r"""
Field `\QQ` of Rational Numbers

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

Interactively, an instance of :class:`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

TESTS::

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

AUTHORS:

- Niles Johnson (2010-08): :trac:`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()``.

"""
from __future__ import print_function, absolute_import

from .rational import Rational
from .integer import Integer

ZZ = None

from sage.structure.parent_gens import ParentWithGens
from sage.structure.sequence import Sequence
import sage.rings.number_field.number_field_base as number_field_base
from sage.misc.fast_methods import Singleton

class RationalField(Singleton, number_field_base.NumberField):
    r"""
    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
        sage: QQ(u'-5/7')
        -5/7

    Conversion from the reals to the rationals 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
    """
    def __new__(cls):
        """
        This method actually is not needed for using :class:`RationalField`.
        But it is used to unpickle some very old pickles.

        TESTS::

            sage: RationalField() in Fields() # indirect doctest
            True

        """
        try:
            from sage.rings.rational_field import QQ
            return QQ
        except BaseException:
            from sage.rings.number_field.number_field_base import NumberField
            return NumberField.__new__(cls)

    def __init__(self):
        r"""
        We create the rational numbers `\QQ`, and call a few functions::

            sage: Q = RationalField(); Q
            Rational Field
            sage: Q.characteristic()
            0
            sage: Q.is_field()
            True
            sage: Q.category()
            Join of Category of quotient fields and Category of metric spaces
            sage: Q.zeta()
            -1

        We next illustrate arithmetic in `\QQ`.

        ::

            sage: Q('49/7')
            7
            sage: type(Q('49/7'))
            <type 'sage.rings.rational.Rational'>
            sage: a = Q('19/374'); a
            19/374
            sage: b = Q('17/371'); b
            17/371
            sage: a + b
            13407/138754
            sage: b + a
            13407/138754
            sage: a * b
            19/8162
            sage: b * a
            19/8162
            sage: a - b
            691/138754
            sage: b - a
            -691/138754
            sage: a / b
            7049/6358
            sage: b / a
            6358/7049
            sage: b < a
            True
            sage: a < b
            False

        Next finally illustrate arithmetic with automatic coercion. The
        types that coerce into the rational field include ``str, int,
        long, Integer``.

        ::

            sage: a + Q('17/371')
            13407/138754
            sage: a * 374
            19
            sage: 374 * a
            19
            sage: a/19
            1/374
            sage: a + 1
            393/374

        TESTS::

            sage: TestSuite(QQ).run()
            sage: QQ.variable_name()
            'x'
            sage: QQ.variable_names()
            ('x',)
        """
        from sage.categories.basic import QuotientFields
        ParentWithGens.__init__(self, self, category=QuotientFields().Metric())
        self._assign_names(('x',),normalize=False) # ???
        self._populate_coercion_lists_(element_constructor=Rational, init_no_parent=True)

    def _repr_(self):
        """
        Return a string representation of ``self``.

        EXAMPLES::

            sage: QQ # indirect doctest
            Rational Field
        """
        return "Rational Field"

    def _repr_option(self, key):
        """
        Metadata about the :meth:`_repr_` output.

        See :meth:`sage.structure.parent._repr_option` for details.

        EXAMPLES::

            sage: QQ._repr_option('element_is_atomic')
            True
        """
        if key == 'element_is_atomic':
            return True
        return super(RationalField, self)._repr_option(key)

    def _latex_(self):
        r"""
        Return a latex representation of ``self``.

        EXAMPLES::

            sage: latex(QQ) # indirect doctest
            \Bold{Q}
        """
        return "\Bold{Q}"

    def __reduce__(self):
        r"""
        Used for pickling `\QQ`.

        EXAMPLES::

           sage: loads(dumps(QQ)) is QQ
           True
        """
        return RationalField, tuple([])

    def __len__(self):
        """
        Return the number of elements in ``self``.

        Since this does not have a size, this throws a ``TypeError``.

        EXAMPLES::

            sage: len(QQ)
            Traceback (most recent call last):
            ...
            TypeError: len() of unsized object
        """
        raise TypeError('len() of unsized object')

    def construction(self):
        r"""
        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)
        """
        from sage.categories.pushout import FractionField
        from . import integer_ring
        return FractionField(), integer_ring.ZZ

    def completion(self, p, prec, extras = {}):
        r"""
        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
        """
        from sage.rings.infinity import Infinity
        if p == Infinity:
            from sage.rings.real_mpfr import create_RealField
            return create_RealField(prec, **extras)
        else:
            from sage.rings.padics.factory import Qp
            return Qp(p, prec, **extras)

    def _coerce_map_from_(self, S):
        """
        Return a coerce map from ``S``.

        EXAMPLES::

            sage: f = QQ.coerce_map_from(ZZ); f # indirect doctest
            Natural morphism:
              From: Integer Ring
              To:   Rational Field
            sage: f(3)
            3
            sage: f(3^99) - 3^99
            0
            sage: f = QQ.coerce_map_from(int); f # indirect doctest
            Native morphism:
              From: Set of Python objects of type 'int'
              To:   Rational Field
            sage: f(44)
            44

        ::

            sage: QQ.coerce_map_from(long) # indirect doctest
            Composite map:
              From: Set of Python objects of type 'long'
              To:   Rational Field
              Defn:   Native morphism:
                      From: Set of Python objects of type 'long'
                      To:   Integer Ring
                    then
                      Natural morphism:
                      From: Integer Ring
                      To:   Rational Field
        """
        global ZZ
        from . import rational
        if ZZ is None:
            from . import integer_ring
            ZZ = integer_ring.ZZ
        if S is ZZ:
            return rational.Z_to_Q()
        elif S is int:
            return rational.int_to_Q()
        elif ZZ.has_coerce_map_from(S):
            return rational.Z_to_Q() * ZZ._internal_coerce_map_from(S)

    def _is_valid_homomorphism_(self, codomain, im_gens):
        """
        Check to see if the map into ``codomain`` determined by ``im_gens`` is
        a valid homomorphism.

        EXAMPLES::

            sage: QQ._is_valid_homomorphism_(ZZ, [1])
            False
            sage: QQ._is_valid_homomorphism_(QQ, [1])
            True
            sage: QQ._is_valid_homomorphism_(RR, [1])
            True
            sage: QQ._is_valid_homomorphism_(RR, [2])
            False
        """
        try:
            return im_gens[0] == codomain._coerce_(self.gen(0))
        except TypeError:
            return False

    def __iter__(self):
        r"""
        Creates an iterator that generates the rational numbers without
        repetition, in order of the height.

        See also :meth:`range_by_height()`.

        EXAMPLES:

        The first 17 rational numbers, ordered by height::

            sage: import itertools
            sage: lst = [a for a in itertools.islice(Rationals(),17)]
            sage: lst
            [0, 1, -1, 1/2, -1/2, 2, -2, 1/3, -1/3, 3, -3, 2/3, -2/3, 3/2, -3/2, 1/4, -1/4]
            sage: [a.height() for a in lst]
            [1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4]
        """
        yield self(0)
        yield self(1)
        yield self(-1)
        height = Integer(1)
        while True:
            height = height + 1
            for other in range(1, height):
                if height.gcd(other) == 1:
                    yield self(other/height)
                    yield self(-other/height)
                    yield self(height/other)
                    yield self(-height/other)

    def range_by_height(self, start, end=None):
        r"""
        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.

        See also ``__iter__()``.

        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))
            []
        """
        if end is None:
            end = start
            start = 1
        if start < 1:
            start = 1
        for height in ZZ.range(start, end):
            if height == 1:
                yield self(0)
                yield self(1)
                yield self(-1)
            for other in ZZ.range(1, height):
                if height.gcd(other) == 1:
                    yield self(other/height)
                    yield self(-other/height)
                    yield self(height/other)
                    yield self(-height/other)

    def primes_of_bounded_norm_iter(self, B):
        r"""
        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))
            []
        """
        try:
            B = ZZ(B.ceil())
        except (TypeError, AttributeError):
            raise TypeError("%s is not valid bound on prime ideals" % B)

        if B<2:
            raise StopIteration

        from sage.arith.all import primes
        for p in primes(B+1):
            yield p

    def discriminant(self):
        """
        Return the discriminant of the field of rational numbers, which is 1.

        EXAMPLES::

            sage: QQ.discriminant()
            1
        """
        return Integer(1)

    def absolute_discriminant(self):
        """
        Return the absolute discriminant, which is 1.

        EXAMPLES::

            sage: QQ.absolute_discriminant()
            1
        """
        return self.discriminant()

    def relative_discriminant(self):
        """
        Return the relative discriminant, which is 1.

        EXAMPLES::

            sage: QQ.relative_discriminant()
            1
        """
        return self.discriminant()

    def class_number(self):
        """
        Return the class number of the field of rational numbers, which is 1.

        EXAMPLES::

            sage: QQ.class_number()
            1
        """
        return Integer(1)

    def signature(self):
        r"""
        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)
        """
        return (Integer(1), Integer(0))

    def embeddings(self, K):
        r"""
        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.
        """
        if K.characteristic() != 0:
            raise ValueError("no embeddings of the rational field into K.")
        return [self.hom(K)]

    def automorphisms(self):
        r"""
        Return all Galois automorphisms of ``self``.

        OUTPUT:

        - a sequence containing just the identity morphism

        EXAMPLES::

            sage: QQ.automorphisms()
            [
            Ring endomorphism of Rational Field
              Defn: 1 |--> 1
            ]
        """
        return Sequence([self.hom(1, self)], cr=True, immutable=False,
                        check=False)

    def places(self, all_complex=False, prec=None):
        r"""
        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]
        """
        import sage.rings.all
        from sage.rings.infinity import Infinity
        if prec is None:
            R = sage.rings.all.RR
            C = sage.rings.all.CC
        elif prec == 53:
            R = sage.rings.all.RDF
            C = sage.rings.all.CDF
        elif prec == Infinity:
            R = sage.rings.all.AA
            C = sage.rings.all.QQbar
        else:
            R = sage.rings.all.RealField(prec)
            C = sage.rings.all.ComplexField(prec)
        domain = C if all_complex else R
        return [self.hom([domain(1)])]

    def complex_embedding(self, 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
        """
        from . import complex_field
        CC = complex_field.ComplexField(prec)
        return self.hom([CC(1)])

    def residue_field(self, p, check=True):
        r"""
        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
        """
        from sage.rings.finite_rings.residue_field import ResidueField
        return ResidueField(ZZ.ideal(p), check=check)

    def gens(self):
        r"""
        Return a tuple of generators of `\QQ` which is only ``(1,)``.

        EXAMPLES::

            sage: QQ.gens()
            (1,)
        """
        return (self(1), )

    def gen(self, n=0):
        r"""
        Return the ``n``-th generator of `\QQ`.

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

        EXAMPLES::

            sage: QQ.gen()
            1
        """
        if n == 0:
            return self(1)
        else:
            raise IndexError("n must be 0")

    def degree(self):
        r"""
        Return the degree of `\QQ` which is 1.

        EXAMPLES::

            sage: QQ.degree()
            1
        """
        return Integer(1)

    def absolute_degree(self):
        r"""
        Return the absolute degree of `\QQ` which is 1.

        EXAMPLES::

            sage: QQ.absolute_degree()
            1
        """
        return Integer(1)

    def ngens(self):
        r"""
        Return the number of generators of `\QQ` which is 1.

        EXAMPLES::

            sage: QQ.ngens()
            1
        """
        return Integer(1)

    def is_absolute(self):
        r"""
        `\QQ` is an absolute extension of `\QQ`.

        EXAMPLES::

            sage: QQ.is_absolute()
            True
        """
        return True

    def is_field(self, proof = True):
        """
        Return ``True``, since the rational field is a field.

        EXAMPLES::

            sage: QQ.is_field()
            True
        """
        return True

    def is_finite(self):
        """
        Return ``False``, since the rational field is not finite.

        EXAMPLES::

            sage: QQ.is_finite()
            False
        """
        return False

    def is_prime_field(self):
        r"""
        Return ``True`` since `\QQ` is a prime field.

        EXAMPLES::

            sage: QQ.is_prime_field()
            True
        """
        return True

    def characteristic(self):
        r"""
        Return 0 since the rational field has characteristic 0.

        EXAMPLES::

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

    def maximal_order(self):
        r"""
        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
        """
        from .integer_ring import ZZ
        return ZZ

    def number_field(self):
        r"""
        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
        """
        return self

    def power_basis(self):
        r"""
        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]
        """
        return [ self.gen() ]

    def extension(self, poly, names, **kwds):
        r"""
        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
        """
        from sage.rings.number_field.all import NumberField
        return NumberField(poly, names=names, **kwds)

    def algebraic_closure(self):
        r"""
        Return the algebraic closure of self (which is `\QQbar`).

        EXAMPLES::

            sage: QQ.algebraic_closure()
            Algebraic Field
        """
        from sage.rings.all import QQbar
        return QQbar

    def order(self):
        r"""
        Return the order of `\QQ` which is `\infty`.

        EXAMPLES::

            sage: QQ.order()
            +Infinity
        """
        from sage.rings.infinity import Infinity
        return Infinity

    def _an_element_(self):
        r"""
        Return an element of `\QQ`.

        EXAMPLES::

            sage: QQ.an_element() # indirect doctest
            1/2
        """
        return Rational((1,2))

    def some_elements(self):
        r"""
        Return some elements of `\QQ`.

        See :func:`TestSuite` for a typical use case.

        OUTPUT:

        An iterator over 100 elements of `\QQ`.

        EXAMPLES::

            sage: tuple(QQ.some_elements())
            (1/2, -1/2, 2, -2,
             0, 1, -1, 42,
             2/3, -2/3, 3/2, -3/2,
             4/5, -4/5, 5/4, -5/4,
             6/7, -6/7, 7/6, -7/6,
             8/9, -8/9, 9/8, -9/8,
             10/11, -10/11, 11/10, -11/10,
             12/13, -12/13, 13/12, -13/12,
             14/15, -14/15, 15/14, -15/14,
             16/17, -16/17, 17/16, -17/16,
             18/19, -18/19, 19/18, -19/18,
             20/441, -20/441, 441/20, -441/20,
             22/529, -22/529, 529/22, -529/22,
             24/625, -24/625, 625/24, -625/24,
             ...)
        """
        yield self.an_element()
        yield -self.an_element()
        yield 1/self.an_element()
        yield -1/self.an_element()
        yield self(0)
        yield self(1)
        yield self(-1)
        yield self(42)
        for n in range(1, 24):
            a = 2*n
            b = (2*n + 1)**(n//10 + 1)
            yield Rational((a, b))
            yield Rational((-a, b))
            yield Rational((b, a))
            yield Rational((-b, a))

    def random_element(self, num_bound=None, den_bound=None, *args, **kwds):
        """
        Return an random element of `\QQ`.

        Elements are constructed by randomly choosing integers
        for the numerator and denominator, not necessarily coprime.

        INPUT:

        -  ``num_bound`` -- a positive integer, specifying a bound
           on the absolute value of the numerator.
           If absent, no bound is enforced.

        -  ``den_bound`` -- a positive integer, specifying a bound
           on the value of the denominator.
           If absent, the bound for the numerator will be reused.

        Any extra positional or keyword arguments are passed through to
        :meth:`sage.rings.integer_ring.IntegerRing_class.random_element`.

        EXAMPLES::

            sage: QQ.random_element()
            -4
            sage: QQ.random_element()
            0
            sage: QQ.random_element()
            -1/2

        In the following example, the resulting numbers range from
        -5/1 to 5/1 (both inclusive),
        while the smallest possible positive value is 1/10::

            sage: QQ.random_element(5, 10)
            -2/7

        Extra positional or keyword arguments are passed through::

            sage: QQ.random_element(distribution='1/n')
            0
            sage: QQ.random_element(distribution='1/n')
            -1

        """
        global ZZ
        if ZZ is None:
            from . import integer_ring
            ZZ = integer_ring.ZZ
        if num_bound is None:
            num = ZZ.random_element(*args, **kwds)
            den = ZZ.random_element(*args, **kwds)
            while den == 0: den = ZZ.random_element(*args, **kwds)
            return self((num, den))
        else:
            if num_bound == 0:
                num_bound = 2
            if den_bound is None:
                den_bound = num_bound
                if den_bound < 1:
                    den_bound = 2
            num = ZZ.random_element(-num_bound, num_bound+1, *args, **kwds)
            den = ZZ.random_element(1, den_bound+1, *args, **kwds)
            while den == 0: den = ZZ.random_element(1, den_bound+1, *args, **kwds)
            return self((num,den))

    def zeta(self, 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
        """
        if n == 1:
            return Rational(1)
        elif n == 2:
            return Rational(-1)
        else:
            raise ValueError("no n-th root of unity in rational field")


    def selmer_group(self, S, m, proof=True, orders=False):
        r"""
        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])
        """
        gens = list(S)
        ords = [ZZ(m)] * len(S)
        if m % 2 == 0:
            gens = [ZZ(-1)] + gens
            ords = [ZZ(2)] + ords
        if orders:
            return gens, ords
        else:
            return gens

    def selmer_group_iterator(self, S, m, proof=True):
        r"""
        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 :meth:`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]
        """
        KSgens, ords = self.selmer_group(S=S, m=m, proof=proof, orders=True)
        one = self.one()
        from sage.misc.all import prod
        from itertools import product
        for ev in product(*[range(o) for o in ords]):
            yield prod((p**e for p,e in zip(KSgens, ev)), one)


    #################################
    ## Coercions to interfaces
    #################################
    def _gap_init_(self):
        r"""
        Return the GAP representation of `\QQ`.

        EXAMPLES::

            sage: gap(QQ) # indirect doctest
            Rationals
        """
        return 'Rationals'

    def _magma_init_(self, magma):
        r"""
        Return the magma representation of `\QQ`.

        EXAMPLES::

            sage: magma(QQ)      # optional - magma # indirect doctest
            Rational Field

        TESTS:

        See :trac:`5521`::

            sage: loads(dumps(QQ)) == QQ  # optional - magma
            True
        """
        return 'RationalField()'

    def _macaulay2_init_(self):
        r"""
        Return the macaulay2 representation of `\QQ`.

        EXAMPLES::

            sage: macaulay2(QQ)   # optional - macaulay2 # indirect doctest
            QQ
        """
        return "QQ"

    def _axiom_init_(self):
        r"""
        Return the axiom/fricas representation of `\QQ`.

        EXAMPLES::

           sage: axiom(QQ)    #optional - axiom # indirect doctest
           Fraction Integer
           sage: fricas(QQ)   #optional - fricas # indirect doctest
           Fraction(Integer)

        """
        return 'Fraction Integer'

    _fricas_init_ = _axiom_init_

    def _polymake_init_(self):
        r"""
        Return the polymake representation of `\QQ`.

        EXAMPLES::

            sage: polymake(QQ)    #optional - polymake # indirect doctest
            Rational

        """
        return '"Rational"'

    def _sage_input_(self, sib, coerced):
        r"""
        Produce an expression which will reproduce this value when evaluated.

        EXAMPLES::

            sage: sage_input(QQ, verify=True)
            # Verified
            QQ
            sage: from sage.misc.sage_input import SageInputBuilder
            sage: QQ._sage_input_(SageInputBuilder(), False)
            {atomic:QQ}
        """
        return sib.name('QQ')

    def _factor_univariate_polynomial(self, f):
        """
        Factor the univariate polynomial ``f``.

        INPUT:

        - ``f`` -- a univariate polynomial defined over the rationals

        OUTPUT:

        - A factorization of ``f`` over the rationals into a unit and monic
          irreducible factors

        .. NOTE::

            This is a helper method for
            :meth:`sage.rings.polynomial.polynomial_element.Polynomial.factor`.

            This method calls PARI to compute the factorization.

        TESTS::

            sage: R.<x> = QQ[]
            sage: QQ._factor_univariate_polynomial( x )
            x
            sage: QQ._factor_univariate_polynomial( 2*x )
            (2) * x
            sage: QQ._factor_univariate_polynomial( (x^2 - 1/4)^4 )
            (x - 1/2)^4 * (x + 1/2)^4
            sage: QQ._factor_univariate_polynomial( (2*x + 1) * (3*x^2 - 5)^2 )
            (18) * (x + 1/2) * (x^2 - 5/3)^2
            sage: f = prod((k^2*x^k + k)^(k-1) for k in primes(10))
            sage: QQ._factor_univariate_polynomial(f)
            (1751787911376562500) * (x^2 + 1/2) * (x^3 + 1/3)^2 * (x^5 + 1/5)^4 * (x^7 + 1/7)^6
            sage: QQ._factor_univariate_polynomial( 10*x^5 - 1 )
            (10) * (x^5 - 1/10)
            sage: QQ._factor_univariate_polynomial( 10*x^5 - 10 )
            (10) * (x - 1) * (x^4 + x^3 + x^2 + x + 1)

        """
        G = list(f._pari_with_name().factor())

        # normalize the leading coefficients
        F = [(f.parent()(g).monic(), int(e)) for (g,e) in zip(*G)]

        from sage.structure.factorization import Factorization
        return Factorization(F, f.leading_coefficient())

QQ = RationalField()
Q = QQ

def 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
    """
    return isinstance(x, RationalField)

def frac(n,d):
    """
    Return the fraction ``n/d``.

    EXAMPLES::

        sage: from sage.rings.rational_field import frac
        sage: frac(1,2)
        1/2
    """
    return Rational(n) / Rational(d)
