# Testing whether elliptic curves over number fields are $$\QQ$$-curves¶

AUTHORS:

• John Cremona (February 2021)

The code here implements the algorithm of Cremona and Najman presented in [CrNa2020].

sage.schemes.elliptic_curves.Qcurves.Step4Test(E, B, oldB=0, verbose=False)

Apply local Q-curve test to E at all primes up to B.

INPUT:

• $$E$$ (elliptic curve): an elliptic curve defined over a number field

• $$B$$ (integer): upper bound on primes to test

• $$oldB$$ (integer, default 0): lower bound on primes to test

• $$verbose$$ (boolean, default False): verbosity flag

OUTPUT:

Either (False, $$p$$), if the local test at $$p$$ proves that $$E$$ is not a $$\QQ$$-curve, or (True, $$0$$) if all local tests at primes between oldB and B fail to prove that $$E$$ is not a $$\QQ$$-curve.

ALGORITHM (see [CrNa2020] for details):

This local test at $$p$$ only applies if $$E$$ has good reduction at all of the primes lying above $$p$$ in the base field $$K$$ of $$E$$. It tests whether (1) $$E$$ is either ordinary at all $$P\mid p$$, or supersingular at all; (2) if ordinary at all, it tests that the squarefree part of $$a_P^2-4N(P)$$ is the same for all $$P\mid p$$.

EXAMPLES:

A non-$$\QQ$$-curve over a quartic field (with LMFDB label ‘4.4.8112.1-12.1-a1’) fails this test at $$p=13$$:

sage: from sage.schemes.elliptic_curves.Qcurves import Step4Test
sage: R.<x> = PolynomialRing(QQ)
sage: K.<a> = NumberField(R([3, 0, -5, 0, 1]))
sage: E = EllipticCurve([K([-3,-4,1,1]),K([4,-1,-1,0]),K([-2,0,1,0]),K([-621,778,138,-178]),K([9509,2046,-24728,10380])])
sage: Step4Test(E, 100, verbose=True)
No: inconsistency at the 2 ordinary primes dividing 13
- Frobenius discriminants mod squares: [-3, -1]
(False, 13)


A $$\QQ$$-curve over a sextic field (with LMFDB label ‘6.6.1259712.1-64.1-a6’) passes this test for all $$p<100$$:

sage: from sage.schemes.elliptic_curves.Qcurves import Step4Test
sage: R.<x> = PolynomialRing(QQ)
sage: K.<a> = NumberField(R([-3, 0, 9, 0, -6, 0, 1]))
sage: E = EllipticCurve([K([1,-3,0,1,0,0]),K([5,-3,-6,1,1,0]),K([1,-3,0,1,0,0]),K([-139,-129,331,277,-76,-63]),K([2466,1898,-5916,-4582,1361,1055])])
sage: Step4Test(E, 100, verbose=True)
(True, 0)

sage.schemes.elliptic_curves.Qcurves.conjugacy_test(jlist, verbose=False)

Test whether a list of algebraic numbers contains a complete conjugacy class of 2-power degree.

INPUT:

• $$jlist$$ (list): a list of algebraic numbers in the same field

• $$verbose$$ (boolean, default False): verbosity flag

OUTPUT:

A possibly empty list of irreducible polynomials over $$\QQ$$ of 2-power degree all of whose roots are in the list.

EXAMPLES:

sage: from sage.schemes.elliptic_curves.Qcurves import conjugacy_test
sage: conjugacy_test([3])
[x - 3]
sage: conjugacy_test([K(3), a])
[x - 3]
sage: conjugacy_test([K(3), 3+a])
[x - 3]
sage: conjugacy_test([3+a])
[]
sage: conjugacy_test([3+a, 3-a])
[x^2 - 6*x + 7]
sage: x = polygen(QQ)
sage: f = x^3-3
sage: K.<a> = f.splitting_field()
sage: js = f.roots(K, multiplicities=False)
sage: conjugacy_test(js)
[]
sage: f = x^4-3
sage: K.<a> = NumberField(f)
sage: js = f.roots(K, multiplicities=False)
sage: conjugacy_test(js)
[]
sage: K.<a> = f.splitting_field()
sage: js = f.roots(K, multiplicities=False)
sage: conjugacy_test(js)
[x^4 - 3]

sage.schemes.elliptic_curves.Qcurves.is_Q_curve(E, maxp=100, certificate=False, verbose=False)

Return whether E is a $$\QQ$$-curve, with optional certificate.

INPUT:

• E (elliptic curve) – an elliptic curve over a number field.

• maxp (int, default 100): bound on primes used for checking necessary local conditions. The result will not depend on this, but using a larger value may return False faster.

• certificate (bool, default False): if True then a second value is returned giving a certificate for the $$\QQ$$-curve property.

OUTPUT:

If certificate is False: either True (if $$E$$ is a $$\QQ$$-curve), or False.

If certificate is True: a tuple consisting of a boolean flag as before and a certificate, defined as follows:

• when the flag is True, so $$E$$ is a $$\QQ$$-curve:

• either {‘CM’:$$D$$} where $$D$$ is a negative discriminant, when $$E$$ has potential CM with discriminant $$D$$;

• otherwise {‘CM’: $$0$$, ‘core_poly’: $$f$$, ‘rho’: $$\rho$$, ‘r’: $$r$$, ‘N’: $$N$$}, when $$E$$ is a non-CM $$\QQ$$-curve, where the core polynomial $$f$$ is an irreducible monic polynomial over $$QQ$$ of degree $$2^\rho$$, all of whose roots are $$j$$-invariants of curves isogenous to $$E$$, the core level $$N$$ is a square-free integer with $$r$$ prime factors which is the LCM of the degrees of the isogenies between these conjugates. For example, if there exists a curve $$E'$$ isogenous to $$E$$ with $$j(E')=j\in\QQ$$, then the certificate is {‘CM’:0, ‘r’:0, ‘rho’:0, ‘core_poly’: x-j, ‘N’:1}.

• when the flag is False, so $$E$$ is not a $$\QQ$$-curve, the certificate is a prime $$p$$ such that the reductions of $$E$$ at the primes dividing $$p$$ are inconsistent with the property of being a $$\QQ$$-curve. See the ALGORITHM section for details.

ALGORITHM:

See [CrNa2020] for details.

1. If $$E$$ has rational $$j$$-invariant, or has CM, then return True.

2. Replace $$E$$ by a curve defined over $$K=\QQ(j(E))$$. Let $$N$$ be the conductor norm.

3. For all primes $$p\mid N$$ check that the valuations of $$j$$ at all $$P\mid p$$ are either all negative or all non-negative; if not, return False.

4. For $$p\le maxp$$, $$p\not\mid N$$, check that either $$E$$ is ordinary mod $$P$$ for all $$P\mid p$$, or $$E$$ is supersingular mod $$P$$ for all $$P\mid p$$; if neither, return False. If all are ordinary, check that the integers $$a_P(E)^2-4N(P)$$ have the same square-free part; if not, return False.

5. Compute the $$K$$-isogeny class of $$E$$ using the “heuristic” option (which is faster, but not guaranteed to be complete). Check whether the set of $$j$$-invariants of curves in the class of $$2$$-power degree contains a complete Galois orbit. If so, return True.

6. Otherwise repeat step 4 for more primes, and if still undecided, repeat Step 5 without the “heuristic” option, to get the complete $$K$$-isogeny class (which will probably be no bigger than before). Now return True if the set of $$j$$-invariants of curves in the class contains a complete Galois orbit, otherwise return False.

EXAMPLES:

A non-CM curve over $$\QQ$$ and a CM curve over $$\QQ$$ are both trivially $$\QQ$$-curves:

sage: from sage.schemes.elliptic_curves.Qcurves import is_Q_curve
sage: E = EllipticCurve([1,2,3,4,5])
sage: flag, cert = is_Q_curve(E, certificate=True)
sage: flag
True
sage: cert
{'CM': 0, 'N': 1, 'core_poly': x, 'r': 0, 'rho': 0}

sage: E = EllipticCurve(j=8000)
sage: flag, cert = is_Q_curve(E, certificate=True)
sage: flag
True
sage: cert
{'CM': -8}


A non-$$\QQ$$-curve over a quartic field. The local data at bad primes above $$3$$ is inconsistent:

sage: from sage.schemes.elliptic_curves.Qcurves import is_Q_curve
sage: R.<x> = PolynomialRing(QQ)
sage: K.<a> = NumberField(R([3, 0, -5, 0, 1]))
sage: E = EllipticCurve([K([-3,-4,1,1]),K([4,-1,-1,0]),K([-2,0,1,0]),K([-621,778,138,-178]),K([9509,2046,-24728,10380])])
sage: is_Q_curve(E, certificate=True, verbose=True)
Checking whether Elliptic Curve defined by y^2 + (a^3+a^2-4*a-3)*x*y + (a^2-2)*y = x^3 + (-a^2-a+4)*x^2 + (-178*a^3+138*a^2+778*a-621)*x + (10380*a^3-24728*a^2+2046*a+9509) over Number Field in a with defining polynomial x^4 - 5*x^2 + 3 is a Q-curve
No: inconsistency at the 2 primes dividing 3
- potentially multiplicative: [True, False]
(False, 3)


A non-$$\QQ$$-curve over a quadratic field. The local data at bad primes is consistent, but the local test at good primes above $$13$$ is not:

sage: K.<a> = NumberField(R([-10, 0, 1]))
sage: E = EllipticCurve([K([0,1]),K([-1,-1]),K([0,0]),K([-236,40]),K([-1840,464])])
sage: is_Q_curve(E, certificate=True, verbose=True)
Checking whether Elliptic Curve defined by y^2 + a*x*y = x^3 + (-a-1)*x^2 + (40*a-236)*x + (464*a-1840) over Number Field in a with defining polynomial x^2 - 10 is a Q-curve
Applying local tests at good primes above p<=100
No: inconsistency at the 2 ordinary primes dividing 13
- Frobenius discriminants mod squares: [-1, -3]
No: local test at p=13 failed
(False, 13)


A quadratic $$\QQ$$-curve with CM discriminant $$-15$$ ($$j$$-invariant not in $$\QQ$$):

sage: from sage.schemes.elliptic_curves.Qcurves import is_Q_curve
sage: R.<x> = PolynomialRing(QQ)
sage: K.<a> = NumberField(R([-1, -1, 1]))
sage: E = EllipticCurve([K([1,0]),K([-1,0]),K([0,1]),K([0,-2]),K([0,1])])
sage: is_Q_curve(E, certificate=True, verbose=True)
Checking whether Elliptic Curve defined by y^2 + x*y + a*y = x^3 + (-1)*x^2 + (-2*a)*x + a over Number Field in a with defining polynomial x^2 - x - 1 is a Q-curve
Yes: E is CM (discriminant -15)
(True, {'CM': -15})


An example over $$\QQ(\sqrt{2},\sqrt{3})$$. The $$j$$-invariant is in $$\QQ(\sqrt{6})$$, so computations will be done over that field, and in fact there is an isogenous curve with rational $$j$$, so we have a so-called rational $$\QQ$$-curve:

sage: K.<a> = NumberField(R([1, 0, -4, 0, 1]))
sage: E = EllipticCurve([K([-2,-4,1,1]),K([0,1,0,0]),K([0,1,0,0]),K([-4780,9170,1265,-2463]),K([163923,-316598,-43876,84852])])
sage: flag, cert = is_Q_curve(E, certificate=True)
sage: flag
True
sage: cert
{'CM': 0, 'N': 1, 'core_degs': [1], 'core_poly': x - 85184/3, 'r': 0, 'rho': 0}


Over the same field, a so-called strict $$\QQ$$-curve which is not isogenous to one with rational $$j$$, but whose core field is quadratic. In fact the isogeny class over $$K$$ consists of $$6$$ curves, four with conjugate quartic $$j$$-invariants and $$2$$ with quadratic conjugate $$j$$-invariants in $$\QQ(\sqrt{3})$$ (but which are not base-changes from the quadratic subfield):

sage: E = EllipticCurve([K([0,-3,0,1]),K([1,4,0,-1]),K([0,0,0,0]),K([-2,-16,0,4]),K([-19,-32,4,8])])
sage: flag, cert = is_Q_curve(E, certificate=True)
sage: flag
True
sage: cert
{'CM': 0,
'N': 2,
'core_degs': [1, 2],
'core_poly': x^2 - 840064*x + 1593413632,
'r': 1,
'rho': 1}