Formal groups of elliptic curves.

AUTHORS:

  • William Stein: original implementations
  • David Harvey: improved asymptotics of some methods
  • Nick Alexander: separation from ell_generic.py, bugfixes and docstrings
class sage.schemes.elliptic_curves.formal_group.EllipticCurveFormalGroup(E)

Bases: sage.structure.sage_object.SageObject

The formal group associated to an elliptic curve.

curve()

The elliptic curve this formal group is associated to.

EXAMPLES:

sage: E = EllipticCurve("37a")
sage: F = E.formal_group()
sage: F.curve()
Elliptic Curve defined by y^2 + y = x^3 - x over Rational Field
differential(prec=20)

Returns the power series f(t) = 1 + \cdots such that f(t) dt is the usual invariant differential dx/(2y + a_1 x + a_3).

INPUT:

  • prec - nonnegative integer (default 20), answer will be returned O(t^{\mathrm{prec}})

OUTPUT: a power series with given precision

DETAILS: Return the formal series

f(t) = 1 + a_1 t + ({a_1}^2 + a_2) t^2 + \cdots

to precision O(t^{prec}) of page 113 of [Silverman AEC1].

The result is cached, and a cached version is returned if possible.

Warning

The resulting series will have precision prec, but its parent PowerSeriesRing will have default precision 20 (or whatever the default default is).

EXAMPLES:

sage: EllipticCurve([-1, 1/4]).formal_group().differential(15)
 1 - 2*t^4 + 3/4*t^6 + 6*t^8 - 5*t^10 - 305/16*t^12 + 105/4*t^14 + O(t^15)
sage: EllipticCurve(Integers(53), [-1, 1/4]).formal_group().differential(15)
 1 + 51*t^4 + 14*t^6 + 6*t^8 + 48*t^10 + 24*t^12 + 13*t^14 + O(t^15)

AUTHOR:

  • David Harvey (2006-09-10): factored out of log
group_law(prec=10)

The formal group law.

INPUT:

  • prec - integer (default 10)

OUTPUT: a power series with given precision in ZZ[[ ZZ[[‘t1’]],’t2’]]

DETAILS: Return the formal power series

F(t_1, t_2) = t_1 + t_2 - a_1 t_1 t_2 - \cdots

to precision O(t^{prec}) of page 115 of [Silverman AEC1].

The result is cached, and a cached version is returned if possible.

Warning

The resulting power series will have precision prec, but its parent PowerSeriesRing will have default precision 20 (or whatever the default default is).

AUTHORS:

  • Nick Alexander: minor fixes, docstring

EXAMPLES:

sage: e = EllipticCurve([1, 2])
sage: F = e.formal_group().group_law(5); F
 t1 + O(t1^5) + (1 - 2*t1^4 + O(t1^5))*t2 + (-4*t1^3 + O(t1^5))*t2^2 + (-4*t1^2 - 30*t1^4 + O(t1^5))*t2^3 + (-2*t1 - 30*t1^3 + O(t1^5))*t2^4 + O(t2^5)
sage: i = e.formal_group().inverse(5)
sage: Fx = F.base_extend(F.base_ring().base_extend(i.parent()))
sage: Fx (i.parent().gen()) (i)
 O(t^5)

Let’s ensure caching with changed precision is working:

sage: e.formal_group().group_law(4)
 t1 + O(t1^4) + (1 + O(t1^4))*t2 + (-4*t1^3 + O(t1^4))*t2^2 + (-4*t1^2 + O(t1^4))*t2^3 + O(t2^4)
inverse(prec=20)

The formal group inverse law i(t), which satisfies F(t, i(t)) = 0.

INPUT:

  • prec - integer (default 20)

OUTPUT: a power series with given precision

DETAILS: Return the formal power series

i(t) = - t + a_1 t^2 + \cdots

to precision O(t^{prec}) of page 114 of [Silverman AEC1].

The result is cached, and a cached version is returned if possible.

Warning

The resulting power series will have precision prec, but its parent PowerSeriesRing will have default precision 20 (or whatever the default default is).

EXAMPLES:

sage: e = EllipticCurve([1, 2])
sage: F = e.formal_group().group_law(5)
sage: i = e.formal_group().inverse(5)
sage: Fx = F.base_extend(F.base_ring().base_extend(i.parent()))
sage: Fx (i) (i.parent().gen())
O(t^5)
log(prec=20)

Returns the power series f(t) = t + \cdots which is an isomorphism to the additive formal group.

Generally this only makes sense in characteristic zero, although the terms before t^p may work in characteristic p.

INPUT:

  • prec - nonnegative integer (default 20)

OUTPUT: a power series with given precision

EXAMPLES:

sage: EllipticCurve([-1, 1/4]).formal_group().log(15)
 t - 2/5*t^5 + 3/28*t^7 + 2/3*t^9 - 5/11*t^11 - 305/208*t^13 + O(t^15)

AUTHORS:

  • David Harvey (2006-09-10): rewrote to use differential
mult_by_n(n, prec=10)

The formal ‘multiplication by n’ endomorphism [n].

INPUT:

  • prec - integer (default 10)

OUTPUT: a power series with given precision

DETAILS: Return the formal power series

[n](t) = n t + \cdots

to precision O(t^{prec}) of Proposition 2.3 of [Silverman AEC1].

Warning

The resulting power series will have precision prec, but its parent PowerSeriesRing will have default precision 20 (or whatever the default default is).

AUTHORS:

  • Nick Alexander: minor fixes, docstring
  • David Harvey (2007-03): faster algorithm for char 0 field case
  • Hamish Ivey-Law (2009-06): double-and-add algorithm for non char 0 field case.
  • Tom Boothby (2009-06): slight improvement to double-and-add

EXAMPLES:

sage: e = EllipticCurve([1, 2, 3, 4, 6])
sage: e.formal_group().mult_by_n(0, 5)
 O(t^5)
sage: e.formal_group().mult_by_n(1, 5)
 t + O(t^5)

We verify an identity of low degree:

sage: none = e.formal_group().mult_by_n(-1, 5)
sage: two = e.formal_group().mult_by_n(2, 5)
sage: ntwo = e.formal_group().mult_by_n(-2, 5)
sage: ntwo - none(two)
 O(t^5)
sage: ntwo - two(none)
 O(t^5)

It’s quite fast:

sage: E = EllipticCurve("37a"); F = E.formal_group()
sage: F.mult_by_n(100, 20)
100*t - 49999950*t^4 + 3999999960*t^5 + 14285614285800*t^7 - 2999989920000150*t^8 + 133333325333333400*t^9 - 3571378571674999800*t^10 + 1402585362624965454000*t^11 - 146666057066712847999500*t^12 + 5336978000014213190385000*t^13 - 519472790950932256570002000*t^14 + 93851927683683567270392002800*t^15 - 6673787211563812368630730325175*t^16 + 320129060335050875009191524993000*t^17 - 45670288869783478472872833214986000*t^18 + 5302464956134111125466184947310391600*t^19 + O(t^20)

TESTS:

sage: F = EllipticCurve(GF(17), [1, 1]).formal_group()
sage: F.mult_by_n(10, 50)
10*t + 5*t^5 + 7*t^7 + 13*t^9 + t^11 + 16*t^13 + 13*t^15 + 9*t^17 + 16*t^19 + 15*t^23 + 15*t^25 + 2*t^27 + 10*t^29 + 8*t^31 + 15*t^33 + 6*t^35 + 7*t^37 + 9*t^39 + 10*t^41 + 5*t^43 + 4*t^45 + 6*t^47 + 13*t^49 + O(t^50)

sage: F = EllipticCurve(GF(101), [1, 1]).formal_group()
sage: F.mult_by_n(100, 20)
100*t + O(t^20)
sigma(prec=10)

EXAMPLE:

sage: E = EllipticCurve('14a')
sage: F = E.formal_group()
sage: F.sigma(5)
t + 1/2*t^2 + (1/2*c + 1/3)*t^3 + (3/4*c + 3/4)*t^4 + O(t^5)
w(prec=20)

The formal group power series w.

INPUT:

  • prec - integer (default 20)

OUTPUT: a power series with given precision

DETAILS: Return the formal power series

w(t) = t^3 + a_1 t^4 + (a_2 + a_1^2) t^5 + \cdots

to precision O(t^{prec}) of Proposition IV.1.1 of [Silverman AEC1]. This is the formal expansion of w = -1/y about the formal parameter t = -x/y at \\infty.

The result is cached, and a cached version is returned if possible.

Warning

The resulting power series will have precision prec, but its parent PowerSeriesRing will have default precision 20 (or whatever the default default is).

ALGORITHM: Uses Newton’s method to solve the elliptic curve equation at the origin. Complexity is roughly O(M(n)) where n is the precision and M(n) is the time required to multiply polynomials of length n over the coefficient ring of E.

AUTHOR:

  • David Harvey (2006-09-09): modified to use Newton’s method instead of a recurrence formula.

EXAMPLES:

sage: e = EllipticCurve([0, 0, 1, -1, 0])
sage: e.formal_group().w(10)
 t^3 + t^6 - t^7 + 2*t^9 + O(t^10)

Check that caching works:

sage: e = EllipticCurve([3, 2, -4, -2, 5])
sage: e.formal_group().w(20)
 t^3 + 3*t^4 + 11*t^5 + 35*t^6 + 101*t^7 + 237*t^8 + 312*t^9 - 949*t^10 - 10389*t^11 - 57087*t^12 - 244092*t^13 - 865333*t^14 - 2455206*t^15 - 4366196*t^16 + 6136610*t^17 + 109938783*t^18 + 688672497*t^19 + O(t^20)
sage: e.formal_group().w(7)
 t^3 + 3*t^4 + 11*t^5 + 35*t^6 + O(t^7)
sage: e.formal_group().w(35)
 t^3 + 3*t^4 + 11*t^5 + 35*t^6 + 101*t^7 + 237*t^8 + 312*t^9 - 949*t^10 - 10389*t^11 - 57087*t^12 - 244092*t^13 - 865333*t^14 - 2455206*t^15 - 4366196*t^16 + 6136610*t^17 + 109938783*t^18 + 688672497*t^19 + 3219525807*t^20 + 12337076504*t^21 + 38106669615*t^22 + 79452618700*t^23 - 33430470002*t^24 - 1522228110356*t^25 - 10561222329021*t^26 - 52449326572178*t^27 - 211701726058446*t^28 - 693522772940043*t^29 - 1613471639599050*t^30 - 421817906421378*t^31 + 23651687753515182*t^32 + 181817896829144595*t^33 + 950887648021211163*t^34 + O(t^35)
x(prec=20)

Return the formal series x(t) = t/w(t) in terms of the local parameter t = -x/y at infinity.

INPUT:

  • prec - integer (default 20)

OUTPUT: a Laurent series with given precision

DETAILS: Return the formal series

x(t) = t^{-2} - a_1 t^{-1} - a_2 - a_3 t - \cdots

to precision O(t^{prec}) of page 113 of [Silverman AEC1].

Warning

The resulting series will have precision prec, but its parent PowerSeriesRing will have default precision 20 (or whatever the default default is).

EXAMPLES:

sage: EllipticCurve([0, 0, 1, -1, 0]).formal_group().x(10)
 t^-2 - t + t^2 - t^4 + 2*t^5 - t^6 - 2*t^7 + 6*t^8 - 6*t^9 + O(t^10)
y(prec=20)

Return the formal series y(t) = -1/w(t) in terms of the local parameter t = -x/y at infinity.

INPUT:

  • prec - integer (default 20)

OUTPUT: a Laurent series with given precision

DETAILS: Return the formal series

y(t) = - t^{-3} + a_1 t^{-2} + a_2 t + a_3 + \cdots

to precision O(t^{prec}) of page 113 of [Silverman AEC1].

The result is cached, and a cached version is returned if possible.

Warning

The resulting series will have precision prec, but its parent PowerSeriesRing will have default precision 20 (or whatever the default default is).

EXAMPLES:

sage: EllipticCurve([0, 0, 1, -1, 0]).formal_group().y(10)
 -t^-3 + 1 - t + t^3 - 2*t^4 + t^5 + 2*t^6 - 6*t^7 + 6*t^8 + 3*t^9 + O(t^10)

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