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# 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 R[[‘t1’,’t2’]], where the curve is defined over R.

    DETAILS: Return the formal power series

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

    to precision \(O(t1,t2)^{prec}\) of page 115 of [Silverman AEC1].

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

    AUTHORS:

    - Nick Alexander: minor fixes, docstring
    - Francis Clarke (2012-08): modified to use two-variable power series ring

    EXAMPLES:

        sage: e = EllipticCurve([1, 2])
        sage: e.formal_group().group_law(6)
        t1 + t2 - 2*t1^4*t2 - 4*t1^3*t2^2 - 4*t1^2*t2^3 - 2*t1*t2^4 + O(t1, t2)^6

        sage: e = EllipticCurve('14a1')
        sage: ehat = e.formal()
        sage: ehat.group_law(3)
        t1 + t2 - t1*t2 + O(t1, t2)^3
        sage: ehat.group_law(5)
        t1 + t2 - t1*t2 - 2*t1^3*t2 - 3*t1^2*t2^2 - 2*t1*t2^3 + O(t1, t2)^5

        sage: e = EllipticCurve(GF(7), [3, 4])
        sage: ehat = e.formal()
        sage: ehat.group_law(3)
        t1 + t2 + O(t1, t2)^3
        sage: F = ehat.group_law(7); F
        t1 + t2 + t1^4*t2 + 2*t1^3*t2^2 + 2*t1^2*t2^3 + t1*t2^4 + O(t1, t2)^7

    TESTS:

        sage: R.<x,y,z> = GF(7)[[]]
        sage: F(x, ehat.inverse()(x))
        0 + O(x, y, z)^7
        sage: F(x, y) == F(y, x)
        True
        sage: F(x, F(y, z)) == F(F(x, y), z)
        True

    Let’s ensure caching with changed precision is working:

        sage: e.formal_group().group_law(4)
        t1 + t2 + O(t1, t2)^4

    Test for trac ticket #9646:

        sage: P.<a1, a2, a3, a4, a6> = PolynomialRing(ZZ, 5)
        sage: E = EllipticCurve(list(P.gens()))
        sage: F = E.formal().group_law(prec=5)
        sage: t1, t2 = F.parent().gens()
        sage: F(t1, 0)
        t1 + O(t1, t2)^5
        sage: F(0, t2)
        t2 + O(t1, t2)^5
        sage: F.coefficients()[t1*t2^2]
        -a2

  `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: P.<a1, a2, a3, a4, a6> = ZZ[]
        sage: E = EllipticCurve(list(P.gens()))
        sage: i = E.formal_group().inverse(6); i
        -t - a1*t^2 - a1^2*t^3 + (-a1^3 - a3)*t^4 + (-a1^4 - 3*a1*a3)*t^5 + O(t^6)
        sage: F = E.formal_group().group_law(6)
        sage: F(i.parent().gen(), i)
        O(t^6)

  `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
    - Francis Clarke (2012-08): adjustments and simplifications using group_law code as modified to yield a two-variable power series.

    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)  # long time (13s on sage.math, 2011)
        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)

        sage: P.<a1, a2, a3, a4, a6> = PolynomialRing(ZZ, 5)
        sage: E = EllipticCurve(list(P.gens()))
        sage: E.formal().mult_by_n(2,prec=5)
        2*t - a1*t^2 - 2*a2*t^3 + (a1*a2 - 7*a3)*t^4 + O(t^5)

        sage: E = EllipticCurve(QQ, [1,2,3,4,6])
        sage: E.formal().mult_by_n(2,prec=5)
        2*t - t^2 - 4*t^3 - 19*t^4 + O(t^5)

  `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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