takes a long time.

EXAMPLES:

```sage: K.<a> = NumberField(x^2 + 23, 'a')
sage: E = EllipticCurve(K, '37')
sage: E == loads(dumps(E))
True
sage: E.rank()
2
```

Here is a curve with two-torsion, so here the bounds given by the algorithm do not uniquely determine the rank:

```sage: Qrt5.<rt5>=NumberField(x^2-5)
sage: E=EllipticCurve([0,5-rt5,0,rt5,0])
sage: E.rank()
Traceback (most recent call last):
...
ValueError: There is insufficient data to determine the rank -
2-descent gave lower bound 1 and upper bound 2
```

IMPLEMENTATION:

Uses Denis Simon’s PARI/GP scripts from http://www.math.unicaen.fr/~simon/.

rank_bounds(verbose=0, lim1=5, lim3=50, limtriv=10, maxprob=20, limbigprime=30)¶

Returns the lower and upper bounds using simon_two_descent(). The results of simon_two_descent() are cached.

Note

The optional parameters control the Simon two descent algorithm; see the documentation of simon_two_descent() for more details.

INPUT:

• verbose – 0, 1, 2, or 3 (default: 0), the verbosity level
• lim1 – (default: 5) limit on trivial points on quartics
• lim3 – (default: 50) limit on points on ELS quartics
• limtriv – (default: 10) limit on trivial points on elliptic curve
• maxprob – (default: 20)
• limbigprime – (default: 30) to distinguish between small and large prime numbers. Use probabilistic tests for large primes. If 0, don’t use probabilistic tests.

OUTPUT:

lower and upper bounds

Note

For non-quadratic number fields, this code does return, but it takes a long time.

EXAMPLES:

```sage: K.<a> = NumberField(x^2 + 23, 'a')
sage: E = EllipticCurve(K, '37')
sage: E == loads(dumps(E))
True
sage: E.rank_bounds()
(2, 2)
```

Here is a curve with two-torsion, so here the algorithm gives bounds on the rank:

```sage: Qrt5.<rt5>=NumberField(x^2-5)
sage: E=EllipticCurve([0,5-rt5,0,rt5,0])
sage: E.rank_bounds()
(1, 2)
```

IMPLEMENTATION:

Uses Denis Simon’s PARI/GP scripts from http://www.math.unicaen.fr/~simon/.

reduction(place)¶

Return the reduction of the elliptic curve at a place of good reduction.

INPUT:

• place – a prime ideal in the base field of the curve

OUTPUT:

An elliptic curve over a finite field, the residue field of the place.

EXAMPLES:

```sage: K.<i> = QuadraticField(-1)
sage: EK = EllipticCurve([0,0,0,i,i+3])
sage: v = K.fractional_ideal(2*i+3)
sage: EK.reduction(v)
Elliptic Curve defined by y^2  = x^3 + 5*x + 8 over Residue field of Fractional ideal (2*i + 3)
sage: EK.reduction(K.ideal(1+i))
Traceback (most recent call last):
...
ValueError: The curve must have good reduction at the place.
sage: EK.reduction(K.ideal(2))
Traceback (most recent call last):
...
ValueError: The ideal must be prime.
sage: K=QQ.extension(x^2+x+1,"a")
sage: E=EllipticCurve([1024*K.0,1024*K.0])
sage: E.reduction(2*K)
Elliptic Curve defined by y^2 + (abar+1)*y = x^3 over Residue field in abar of Fractional ideal (2)
```
regulator_of_points(points=[], precision=None)¶

Returns the regulator of the given points on this curve.

INPUT:

• points -(default: empty list) a list of points on this curve
• precision - int or None (default: None): the precision in bits of the result (default real precision if None)

EXAMPLES:

```sage: E = EllipticCurve('37a1')
sage: P = E(0,0)
sage: Q = E(1,0)
sage: E.regulator_of_points([P,Q])
0.000000000000000
sage: 2*P==Q
True
```
```sage: E = EllipticCurve('5077a1')
sage: points = [E.lift_x(x) for x in [-2,-7/4,1]]
sage: E.regulator_of_points(points)
0.417143558758384
sage: E.regulator_of_points(points,precision=100)
0.41714355875838396981711954462
```
```sage: E = EllipticCurve('389a')
sage: E.regulator_of_points()
1.00000000000000
sage: points = [P,Q] = [E(-1,1),E(0,-1)]
sage: E.regulator_of_points(points)
0.152460177943144
sage: E.regulator_of_points(points, precision=100)
0.15246017794314375162432475705
sage: E.regulator_of_points(points, precision=200)
0.15246017794314375162432475704945582324372707748663081784028
sage: E.regulator_of_points(points, precision=300)
0.152460177943143751624324757049455823243727077486630817840280980046053225683562463604114816
```

Examples over number fields:

```sage: K.<a> = QuadraticField(97)
sage: E = EllipticCurve(K,[1,1])
sage: P = E(0,1)
sage: P.height()
0.476223106404866
sage: E.regulator_of_points([P])
0.476223106404866
```
```sage: E = EllipticCurve('11a1')
sage: x = polygen(QQ)
sage: K.<t> = NumberField(x^2+47)
sage: EK = E.base_extend(K)
sage: T = EK(5,5)
sage: T.order()
5
sage: P = EK(-2, -1/2*t - 1/2)
sage: P.order()
+Infinity
sage: EK.regulator_of_points([P,T]) # random very small output
-1.23259516440783e-32
sage: EK.regulator_of_points([P,T]).abs() < 1e-30
True
```
```sage: E = EllipticCurve('389a1')
sage: P,Q = E.gens()
sage: E.regulator_of_points([P,Q])
0.152460177943144
sage: K.<t> = NumberField(x^2+47)
sage: EK = E.base_extend(K)
sage: EK.regulator_of_points([EK(P),EK(Q)])
0.152460177943144
```
```sage: K.<i> = QuadraticField(-1)
sage: E = EllipticCurve([0,0,0,i,i])
sage: P = E(-9+4*i,-18-25*i)
sage: Q = E(i,-i)
sage: E.height_pairing_matrix([P,Q])
[  2.16941934493768 -0.870059380421505]
[-0.870059380421505  0.424585837470709]
sage: E.regulator_of_points([P,Q])
0.164101403936070
```
simon_two_descent(verbose=0, lim1=5, lim3=50, limtriv=10, maxprob=20, limbigprime=30)¶

Computes lower and upper bounds on the rank of the Mordell-Weil group, and a list of independent points. Used internally by the rank(), rank_bounds() and gens() methods.

INPUT:

• verbose – 0, 1, 2, or 3 (default: 0), the verbosity level
• lim1 – (default: 5) limit on trivial points on quartics
• lim3 – (default: 50) limit on points on ELS quartics
• limtriv – (default: 10) limit on trivial points on elliptic curve
• maxprob – (default: 20)
• limbigprime – (default: 30) to distinguish between small and large prime numbers. Use probabilistic tests for large primes. If 0, don’t use probabilistic tests.

OUTPUT:

(lower, upper, list) where lower is a lower bound on the rank, upper is an upper bound (the 2-Selmer rank) and list is a list of independent points on the Weierstrass model. The length of list is equal to either lower, or lower-1, since when lower is less than upper and of different parity, the value of lower is increased by 1.

Note

For non-quadratic number fields, this code does return, but it takes a long time.

ALGORITHM:

Uses Denis Simon’s PARI/GP scripts from http://www.math.unicaen.fr/~simon/.

EXAMPLES:

```sage: K.<a> = NumberField(x^2 + 23, 'a')
sage: E = EllipticCurve(K, '37')
sage: E == loads(dumps(E))
True
sage: E.simon_two_descent()
(2, 2, [(-1 : 0 : 1), (1/2*a - 5/2 : -1/2*a - 13/2 : 1)])
sage: E.simon_two_descent(lim1=3, lim3=20, limtriv=5, maxprob=7, limbigprime=10)
(2, 2, [(-1 : 0 : 1), (-1/8*a + 5/8 : -3/16*a - 9/16 : 1)])
```
```sage: K.<a> = NumberField(x^2 + 7, 'a')
sage: E = EllipticCurve(K, [0,0,0,1,a]); E
Elliptic Curve defined by y^2  = x^3 + x + a over Number Field in a with defining polynomial x^2 + 7

sage: v = E.simon_two_descent(verbose=1); v
courbe elliptique : Y^2 = x^3 + Mod(1, y^2 + 7)*x + Mod(y, y^2 + 7)
points triviaux sur la courbe = [[1, 1, 0], [Mod(1/2*y + 3/2, y^2 + 7), Mod(-y - 2, y^2 + 7), 1]]
#S(E/K)[2]    = 2
#E(K)/2E(K)   = 2
#III(E/K)[2]  = 1
rang(E/K)     = 1
listpointsmwr = [[Mod(1/2*y + 3/2, y^2 + 7), Mod(-y - 2, y^2 + 7), 1]]
(1, 1, [(1/2*a + 3/2 : -a - 2 : 1)])

sage: v = E.simon_two_descent(verbose=2)  # random output
K = bnfinit(y^2 + 7);
a = Mod(y,K.pol);
bnfellrank(K, [0,0,0,1,a]);
courbe elliptique : Y^2 = x^3 + Mod(1, y^2 + 7)*x + Mod(y, y^2 + 7)
A = 0
B = Mod(1, y^2 + 7)
C = Mod(y, y^2 + 7)
LS2gen = [Mod(Mod(-5, y^2 + 7)*x^2 + Mod(-3*y, y^2 + 7)*x + Mod(8, y^2 + 7), x^3 + Mod(1, y^2 + 7)*x + Mod(y, y^2 + 7)), Mod(Mod(1, y^2 + 7)*x^2 + Mod(1/2*y - 1/2, y^2 + 7)*x - 1, x^3 + Mod(1, y^2 + 7)*x + Mod(y, y^2 + 7))]
#LS2gen = 2
Recherche de points triviaux sur la courbe
points triviaux sur la courbe = [[1, 1, 0], [Mod(1/2*y + 3/2, y^2 + 7), Mod(-y - 2, y^2 + 7), 1]]
zc = Mod(Mod(-5, y^2 + 7)*x^2 + Mod(-3*y, y^2 + 7)*x + Mod(8, y^2 + 7), x^3 + Mod(1, y^2 + 7)*x + Mod(y, y^2 + 7))
symbole de Hilbert (Mod(2, y^2 + 7),Mod(-5, y^2 + 7)) = -1
zc = Mod(Mod(1, y^2 + 7)*x^2 + Mod(1/2*y - 1/2, y^2 + 7)*x + Mod(-1, y^2 + 7), x^3 + Mod(1, y^2 + 7)*x + Mod(y, y^2 + 7))
symbole de Hilbert (Mod(-2*y + 2, y^2 + 7),Mod(1, y^2 + 7)) = 0
sol de Legendre = [1, 0, 1]~
zc*z1^2 = Mod(Mod(2*y - 2, y^2 + 7)*x + Mod(2*y + 10, y^2 + 7), x^3 + Mod(1, y^2 + 7)*x + Mod(y, y^2 + 7))
quartique : (-1/2*y + 1/2)*Y^2 = x^4 + (-3*y - 15)*x^2 + (-8*y - 16)*x + (-11/2*y - 15/2)
reduite: Y^2 = (-1/2*y + 1/2)*x^4 - 4*x^3 + (-3*y + 3)*x^2 + (2*y - 2)*x + (1/2*y + 3/2)
non ELS en [2, [0, 1]~, 1, 1, [1, 1]~]
zc = Mod(Mod(1, y^2 + 7)*x^2 + Mod(1/2*y + 1/2, y^2 + 7)*x + Mod(-1, y^2 + 7), x^3 + Mod(1, y^2 + 7)*x + Mod(y, y^2 + 7))
vient du point trivial [Mod(1/2*y + 3/2, y^2 + 7), Mod(-y - 2, y^2 + 7), 1]
m1 = 1
m2 = 1
#S(E/K)[2]    = 2
#E(K)/2E(K)   = 2
#III(E/K)[2]  = 1
rang(E/K)     = 1
listpointsmwr = [[Mod(1/2*y + 3/2, y^2 + 7), Mod(-y - 2, y^2 + 7), 1]]
v =  [1, 1, [[Mod(1/2*y + 3/2, y^2 + 7), Mod(-y - 2, y^2 + 7)]]]
sage: v
(1, 1, [(1/2*a + 3/2 : -a - 2 : 1)])
```

A curve with 2-torsion:

```sage: K.<a> = NumberField(x^2 + 7, 'a')
sage: E = EllipticCurve(K, '15a')
sage: v = E.simon_two_descent(); v  # long time (about 10 seconds), points can vary
(1, 3, [...])
```

A failure in the PARI/GP script ell.gp (VERSION 25/03/2009) is reported:

```sage: K = CyclotomicField(43).subfields(3)[0][0]
sage: E = EllipticCurve(K, '37')
sage: E.simon_two_descent()
Traceback (most recent call last):
...
RuntimeError: 
  ***   at top-level: ans=bnfellrank(K,[0,0,1,
  ***                     ^--------------------
  ***   in function bnfellrank: ...eqtheta,rnfeq,bbnf];rang=
  ***   bnfell2descent_gen(b
  ***   ^--------------------
  ***   in function bnfell2descent_gen: ...riv,r=nfsqrt(nf,norm(zc))
  ***   [1];if(DEBUGLEVEL_el
  ***   ^--------------------
  ***   array index (1) out of allowed range [none].
An error occurred while running Simon's 2-descent program```
tamagawa_exponent(P, proof=None)¶

Returns the Tamagawa index of this elliptic curve at the prime $$P$$.

INPUT:

• P – either None or a prime ideal of the base field of self.
• proof – whether to only use provably correct methods (default controlled by global proof module). Note that the proof module is number_field, not elliptic_curves, since the functions that actually need the flag are in number fields.

OUTPUT:

(positive integer) The Tamagawa index of the curve at P.

EXAMPLES:

```sage: K.<a>=NumberField(x^2-5)
sage: E=EllipticCurve([20, 225, 750, 625*a + 6875, 31250*a + 46875])
sage: [E.tamagawa_exponent(P) for P in E.discriminant().support()]
[1, 1, 1, 1]
sage: K.<a> = QuadraticField(-11)
sage: E = EllipticCurve('11a1').change_ring(K)
sage: [E.tamagawa_exponent(P) for P in K(11).support()]
[10]
```
tamagawa_number(P, proof=None)¶

Returns the Tamagawa number of this elliptic curve at the prime $$P$$.

INPUT:

• P – either None or a prime ideal of the base field of self.
• proof – whether to only use provably correct methods (default controlled by global proof module). Note that the proof module is number_field, not elliptic_curves, since the functions that actually need the flag are in number fields.

OUTPUT:

(positive integer) The Tamagawa number of the curve at $$P$$.

EXAMPLES:

```sage: K.<a>=NumberField(x^2-5)
sage: E=EllipticCurve([20, 225, 750, 625*a + 6875, 31250*a + 46875])
sage: [E.tamagawa_number(P) for P in E.discriminant().support()]
[1, 1, 1, 1]
sage: K.<a> = QuadraticField(-11)
sage: E = EllipticCurve('11a1').change_ring(K)
sage: [E.tamagawa_number(P) for P in K(11).support()]
[10]
```
tamagawa_numbers()¶

Return a list of all Tamagawa numbers for all prime divisors of the conductor (in order).

EXAMPLES:

```sage: e = EllipticCurve('30a1')
sage: e.tamagawa_numbers()
[2, 3, 1]
sage: vector(e.tamagawa_numbers())
(2, 3, 1)
sage: K.<a>=NumberField(x^2+3)
sage: eK = e.base_extend(K)
sage: eK.tamagawa_numbers()
[4, 6, 1]
```
tamagawa_product_bsd()¶

Given an elliptic curve $$E$$ over a number field $$K$$, this function returns the integer $$C(E/K)$$ that appears in the Birch and Swinnerton-Dyer conjecture accounting for the local information at finite places. If the model is a global minimal model then $$C(E/K)$$ is simply the product of the Tamagawa numbers $$c_v$$ where $$v$$ runs over all prime ideals of $$K$$. Otherwise, if the model has to be changed at a place $$v$$ a correction factor appears. The definition is such that $$C(E/K)$$ times the periods at the infinite places is invariant under change of the Weierstrass model. See [Ta2] and [Do] for details.

Note

This definition is slightly different from the definition of tamagawa_product for curves defined over $$\QQ$$. Over the rational number it is always defined to be the product of the Tamagawa numbers, so the two definitions only agree when the model is global minimal.

OUTPUT:

A rational number

EXAMPLES:

```sage: K.<i> = NumberField(x^2+1)
sage: E = EllipticCurve([0,2+i])
sage: E.tamagawa_product_bsd()
1

sage: E = EllipticCurve([(2*i+1)^2,i*(2*i+1)^7])
sage: E.tamagawa_product_bsd()
4
```

An example where the Neron model changes over K:

```sage: K.<t> = NumberField(x^5-10*x^3+5*x^2+10*x+1)
sage: E = EllipticCurve(K,'75a1')
sage: E.tamagawa_product_bsd()
5
sage: da = E.local_data()
sage: [dav.tamagawa_number() for dav in da]
[1, 1]
```

An example over $$\mathbb{Q}$$ (trac #9413):

```sage: E = EllipticCurve('30a')
sage: E.tamagawa_product_bsd()
6
```

REFERENCES:

• [Ta2] Tate, John, On the conjectures of Birch and Swinnerton-Dyer and a geometric analog. Seminaire Bourbaki, Vol. 9, Exp. No. 306.
• [Do] Dokchitser, Tim and Vladimir, On the Birch-Swinnerton-Dyer quotients modulo squares, Annals of Math., 2010.
torsion_order()¶

Returns the order of the torsion subgroup of this elliptic curve.

OUTPUT:

(integer) the order of the torsion subgroup of this elliptic curve.

EXAMPLES:

```sage: E = EllipticCurve('11a1')
sage: K.<t> = NumberField(x^4 + x^3 + 11*x^2 + 41*x + 101)
sage: EK = E.base_extend(K)
sage: EK.torsion_order()
25
```
```sage: E = EllipticCurve('15a1')
sage: K.<t> = NumberField(x^2 + 2*x + 10)
sage: EK = E.base_extend(K)
sage: EK.torsion_order()
16
```
```sage: E = EllipticCurve('19a1')
sage: K.<t> = NumberField(x^9-3*x^8-4*x^7+16*x^6-3*x^5-21*x^4+5*x^3+7*x^2-7*x+1)
sage: EK = E.base_extend(K)
sage: EK.torsion_order()
9
```
```sage: K.<i> = QuadraticField(-1)
sage: EK = EllipticCurve([0,0,0,i,i+3])
sage: EK.torsion_order()
1
```
torsion_points()¶

Returns a list of the torsion points of this elliptic curve.

OUTPUT:

(list) A sorted list of the torsion points.

EXAMPLES:

```sage: E = EllipticCurve('11a1')
sage: E.torsion_points()
[(0 : 1 : 0), (5 : -6 : 1), (5 : 5 : 1), (16 : -61 : 1), (16 : 60 : 1)]
sage: K.<t> = NumberField(x^4 + x^3 + 11*x^2 + 41*x + 101)
sage: EK = E.base_extend(K)
sage: EK.torsion_points()
[(16 : 60 : 1), 
 (5 : 5 : 1), 
 (5 : -6 : 1),
 (16 : -61 : 1),
 (t : 1/11*t^3 + 6/11*t^2 + 19/11*t + 48/11 : 1), 
 (-3/55*t^3 - 7/55*t^2 - 2/55*t - 133/55 : 6/55*t^3 + 3/55*t^2 + 25/11*t + 156/55 : 1), 
 (-9/121*t^3 - 21/121*t^2 - 127/121*t - 377/121 : -7/121*t^3 + 24/121*t^2 + 197/121*t + 16/121 : 1), 
 (5/121*t^3 - 14/121*t^2 - 158/121*t - 453/121 : -49/121*t^3 - 129/121*t^2 - 315/121*t - 207/121 : 1),
 (10/121*t^3 + 49/121*t^2 + 168/121*t + 73/121 : 32/121*t^3 + 60/121*t^2 - 261/121*t - 807/121 : 1), 
 (1/11*t^3 - 5/11*t^2 + 19/11*t - 40/11 : -6/11*t^3 - 3/11*t^2 - 26/11*t - 321/11 : 1), 
 (14/121*t^3 - 15/121*t^2 + 90/121*t + 232/121 : 16/121*t^3 - 69/121*t^2 + 293/121*t - 46/121 : 1), 
 (3/55*t^3 + 7/55*t^2 + 2/55*t + 78/55 : 7/55*t^3 - 24/55*t^2 + 9/11*t + 17/55 : 1), 
 (-5/121*t^3 + 36/121*t^2 - 84/121*t + 24/121 : 34/121*t^3 - 27/121*t^2 + 305/121*t + 708/121 : 1),
 (-26/121*t^3 + 20/121*t^2 - 219/121*t - 995/121 : 15/121*t^3 + 156/121*t^2 - 232/121*t + 2766/121 : 1), 
 (1/11*t^3 - 5/11*t^2 + 19/11*t - 40/11 : 6/11*t^3 + 3/11*t^2 + 26/11*t + 310/11 : 1), 
 (-26/121*t^3 + 20/121*t^2 - 219/121*t - 995/121 : -15/121*t^3 - 156/121*t^2 + 232/121*t - 2887/121 : 1), 
 (-5/121*t^3 + 36/121*t^2 - 84/121*t + 24/121 : -34/121*t^3 + 27/121*t^2 - 305/121*t - 829/121 : 1), 
 (3/55*t^3 + 7/55*t^2 + 2/55*t + 78/55 : -7/55*t^3 + 24/55*t^2 - 9/11*t - 72/55 : 1), 
 (14/121*t^3 - 15/121*t^2 + 90/121*t + 232/121 : -16/121*t^3 + 69/121*t^2 - 293/121*t - 75/121 : 1), 
 (t : -1/11*t^3 - 6/11*t^2 - 19/11*t - 59/11 : 1),
 (10/121*t^3 + 49/121*t^2 + 168/121*t + 73/121 : -32/121*t^3 - 60/121*t^2 + 261/121*t + 686/121 : 1), 
 (5/121*t^3 - 14/121*t^2 - 158/121*t - 453/121 : 49/121*t^3 + 129/121*t^2 + 315/121*t + 86/121 : 1), 
 (-9/121*t^3 - 21/121*t^2 - 127/121*t - 377/121 : 7/121*t^3 - 24/121*t^2 - 197/121*t - 137/121 : 1), 
 (-3/55*t^3 - 7/55*t^2 - 2/55*t - 133/55 : -6/55*t^3 - 3/55*t^2 - 25/11*t - 211/55 : 1), 
 (0 : 1 : 0)]
```
```sage: E = EllipticCurve('15a1')
sage: K.<t> = NumberField(x^2 + 2*x + 10)
sage: EK = E.base_extend(K)
sage: EK.torsion_points()
[(8 : 18 : 1), 
 (3 : -2 : 1), 
 (8 : -27 : 1), 
 (t : t - 5 : 1), 
 (1/2 : 5/4*t + 1/2 : 1), 
 (-t - 2 : 2*t + 8 : 1), 
 (-7 : -5*t - 2 : 1), 
 (-1 : 0 : 1), 
 (-2 : 3 : 1), 
 (-13/4 : 9/8 : 1), 
 (-2 : -2 : 1), 
 (t : -2*t + 4 : 1),
 (-7 : 5*t + 8 : 1),
 (-t - 2 : -t - 7 : 1), 
 (1/2 : -5/4*t - 2 : 1),
 (0 : 1 : 0)]
```
```sage: K.<i> = QuadraticField(-1)
sage: EK = EllipticCurve(K,[0,0,0,0,-1])
sage: EK.torsion_points ()
[(-2 : -3*i : 1), (0 : -i : 1), (1 : 0 : 1), (0 : i : 1), (-2 : 3*i : 1), (0 : 1 : 0)]
```
torsion_subgroup()¶

Returns the torsion subgroup of this elliptic curve.

OUTPUT:

(EllipticCurveTorsionSubgroup) The EllipticCurveTorsionSubgroup associated to this elliptic curve.

EXAMPLES:

```sage: E = EllipticCurve('11a1')
sage: K.<t>=NumberField(x^4 + x^3 + 11*x^2 + 41*x + 101)
sage: EK=E.base_extend(K)
sage: tor = EK.torsion_subgroup()
sage: tor
Torsion Subgroup isomorphic to Z/5 + Z/5 associated to the Elliptic Curve defined by y^2 + y = x^3 + (-1)*x^2 + (-10)*x + (-20) over Number Field in t with defining polynomial x^4 + x^3 + 11*x^2 + 41*x + 101
sage: tor.gens()
((16 : 60 : 1), (t : 1/11*t^3 + 6/11*t^2 + 19/11*t + 48/11 : 1))
```
```sage: E = EllipticCurve('15a1')
sage: K.<t>=NumberField(x^2 + 2*x + 10)
sage: EK=E.base_extend(K)
sage: EK.torsion_subgroup()
Torsion Subgroup isomorphic to Z/4 + Z/4 associated to the Elliptic Curve defined by y^2 + x*y + y = x^3 + x^2 + (-10)*x + (-10) over Number Field in t with defining polynomial x^2 + 2*x + 10
```
```sage: E = EllipticCurve('19a1')
sage: K.<t>=NumberField(x^9-3*x^8-4*x^7+16*x^6-3*x^5-21*x^4+5*x^3+7*x^2-7*x+1)
sage: EK=E.base_extend(K)
sage: EK.torsion_subgroup()
Torsion Subgroup isomorphic to Z/9 associated to the Elliptic Curve defined by y^2 + y = x^3 + x^2 + (-9)*x + (-15) over Number Field in t with defining polynomial x^9 - 3*x^8 - 4*x^7 + 16*x^6 - 3*x^5 - 21*x^4 + 5*x^3 + 7*x^2 - 7*x + 1
```
```sage: K.<i> = QuadraticField(-1)
sage: EK = EllipticCurve([0,0,0,i,i+3])
sage: EK.torsion_subgroup ()
Torsion Subgroup isomorphic to Trivial group associated to the Elliptic Curve defined by y^2  = x^3 + i*x + (i+3) over Number Field in i with defining polynomial x^2 + 1
```

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