def _eval_(self, n, x):
        """
        EXAMPLES::

            sage: y=var('y')
            sage: bessel_I(y,x)
            bessel_I(y, x)
            sage: bessel_I(0.0, 1.0)
            1.26606587775201
            sage: bessel_I(1/2, 1)
            sqrt(2)*sinh(1)/sqrt(pi)
            sage: bessel_I(-1/2, pi)
            sqrt(2)*cosh(pi)/pi
        """
        if (not isinstance(n, Expression) and not isinstance(x, Expression) and
                (is_inexact(n) or is_inexact(x))):
            coercion_model = get_coercion_model()
            n, x = coercion_model.canonical_coercion(n, x)
            return self._evalf_(n, x, parent(n))

        # special identities
        if n == Integer(1) / Integer(2):
            return sqrt(2 / (pi * x)) * sinh(x)
        elif n == -Integer(1) / Integer(2):
            return sqrt(2 / (pi * x)) * cosh(x)

        return None  # leaves the expression unevaluated

    def _eval_(self, x, y):
        """
        EXAMPLES::

            sage: gamma_inc(2.,0)
            1.00000000000000
            sage: gamma_inc(2,0)
            1
            sage: gamma_inc(1/2,2)
            -(erf(sqrt(2)) - 1)*sqrt(pi)
            sage: gamma_inc(1/2,1)
            -(erf(1) - 1)*sqrt(pi)
            sage: gamma_inc(1/2,0)
            sqrt(pi)
            sage: gamma_inc(x,0)
            gamma(x)
            sage: gamma_inc(1,2)
            e^(-2)
            sage: gamma_inc(0,2)
            -Ei(-2)
        """
        if not isinstance(x, Expression) and not isinstance(y, Expression) and \
               (is_inexact(x) or is_inexact(y)):
            x, y = coercion_model.canonical_coercion(x, y)
            return self._evalf_(x, y, parent(x))

        if y == 0:
            return gamma(x)
        if x == 1:
            return exp(-y)
        if x == 0:
            return -Ei(-y)
        if x == Rational(1)/2: #only for x>0
            return sqrt(pi)*(1-erf(sqrt(y)))
        return None

    def _eval_(self, n, m, theta, phi, **kwargs):
        r"""
        TESTS::

            sage: x, y = var('x y')
            sage: spherical_harmonic(1, 2, x, y)
            0
            sage: spherical_harmonic(1, -2, x, y)
            0
            sage: spherical_harmonic(1/2, 2, x, y)
            spherical_harmonic(1/2, 2, x, y)
            sage: spherical_harmonic(3, 2, x, y)
            15/4*sqrt(7/30)*cos(x)*e^(2*I*y)*sin(x)^2/sqrt(pi)
            sage: spherical_harmonic(3, 2, 1, 2)
            15/4*sqrt(7/30)*cos(1)*e^(4*I)*sin(1)^2/sqrt(pi)
            sage: spherical_harmonic(3 + I, 2., 1, 2)
            -0.351154337307488 - 0.415562233975369*I
        """
        from sage.structure.coerce import parent

        cc = get_coercion_model().canonical_coercion
        coerced = cc(phi, cc(theta, cc(n, m)[0])[0])[0]
        if is_inexact(coerced) and not isinstance(coerced, Expression):
            return self._evalf_(n, m, theta, phi, parent=parent(coerced))
        elif n in ZZ and m in ZZ and n > -1:
            if abs(m) > n:
                return ZZ(0)
            return meval("spherical_harmonic({},{},{},{})".format(ZZ(n), ZZ(m), maxima(theta), maxima(phi)))
        return

    def _eval_(self, z):
        """
        EXAMPLES::

            sage: z = var('z')
            sage: sin_integral(z)
            sin_integral(z)
            sage: sin_integral(3.0)
            1.84865252799947
            sage: sin_integral(0)
            0

        """
        if not isinstance(z, Expression) and is_inexact(z):
            return self._evalf_(z, parent(z))

        # special case: z = 0
        if isinstance(z, Expression):
            if z.is_trivial_zero():
                return z
        else:
            if not z:
                return z

        return None # leaves the expression unevaluated

 def _eval_(self, n, z ):
     """
     EXAMPLES::
     """
     # howto find a common parent for n and z here? 
     if not isinstance(z, Expression) and is_inexact(z):
         return self._evalf_(n, z, parent(z))
     return None

    def _eval_(self, n, z):
        """
        EXAMPLES::

            sage: lambert_w(6.0)
            1.43240477589830
            sage: lambert_w(1)
            lambert_w(1)
            sage: lambert_w(x+1)
            lambert_w(x + 1)

        There are three special values which are automatically simplified::

            sage: lambert_w(0)
            0
            sage: lambert_w(e)
            1
            sage: lambert_w(-1/e)
            -1
            sage: lambert_w(SR(0))
            0

        The special values only hold on the principal branch::

            sage: lambert_w(1,e)
            lambert_w(1, e)
            sage: lambert_w(1, e.n())
            -0.532092121986380 + 4.59715801330257*I

        TESTS:

        When automatic simplication occurs, the parent of the output value should be
        either the same as the parent of the input, or a Sage type::

            sage: parent(lambert_w(int(0)))
            <type 'int'>
            sage: parent(lambert_w(Integer(0)))
            Integer Ring
            sage: parent(lambert_w(e))
            Integer Ring
        """
        if not isinstance(z, Expression):
            if is_inexact(z):
                return self._evalf_(n, z, parent=sage_structure_coerce_parent(z))
            elif n == 0 and z == 0:
                return sage_structure_coerce_parent(z)(Integer(0))
        elif n == 0:
            if z.is_trivial_zero():
                return sage_structure_coerce_parent(z)(Integer(0))
            elif (z-const_e).is_trivial_zero():
                return sage_structure_coerce_parent(z)(Integer(1))
            elif (z+1/const_e).is_trivial_zero():
                return sage_structure_coerce_parent(z)(Integer(-1))
        return None

 def _eval_(self, a, b, z, **kwargs):
     """
     EXAMPLES::
     
         sage: hypergeometric([], [], 0)
         1
     """
     if not isinstance(a,tuple) or not isinstance(b,tuple):
         raise ValueError('First two parameters must be of type list.')
     coercion_model = get_coercion_model()
     co = reduce(lambda x, y: coercion_model.canonical_coercion(x, y)[0],
                 a + b + (z,))
     if is_inexact(co) and not isinstance(co, Expression):
         from sage.structure.coerce import parent
         return self._evalf_(a, b, z, parent=parent(co))
     if not isinstance(z, Expression) and z == 0:  # Expression is excluded
         return Integer(1)                         # to avoid call to Maxima
     return

    def _eval_(self, s, x):
        r"""
        TESTS::

            sage: hurwitz_zeta(x, 1)
            zeta(x)
            sage: hurwitz_zeta(4, 3)
            1/90*pi^4 - 17/16
            sage: hurwitz_zeta(-4, x)
            -1/5*x^5 + 1/2*x^4 - 1/3*x^3 + 1/30*x
            sage: hurwitz_zeta(3, 0.5)
            8.41439832211716
        """
        co = get_coercion_model().canonical_coercion(s, x)[0]
        if is_inexact(co) and not isinstance(co, Expression):
            return self._evalf_(s, x, parent=parent(co))
        if x == 1:
            return zeta(s)
        if s in ZZ and s > 1:
            return ((-1) ** s) * psi(s - 1, x) / factorial(s - 1)
        elif s in ZZ and s < 0:
            return -bernoulli_polynomial(x, -s + 1) / (-s + 1)
        else:
            return

def exponential_integral_1(x, n=0):
    r"""
    Returns the exponential integral `E_1(x)`. If the optional
    argument `n` is given, computes list of the first
    `n` values of the exponential integral
    `E_1(x m)`.

    The exponential integral `E_1(x)` is

    .. math::

                      E_1(x) = \int_{x}^{\infty} e^{-t}/t dt

    INPUT:

    -  ``x`` - a positive real number

    -  ``n`` - (default: 0) a nonnegative integer; if
       nonzero, then return a list of values E_1(x\*m) for m =
       1,2,3,...,n. This is useful, e.g., when computing derivatives of
       L-functions.


    OUTPUT:

    -  ``float`` - if n is 0 (the default) or

    -  ``list`` - list of floats if n 0


    EXAMPLES::

        sage: exponential_integral_1(2)
        0.04890051070806112
        sage: exponential_integral_1(2,4)    # rel tol 1e-10
        [0.04890051070806112, 0.0037793524098489067, 0.00036008245216265873, 3.7665622843924751e-05]
        sage: exponential_integral_1(0)
        +Infinity

    IMPLEMENTATION: We use the PARI C-library functions eint1 and
    veceint1.

    REFERENCE:

    - See page 262, Prop 5.6.12, of Cohen's book "A Course in
      Computational Algebraic Number Theory".

    REMARKS: When called with the optional argument n, the PARI
    C-library is fast for values of n up to some bound, then very very
    slow. For example, if x=5, then the computation takes less than a
    second for n=800000, and takes "forever" for n=900000.
    """
    if isinstance(x, Expression):
        if x.is_trivial_zero():
            from sage.rings.infinity import Infinity
            return Infinity
        else:
            raise NotImplementedError("Use the symbolic exponential integral " +
                                      "function: exp_integral_e1.")
    elif not is_inexact(x): # x is exact and not an expression
        if not x: # test if exact x == 0 quickly
            from sage.rings.infinity import Infinity
            return Infinity

    # else x is in not an exact 0
    from sage.libs.pari.all import pari
    if n <= 0:
        return float(pari(x).eint1())
    else:
        return [float(z) for z in pari(x).eint1(n)]

def exponential_integral_1(x, n=0):
    r"""
    Returns the exponential integral `E_1(x)`. If the optional
    argument `n` is given, computes list of the first
    `n` values of the exponential integral
    `E_1(x m)`.

    The exponential integral `E_1(x)` is

    .. math::

                      E_1(x) = \int_{x}^{\infty} e^{-t}/t dt

    INPUT:

    - ``x`` -- a positive real number

    - ``n`` -- (default: 0) a nonnegative integer; if
      nonzero, then return a list of values ``E_1(x*m)`` for m =
      1,2,3,...,n. This is useful, e.g., when computing derivatives of
      L-functions.


    OUTPUT:

    A real number if n is 0 (the default) or a list of reals if n > 0.
    The precision is the same as the input, with a default of 53 bits
    in case the input is exact.

    EXAMPLES::

        sage: exponential_integral_1(2)
        0.0489005107080611
        sage: exponential_integral_1(2,4)  # abs tol 1e-18
        [0.0489005107080611, 0.00377935240984891, 0.000360082452162659, 0.0000376656228439245]
        sage: exponential_integral_1(40,5)
        [1.03677326145166e-19, 2.22854325868847e-37, 6.33732515501151e-55, 2.02336191509997e-72, 6.88522610630764e-90]
        sage: exponential_integral_1(0)
        +Infinity
        sage: r = exponential_integral_1(RealField(150)(1))
        sage: r
        0.21938393439552027367716377546012164903104729
        sage: parent(r)
        Real Field with 150 bits of precision
        sage: exponential_integral_1(RealField(150)(100))
        3.6835977616820321802351926205081189876552201e-46

    TESTS:

    The relative error for a single value should be less than 1 ulp::

        sage: for prec in [20..1000]:  # long time (22s on sage.math, 2013)
        ....:     R = RealField(prec)
        ....:     S = RealField(prec+64)
        ....:     for t in range(8):  # Try 8 values for each precision
        ....:         a = R.random_element(-15,10).exp()
        ....:         x = exponential_integral_1(a)
        ....:         y = exponential_integral_1(S(a))
        ....:         e = float(abs(S(x) - y)/x.ulp())
        ....:         if e >= 1.0:
        ....:             print "exponential_integral_1(%s) with precision %s has error of %s ulp"%(a, prec, e)

    The absolute error for a vector should be less than `c 2^{-p}`, where
    `p` is the precision in bits of `x` and `c = 2 max(1, exponential_integral_1(x))`::

        sage: for prec in [20..128]:  # long time (15s on sage.math, 2013)
        ....:     R = RealField(prec)
        ....:     S = RealField(prec+64)
        ....:     a = R.random_element(-15,10).exp()
        ....:     n = 2^ZZ.random_element(14)
        ....:     x = exponential_integral_1(a, n)
        ....:     y = exponential_integral_1(S(a), n)
        ....:     c = RDF(2 * max(1.0, y[0]))
        ....:     for i in range(n):
        ....:         e = float(abs(S(x[i]) - y[i]) << prec)
        ....:         if e >= c:
        ....:             print "exponential_integral_1(%s, %s)[%s] with precision %s has error of %s >= %s"%(a, n, i, prec, e, c)

    ALGORITHM: use the PARI C-library function ``eint1``.

    REFERENCE:

    - See Proposition 5.6.12 of Cohen's book "A Course in
      Computational Algebraic Number Theory".
    """
    if isinstance(x, Expression):
        if x.is_trivial_zero():
            from sage.rings.infinity import Infinity
            return Infinity
        else:
            raise NotImplementedError("Use the symbolic exponential integral " +
                                      "function: exp_integral_e1.")
    elif not is_inexact(x): # x is exact and not an expression
        if not x: # test if exact x == 0 quickly
            from sage.rings.infinity import Infinity
            return Infinity

    # else x is not an exact 0
    from sage.libs.pari.all import pari
    # Figure out output precision
#.........这里部分代码省略.........