# Field $$\QQ$$ of Rational Numbers¶

The class RationalField represents the field $$\QQ$$ of (arbitrary precision) rational numbers. Each rational number is an instance of the class Rational.

Interactively, an instance of RationalField is available as QQ:

sage: QQ
Rational Field


Values of various types can be converted to rational numbers by using the __call__ method of RationalField (that is, by treating QQ as a function).

sage: RealField(9).pi()
3.1
sage: QQ(RealField(9).pi())
22/7
sage: QQ(RealField().pi())
245850922/78256779
sage: QQ(35)
35
sage: QQ('12/347')
12/347
sage: QQ(exp(pi*I))
-1
sage: x = polygen(ZZ)
sage: QQ((3*x)/(4*x))
3/4


TEST:

sage: Q = RationalField()
True
sage: RationalField() is RationalField()
True


AUTHORS:

• Niles Johnson (2010-08): trac ticket #3893: random_element() should pass on *args and **kwds.
• Travis Scrimshaw (2012-10-18): Added additional docstrings for full coverage. Removed duplicates of discriminant() and signature().
class sage.rings.rational_field.RationalField

The class RationalField represents the field $$\QQ$$ of rational numbers.

EXAMPLES:

sage: a = long(901824309821093821093812093810928309183091832091)
sage: b = QQ(a); b
901824309821093821093812093810928309183091832091
sage: QQ(b)
901824309821093821093812093810928309183091832091
sage: QQ(int(93820984323))
93820984323
sage: QQ(ZZ(901824309821093821093812093810928309183091832091))
901824309821093821093812093810928309183091832091
sage: QQ('-930482/9320842317')
-930482/9320842317
sage: QQ((-930482, 9320842317))
-930482/9320842317
sage: QQ([9320842317])
9320842317
sage: QQ(pari(39029384023840928309482842098430284398243982394))
39029384023840928309482842098430284398243982394
sage: QQ('sage')
Traceback (most recent call last):
...
TypeError: unable to convert sage to a rational


Coercion from the reals to the rational is done by default using continued fractions.

sage: QQ(RR(3929329/32))
3929329/32
sage: QQ(-RR(3929329/32))
-3929329/32
sage: QQ(RR(1/7)) - 1/7
0


If you specify an optional second base argument, then the string representation of the float is used.

sage: QQ(23.2, 2)
6530219459687219/281474976710656
sage: 6530219459687219.0/281474976710656
23.20000000000000
sage: a = 23.2; a
23.2000000000000
sage: QQ(a, 10)
116/5


Here’s a nice example involving elliptic curves:

sage: E = EllipticCurve('11a')
sage: L = E.lseries().at1(300)[0]; L
0.2538418608559106843377589233...
sage: O = E.period_lattice().omega(); O
1.26920930427955
sage: t = L/O; t
0.200000000000000
sage: QQ(RealField(45)(t))
1/5

absolute_degree()

Return the absolute degree of $$\QQ$$ which is 1.

EXAMPLES:

sage: QQ.absolute_degree()
1

absolute_discriminant()

Return the absolute discriminant, which is 1.

EXAMPLES:

sage: QQ.absolute_discriminant()
1

algebraic_closure()

Return the algebraic closure of self (which is $$\QQbar$$).

EXAMPLES:

sage: QQ.algebraic_closure()
Algebraic Field

characteristic()

Return 0 since the rational field has characteristic 0.

EXAMPLES:

sage: c = QQ.characteristic(); c
0
sage: parent(c)
Integer Ring

class_number()

Return the class number of the field of rational numbers, which is 1.

EXAMPLES:

sage: QQ.class_number()
1

completion(p, prec, extras={})

Return the completion of $$\QQ$$ at $$p$$.

EXAMPLES:

sage: QQ.completion(infinity, 53)
Real Field with 53 bits of precision
sage: QQ.completion(5, 15, {'print_mode': 'bars'})
5-adic Field with capped relative precision 15

complex_embedding(prec=53)

Return embedding of the rational numbers into the complex numbers.

EXAMPLES:

sage: QQ.complex_embedding()
Ring morphism:
From: Rational Field
To:   Complex Field with 53 bits of precision
Defn: 1 |--> 1.00000000000000
sage: QQ.complex_embedding(20)
Ring morphism:
From: Rational Field
To:   Complex Field with 20 bits of precision
Defn: 1 |--> 1.0000

construction()

Returns a pair (functor, parent) such that functor(parent) returns self.

This is the construction of $$\QQ$$ as the fraction field of $$\ZZ$$.

EXAMPLES:

sage: QQ.construction()
(FractionField, Integer Ring)

degree()

Return the degree of $$\QQ$$ which is 1.

EXAMPLES:

sage: QQ.degree()
1

discriminant()

Return the discriminant of the field of rational numbers, which is 1.

EXAMPLES:

sage: QQ.discriminant()
1

embeddings(K)

Return list of the one embedding of $$\QQ$$ into $$K$$, if it exists.

EXAMPLES:

sage: QQ.embeddings(QQ)
[Ring Coercion endomorphism of Rational Field]
sage: QQ.embeddings(CyclotomicField(5))
[Ring Coercion morphism:
From: Rational Field
To:   Cyclotomic Field of order 5 and degree 4]


$$K$$ must have characteristic 0:

sage: QQ.embeddings(GF(3))
Traceback (most recent call last):
...
ValueError: no embeddings of the rational field into K.

extension(poly, names, check=True, embedding=None)

Create a field extension of $$\QQ$$.

EXAMPLES:

We make a single absolute extension:

sage: K.<a> = QQ.extension(x^3 + 5); K
Number Field in a with defining polynomial x^3 + 5


We make an extension generated by roots of two polynomials:

sage: K.<a,b> = QQ.extension([x^3 + 5, x^2 + 3]); K
Number Field in a with defining polynomial x^3 + 5 over its base field
sage: b^2
-3
sage: a^3
-5

gen(n=0)

Return the n-th generator of $$\QQ$$.

There is only the 0-th generator which is 1.

EXAMPLES:

sage: QQ.gen()
1

gens()

Return a tuple of generators of $$\QQ$$ which is only (1,).

EXAMPLES:

sage: QQ.gens()
(1,)

is_absolute()

$$\QQ$$ is an absolute extension of $$\QQ$$.

EXAMPLES:

sage: QQ.is_absolute()
True

is_field(proof=True)

Return True, since the rational field is a field.

EXAMPLES:

sage: QQ.is_field()
True

is_finite()

Return False, since the rational field is not finite.

EXAMPLES:

sage: QQ.is_finite()
False

is_prime_field()

Return True since $$\QQ$$ is a prime field.

EXAMPLES:

sage: QQ.is_prime_field()
True

is_subring(K)

Return True if $$\QQ$$ is a subring of $$K$$.

We are only able to determine this in some cases, e.g., when $$K$$ is a field or of positive characteristic.

EXAMPLES:

sage: QQ.is_subring(QQ)
True
sage: QQ.is_subring(QQ['x'])
True
sage: QQ.is_subring(GF(7))
False
sage: QQ.is_subring(CyclotomicField(7))
True
sage: QQ.is_subring(ZZ)
False
sage: QQ.is_subring(Frac(ZZ))
True

maximal_order()

Return the maximal order of the rational numbers, i.e., the ring $$\ZZ$$ of integers.

EXAMPLES:

sage: QQ.maximal_order()
Integer Ring
sage: QQ.ring_of_integers ()
Integer Ring

ngens()

Return the number of generators of $$\QQ$$ which is 1.

EXAMPLES:

sage: QQ.ngens()
1

number_field()

Return the number field associated to $$\QQ$$. Since $$\QQ$$ is a number field, this just returns $$\QQ$$ again.

EXAMPLES:

sage: QQ.number_field() is QQ
True

order()

Return the order of $$\QQ$$ which is $$\infty$$.

EXAMPLES:

sage: QQ.order()
+Infinity

places(all_complex=False, prec=None)

Return the collection of all infinite places of self, which in this case is just the embedding of self into $$\RR$$.

By default, this returns homomorphisms into RR. If prec is not None, we simply return homomorphisms into RealField(prec) (or RDF if prec=53).

There is an optional flag all_complex, which defaults to False. If all_complex is True, then the real embeddings are returned as embeddings into the corresponding complex field.

For consistency with non-trivial number fields.

EXAMPLES:

sage: QQ.places()
[Ring morphism:
From: Rational Field
To:   Real Field with 53 bits of precision
Defn: 1 |--> 1.00000000000000]
sage: QQ.places(prec=53)
[Ring morphism:
From: Rational Field
To:   Real Double Field
Defn: 1 |--> 1.0]
sage: QQ.places(prec=200, all_complex=True)
[Ring morphism:
From: Rational Field
To:   Complex Field with 200 bits of precision
Defn: 1 |--> 1.0000000000000000000000000000000000000000000000000000000000]

power_basis()

Return a power basis for this number field over its base field.

The power basis is always [1] for the rational field. This method is defined to make the rational field behave more like a number field.

EXAMPLES:

sage: QQ.power_basis()
[1]

primes_of_bounded_norm_iter(B)

Iterator yielding all primes less than or equal to $$B$$.

INPUT:

• B – a positive integer; upper bound on the primes generated.

OUTPUT:

An iterator over all integer primes less than or equal to $$B$$.

Note

This function exists for compatibility with the related number field method, though it returns prime integers, not ideals.

EXAMPLES:

sage: it = QQ.primes_of_bounded_norm_iter(10)
sage: list(it)
[2, 3, 5, 7]
sage: list(QQ.primes_of_bounded_norm_iter(1))
[]

random_element(num_bound=None, den_bound=None, *args, **kwds)

Return an random element of $$\QQ$$.

EXAMPLES:

sage: QQ.random_element(10,10)
1/4


Passes extra positional or keyword arguments through:

sage: QQ.random_element(10,10, distribution='1/n')
-1

range_by_height(start, end=None)

Range function for rational numbers, ordered by height.

Returns a Python generator for the list of rational numbers with heights in range(start, end). Follows the same convention as Python range, see range? for details.

EXAMPLES:

All rational numbers with height strictly less than 4:

sage: list(QQ.range_by_height(4))
[0, 1, -1, 1/2, -1/2, 2, -2, 1/3, -1/3, 3, -3, 2/3, -2/3, 3/2, -3/2]
sage: [a.height() for a in QQ.range_by_height(4)]
[1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3]


All rational numbers with height 2:

sage: list(QQ.range_by_height(2, 3))
[1/2, -1/2, 2, -2]


Nonsensical integer arguments will return an empty generator:

sage: list(QQ.range_by_height(3, 3))
[]
sage: list(QQ.range_by_height(10, 1))
[]


There are no rational numbers with height $$\leq 0$$:

sage: list(QQ.range_by_height(-10, 1))
[]

relative_discriminant()

Return the relative discriminant, which is 1.

EXAMPLES:

sage: QQ.relative_discriminant()
1

residue_field(p, check=True)

Return the residue field of $$\QQ$$ at the prime $$p$$, for consistency with other number fields.

INPUT:

• p - a prime integer.
• check (default True) - if True check the primality of $$p$$, else do not.

OUTPUT: The residue field at this prime.

EXAMPLES:

sage: QQ.residue_field(5)
Residue field of Integers modulo 5
sage: QQ.residue_field(next_prime(10^9))
Residue field of Integers modulo 1000000007

selmer_group(S, m, proof=True, orders=False)

Compute the group $$\QQ(S,m)$$.

INPUT:

• S – a set of primes
• m – a positive integer
• proof – ignored
• orders (default False) – if True, output two lists, the generators and their orders

OUTPUT:

A list of generators of $$\QQ(S,m)$$ (and, optionally, their orders in $$\QQ^\times/(\QQ^\times)^m$$). This is the subgroup of $$\QQ^\times/(\QQ^\times)^m$$ consisting of elements $$a$$ such that the valuation of $$a$$ is divisible by $$m$$ at all primes not in $$S$$. It is equal to the group of $$S$$-units modulo $$m$$-th powers. The group $$\QQ(S,m)$$ contains the subgroup of those $$a$$ such that $$\QQ(\sqrt[m]{a})/\QQ$$ is unramified at all primes of $$\QQ$$ outside of $$S$$, but may contain it properly when not all primes dividing $$m$$ are in $$S$$.

EXAMPLES:

sage: QQ.selmer_group((), 2)
[-1]
sage: QQ.selmer_group((3,), 2)
[-1, 3]
sage: QQ.selmer_group((5,), 2)
[-1, 5]


The previous examples show that the group generated by the output may be strictly larger than the ‘true’ Selmer group of elements giving extensions unramified outside $$S$$.

When $$m$$ is even, $$-1$$ is a generator of order $$2$$:

sage: QQ.selmer_group((2,3,5,7,), 2, orders=True)
([-1, 2, 3, 5, 7], [2, 2, 2, 2, 2])
sage: QQ.selmer_group((2,3,5,7,), 3, orders=True)
([2, 3, 5, 7], [3, 3, 3, 3])

selmer_group_iterator(S, m, proof=True)

Return an iterator through elements of the finite group $$\QQ(S,m)$$.

INPUT:

• S – a set of primes
• m – a positive integer
• proof – ignored

OUTPUT:

An iterator yielding the distinct elements of $$\QQ(S,m)$$. See the docstring for selmer_group() for more information.

EXAMPLES:

sage: list(QQ.selmer_group_iterator((), 2))
[1, -1]
sage: list(QQ.selmer_group_iterator((2,), 2))
[1, 2, -1, -2]
sage: list(QQ.selmer_group_iterator((2,3), 2))
[1, 3, 2, 6, -1, -3, -2, -6]
sage: list(QQ.selmer_group_iterator((5,), 2))
[1, 5, -1, -5]

signature()

Return the signature of the rational field, which is $$(1,0)$$, since there are 1 real and no complex embeddings.

EXAMPLES:

sage: QQ.signature()
(1, 0)

zeta(n=2)

Return a root of unity in self.

INPUT:

• n – integer (default: 2) order of the root of unity

EXAMPLES:

sage: QQ.zeta()
-1
sage: QQ.zeta(2)
-1
sage: QQ.zeta(1)
1
sage: QQ.zeta(3)
Traceback (most recent call last):
...
ValueError: no n-th root of unity in rational field

sage.rings.rational_field.frac(n, d)

Return the fraction n/d.

EXAMPLES:

sage: from sage.rings.rational_field import frac
sage: frac(1,2)
1/2

sage.rings.rational_field.is_RationalField(x)

Check to see if x is the rational field.

EXAMPLES:

sage: from sage.rings.rational_field import is_RationalField as is_RF
sage: is_RF(QQ)
True
sage: is_RF(ZZ)
False


#### Previous topic

Elements of $$\ZZ/n\ZZ$$

Rational Numbers