# Projective plane conics over a number field¶

AUTHORS:

• Marco Streng (2010-07-20)
class sage.schemes.plane_conics.con_number_field.ProjectiveConic_number_field(A, f)

Create a projective plane conic curve over a number field. See Conic for full documentation.

EXAMPLES:

sage: K.<a> = NumberField(x^3 - 2, 'a')
sage: P.<X, Y, Z> = K[]
sage: Conic(X^2 + Y^2 - a*Z^2)
Projective Conic Curve over Number Field in a with defining polynomial x^3 - 2 defined by X^2 + Y^2 + (-a)*Z^2


TESTS:

sage: K.<a> = NumberField(x^3 - 3, 'a')
sage: Conic([a, 1, -1])._test_pickling()


Returns True if and only if self has a point defined over its base field $$B$$.

If point and obstruction are both False (default), then the output is a boolean out saying whether self has a rational point.

If point or obstruction is True, then the output is a pair (out, S), where out is as above and:

• if point is True and self has a rational point, then S is a rational point,
• if obstruction is True, self has no rational point, then S is a prime or infinite place of $$B$$ such that no rational point exists over the completion at S.

Points and obstructions are cached whenever they are found. Cached information is used for the output if available, but only if read_cache is True.

ALGORITHM:

The parameter algorithm specifies the algorithm to be used:

• 'rnfisnorm' – Use PARI’s rnfisnorm (cannot be combined with obstruction = True)
• 'local' – Check if a local solution exists for all primes and infinite places of $$B$$ and apply the Hasse principle. (Cannot be combined with point = True.)
• 'default' – Use algorithm 'rnfisnorm' first. Then, if no point exists and obstructions are requested, use algorithm 'local' to find an obstruction.
• 'magma' (requires Magma to be installed) – delegates the task to the Magma computer algebra system.

EXAMPLES:

An example over $$\QQ$$

sage: C = Conic(QQ, [1, 113922743, -310146482690273725409])
sage: C.has_rational_point(point = True)
(True, (-76842858034579/5424 : -5316144401/5424 : 1))
sage: C.has_rational_point(algorithm = 'local', read_cache = False)
True


Examples over number fields:

sage: K.<i> = QuadraticField(-1)
sage: C = Conic(K, [1, 3, -5])
sage: C.has_rational_point(point = True, obstruction = True)
(False, Fractional ideal (-i - 2))
sage: C.has_rational_point(algorithm = "rnfisnorm")
False
sage: C.has_rational_point(algorithm = "rnfisnorm", obstruction = True, read_cache=False)
Traceback (most recent call last):
...
ValueError: Algorithm rnfisnorm cannot be combined with obstruction = True in has_rational_point

sage: P.<x> = QQ[]
sage: L.<b> = NumberField(x^3-5)
sage: C = Conic(L, [1, 2, -3])
sage: C.has_rational_point(point = True, algorithm = 'rnfisnorm')
(True, (5/3 : -1/3 : 1))

sage: K.<a> = NumberField(x^4+2)
sage: Conic(QQ, [4,5,6]).has_rational_point()
False
sage: Conic(K, [4,5,6]).has_rational_point()
True
sage: Conic(K, [4,5,6]).has_rational_point(algorithm='magma', read_cache=False) # optional - magma
True


TESTS:

Create a bunch of conics over number fields and check whether has_rational_point runs without errors for algorithms 'rnfisnorm' and 'local'. Check if all points returned are valid. If Magma is available, then also check if the output agrees with Magma.

sage: P.<X> = QQ[]
sage: Q = P.fraction_field()
sage: c = [1, X/2, 1/X]
sage: l = Sequence(cartesian_product_iterator([c for i in range(3)]))
sage: l = l + [[X, 1, 1, 1, 1, 1]] + [[X, 1/5, 1, 1, 2, 1]]
sage: M.<c> = NumberField(X^3+3*X+1)
sage: m = [[Q(b)(F.gen()) for b in a] for a in l for F in [K, L, M]]
sage: d = []
sage: c = []
sage: c = [Conic(a) for a in m if a != [0,0,0]]
sage: d = [C.has_rational_point(algorithm = 'rnfisnorm', point = True) for C in c] # long time: 3.3 seconds
sage: all([c[k].defining_polynomial()(Sequence(d[k][1])) == 0 for k in range(len(d)) if d[k][0]])
True
sage: [C.has_rational_point(algorithm='local', read_cache=False) for C in c] == [o[0] for o in d] # long time: 5 seconds
True
sage: [C.has_rational_point(algorithm = 'magma', read_cache=False) for C in c] == [o[0] for o in d] # long time: 3 seconds, optional - magma
True


Create a bunch of conics that are known to have rational points already over $$\QQ$$ and check if points are found by has_rational_point.

sage: l = Sequence(cartesian_product_iterator([[-1, 0, 1] for i in range(3)]))
sage: M.<c> = NumberField(x^5+3*x+1)
sage: m = [[F(b) for b in a] for a in l for F in [K, L, M]]
sage: c = [Conic(a) for a in m if a != [0,0,0] and a != [1,1,1] and a != [-1,-1,-1]]
sage: assert all([C.has_rational_point(algorithm = 'rnfisnorm') for C in c])
sage: assert all([C.defining_polynomial()(Sequence(C.has_rational_point(point = True)[1])) == 0 for C in c])
sage: assert all([C.has_rational_point(algorithm='local', read_cache=False) for C in c]) # long time: 1 second

is_locally_solvable(p)

Returns True if and only if self has a solution over the completion of the base field $$B$$ of self at p. Here p is a finite prime or infinite place of $$B$$.

EXAMPLES:

sage: P.<x> = QQ[]
sage: K.<a> = NumberField(x^3 + 5)
sage: C = Conic(K, [1, 2, 3 - a])
sage: [p1, p2] = K.places()
sage: C.is_locally_solvable(p1)
False

sage: C.is_locally_solvable(p2)
True

sage: O = K.maximal_order()
sage: f = (2*O).factor()
sage: C.is_locally_solvable(f[0][0])
True

sage: C.is_locally_solvable(f[1][0])
False


Returns the sequence of finite primes and/or infinite places such that self is locally solvable at those primes and places.

If the base field is $$\QQ$$, then the infinite place is denoted $$-1$$.

The parameters finite and infinite (both True by default) are used to specify whether to look at finite and/or infinite places. Note that finite = True involves factorization of the determinant of self, hence may be slow.

Local obstructions are cached. The parameter read_cache specifies whether to look at the cache before computing anything.

EXAMPLES

sage: K.<i> = QuadraticField(-1)
sage: Conic(K, [1, 2, 3]).local_obstructions()
[]

sage: Conic(L, [1, 2, 3]).local_obstructions()
[Ring morphism:
From: Number Field in a with defining polynomial x^2 - 5
To:   Algebraic Real Field
Defn: a |--> -2.236067977499790?, Ring morphism:
From: Number Field in a with defining polynomial x^2 - 5
To:   Algebraic Real Field
Defn: a |--> 2.236067977499790?]


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