# Relative Number Field Ideals¶

AUTHORS:

• Steven Sivek (2005-05-16)
• William Stein (2007-09-06)
• Nick Alexander (2009-01)

EXAMPLES:

sage: K.<a,b> = NumberField([x^2 + 1, x^2 + 2])
sage: A = K.absolute_field('z')
sage: I = A.factor(7)[0][0]
sage: from_A, to_A = A.structure()
sage: G = [from_A(z) for z in I.gens()]; G
[7, -2*b*a - 1]
sage: K.fractional_ideal(G)
Fractional ideal (2*b*a + 1)
sage: K.fractional_ideal(G).absolute_norm().factor()
7^2

class sage.rings.number_field.number_field_ideal_rel.NumberFieldFractionalIdeal_rel(field, gens, coerce=True)

An ideal of a relative number field.

EXAMPLES:

sage: K.<a> = NumberField([x^2 + 1, x^2 + 2]); K
Number Field in a0 with defining polynomial x^2 + 1 over its base field
sage: i = K.ideal(38); i
Fractional ideal (38)

sage: K.<a0, a1> = NumberField([x^2 + 1, x^2 + 2]); K
Number Field in a0 with defining polynomial x^2 + 1 over its base field
sage: i = K.ideal([a0+1]); i # random
Fractional ideal (-a1*a0)
sage: (g, ) = i.gens_reduced(); g # random
-a1*a0
sage: (g / (a0 + 1)).is_integral()
True
sage: ((a0 + 1) / g).is_integral()
True


TESTS: one test fails, because ideals aren’t fully integrated into the categories framework yet:

sage: TestSuite(i).run()
Failure in _test_category:
...
The following tests failed: _test_category

absolute_ideal(names='a')

If this is an ideal in the extension $$L/K$$, return the ideal with the same generators in the absolute field $$L/\QQ$$.

INPUT:

• names (optional) – string; name of generator of the absolute field

EXAMPLES:

sage: x = ZZ['x'].0
sage: K.<b> = NumberField(x^2 - 2)
sage: L.<c> = K.extension(x^2 - b)
sage: F.<m> = L.absolute_field()


An example of an inert ideal:

sage: P = F.factor(13)[0][0]; P
Fractional ideal (13)
sage: J = L.ideal(13)
sage: J.absolute_ideal()
Fractional ideal (13)


Now a non-trivial ideal in $$L$$ that is principal in the subfield $$K$$. Since the optional ‘names’ argument is not passed, the generators of the absolute ideal J are returned in terms of the default field generator ‘a’. This does not agree with the generator ‘m’ of the absolute field F defined above:

sage: J = L.ideal(b); J
Fractional ideal (b)
sage: J.absolute_ideal()
Fractional ideal (a^2)
sage: J.relative_norm()
Fractional ideal (2)
sage: J.absolute_norm()
4
sage: J.absolute_ideal().norm()
4


Now pass ‘m’ as the name for the generator of the absolute field:

sage: J.absolute_ideal(‘m’) Fractional ideal (m^2)

Now an ideal not generated by an element of $$K$$:

sage: J = L.ideal(c); J
Fractional ideal (c)
sage: J.absolute_ideal()
Fractional ideal (a)
sage: J.absolute_norm()
2
sage: J.ideal_below()
Fractional ideal (b)
sage: J.ideal_below().norm()
2

absolute_norm()

Compute the absolute norm of this fractional ideal in a relative number field, returning a positive integer.

EXAMPLES:

sage: L.<a, b, c> = QQ.extension([x^2 - 23, x^2 - 5, x^2 - 7])
sage: I = L.ideal(a + b)
sage: I.absolute_norm()
104976
sage: I.relative_norm().relative_norm().relative_norm()
104976

absolute_ramification_index()

Return the absolute ramification index of this fractional ideal, assuming it is prime. Otherwise, raise a ValueError.

The absolute ramification index is the power of this prime appearing in the factorization of the rational prime that this prime lies over.

Use relative_ramification_index to obtain the power of this prime occurring in the factorization of the prime ideal of the base field that this prime lies over.

EXAMPLES:

sage: PQ.<X> = QQ[]
sage: F.<a, b> = NumberFieldTower([X^2 - 2, X^2 - 3])
sage: PF.<Y> = F[]
sage: K.<c> = F.extension(Y^2 - (1 + a)*(a + b)*a*b)
sage: I = K.ideal(3, c)
sage: I.absolute_ramification_index()
4
sage: I.smallest_integer()
3
sage: K.ideal(3) == I^4
True

element_1_mod(other)

Returns an element $$r$$ in this ideal such that $$1-r$$ is in other.

An error is raised if either ideal is not integral of if they are not coprime.

INPUT:

• other – another ideal of the same field, or generators of an ideal.

OUTPUT:

an element $$r$$ of the ideal self such that $$1-r$$ is in the ideal other.

EXAMPLES:

sage: K.<a, b> = NumberFieldTower([x^2 - 23, x^2 + 1])
sage: I = Ideal(2, (a - 3*b + 2)/2)
sage: J = K.ideal(a)
sage: z = I.element_1_mod(J)
sage: z in I
True
sage: 1 - z in J
True

factor()

Factor the ideal by factoring the corresponding ideal in the absolute number field.

EXAMPLES:

sage: K.<a, b> = QQ.extension([x^2 + 11, x^2 - 5])
sage: K.factor(5)
(Fractional ideal (5, 1/2*a - 1/2*b - 1))^2 * (Fractional ideal (5, 1/2*a - 1/2*b + 1))^2
sage: K.ideal(5).factor()
(Fractional ideal (5, 1/2*a - 1/2*b - 1))^2 * (Fractional ideal (5, 1/2*a - 1/2*b + 1))^2
sage: K.ideal(5).prime_factors()
[Fractional ideal (5, 1/2*a - 1/2*b - 1),
Fractional ideal (5, 1/2*a - 1/2*b + 1)]

sage: PQ.<X> = QQ[]
sage: F.<a, b> = NumberFieldTower([X^2 - 2, X^2 - 3])
sage: PF.<Y> = F[]
sage: K.<c> = F.extension(Y^2 - (1 + a)*(a + b)*a*b)
sage: I = K.ideal(c)
sage: P = K.ideal((b*a - b - 1)*c/2 + a - 1)
sage: Q = K.ideal((b*a - b - 1)*c/2)
sage: list(I.factor()) == [(P, 2), (Q, 1)]
True
sage: I == P^2*Q
True
sage: [p.is_prime() for p in [P, Q]]
[True, True]

free_module()

Return this ideal as a $$\ZZ$$-submodule of the $$\QQ$$-vector space corresponding to the ambient number field.

EXAMPLES:

sage: K.<a, b> = NumberField([x^3 - x + 1, x^2 + 23])
sage: I = K.ideal(a*b - 1)
sage: I.free_module()
Free module of degree 6 and rank 6 over Integer Ring
User basis matrix:
...
sage: I.free_module().is_submodule(K.maximal_order().free_module())
True

gens_reduced()

Return a small set of generators for this ideal. This will always return a single generator if one exists (i.e. if the ideal is principal), and otherwise two generators.

EXAMPLE:

sage: K.<a, b> = NumberField([x^2 + 1, x^2 - 2])
sage: I = K.ideal((a + 1)*b/2 + 1)
sage: I.gens_reduced()
(1/2*b*a + 1/2*b + 1,)

ideal_below()

Compute the ideal of $$K$$ below this ideal of $$L$$.

EXAMPLES:

sage: R.<x> = QQ[]
sage: K.<a> = NumberField(x^2+6)
sage: L.<b> = K.extension(K['x'].gen()^4 + a)
sage: N = L.ideal(b)
sage: M = N.ideal_below(); M == K.ideal([-a])
True
sage: Np = L.ideal( [ L(t) for t in M.gens() ])
sage: Np.ideal_below() == M
True
sage: M.parent()
Monoid of ideals of Number Field in a with defining polynomial x^2 + 6
sage: M.ring()
Number Field in a with defining polynomial x^2 + 6
sage: M.ring() is K
True


This example concerns an inert ideal:

sage: K = NumberField(x^4 + 6*x^2 + 24, 'a')
sage: K.factor(7)
Fractional ideal (7)
sage: K0, K0_into_K, _ = K.subfields(2)[0]
sage: K0
Number Field in a0 with defining polynomial x^2 - 6*x + 24
sage: L = K.relativize(K0_into_K, 'c'); L
Number Field in c0 with defining polynomial x^2 + a0 over its base field
sage: L.base_field() is K0
True
sage: L.ideal(7)
Fractional ideal (7)
sage: L.ideal(7).ideal_below()
Fractional ideal (7)
sage: L.ideal(7).ideal_below().number_field() is K0
True


This example concerns an ideal that splits in the quadratic field but each factor ideal remains inert in the extension:

sage: len(K.factor(19))
2
sage: K0 = L.base_field(); a0 = K0.gen()
sage: len(K0.factor(19))
2
sage: w1 = -a0 + 1; P1 = K0.ideal([w1])
sage: P1.norm().factor(), P1.is_prime()
(19, True)
sage: L_into_K, K_into_L = L.structure()
sage: L.ideal(K_into_L(K0_into_K(w1))).ideal_below() == P1
True


The choice of embedding of quadratic field into quartic field matters:

sage: rho, tau = K0.embeddings(K)
sage: L1 = K.relativize(rho, 'b')
sage: L2 = K.relativize(tau, 'b')
sage: L1_into_K, K_into_L1 = L1.structure()
sage: L2_into_K, K_into_L2 = L2.structure()
sage: a = K.gen()
sage: P = K.ideal([a^2 + 5])
sage: K_into_L1(P).ideal_below() == K0.ideal([-a0 + 1])
True
sage: K_into_L2(P).ideal_below() == K0.ideal([-a0 + 5])
True
sage: K0.ideal([-a0 + 1]) == K0.ideal([-a0 + 5])
False


It works when the base_field is itself a relative number field:

sage: PQ.<X> = QQ[]
sage: F.<a, b> = NumberFieldTower([X^2 - 2, X^2 - 3])
sage: PF.<Y> = F[]
sage: K.<c> = F.extension(Y^2 - (1 + a)*(a + b)*a*b)
sage: I = K.ideal(3, c)
sage: J = I.ideal_below(); J
Fractional ideal (-b)
sage: J.number_field() == F
True

integral_basis()

Return a basis for self as a $$\ZZ$$-module.

EXAMPLES:

sage: K.<a,b> = NumberField([x^2 + 1, x^2 - 3])
sage: I = K.ideal(17*b - 3*a)
sage: x = I.integral_basis(); x # random
[438, -b*a + 309, 219*a - 219*b, 156*a - 154*b]


The exact results are somewhat unpredictable, hence the # random flag, but we can test that they are indeed a basis:

sage: V, _, phi = K.absolute_vector_space()
sage: V.span([phi(u) for u in x], ZZ) == I.free_module()
True

integral_split()

Return a tuple $$(I, d)$$, where $$I$$ is an integral ideal, and $$d$$ is the smallest positive integer such that this ideal is equal to $$I/d$$.

EXAMPLES:

sage: K.<a, b> = NumberFieldTower([x^2 - 23, x^2 + 1])
sage: I = K.ideal([a + b/3])
sage: J, d = I.integral_split()
sage: J.is_integral()
True
sage: J == d*I
True

is_integral()

Return True if this ideal is integral.

EXAMPLES:

sage: K.<a, b> = QQ.extension([x^2 + 11, x^2 - 5])
sage: I = K.ideal(7).prime_factors()[0]
sage: I.is_integral()
True
sage: (I/2).is_integral()
False

is_prime()

Return True if this ideal of a relative number field is prime.

EXAMPLES:

sage: K.<a, b> = NumberField([x^2 - 17, x^3 - 2])
sage: K.ideal(a + b).is_prime()
True
sage: K.ideal(13).is_prime()
False

is_principal(proof=None)

Return True if this ideal is principal. If so, set self.__reduced_generators, with length one.

EXAMPLES:

sage: K.<a, b> = NumberField([x^2 - 23, x^2 + 1])
sage: I = K.ideal([7, (-1/2*b - 3/2)*a + 3/2*b + 9/2])
sage: I.is_principal()
True
sage: I # random
Fractional ideal ((1/2*b + 1/2)*a - 3/2*b - 3/2)

is_zero()

Return True if this is the zero ideal.

EXAMPLE:

sage: K.<a, b> = NumberField([x^2 + 3, x^3 + 4])
sage: K.ideal(17).is_zero()
False
sage: K.ideal(0).is_zero()
True

norm()

The norm of a fractional ideal in a relative number field is deliberately unimplemented, so that a user cannot mistake the absolute norm for the relative norm, or vice versa.

EXAMPLE:

sage: K.<a, b> = NumberField([x^2 + 1, x^2 - 2])
sage: K.ideal(2).norm()
Traceback (most recent call last):
...
NotImplementedError: For a fractional ideal in a relative number field you must use relative_norm or absolute_norm as appropriate

pari_rhnf()

Return PARI’s representation of this relative ideal in Hermite normal form.

EXAMPLE:

sage: K.<a, b> = NumberField([x^2 + 23, x^2 - 7])
sage: I = K.ideal(2, (a + 2*b + 3)/2)
sage: I.pari_rhnf()
[[1, -2; 0, 1], [[2, 1; 0, 1], [1/2, 0; 0, 1/2]]]

ramification_index()

For ideals in relative number fields, ramification_index is deliberately not implemented in order to avoid ambiguity. Either relative_ramification_index() or absolute_ramification_index() should be used instead.

EXAMPLE:

sage: K.<a, b> = NumberField([x^2 + 1, x^2 - 2])
sage: K.ideal(2).ramification_index()
Traceback (most recent call last):
...
NotImplementedError: For an ideal in a relative number field you must use relative_ramification_index or absolute_ramification_index as appropriate

relative_norm()

Compute the relative norm of this fractional ideal in a relative number field, returning an ideal in the base field.

EXAMPLES:

sage: R.<x> = QQ[]
sage: K.<a> = NumberField(x^2+6)
sage: L.<b> = K.extension(K['x'].gen()^4 + a)
sage: N = L.ideal(b).relative_norm(); N
Fractional ideal (-a)
sage: N.parent()
Monoid of ideals of Number Field in a with defining polynomial x^2 + 6
sage: N.ring()
Number Field in a with defining polynomial x^2 + 6
sage: PQ.<X> = QQ[]
sage: F.<a, b> = NumberField([X^2 - 2, X^2 - 3])
sage: PF.<Y> = F[]
sage: K.<c> = F.extension(Y^2 - (1 + a)*(a + b)*a*b)
sage: K.ideal(1).relative_norm()
Fractional ideal (1)
sage: K.ideal(13).relative_norm().relative_norm()
Fractional ideal (28561)
sage: K.ideal(13).relative_norm().relative_norm().relative_norm()
815730721
sage: K.ideal(13).absolute_norm()
815730721

relative_ramification_index()

Return the relative ramification index of this fractional ideal, assuming it is prime. Otherwise, raise a ValueError.

The relative ramification index is the power of this prime appearing in the factorization of the prime ideal of the base field that this prime lies over.

Use absolute_ramification_index to obtain the power of this prime occurring in the factorization of the rational prime that this prime lies over.

EXAMPLES:

sage: PQ.<X> = QQ[]
sage: F.<a, b> = NumberFieldTower([X^2 - 2, X^2 - 3])
sage: PF.<Y> = F[]
sage: K.<c> = F.extension(Y^2 - (1 + a)*(a + b)*a*b)
sage: I = K.ideal(3, c)
sage: I.relative_ramification_index()
2
sage: I.ideal_below()  # random sign
Fractional ideal (b)
sage: I.ideal_below() == K.ideal(b)
True
sage: K.ideal(b) == I^2
True

residue_class_degree()

Return the residue class degree of this prime.

EXAMPLES:

sage: PQ.<X> = QQ[]
sage: F.<a, b> = NumberFieldTower([X^2 - 2, X^2 - 3])
sage: PF.<Y> = F[]
sage: K.<c> = F.extension(Y^2 - (1 + a)*(a + b)*a*b)
sage: [I.residue_class_degree() for I in K.ideal(c).prime_factors()]
[1, 2]

residues()

Returns a iterator through a complete list of residues modulo this integral ideal.

An error is raised if this fractional ideal is not integral.

EXAMPLES:

sage: K.<a, w> = NumberFieldTower([x^2 - 3, x^2 + x + 1])
sage: I = K.ideal(6, -w*a - w + 4)
sage: list(I.residues())[:5]
[(25/3*w - 1/3)*a + 22*w + 1,
(16/3*w - 1/3)*a + 13*w,
(7/3*w - 1/3)*a + 4*w - 1,
(-2/3*w - 1/3)*a - 5*w - 2,
(-11/3*w - 1/3)*a - 14*w - 3]

smallest_integer()

Return the smallest non-negative integer in $$I \cap \ZZ$$, where $$I$$ is this ideal. If $$I = 0$$, returns $$0$$.

EXAMPLES:

sage: K.<a, b> = NumberFieldTower([x^2 - 23, x^2 + 1])
sage: I = K.ideal([a + b])
sage: I.smallest_integer()
12
sage: [m for m in range(13) if m in I]
[0, 12]

valuation(p)

Return the valuation of this fractional ideal at p.

INPUT:

• p – a prime ideal $$\mathfrak{p}$$ of this relative number field.

OUTPUT:

(integer) The valuation of this fractional ideal at the prime $$\mathfrak{p}$$. If $$\mathfrak{p}$$ is not prime, raise a ValueError.

EXAMPLES:

sage: K.<a, b> = NumberField([x^2 - 17, x^3 - 2])
sage: A = K.ideal(a + b)
sage: A.is_prime()
True
sage: (A*K.ideal(3)).valuation(A)
1
sage: K.ideal(25).valuation(5)
Traceback (most recent call last):
...
ValueError: p (= Fractional ideal (5)) must be a prime

sage.rings.number_field.number_field_ideal_rel.is_NumberFieldFractionalIdeal_rel(x)

Return True if x is a fractional ideal of a relative number field.

EXAMPLES:

sage: from sage.rings.number_field.number_field_ideal_rel import is_NumberFieldFractionalIdeal_rel
sage: from sage.rings.number_field.number_field_ideal import is_NumberFieldFractionalIdeal
sage: is_NumberFieldFractionalIdeal_rel(2/3)
False
sage: is_NumberFieldFractionalIdeal_rel(ideal(5))
False
sage: k.<a> = NumberField(x^2 + 2)
sage: I = k.ideal([a + 1]); I
Fractional ideal (a + 1)
sage: is_NumberFieldFractionalIdeal_rel(I)
False
sage: R.<x> = QQ[]
sage: K.<a> = NumberField(x^2+6)
sage: L.<b> = K.extension(K['x'].gen()^4 + a)
sage: I = L.ideal(b); I
Fractional ideal (6, b)
sage: is_NumberFieldFractionalIdeal_rel(I)
True
sage: N = I.relative_norm(); N
Fractional ideal (-a)
sage: is_NumberFieldFractionalIdeal_rel(N)
False
sage: is_NumberFieldFractionalIdeal(N)
True


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