Let \(E\) be an elliptic curve over a number field \(K\) (including \(\QQ\)). There are several local invariants at a finite place \(v\) that can be computed via Tate’s algorithm (see [Sil2] IV.9.4 or [Ta]).
These include the type of reduction (good, additive, multiplicative), a minimal equation of \(E\) over \(K_v\), the Tamagawa number \(c_v\), defined to be the index \([E(K_v):E^0(K_v)]\) of the points with good reduction among the local points, and the exponent of the conductor \(f_v\).
The functions in this file will typically be called by using local_data.
EXAMPLES:
sage: K.<i> = NumberField(x^2+1)
sage: E = EllipticCurve([(2+i)^2,(2+i)^7])
sage: pp = K.fractional_ideal(2+i)
sage: da = E.local_data(pp)
sage: da.has_bad_reduction()
True
sage: da.has_multiplicative_reduction()
False
sage: da.kodaira_symbol()
I0*
sage: da.tamagawa_number()
4
sage: da.minimal_model()
Elliptic Curve defined by y^2 = x^3 + (4*i+3)*x + (-29*i-278) over Number Field in i with defining polynomial x^2 + 1
An example to show how the Neron model can change as one extends the field:
sage: E = EllipticCurve([0,-1])
sage: E.local_data(2)
Local data at Principal ideal (2) of Integer Ring:
Reduction type: bad additive
Local minimal model: Elliptic Curve defined by y^2 = x^3 - 1 over Rational Field
Minimal discriminant valuation: 4
Conductor exponent: 4
Kodaira Symbol: II
Tamagawa Number: 1
sage: EK = E.base_extend(K)
sage: EK.local_data(1+i)
Local data at Fractional ideal (i + 1):
Reduction type: bad additive
Local minimal model: Elliptic Curve defined by y^2 = x^3 + (-1) over Number Field in i with defining polynomial x^2 + 1
Minimal discriminant valuation: 8
Conductor exponent: 2
Kodaira Symbol: IV*
Tamagawa Number: 3
Or how the minimal equation changes:
sage: E = EllipticCurve([0,8])
sage: E.is_minimal()
True
sage: EK = E.base_extend(K)
sage: da = EK.local_data(1+i)
sage: da.minimal_model()
Elliptic Curve defined by y^2 = x^3 + (-i) over Number Field in i with defining polynomial x^2 + 1
REFERENCES:
AUTHORS:
Bases: sage.structure.sage_object.SageObject
The class for the local reduction data of an elliptic curve.
Currently supported are elliptic curves defined over \(\QQ\), and elliptic curves defined over a number field, at an arbitrary prime or prime ideal.
INPUT:
Note
This function is not normally called directly by users, who may access the data via methods of the EllipticCurve classes.
EXAMPLES:
sage: from sage.schemes.elliptic_curves.ell_local_data import EllipticCurveLocalData
sage: E = EllipticCurve('14a1')
sage: EllipticCurveLocalData(E,2)
Local data at Principal ideal (2) of Integer Ring:
Reduction type: bad non-split multiplicative
Local minimal model: Elliptic Curve defined by y^2 + x*y + y = x^3 + 4*x - 6 over Rational Field
Minimal discriminant valuation: 6
Conductor exponent: 1
Kodaira Symbol: I6
Tamagawa Number: 2
Return the type of bad reduction of this reduction data.
OUTPUT:
(int or None):
EXAMPLES:
sage: E=EllipticCurve('14a1')
sage: [(p,E.local_data(p).bad_reduction_type()) for p in prime_range(15)]
[(2, -1), (3, None), (5, None), (7, 1), (11, None), (13, None)]
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.local_data(p).bad_reduction_type()) for p in [P17a,P17b]]
[(Fractional ideal (4*a^2 - 2*a + 1), None), (Fractional ideal (2*a + 1), 0)]
Return the valuation of the conductor from this local reduction data.
EXAMPLES:
sage: from sage.schemes.elliptic_curves.ell_local_data import EllipticCurveLocalData
sage: E = EllipticCurve([0,0,0,0,64]); E
Elliptic Curve defined by y^2 = x^3 + 64 over Rational Field
sage: data = EllipticCurveLocalData(E,2)
sage: data.conductor_valuation()
2
Return the valuation of the minimal discriminant from this local reduction data.
EXAMPLES:
sage: from sage.schemes.elliptic_curves.ell_local_data import EllipticCurveLocalData
sage: E = EllipticCurve([0,0,0,0,64]); E
Elliptic Curve defined by y^2 = x^3 + 64 over Rational Field
sage: data = EllipticCurveLocalData(E,2)
sage: data.discriminant_valuation()
4
Return True if there is additive reduction.
EXAMPLES:
sage: E = EllipticCurve('27a1')
sage: [(p,E.local_data(p).has_additive_reduction()) 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.local_data(p).has_additive_reduction()) for p in [P17a,P17b]]
[(Fractional ideal (4*a^2 - 2*a + 1), False),
(Fractional ideal (2*a + 1), True)]
Return True if there is bad reduction.
EXAMPLES:
sage: E = EllipticCurve('14a1')
sage: [(p,E.local_data(p).has_bad_reduction()) 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.local_data(p).has_bad_reduction()) for p in [P17a,P17b]]
[(Fractional ideal (4*a^2 - 2*a + 1), False),
(Fractional ideal (2*a + 1), True)]
Return True if there is good reduction.
EXAMPLES:
sage: E = EllipticCurve('14a1')
sage: [(p,E.local_data(p).has_good_reduction()) 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.local_data(p).has_good_reduction()) for p in [P17a,P17b]]
[(Fractional ideal (4*a^2 - 2*a + 1), True),
(Fractional ideal (2*a + 1), False)]
Return True if there is multiplicative reduction.
Note
See also has_split_multiplicative_reduction() and has_nonsplit_multiplicative_reduction().
EXAMPLES:
sage: E = EllipticCurve('14a1')
sage: [(p,E.local_data(p).has_multiplicative_reduction()) 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.local_data(p).has_multiplicative_reduction()) for p in [P17a,P17b]]
[(Fractional ideal (4*a^2 - 2*a + 1), False), (Fractional ideal (2*a + 1), False)]
Return True if there is non-split multiplicative reduction.
EXAMPLES:
sage: E = EllipticCurve('14a1')
sage: [(p,E.local_data(p).has_nonsplit_multiplicative_reduction()) 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.local_data(p).has_nonsplit_multiplicative_reduction()) for p in [P17a,P17b]]
[(Fractional ideal (4*a^2 - 2*a + 1), False), (Fractional ideal (2*a + 1), False)]
Return True if there is split multiplicative reduction.
EXAMPLES:
sage: E = EllipticCurve('14a1')
sage: [(p,E.local_data(p).has_split_multiplicative_reduction()) 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.local_data(p).has_split_multiplicative_reduction()) for p in [P17a,P17b]]
[(Fractional ideal (4*a^2 - 2*a + 1), False),
(Fractional ideal (2*a + 1), False)]
Return the Kodaira symbol from this local reduction data.
EXAMPLES:
sage: from sage.schemes.elliptic_curves.ell_local_data import EllipticCurveLocalData
sage: E = EllipticCurve([0,0,0,0,64]); E
Elliptic Curve defined by y^2 = x^3 + 64 over Rational Field
sage: data = EllipticCurveLocalData(E,2)
sage: data.kodaira_symbol()
IV
Return the (local) minimal model from this local reduction data.
INPUT:
EXAMPLES:
sage: from sage.schemes.elliptic_curves.ell_local_data import EllipticCurveLocalData
sage: E = EllipticCurve([0,0,0,0,64]); E
Elliptic Curve defined by y^2 = x^3 + 64 over Rational Field
sage: data = EllipticCurveLocalData(E,2)
sage: data.minimal_model()
Elliptic Curve defined by y^2 = x^3 + 1 over Rational Field
sage: data.minimal_model() == E.local_minimal_model(2)
True
To demonstrate the behaviour of the parameter reduce:
sage: K.<a> = NumberField(x^3+x+1)
sage: E = EllipticCurve(K, [0, 0, a, 0, 1])
sage: E.local_data(K.ideal(a-1)).minimal_model()
Elliptic Curve defined by y^2 + a*y = x^3 + 1 over Number Field in a with defining polynomial x^3 + x + 1
sage: E.local_data(K.ideal(a-1)).minimal_model(reduce=False)
Elliptic Curve defined by y^2 + (a+2)*y = x^3 + 3*x^2 + 3*x + (-a+1) over Number Field in a with defining polynomial x^3 + x + 1
sage: E = EllipticCurve([2, 1, 0, -2, -1])
sage: E.local_data(ZZ.ideal(2), algorithm="generic").minimal_model(reduce=False)
Elliptic Curve defined by y^2 + 2*x*y + 2*y = x^3 + x^2 - 4*x - 2 over Rational Field
sage: E.local_data(ZZ.ideal(2), algorithm="pari").minimal_model(reduce=False)
Traceback (most recent call last):
...
ValueError: the argument reduce must not be False if algorithm=pari is used
sage: E.local_data(ZZ.ideal(2), algorithm="generic").minimal_model()
Elliptic Curve defined by y^2 = x^3 - x^2 - 3*x + 2 over Rational Field
sage: E.local_data(ZZ.ideal(2), algorithm="pari").minimal_model()
Elliptic Curve defined by y^2 = x^3 - x^2 - 3*x + 2 over Rational Field
sage: t = QQ['t'].0
sage: K.<g> = NumberField(t^4 - t^3-3*t^2 - t +1)
sage: E = EllipticCurve([-2*g^3 + 10/3*g^2 + 3*g - 2/3, -11/9*g^3 + 34/9*g^2 - 7/3*g + 4/9, -11/9*g^3 + 34/9*g^2 - 7/3*g + 4/9, 0, 0])
sage: vv = K.fractional_ideal(g^2 - g - 2)
sage: E.local_data(vv).minimal_model()
Elliptic Curve defined by y^2 + (-2*g^3+10/3*g^2+3*g-2/3)*x*y + (-11/9*g^3+34/9*g^2-7/3*g+4/9)*y = x^3 + (-11/9*g^3+34/9*g^2-7/3*g+4/9)*x^2 over Number Field in g with defining polynomial t^4 - t^3 - 3*t^2 - t + 1
Return the prime ideal associated with this local reduction data.
EXAMPLES:
sage: from sage.schemes.elliptic_curves.ell_local_data import EllipticCurveLocalData
sage: E = EllipticCurve([0,0,0,0,64]); E
Elliptic Curve defined by y^2 = x^3 + 64 over Rational Field
sage: data = EllipticCurveLocalData(E,2)
sage: data.prime()
Principal ideal (2) of Integer Ring
Return the Tamagawa index from this local reduction data.
This is the exponent of \(E(K_v)/E^0(K_v)\); in most cases it is the same as the Tamagawa index.
EXAMPLES:
sage: from sage.schemes.elliptic_curves.ell_local_data import EllipticCurveLocalData
sage: E = EllipticCurve('816a1')
sage: data = EllipticCurveLocalData(E,2)
sage: data.kodaira_symbol()
I2*
sage: data.tamagawa_number()
4
sage: data.tamagawa_exponent()
2
sage: E = EllipticCurve('200c4')
sage: data = EllipticCurveLocalData(E,5)
sage: data.kodaira_symbol()
I4*
sage: data.tamagawa_number()
4
sage: data.tamagawa_exponent()
2
Return the Tamagawa number from this local reduction data.
This is the index \([E(K_v):E^0(K_v)]\).
EXAMPLES:
sage: from sage.schemes.elliptic_curves.ell_local_data import EllipticCurveLocalData
sage: E = EllipticCurve([0,0,0,0,64]); E
Elliptic Curve defined by y^2 = x^3 + 64 over Rational Field
sage: data = EllipticCurveLocalData(E,2)
sage: data.tamagawa_number()
3
Function to check that \(P\) determines a prime of \(K\), and return that ideal.
INPUT:
OUTPUT:
Note
If \(P\) is not a prime and does not generate a prime, a TypeError is raised.
EXAMPLES:
sage: from sage.schemes.elliptic_curves.ell_local_data import check_prime
sage: check_prime(QQ,3)
3
sage: check_prime(QQ,ZZ.ideal(31))
31
sage: K.<a>=NumberField(x^2-5)
sage: check_prime(K,a)
Fractional ideal (a)
sage: check_prime(K,a+1)
Fractional ideal (a + 1)
sage: [check_prime(K,P) for P in K.primes_above(31)]
[Fractional ideal (5/2*a + 1/2), Fractional ideal (5/2*a - 1/2)]