Mestre’s algorithm¶

This file contains functions that:

• create hyperelliptic curves from the Igusa-Clebsch invariants (over $$\QQ$$ and finite fields)
• create Mestre’s conic from the Igusa-Clebsch invariants

AUTHORS:

• Florian Bouyer
• Marco Streng
sage.schemes.hyperelliptic_curves.mestre.HyperellipticCurve_from_invariants(i, reduced=True, precision=None, algorithm='default')

Returns a hyperelliptic curve with the given Igusa-Clebsch invariants up to scaling.

The output is a curve over the field in which the Igusa-Clebsch invariants are given. The output curve is unique up to isomorphism over the algebraic closure. If no such curve exists over the given field, then raise a ValueError.

INPUT:

• i - list or tuple of length 4 containing the four Igusa-Clebsch invariants: I2,I4,I6,I10.
• reduced - Boolean (default = True) If True, tries to reduce the polynomial defining the hyperelliptic curve using the function reduce_polynomial() (see the reduce_polynomial() documentation for more details).
• precision - integer (default = None) Which precision for real and complex numbers should the reduction use. This only affects the reduction, not the correctness. If None, the algorithm uses the default 53 bit precision.
• algorithm - 'default' or 'magma'. If set to 'magma', uses Magma to parameterize Mestre’s conic (needs Magma to be installed).

OUTPUT:

A hyperelliptic curve object.

EXAMPLE:

Examples over the rationals:

sage: HyperellipticCurve_from_invariants([3840,414720,491028480,2437709561856])
Traceback (most recent call last):
...
NotImplementedError: Reduction of hyperelliptic curves not yet implemented. See trac #14755 and #14756.
sage: HyperellipticCurve_from_invariants([3840,414720,491028480,2437709561856],reduced = False)
Hyperelliptic Curve over Rational Field defined by y^2 = -46656*x^6 + 46656*x^5 - 19440*x^4 + 4320*x^3 - 540*x^2 + 4410*x - 1
sage: HyperellipticCurve_from_invariants([21, 225/64, 22941/512, 1])
Traceback (most recent call last):
...
NotImplementedError: Reduction of hyperelliptic curves not yet implemented. See trac #14755 and #14756.


An example over a finite field:

sage: HyperellipticCurve_from_invariants([GF(13)(1),3,7,5])
Hyperelliptic Curve over Finite Field of size 13 defined by y^2 = 8*x^5 + 5*x^4 + 5*x^2 + 9*x + 3


An example over a number field:

sage: K = QuadraticField(353, 'a')
sage: H = HyperellipticCurve_from_invariants([21, 225/64, 22941/512, 1], reduced = false)
sage: f = K['x'](H.hyperelliptic_polynomials()[0])


If the Mestre Conic defined by the Igusa-Clebsch invariants has no rational points, then there exists no hyperelliptic curve over the base field with the given invariants.:

sage: HyperellipticCurve_from_invariants([1,2,3,4])
Traceback (most recent call last):
...
ValueError: No such curve exists over Rational Field as there are no rational points on Projective Conic Curve over Rational Field defined by -2572155000*u^2 - 317736000*u*v + 1250755459200*v^2 + 2501510918400*u*w + 39276887040*v*w + 2736219686912*w^2


Mestre’s algorithm only works for generic curves of genus two, so another algorithm is needed for those curves with extra automorphism. See also trac ticket #12199:

sage: P.<x> = QQ[]
sage: C = HyperellipticCurve(x^6+1)
sage: i = C.igusa_clebsch_invariants()
sage: HyperellipticCurve_from_invariants(i)
Traceback (most recent call last):
...
TypeError: F (=0) must have degree 2


Igusa-Clebsch invariants also only work over fields of charateristic different from 2, 3, and 5, so another algorithm will be needed for fields of those characteristics. See also trac ticket #12200:

sage: P.<x> = GF(3)[]
sage: HyperellipticCurve(x^6+x+1).igusa_clebsch_invariants()
Traceback (most recent call last):
...
NotImplementedError: Invariants of binary sextics/genus 2 hyperelliptic curves not implemented in characteristics 2, 3, and 5
sage: HyperellipticCurve_from_invariants([GF(5)(1),1,0,1])
Traceback (most recent call last):
...
ZeroDivisionError: Inverse does not exist.


ALGORITHM:

This is Mestre’s algorithm [M1991]. Our implementation is based on the formulae on page 957 of [LY2001], cross-referenced with [W1999] to correct typos.

First construct Mestre’s conic using the Mestre_conic() function. Parametrize the conic if possible. Let $$f_1, f_2, f_3$$ be the three coordinates of the parametrization of the conic by the projective line, and change them into one variable by letting $$F_i = f_i(t, 1)$$. Note that each $$F_i$$ has degree at most 2.

Then construct a sextic polynomial $$f = \sum_{0<=i,j,k<=3}{c_{ijk}*F_i*F_j*F_k}$$, where $$c_{ijk}$$ are defined as rational functions in the invariants (see the source code for detailed formulae for $$c_{ijk}$$). The output is the hyperelliptic curve $$y^2 = f$$.

REFERENCES:

 [LY2001] (1, 2, 3) K. Lauter and T. Yang, “Computing genus 2 curves from invariants on the Hilbert moduli space”, Journal of Number Theory 131 (2011), pages 936 - 958
 [M1991] (1, 2) J.-F. Mestre, “Construction de courbes de genre 2 a partir de leurs modules”, in Effective methods in algebraic geometry (Castiglioncello, 1990), volume 94 of Progr. Math., pages 313 - 334
 [W1999] P. van Wamelen, Pari-GP code, section “thecubic” https://www.math.lsu.edu/~wamelen/Genus2/FindCurve/igusa2curve.gp
sage.schemes.hyperelliptic_curves.mestre.Mestre_conic(i, xyz=False, names='u, v, w')

Return the conic equation from Mestre’s algorithm given the Igusa-Clebsch invariants.

It has a rational point if and only if a hyperelliptic curve corresponding to the invariants exists.

INPUT:

• i - list or tuple of length 4 containing the four Igusa-Clebsch invariants: I2, I4, I6, I10
• xyz - Boolean (default: False) if True, the algorithm also returns three invariants x,y,z used in Mestre’s algorithm
• names (default: ‘u,v,w’) - the variable names for the Conic

OUTPUT:

A Conic object

EXAMPLES:

A standard example:

sage: Mestre_conic([1,2,3,4])
Projective Conic Curve over Rational Field defined by -2572155000*u^2 - 317736000*u*v + 1250755459200*v^2 + 2501510918400*u*w + 39276887040*v*w + 2736219686912*w^2


Note that the algorithm works over number fields as well:

sage: k = NumberField(x^2-41,'a')
sage: a = k.an_element()
sage: Mestre_conic([1,2+a,a,4+a])
Projective Conic Curve over Number Field in a with defining polynomial x^2 - 41 defined by (-801900000*a + 343845000)*u^2 + (855360000*a + 15795864000)*u*v + (312292800000*a + 1284808579200)*v^2 + (624585600000*a + 2569617158400)*u*w + (15799910400*a + 234573143040)*v*w + (2034199306240*a + 16429854656512)*w^2


And over finite fields:

sage: Mestre_conic([GF(7)(10),GF(7)(1),GF(7)(2),GF(7)(3)])
Projective Conic Curve over Finite Field of size 7 defined by -2*u*v - v^2 - 2*u*w + 2*v*w - 3*w^2


An example with xyz:

sage: Mestre_conic([5,6,7,8], xyz=True)
(Projective Conic Curve over Rational Field defined by -415125000*u^2 + 608040000*u*v + 33065136000*v^2 + 66130272000*u*w + 240829440*v*w + 10208835584*w^2, 232/1125, -1072/16875, 14695616/2109375)


ALGORITHM:

The formulas are taken from pages 956 - 957 of [LY2001] and based on pages 321 and 332 of [M1991].

See the code or [LY2001] for the detailed formulae defining x, y, z and L.

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