from __future__ import print_function, division

from sympy.core.add import Add
from sympy.core.basic import sympify, cacheit
from sympy.core.function import Function, ArgumentIndexError
from sympy.core.numbers import igcdex, Rational
from sympy.core.singleton import S
from sympy.core.symbol import Symbol
from sympy.functions.combinatorial.factorials import factorial, RisingFactorial
from sympy.functions.elementary.miscellaneous import sqrt
from sympy.functions.elementary.exponential import log, exp
from sympy.functions.elementary.hyperbolic import acoth, asinh, atanh, cosh, coth, HyperbolicFunction, sinh, tanh
from sympy.utilities.iterables import numbered_symbols
from sympy.core.compatibility import range

###############################################################################
########################## TRIGONOMETRIC FUNCTIONS ############################
###############################################################################


class TrigonometricFunction(Function):
    """Base class for trigonometric functions. """

    unbranched = True

    def _eval_is_rational(self):
        s = self.func(*self.args)
        if s.func == self.func:
            if s.args[0].is_rational and s.args[0].is_nonzero:
                return False
        else:
            return s.is_rational

    def _eval_is_algebraic(self):
        s = self.func(*self.args)
        if s.func == self.func:
            if self.args[0].is_nonzero and self.args[0].is_algebraic:
                return False
            pi_coeff = _pi_coeff(self.args[0])
            if pi_coeff is not None and pi_coeff.is_rational:
                return True
        else:
            return s.is_algebraic

    def _eval_expand_complex(self, deep=True, **hints):
        re_part, im_part = self.as_real_imag(deep=deep, **hints)
        return re_part + im_part*S.ImaginaryUnit

    def _as_real_imag(self, deep=True, **hints):
        if self.args[0].is_real:
            if deep:
                hints['complex'] = False
                return (self.args[0].expand(deep, **hints), S.Zero)
            else:
                return (self.args[0], S.Zero)
        if deep:
            re, im = self.args[0].expand(deep, **hints).as_real_imag()
        else:
            re, im = self.args[0].as_real_imag()
        return (re, im)


def _peeloff_pi(arg):
    """
    Split ARG into two parts, a "rest" and a multiple of pi/2.
    This assumes ARG to be an Add.
    The multiple of pi returned in the second position is always a Rational.

    Examples:
    >>> from sympy.functions.elementary.trigonometric import _peeloff_pi as peel
    >>> from sympy import pi
    >>> from sympy.abc import x, y
    >>> peel(x + pi/2)
    (x, pi/2)
    >>> peel(x + 2*pi/3 + pi*y)
    (x + pi*y + pi/6, pi/2)
    """
    for a in Add.make_args(arg):
        if a is S.Pi:
            K = S.One
            break
        elif a.is_Mul:
            K, p = a.as_two_terms()
            if p is S.Pi and K.is_Rational:
                break
    else:
        return arg, S.Zero

    m1 = (K % S.Half) * S.Pi
    m2 = K*S.Pi - m1
    return arg - m2, m2


def _pi_coeff(arg, cycles=1):
    """
    When arg is a Number times pi (e.g. 3*pi/2) then return the Number
    normalized to be in the range [0, 2], else None.

    When an even multiple of pi is encountered, if it is multiplying
    something with known parity then the multiple is returned as 0 otherwise
    as 2.

    Examples
    ========

    >>> from sympy.functions.elementary.trigonometric import _pi_coeff as coeff
    >>> from sympy import pi, Dummy
    >>> from sympy.abc import x, y
    >>> coeff(3*x*pi)
    3*x
    >>> coeff(11*pi/7)
    11/7
    >>> coeff(-11*pi/7)
    3/7
    >>> coeff(4*pi)
    0
    >>> coeff(5*pi)
    1
    >>> coeff(5.0*pi)
    1
    >>> coeff(5.5*pi)
    3/2
    >>> coeff(2 + pi)

    >>> coeff(2*Dummy(integer=True)*pi)
    2
    >>> coeff(2*Dummy(even=True)*pi)
    0
    """
    arg = sympify(arg)
    if arg is S.Pi:
        return S.One
    elif not arg:
        return S.Zero
    elif arg.is_Mul:
        cx = arg.coeff(S.Pi)
        if cx:
            c, x = cx.as_coeff_Mul()  # pi is not included as coeff
            if c.is_Float:
                # recast exact binary fractions to Rationals
                f = abs(c) % 1
                if f != 0:
                    p = -int(round(log(f, 2).evalf()))
                    m = 2**p
                    cm = c*m
                    i = int(cm)
                    if i == cm:
                        c = Rational(i, m)
                        cx = c*x
                else:
                    c = Rational(int(c))
                    cx = c*x
            if x.is_integer:
                c2 = c % 2
                if c2 == 1:
                    return x
                elif not c2:
                    if x.is_even is not None:  # known parity
                        return S.Zero
                    return S(2)
                else:
                    return c2*x
            return cx


class sin(TrigonometricFunction):
    """
    The sine function.

    Returns the sine of x (measured in radians).

    Notes
    =====

    This function will evaluate automatically in the
    case x/pi is some rational number [4]_.  For example,
    if x is a multiple of pi, pi/2, pi/3, pi/4 and pi/6.

    Examples
    ========

    >>> from sympy import sin, pi
    >>> from sympy.abc import x
    >>> sin(x**2).diff(x)
    2*x*cos(x**2)
    >>> sin(1).diff(x)
    0
    >>> sin(pi)
    0
    >>> sin(pi/2)
    1
    >>> sin(pi/6)
    1/2
    >>> sin(pi/12)
    -sqrt(2)/4 + sqrt(6)/4


    See Also
    ========

    csc, cos, sec, tan, cot
    asin, acsc, acos, asec, atan, acot, atan2

    References
    ==========

    .. [1] http://en.wikipedia.org/wiki/Trigonometric_functions
    .. [2] http://dlmf.nist.gov/4.14
    .. [3] http://functions.wolfram.com/ElementaryFunctions/Sin
    .. [4] http://mathworld.wolfram.com/TrigonometryAngles.html
    """

    def fdiff(self, argindex=1):
        if argindex == 1:
            return cos(self.args[0])
        else:
            raise ArgumentIndexError(self, argindex)

    @classmethod
    def eval(cls, arg):
        if arg.is_Number:
            if arg is S.NaN:
                return S.NaN
            elif arg is S.Zero:
                return S.Zero
            elif arg is S.Infinity or arg is S.NegativeInfinity:
                return

        if arg.could_extract_minus_sign():
            return -cls(-arg)

        i_coeff = arg.as_coefficient(S.ImaginaryUnit)
        if i_coeff is not None:
            return S.ImaginaryUnit * sinh(i_coeff)

        pi_coeff = _pi_coeff(arg)
        if pi_coeff is not None:
            if pi_coeff.is_integer:
                return S.Zero

            if (2*pi_coeff).is_integer:
                if pi_coeff.is_even:
                    return S.Zero
                elif pi_coeff.is_even is False:
                    return S.NegativeOne**(pi_coeff - S.Half)

            if not pi_coeff.is_Rational:
                narg = pi_coeff*S.Pi
                if narg != arg:
                    return cls(narg)
                return None

            # https://github.com/sympy/sympy/issues/6048
            # transform a sine to a cosine, to avoid redundant code
            if pi_coeff.is_Rational:
                x = pi_coeff % 2
                if x > 1:
                    return -cls((x % 1)*S.Pi)
                if 2*x > 1:
                    return cls((1 - x)*S.Pi)
                narg = ((pi_coeff + Rational(3, 2)) % 2)*S.Pi
                result = cos(narg)
                if not isinstance(result, cos):
                    return result
                if pi_coeff*S.Pi != arg:
                    return cls(pi_coeff*S.Pi)
                return None

        if arg.is_Add:
            x, m = _peeloff_pi(arg)
            if m:
                return sin(m)*cos(x) + cos(m)*sin(x)

        if arg.func is asin:
            return arg.args[0]

        if arg.func is atan:
            x = arg.args[0]
            return x / sqrt(1 + x**2)

        if arg.func is atan2:
            y, x = arg.args
            return y / sqrt(x**2 + y**2)

        if arg.func is acos:
            x = arg.args[0]
            return sqrt(1 - x**2)

        if arg.func is acot:
            x = arg.args[0]
            return 1 / (sqrt(1 + 1 / x**2) * x)

    @staticmethod
    @cacheit
    def taylor_term(n, x, *previous_terms):
        if n < 0 or n % 2 == 0:
            return S.Zero
        else:
            x = sympify(x)

            if len(previous_terms) > 2:
                p = previous_terms[-2]
                return -p * x**2 / (n*(n - 1))
            else:
                return (-1)**(n//2) * x**(n)/factorial(n)

    def _eval_rewrite_as_exp(self, arg):
        I = S.ImaginaryUnit
        if isinstance(arg, TrigonometricFunction) or isinstance(arg, HyperbolicFunction):
            arg = arg.func(arg.args[0]).rewrite(exp)
        return (exp(arg*I) - exp(-arg*I)) / (2*I)

    def _eval_rewrite_as_Pow(self, arg):
        if arg.func is log:
            I = S.ImaginaryUnit
            x = arg.args[0]
            return I*x**-I / 2 - I*x**I /2

    def _eval_rewrite_as_cos(self, arg):
        return -cos(arg + S.Pi/2)

    def _eval_rewrite_as_tan(self, arg):
        tan_half = tan(S.Half*arg)
        return 2*tan_half/(1 + tan_half**2)

    def _eval_rewrite_as_sincos(self, arg):
        return sin(arg)*cos(arg)/cos(arg)

    def _eval_rewrite_as_cot(self, arg):
        cot_half = cot(S.Half*arg)
        return 2*cot_half/(1 + cot_half**2)

    def _eval_rewrite_as_pow(self, arg):
        return self.rewrite(cos).rewrite(pow)

    def _eval_rewrite_as_sqrt(self, arg):
        return self.rewrite(cos).rewrite(sqrt)

    def _eval_rewrite_as_csc(self, arg):
        return 1/csc(arg)

    def _eval_conjugate(self):
        return self.func(self.args[0].conjugate())

    def as_real_imag(self, deep=True, **hints):
        re, im = self._as_real_imag(deep=deep, **hints)
        return (sin(re)*cosh(im), cos(re)*sinh(im))

    def _eval_expand_trig(self, **hints):
        from sympy import expand_mul
        from sympy.functions.special.polynomials import chebyshevt, chebyshevu
        arg = self.args[0]
        x = None
        if arg.is_Add:  # TODO, implement more if deep stuff here
            # TODO: Do this more efficiently for more than two terms
            x, y = arg.as_two_terms()
            sx = sin(x, evaluate=False)._eval_expand_trig()
            sy = sin(y, evaluate=False)._eval_expand_trig()
            cx = cos(x, evaluate=False)._eval_expand_trig()
            cy = cos(y, evaluate=False)._eval_expand_trig()
            return sx*cy + sy*cx
        else:
            n, x = arg.as_coeff_Mul(rational=True)
            if n.is_Integer:  # n will be positive because of .eval
                # canonicalization

                # See http://mathworld.wolfram.com/Multiple-AngleFormulas.html
                if n.is_odd:
                    return (-1)**((n - 1)/2)*chebyshevt(n, sin(x))
                else:
                    return expand_mul((-1)**(n/2 - 1)*cos(x)*chebyshevu(n -
                        1, sin(x)), deep=False)
            pi_coeff = _pi_coeff(arg)
            if pi_coeff is not None:
                if pi_coeff.is_Rational:
                    return self.rewrite(sqrt)
        return sin(arg)

    def _eval_as_leading_term(self, x):
        from sympy import Order
        arg = self.args[0].as_leading_term(x)

        if x in arg.free_symbols and Order(1, x).contains(arg):
            return arg
        else:
            return self.func(arg)

    def _eval_is_real(self):
        return self.args[0].is_real

    def _eval_is_finite(self):
        arg = self.args[0]
        if arg.is_real:
            return True


class cos(TrigonometricFunction):
    """
    The cosine function.

    Returns the cosine of x (measured in radians).

    Notes
    =====

    See :func:`sin` for notes about automatic evaluation.

    Examples
    ========

    >>> from sympy import cos, pi
    >>> from sympy.abc import x
    >>> cos(x**2).diff(x)
    -2*x*sin(x**2)
    >>> cos(1).diff(x)
    0
    >>> cos(pi)
    -1
    >>> cos(pi/2)
    0
    >>> cos(2*pi/3)
    -1/2
    >>> cos(pi/12)
    sqrt(2)/4 + sqrt(6)/4

    See Also
    ========

    sin, csc, sec, tan, cot
    asin, acsc, acos, asec, atan, acot, atan2

    References
    ==========

    .. [1] http://en.wikipedia.org/wiki/Trigonometric_functions
    .. [2] http://dlmf.nist.gov/4.14
    .. [3] http://functions.wolfram.com/ElementaryFunctions/Cos
    """

    def fdiff(self, argindex=1):
        if argindex == 1:
            return -sin(self.args[0])
        else:
            raise ArgumentIndexError(self, argindex)

    @classmethod
    def eval(cls, arg):
        from sympy.functions.special.polynomials import chebyshevt
        if arg.is_Number:
            if arg is S.NaN:
                return S.NaN
            elif arg is S.Zero:
                return S.One
            elif arg is S.Infinity or arg is S.NegativeInfinity:
                # In this cases, it is unclear if we should
                # return S.NaN or leave un-evaluated.  One
                # useful test case is how "limit(sin(x)/x,x,oo)"
                # is handled.
                # See test_sin_cos_with_infinity() an
                # Test for issue 3308
                # https://github.com/sympy/sympy/issues/5196
                # For now, we return un-evaluated.
                return

        if arg.could_extract_minus_sign():
            return cls(-arg)

        i_coeff = arg.as_coefficient(S.ImaginaryUnit)
        if i_coeff is not None:
            return cosh(i_coeff)

        pi_coeff = _pi_coeff(arg)
        if pi_coeff is not None:
            if pi_coeff.is_integer:
                return (S.NegativeOne)**pi_coeff

            if (2*pi_coeff).is_integer:
                if pi_coeff.is_even:
                    return (S.NegativeOne)**(pi_coeff/2)
                elif pi_coeff.is_even is False:
                    return S.Zero

            if not pi_coeff.is_Rational:
                narg = pi_coeff*S.Pi
                if narg != arg:
                    return cls(narg)
                return None

            # cosine formula #####################
            # https://github.com/sympy/sympy/issues/6048
            # explicit calculations are preformed for
            # cos(k pi/n) for n = 8,10,12,15,20,24,30,40,60,120
            # Some other exact values like cos(k pi/240) can be
            # calculated using a partial-fraction decomposition
            # by calling cos( X ).rewrite(sqrt)
            cst_table_some = {
                3: S.Half,
                5: (sqrt(5) + 1)/4,
            }
            if pi_coeff.is_Rational:
                q = pi_coeff.q
                p = pi_coeff.p % (2*q)
                if p > q:
                    narg = (pi_coeff - 1)*S.Pi
                    return -cls(narg)
                if 2*p > q:
                    narg = (1 - pi_coeff)*S.Pi
                    return -cls(narg)

                # If nested sqrt's are worse than un-evaluation
                # you can require q to be in (1, 2, 3, 4, 6, 12)
                # q <= 12, q=15, q=20, q=24, q=30, q=40, q=60, q=120 return
                # expressions with 2 or fewer sqrt nestings.
                table2 = {
                    12: (3, 4),
                    20: (4, 5),
                    30: (5, 6),
                    15: (6, 10),
                    24: (6, 8),
                    40: (8, 10),
                    60: (20, 30),
                    120: (40, 60)
                    }
                if q in table2:
                    a, b = p*S.Pi/table2[q][0], p*S.Pi/table2[q][1]
                    nvala, nvalb = cls(a), cls(b)
                    if None == nvala or None == nvalb:
                        return None
                    return nvala*nvalb + cls(S.Pi/2 - a)*cls(S.Pi/2 - b)

                if q > 12:
                    return None

                if q in cst_table_some:
                    cts = cst_table_some[pi_coeff.q]
                    return chebyshevt(pi_coeff.p, cts).expand()

                if 0 == q % 2:
                    narg = (pi_coeff*2)*S.Pi
                    nval = cls(narg)
                    if None == nval:
                        return None
                    x = (2*pi_coeff + 1)/2
                    sign_cos = (-1)**((-1 if x < 0 else 1)*int(abs(x)))
                    return sign_cos*sqrt( (1 + nval)/2 )
            return None

        if arg.is_Add:
            x, m = _peeloff_pi(arg)
            if m:
                return cos(m)*cos(x) - sin(m)*sin(x)

        if arg.func is acos:
            return arg.args[0]

        if arg.func is atan:
            x = arg.args[0]
            return 1 / sqrt(1 + x**2)

        if arg.func is atan2:
            y, x = arg.args
            return x / sqrt(x**2 + y**2)

        if arg.func is asin:
            x = arg.args[0]
            return sqrt(1 - x ** 2)

        if arg.func is acot:
            x = arg.args[0]
            return 1 / sqrt(1 + 1 / x**2)

    @staticmethod
    @cacheit
    def taylor_term(n, x, *previous_terms):
        if n < 0 or n % 2 == 1:
            return S.Zero
        else:
            x = sympify(x)

            if len(previous_terms) > 2:
                p = previous_terms[-2]
                return -p * x**2 / (n*(n - 1))
            else:
                return (-1)**(n//2)*x**(n)/factorial(n)

    def _eval_rewrite_as_exp(self, arg):
        I = S.ImaginaryUnit
        if isinstance(arg, TrigonometricFunction) or isinstance(arg, HyperbolicFunction):
            arg = arg.func(arg.args[0]).rewrite(exp)
        return (exp(arg*I) + exp(-arg*I)) / 2

    def _eval_rewrite_as_Pow(self, arg):
        if arg.func is log:
            I = S.ImaginaryUnit
            x = arg.args[0]
            return x**I/2 + x**-I/2

    def _eval_rewrite_as_sin(self, arg):
        return sin(arg + S.Pi/2)

    def _eval_rewrite_as_tan(self, arg):
        tan_half = tan(S.Half*arg)**2
        return (1 - tan_half)/(1 + tan_half)

    def _eval_rewrite_as_sincos(self, arg):
        return sin(arg)*cos(arg)/sin(arg)

    def _eval_rewrite_as_cot(self, arg):
        cot_half = cot(S.Half*arg)**2
        return (cot_half - 1)/(cot_half + 1)

    def _eval_rewrite_as_pow(self, arg):
        return self._eval_rewrite_as_sqrt(arg)

    def _eval_rewrite_as_sqrt(self, arg):
        from sympy.functions.special.polynomials import chebyshevt

        def migcdex(x):
            # recursive calcuation of gcd and linear combination
            # for a sequence of integers.
            # Given  (x1, x2, x3)
            # Returns (y1, y1, y3, g)
            # such that g is the gcd and x1*y1+x2*y2+x3*y3 - g = 0
            # Note, that this is only one such linear combination.
            if len(x) == 1:
                return (1, x[0])
            if len(x) == 2:
                return igcdex(x[0], x[-1])
            g = migcdex(x[1:])
            u, v, h = igcdex(x[0], g[-1])
            return tuple([u] + [v*i for i in g[0:-1] ] + [h])

        def ipartfrac(r, factors=None):
            from sympy.ntheory import factorint
            if isinstance(r, int):
                return r
            if not isinstance(r, Rational):
                raise TypeError("r is not rational")
            n = r.q
            if 2 > r.q*r.q:
                return r.q

            if None == factors:
                a = [n//x**y for x, y in factorint(r.q).items()]
            else:
                a = [n//x for x in factors]
            if len(a) == 1:
                return [ r ]
            h = migcdex(a)
            ans = [ r.p*Rational(i*j, r.q) for i, j in zip(h[:-1], a) ]
            assert r == sum(ans)
            return ans
        pi_coeff = _pi_coeff(arg)
        if pi_coeff is None:
            return None

        if pi_coeff.is_integer:
            # it was unevaluated
            return self.func(pi_coeff*S.Pi)

        if not pi_coeff.is_Rational:
            return None

        def _cospi257():
            """ Express cos(pi/257) explicitly as a function of radicals
                Based upon the equations in
                http://math.stackexchange.com/questions/516142/how-does-cos2-pi-257-look-like-in-real-radicals
                See also http://www.susqu.edu/brakke/constructions/257-gon.m.txt
            """
            def f1(a, b):
                return (a + sqrt(a**2 + b))/2, (a - sqrt(a**2 + b))/2

            def f2(a, b):
                return (a - sqrt(a**2 + b))/2

            t1, t2 = f1(-1, 256)
            z1, z3 = f1(t1, 64)
            z2, z4 = f1(t2, 64)
            y1, y5 = f1(z1, 4*(5 + t1 + 2*z1))
            y6, y2 = f1(z2, 4*(5 + t2 + 2*z2))
            y3, y7 = f1(z3, 4*(5 + t1 + 2*z3))
            y8, y4 = f1(z4, 4*(5 + t2 + 2*z4))
            x1, x9 = f1(y1, -4*(t1 + y1 + y3 + 2*y6))
            x2, x10 = f1(y2, -4*(t2 + y2 + y4 + 2*y7))
            x3, x11 = f1(y3, -4*(t1 + y3 + y5 + 2*y8))
            x4, x12 = f1(y4, -4*(t2 + y4 + y6 + 2*y1))
            x5, x13 = f1(y5, -4*(t1 + y5 + y7 + 2*y2))
            x6, x14 = f1(y6, -4*(t2 + y6 + y8 + 2*y3))
            x15, x7 = f1(y7, -4*(t1 + y7 + y1 + 2*y4))
            x8, x16 = f1(y8, -4*(t2 + y8 + y2 + 2*y5))
            v1 = f2(x1, -4*(x1 + x2 + x3 + x6))
            v2 = f2(x2, -4*(x2 + x3 + x4 + x7))
            v3 = f2(x8, -4*(x8 + x9 + x10 + x13))
            v4 = f2(x9, -4*(x9 + x10 + x11 + x14))
            v5 = f2(x10, -4*(x10 + x11 + x12 + x15))
            v6 = f2(x16, -4*(x16 + x1 + x2 + x5))
            u1 = -f2(-v1, -4*(v2 + v3))
            u2 = -f2(-v4, -4*(v5 + v6))
            w1 = -2*f2(-u1, -4*u2)
            return sqrt(sqrt(2)*sqrt(w1 + 4)/8 + S.Half)

        cst_table_some = {
            3: S.Half,
            5: (sqrt(5) + 1)/4,
            17: sqrt((15 + sqrt(17))/32 + sqrt(2)*(sqrt(17 - sqrt(17)) +
                sqrt(sqrt(2)*(-8*sqrt(17 + sqrt(17)) - (1 - sqrt(17))
                *sqrt(17 - sqrt(17))) + 6*sqrt(17) + 34))/32),
            257: _cospi257()
            # 65537 is the only other known Fermat prime and the very
            # large expression is intentionally omitted from SymPy; see
            # http://www.susqu.edu/brakke/constructions/65537-gon.m.txt
        }

        def _fermatCoords(n):
            # if n can be factored in terms of Fermat primes with
            # multiplicity of each being 1, return those primes, else
            # False
            primes = []
            for p_i in cst_table_some:
                n, r = divmod(n, p_i)
                if not r:
                    primes.append(p_i)
                    if n == 1:
                        return tuple(primes)
            return False

        if pi_coeff.q in cst_table_some:
            rv = chebyshevt(pi_coeff.p, cst_table_some[pi_coeff.q])
            if pi_coeff.q < 257:
                rv = rv.expand()
            return rv

        if not pi_coeff.q % 2:  # recursively remove factors of 2
            pico2 = pi_coeff*2
            nval = cos(pico2*S.Pi).rewrite(sqrt)
            x = (pico2 + 1)/2
            sign_cos = -1 if int(x) % 2 else 1
            return sign_cos*sqrt( (1 + nval)/2 )

        FC = _fermatCoords(pi_coeff.q)
        if FC:
            decomp = ipartfrac(pi_coeff, FC)
            X = [(x[1], x[0]*S.Pi) for x in zip(decomp, numbered_symbols('z'))]
            pcls = cos(sum([x[0] for x in X]))._eval_expand_trig().subs(X)
            return pcls.rewrite(sqrt)
        else:
            decomp = ipartfrac(pi_coeff)
            X = [(x[1], x[0]*S.Pi) for x in zip(decomp, numbered_symbols('z'))]
            pcls = cos(sum([x[0] for x in X]))._eval_expand_trig().subs(X)
            return pcls

    def _eval_rewrite_as_sec(self, arg):
        return 1/sec(arg)

    def _eval_conjugate(self):
        return self.func(self.args[0].conjugate())

    def as_real_imag(self, deep=True, **hints):
        re, im = self._as_real_imag(deep=deep, **hints)
        return (cos(re)*cosh(im), -sin(re)*sinh(im))

    def _eval_expand_trig(self, **hints):
        from sympy.functions.special.polynomials import chebyshevt
        arg = self.args[0]
        x = None
        if arg.is_Add:  # TODO: Do this more efficiently for more than two terms
            x, y = arg.as_two_terms()
            sx = sin(x, evaluate=False)._eval_expand_trig()
            sy = sin(y, evaluate=False)._eval_expand_trig()
            cx = cos(x, evaluate=False)._eval_expand_trig()
            cy = cos(y, evaluate=False)._eval_expand_trig()
            return cx*cy - sx*sy
        else:
            coeff, terms = arg.as_coeff_Mul(rational=True)
            if coeff.is_Integer:
                return chebyshevt(coeff, cos(terms))
            pi_coeff = _pi_coeff(arg)
            if pi_coeff is not None:
                if pi_coeff.is_Rational:
                    return self.rewrite(sqrt)
        return cos(arg)

    def _eval_as_leading_term(self, x):
        from sympy import Order
        arg = self.args[0].as_leading_term(x)

        if x in arg.free_symbols and Order(1, x).contains(arg):
            return S.One
        else:
            return self.func(arg)

    def _eval_is_real(self):
        return self.args[0].is_real

    def _eval_is_finite(self):
        arg = self.args[0]

        if arg.is_real:
            return True


class tan(TrigonometricFunction):
    """
    The tangent function.

    Returns the tangent of x (measured in radians).

    Notes
    =====

    See :func:`sin` for notes about automatic evaluation.

    Examples
    ========

    >>> from sympy import tan, pi
    >>> from sympy.abc import x
    >>> tan(x**2).diff(x)
    2*x*(tan(x**2)**2 + 1)
    >>> tan(1).diff(x)
    0
    >>> tan(pi/8).expand()
    -1 + sqrt(2)

    See Also
    ========

    sin, csc, cos, sec, cot
    asin, acsc, acos, asec, atan, acot, atan2

    References
    ==========

    .. [1] http://en.wikipedia.org/wiki/Trigonometric_functions
    .. [2] http://dlmf.nist.gov/4.14
    .. [3] http://functions.wolfram.com/ElementaryFunctions/Tan
    """

    def fdiff(self, argindex=1):
        if argindex == 1:
            return S.One + self**2
        else:
            raise ArgumentIndexError(self, argindex)

    def inverse(self, argindex=1):
        """
        Returns the inverse of this function.
        """
        return atan

    @classmethod
    def eval(cls, arg):
        if arg.is_Number:
            if arg is S.NaN:
                return S.NaN
            elif arg is S.Zero:
                return S.Zero

        if arg.could_extract_minus_sign():
            return -cls(-arg)

        i_coeff = arg.as_coefficient(S.ImaginaryUnit)
        if i_coeff is not None:
            return S.ImaginaryUnit * tanh(i_coeff)

        pi_coeff = _pi_coeff(arg, 2)
        if pi_coeff is not None:
            if pi_coeff.is_integer:
                return S.Zero

            if not pi_coeff.is_Rational:
                narg = pi_coeff*S.Pi
                if narg != arg:
                    return cls(narg)
                return None

            if pi_coeff.is_Rational:
                if not pi_coeff.q % 2:
                    narg = pi_coeff*S.Pi*2
                    cresult, sresult = cos(narg), cos(narg - S.Pi/2)
                    if not isinstance(cresult, cos) \
                            and not isinstance(sresult, cos):
                        if sresult == 0:
                            return S.ComplexInfinity
                        return (1 - cresult)/sresult
                table2 = {
                    12: (3, 4),
                    20: (4, 5),
                    30: (5, 6),
                    15: (6, 10),
                    24: (6, 8),
                    40: (8, 10),
                    60: (20, 30),
                    120: (40, 60)
                    }
                q = pi_coeff.q
                p = pi_coeff.p % q
                if q in table2:
                    nvala, nvalb = cls(p*S.Pi/table2[q][0]), cls(p*S.Pi/table2[q][1])
                    if None == nvala or None == nvalb:
                        return None
                    return (nvala - nvalb)/(1 + nvala*nvalb)
                narg = ((pi_coeff + S.Half) % 1 - S.Half)*S.Pi
                # see cos() to specify which expressions should  be
                # expanded automatically in terms of radicals
                cresult, sresult = cos(narg), cos(narg - S.Pi/2)
                if not isinstance(cresult, cos) \
                        and not isinstance(sresult, cos):
                    if cresult == 0:
                        return S.ComplexInfinity
                    return (sresult/cresult)
                if narg != arg:
                    return cls(narg)

        if arg.is_Add:
            x, m = _peeloff_pi(arg)
            if m:
                tanm = tan(m)
                tanx = tan(x)
                if tanm is S.ComplexInfinity:
                    return -cot(x)
                return (tanm + tanx)/(1 - tanm*tanx)

        if arg.func is atan:
            return arg.args[0]

        if arg.func is atan2:
            y, x = arg.args
            return y/x

        if arg.func is asin:
            x = arg.args[0]
            return x / sqrt(1 - x**2)

        if arg.func is acos:
            x = arg.args[0]
            return sqrt(1 - x**2) / x

        if arg.func is acot:
            x = arg.args[0]
            return 1 / x

    @staticmethod
    @cacheit
    def taylor_term(n, x, *previous_terms):
        from sympy import bernoulli
        if n < 0 or n % 2 == 0:
            return S.Zero
        else:
            x = sympify(x)

            a, b = ((n - 1)//2), 2**(n + 1)

            B = bernoulli(n + 1)
            F = factorial(n + 1)

            return (-1)**a * b*(b - 1) * B/F * x**n

    def _eval_nseries(self, x, n, logx):
        i = self.args[0].limit(x, 0)*2/S.Pi
        if i and i.is_Integer:
            return self.rewrite(cos)._eval_nseries(x, n=n, logx=logx)
        return Function._eval_nseries(self, x, n=n, logx=logx)

    def _eval_rewrite_as_Pow(self, arg):
        if arg.func is log:
            I = S.ImaginaryUnit
            x = arg.args[0]
            return I*(x**-I - x**I)/(x**-I + x**I)

    def _eval_conjugate(self):
        return self.func(self.args[0].conjugate())

    def as_real_imag(self, deep=True, **hints):
        re, im = self._as_real_imag(deep=deep, **hints)
        if im:
            denom = cos(2*re) + cosh(2*im)
            return (sin(2*re)/denom, sinh(2*im)/denom)
        else:
            return (self.func(re), S.Zero)

    def _eval_expand_trig(self, **hints):
        from sympy import im, re
        arg = self.args[0]
        x = None
        if arg.is_Add:
            from sympy import symmetric_poly
            n = len(arg.args)
            TX = []
            for x in arg.args:
                tx = tan(x, evaluate=False)._eval_expand_trig()
                TX.append(tx)

            Yg = numbered_symbols('Y')
            Y = [ next(Yg) for i in range(n) ]

            p = [0, 0]
            for i in range(n + 1):
                p[1 - i % 2] += symmetric_poly(i, Y)*(-1)**((i % 4)//2)
            return (p[0]/p[1]).subs(list(zip(Y, TX)))

        else:
            coeff, terms = arg.as_coeff_Mul(rational=True)
            if coeff.is_Integer and coeff > 1:
                I = S.ImaginaryUnit
                z = Symbol('dummy', real=True)
                P = ((1 + I*z)**coeff).expand()
                return (im(P)/re(P)).subs([(z, tan(terms))])
        return tan(arg)

    def _eval_rewrite_as_exp(self, arg):
        I = S.ImaginaryUnit
        if isinstance(arg, TrigonometricFunction) or isinstance(arg, HyperbolicFunction):
            arg = arg.func(arg.args[0]).rewrite(exp)
        neg_exp, pos_exp = exp(-arg*I), exp(arg*I)
        return I*(neg_exp - pos_exp)/(neg_exp + pos_exp)

    def _eval_rewrite_as_sin(self, x):
        return 2*sin(x)**2/sin(2*x)

    def _eval_rewrite_as_cos(self, x):
        return -cos(x + S.Pi/2)/cos(x)

    def _eval_rewrite_as_sincos(self, arg):
        return sin(arg)/cos(arg)

    def _eval_rewrite_as_cot(self, arg):
        return 1/cot(arg)

    def _eval_rewrite_as_pow(self, arg):
        y = self.rewrite(cos).rewrite(pow)
        if y.has(cos):
            return None
        return y

    def _eval_rewrite_as_sqrt(self, arg):
        y = self.rewrite(cos).rewrite(sqrt)
        if y.has(cos):
            return None
        return y

    def _eval_as_leading_term(self, x):
        from sympy import Order
        arg = self.args[0].as_leading_term(x)

        if x in arg.free_symbols and Order(1, x).contains(arg):
            return arg
        else:
            return self.func(arg)

    def _eval_is_real(self):
        return self.args[0].is_real

    def _eval_is_finite(self):
        arg = self.args[0]

        if arg.is_imaginary:
            return True


class cot(TrigonometricFunction):
    """
    The cotangent function.

    Returns the cotangent of x (measured in radians).

    Notes
    =====

    See :func:`sin` for notes about automatic evaluation.

    Examples
    ========

    >>> from sympy import cot, pi
    >>> from sympy.abc import x
    >>> cot(x**2).diff(x)
    2*x*(-cot(x**2)**2 - 1)
    >>> cot(1).diff(x)
    0
    >>> cot(pi/12)
    sqrt(3) + 2

    See Also
    ========

    sin, csc, cos, sec, tan
    asin, acsc, acos, asec, atan, acot, atan2

    References
    ==========

    .. [1] http://en.wikipedia.org/wiki/Trigonometric_functions
    .. [2] http://dlmf.nist.gov/4.14
    .. [3] http://functions.wolfram.com/ElementaryFunctions/Cot
    """

    def fdiff(self, argindex=1):
        if argindex == 1:
            return S.NegativeOne - self**2
        else:
            raise ArgumentIndexError(self, argindex)

    def inverse(self, argindex=1):
        """
        Returns the inverse of this function.
        """
        return acot

    @classmethod
    def eval(cls, arg):
        if arg.is_Number:
            if arg is S.NaN:
                return S.NaN
            if arg is S.Zero:
                return S.ComplexInfinity

        if arg.could_extract_minus_sign():
            return -cls(-arg)

        i_coeff = arg.as_coefficient(S.ImaginaryUnit)
        if i_coeff is not None:
            return -S.ImaginaryUnit * coth(i_coeff)

        pi_coeff = _pi_coeff(arg, 2)
        if pi_coeff is not None:
            if pi_coeff.is_integer:
                return S.ComplexInfinity

            if not pi_coeff.is_Rational:
                narg = pi_coeff*S.Pi
                if narg != arg:
                    return cls(narg)
                return None

            if pi_coeff.is_Rational:
                if pi_coeff.q > 2 and not pi_coeff.q % 2:
                    narg = pi_coeff*S.Pi*2
                    cresult, sresult = cos(narg), cos(narg - S.Pi/2)
                    if not isinstance(cresult, cos) \
                            and not isinstance(sresult, cos):
                        return (1 + cresult)/sresult
                table2 = {
                    12: (3, 4),
                    20: (4, 5),
                    30: (5, 6),
                    15: (6, 10),
                    24: (6, 8),
                    40: (8, 10),
                    60: (20, 30),
                    120: (40, 60)
                    }
                q = pi_coeff.q
                p = pi_coeff.p % q
                if q in table2:
                    nvala, nvalb = cls(p*S.Pi/table2[q][0]), cls(p*S.Pi/table2[q][1])
                    if None == nvala or None == nvalb:
                        return None
                    return (1 + nvala*nvalb)/(nvalb - nvala)
                narg = (((pi_coeff + S.Half) % 1) - S.Half)*S.Pi
                # see cos() to specify which expressions should be
                # expanded automatically in terms of radicals
                cresult, sresult = cos(narg), cos(narg - S.Pi/2)
                if not isinstance(cresult, cos) \
                        and not isinstance(sresult, cos):
                    if sresult == 0:
                        return S.ComplexInfinity
                    return cresult / sresult
                if narg != arg:
                    return cls(narg)

        if arg.is_Add:
            x, m = _peeloff_pi(arg)
            if m:
                cotm = cot(m)
                if cotm == 0:
                    return -tan(x)
                cotx = cot(x)
                if cotm is S.ComplexInfinity:
                    return cotx
                if cotm.is_Rational:
                    return (cotm*cotx - 1) / (cotm + cotx)
            return None

        if arg.func is acot:
            return arg.args[0]

        if arg.func is atan:
            x = arg.args[0]
            return 1 / x

        if arg.func is atan2:
            y, x = arg.args
            return x/y

        if arg.func is asin:
            x = arg.args[0]
            return sqrt(1 - x**2) / x

        if arg.func is acos:
            x = arg.args[0]
            return x / sqrt(1 - x**2)

    @staticmethod
    @cacheit
    def taylor_term(n, x, *previous_terms):
        from sympy import bernoulli
        if n == 0:
            return 1 / sympify(x)
        elif n < 0 or n % 2 == 0:
            return S.Zero
        else:
            x = sympify(x)

            B = bernoulli(n + 1)
            F = factorial(n + 1)

            return (-1)**((n + 1)//2) * 2**(n + 1) * B/F * x**n

    def _eval_nseries(self, x, n, logx):
        i = self.args[0].limit(x, 0)/S.Pi
        if i and i.is_Integer:
            return self.rewrite(cos)._eval_nseries(x, n=n, logx=logx)
        return self.rewrite(tan)._eval_nseries(x, n=n, logx=logx)

    def _eval_conjugate(self):
        return self.func(self.args[0].conjugate())

    def as_real_imag(self, deep=True, **hints):
        re, im = self._as_real_imag(deep=deep, **hints)
        if im:
            denom = cos(2*re) - cosh(2*im)
            return (-sin(2*re)/denom, -sinh(2*im)/denom)
        else:
            return (self.func(re), S.Zero)

    def _eval_rewrite_as_exp(self, arg):
        I = S.ImaginaryUnit
        if isinstance(arg, TrigonometricFunction) or isinstance(arg, HyperbolicFunction):
            arg = arg.func(arg.args[0]).rewrite(exp)
        neg_exp, pos_exp = exp(-arg*I), exp(arg*I)
        return I*(pos_exp + neg_exp)/(pos_exp - neg_exp)

    def _eval_rewrite_as_Pow(self, arg):
        if arg.func is log:
            I = S.ImaginaryUnit
            x = arg.args[0]
            return -I*(x**-I + x**I)/(x**-I - x**I)

    def _eval_rewrite_as_sin(self, x):
        return 2*sin(2*x)/sin(x)**2

    def _eval_rewrite_as_cos(self, x):
        return -cos(x)/cos(x + S.Pi/2)

    def _eval_rewrite_as_sincos(self, arg):
        return cos(arg)/sin(arg)

    def _eval_rewrite_as_tan(self, arg):
        return 1/tan(arg)

    def _eval_rewrite_as_pow(self, arg):
        y = self.rewrite(cos).rewrite(pow)
        if y.has(cos):
            return None
        return y

    def _eval_rewrite_as_sqrt(self, arg):
        y = self.rewrite(cos).rewrite(sqrt)
        if y.has(cos):
            return None
        return y

    def _eval_as_leading_term(self, x):
        from sympy import Order
        arg = self.args[0].as_leading_term(x)

        if x in arg.free_symbols and Order(1, x).contains(arg):
            return 1/arg
        else:
            return self.func(arg)

    def _eval_is_real(self):
        return self.args[0].is_real

    def _eval_expand_trig(self, **hints):
        from sympy import im, re
        arg = self.args[0]
        x = None
        if arg.is_Add:
            from sympy import symmetric_poly
            n = len(arg.args)
            CX = []
            for x in arg.args:
                cx = cot(x, evaluate=False)._eval_expand_trig()
                CX.append(cx)

            Yg = numbered_symbols('Y')
            Y = [ next(Yg) for i in range(n) ]

            p = [0, 0]
            for i in range(n, -1, -1):
                p[(n - i) % 2] += symmetric_poly(i, Y)*(-1)**(((n - i) % 4)//2)
            return (p[0]/p[1]).subs(list(zip(Y, CX)))
        else:
            coeff, terms = arg.as_coeff_Mul(rational=True)
            if coeff.is_Integer and coeff > 1:
                I = S.ImaginaryUnit
                z = Symbol('dummy', real=True)
                P = ((z + I)**coeff).expand()
                return (re(P)/im(P)).subs([(z, cot(terms))])
        return cot(arg)

    def _eval_is_finite(self):
        arg = self.args[0]
        if arg.is_imaginary:
            return True

    def _eval_subs(self, old, new):
        if self == old:
            return new
        arg = self.args[0]
        argnew = arg.subs(old, new)
        if arg != argnew and (argnew/S.Pi).is_integer:
            return S.ComplexInfinity
        return cot(argnew)


class ReciprocalTrigonometricFunction(TrigonometricFunction):
    """Base class for reciprocal functions of trigonometric functions. """

    _reciprocal_of = None       # mandatory, to be defined in subclass

    # _is_even and _is_odd are used for correct evaluation of csc(-x), sec(-x)
    # TODO refactor into TrigonometricFunction common parts of
    # trigonometric functions eval() like even/odd, func(x+2*k*pi), etc.
    _is_even = None  # optional, to be defined in subclass
    _is_odd = None   # optional, to be defined in subclass

    @classmethod
    def eval(cls, arg):
        if arg.could_extract_minus_sign():
            if cls._is_even:
                return cls(-arg)
            if cls._is_odd:
                return -cls(-arg)

        pi_coeff = _pi_coeff(arg)
        if (pi_coeff is not None
            and not (2*pi_coeff).is_integer
            and pi_coeff.is_Rational):
                q = pi_coeff.q
                p = pi_coeff.p % (2*q)
                if p > q:
                    narg = (pi_coeff - 1)*S.Pi
                    return -cls(narg)
                if 2*p > q:
                    narg = (1 - pi_coeff)*S.Pi
                    if cls._is_odd:
                        return cls(narg)
                    elif cls._is_even:
                        return -cls(narg)

        t = cls._reciprocal_of.eval(arg)
        if hasattr(arg, 'inverse') and arg.inverse() == cls:
            return arg.args[0]
        return 1/t if t != None else t

    def _call_reciprocal(self, method_name, *args, **kwargs):
        # Calls method_name on _reciprocal_of
        o = self._reciprocal_of(self.args[0])
        return getattr(o, method_name)(*args, **kwargs)

    def _calculate_reciprocal(self, method_name, *args, **kwargs):
        # If calling method_name on _reciprocal_of returns a value != None
        # then return the reciprocal of that value
        t = self._call_reciprocal(method_name, *args, **kwargs)
        return 1/t if t != None else t

    def _rewrite_reciprocal(self, method_name, arg):
        # Special handling for rewrite functions. If reciprocal rewrite returns
        # unmodified expression, then return None
        t = self._call_reciprocal(method_name, arg)
        if t != None and t != self._reciprocal_of(arg):
            return 1/t

    def fdiff(self, argindex=1):
        return -self._calculate_reciprocal("fdiff", argindex)/self**2

    def _eval_rewrite_as_exp(self, arg):
        return self._rewrite_reciprocal("_eval_rewrite_as_exp", arg)

    def _eval_rewrite_as_Pow(self, arg):
        return self._rewrite_reciprocal("_eval_rewrite_as_Pow", arg)

    def _eval_rewrite_as_sin(self, arg):
        return self._rewrite_reciprocal("_eval_rewrite_as_sin", arg)

    def _eval_rewrite_as_cos(self, arg):
        return self._rewrite_reciprocal("_eval_rewrite_as_cos", arg)

    def _eval_rewrite_as_tan(self, arg):
        return self._rewrite_reciprocal("_eval_rewrite_as_tan", arg)

    def _eval_rewrite_as_pow(self, arg):
        return self._rewrite_reciprocal("_eval_rewrite_as_pow", arg)

    def _eval_rewrite_as_sqrt(self, arg):
        return self._rewrite_reciprocal("_eval_rewrite_as_sqrt", arg)

    def _eval_conjugate(self):
        return self.func(self.args[0].conjugate())

    def as_real_imag(self, deep=True, **hints):
        return (1/self._reciprocal_of(self.args[0])).as_real_imag(deep,
                                                                  **hints)

    def _eval_expand_trig(self, **hints):
        return self._calculate_reciprocal("_eval_expand_trig", **hints)

    def _eval_is_real(self):
        return self._reciprocal_of(self.args[0])._eval_is_real()

    def _eval_as_leading_term(self, x):
        return (1/self._reciprocal_of(self.args[0]))._eval_as_leading_term(x)

    def _eval_is_finite(self):
        return (1/self._reciprocal_of(self.args[0])).is_finite

    def _eval_nseries(self, x, n, logx):
        return (1/self._reciprocal_of(self.args[0]))._eval_nseries(x, n, logx)


class sec(ReciprocalTrigonometricFunction):
    """
    The secant function.

    Returns the secant of x (measured in radians).

    Notes
    =====

    See :func:`sin` for notes about automatic evaluation.

    Examples
    ========

    >>> from sympy import sec
    >>> from sympy.abc import x
    >>> sec(x**2).diff(x)
    2*x*tan(x**2)*sec(x**2)
    >>> sec(1).diff(x)
    0

    See Also
    ========

    sin, csc, cos, tan, cot
    asin, acsc, acos, asec, atan, acot, atan2

    References
    ==========

    .. [1] http://en.wikipedia.org/wiki/Trigonometric_functions
    .. [2] http://dlmf.nist.gov/4.14
    .. [2] http://functions.wolfram.com/ElementaryFunctions/Sec
    """

    _reciprocal_of = cos
    _is_even = True

    def _eval_rewrite_as_cot(self, arg):
        cot_half_sq = cot(arg/2)**2
        return (cot_half_sq + 1)/(cot_half_sq - 1)

    def _eval_rewrite_as_cos(self, arg):
        return (1/cos(arg))

    def _eval_rewrite_as_sincos(self, arg):
        return sin(arg)/(cos(arg)*sin(arg))

    def fdiff(self, argindex=1):
        if argindex == 1:
            return tan(self.args[0])*sec(self.args[0])
        else:
            raise ArgumentIndexError(self, argindex)

    @staticmethod
    @cacheit
    def taylor_term(n, x, *previous_terms):
        # Reference Formula:
        # http://functions.wolfram.com/ElementaryFunctions/Sec/06/01/02/01/
        from sympy.functions.combinatorial.numbers import euler
        if n < 0 or n % 2 == 1:
            return S.Zero
        else:
            x = sympify(x)
            k = n//2
            return (-1)**k*euler(2*k)/factorial(2*k)*x**(2*k)


class csc(ReciprocalTrigonometricFunction):
    """
    The cosecant function.

    Returns the cosecant of x (measured in radians).

    Notes
    =====

    See :func:`sin` for notes about automatic evaluation.

    Examples
    ========

    >>> from sympy import csc
    >>> from sympy.abc import x
    >>> csc(x**2).diff(x)
    -2*x*cot(x**2)*csc(x**2)
    >>> csc(1).diff(x)
    0

    See Also
    ========

    sin, cos, sec, tan, cot
    asin, acsc, acos, asec, atan, acot, atan2

    References
    ==========

    .. [1] http://en.wikipedia.org/wiki/Trigonometric_functions
    .. [2] http://dlmf.nist.gov/4.14
    .. [3] http://functions.wolfram.com/ElementaryFunctions/Csc
    """

    _reciprocal_of = sin
    _is_odd = True

    def _eval_rewrite_as_sin(self, arg):
        return (1/sin(arg))

    def _eval_rewrite_as_sincos(self, arg):
        return cos(arg)/(sin(arg)*cos(arg))

    def _eval_rewrite_as_cot(self, arg):
        cot_half = cot(arg/2)
        return (1 + cot_half**2)/(2*cot_half)

    def fdiff(self, argindex=1):
        if argindex == 1:
            return -cot(self.args[0])*csc(self.args[0])
        else:
            raise ArgumentIndexError(self, argindex)

    @staticmethod
    @cacheit
    def taylor_term(n, x, *previous_terms):
        from sympy import bernoulli
        if n == 0:
            return 1/sympify(x)
        elif n < 0 or n % 2 == 0:
            return S.Zero
        else:
            x = sympify(x)
            k = n//2 + 1
            return ((-1)**(k - 1)*2*(2**(2*k - 1) - 1)*
                    bernoulli(2*k)*x**(2*k - 1)/factorial(2*k))


###############################################################################
########################### TRIGONOMETRIC INVERSES ############################
###############################################################################


class InverseTrigonometricFunction(Function):
    """Base class for inverse trigonometric functions."""

    pass


class asin(InverseTrigonometricFunction):
    """
    The inverse sine function.

    Returns the arcsine of x in radians.

    Notes
    =====

    asin(x) will evaluate automatically in the cases oo, -oo, 0, 1,
    -1 and for some instances when the result is a rational multiple
    of pi (see the eval class method).

    Examples
    ========

    >>> from sympy import asin, oo, pi
    >>> asin(1)
    pi/2
    >>> asin(-1)
    -pi/2

    See Also
    ========

    sin, csc, cos, sec, tan, cot
    acsc, acos, asec, atan, acot, atan2

    References
    ==========

    .. [1] http://en.wikipedia.org/wiki/Inverse_trigonometric_functions
    .. [2] http://dlmf.nist.gov/4.23
    .. [3] http://functions.wolfram.com/ElementaryFunctions/ArcSin
    """

    def fdiff(self, argindex=1):
        if argindex == 1:
            return 1/sqrt(1 - self.args[0]**2)
        else:
            raise ArgumentIndexError(self, argindex)

    def _eval_is_rational(self):
        s = self.func(*self.args)
        if s.func == self.func:
            if s.args[0].is_rational:
                return False
        else:
            return s.is_rational

    def _eval_is_positive(self):
        if self.args[0].is_positive:
            return (self.args[0] - 1).is_negative
        if self.args[0].is_negative:
            return not (self.args[0] + 1).is_positive

    @classmethod
    def eval(cls, arg):
        if arg.is_Number:
            if arg is S.NaN:
                return S.NaN
            elif arg is S.Infinity:
                return S.NegativeInfinity * S.ImaginaryUnit
            elif arg is S.NegativeInfinity:
                return S.Infinity * S.ImaginaryUnit
            elif arg is S.Zero:
                return S.Zero
            elif arg is S.One:
                return S.Pi / 2
            elif arg is S.NegativeOne:
                return -S.Pi / 2

        if arg.could_extract_minus_sign():
            return -cls(-arg)

        if arg.is_number:
            cst_table = {
                sqrt(3)/2: 3,
                -sqrt(3)/2: -3,
                sqrt(2)/2: 4,
                -sqrt(2)/2: -4,
                1/sqrt(2): 4,
                -1/sqrt(2): -4,
                sqrt((5 - sqrt(5))/8): 5,
                -sqrt((5 - sqrt(5))/8): -5,
                S.Half: 6,
                -S.Half: -6,
                sqrt(2 - sqrt(2))/2: 8,
                -sqrt(2 - sqrt(2))/2: -8,
                (sqrt(5) - 1)/4: 10,
                (1 - sqrt(5))/4: -10,
                (sqrt(3) - 1)/sqrt(2**3): 12,
                (1 - sqrt(3))/sqrt(2**3): -12,
                (sqrt(5) + 1)/4: S(10)/3,
                -(sqrt(5) + 1)/4: -S(10)/3
            }

            if arg in cst_table:
                return S.Pi / cst_table[arg]

        i_coeff = arg.as_coefficient(S.ImaginaryUnit)
        if i_coeff is not None:
            return S.ImaginaryUnit * asinh(i_coeff)

    @staticmethod
    @cacheit
    def taylor_term(n, x, *previous_terms):
        if n < 0 or n % 2 == 0:
            return S.Zero
        else:
            x = sympify(x)
            if len(previous_terms) >= 2 and n > 2:
                p = previous_terms[-2]
                return p * (n - 2)**2/(n*(n - 1)) * x**2
            else:
                k = (n - 1) // 2
                R = RisingFactorial(S.Half, k)
                F = factorial(k)
                return R / F * x**n / n

    def _eval_as_leading_term(self, x):
        from sympy import Order
        arg = self.args[0].as_leading_term(x)

        if x in arg.free_symbols and Order(1, x).contains(arg):
            return arg
        else:
            return self.func(arg)

    def _eval_rewrite_as_acos(self, x):
        return S.Pi/2 - acos(x)

    def _eval_rewrite_as_atan(self, x):
        return 2*atan(x/(1 + sqrt(1 - x**2)))

    def _eval_rewrite_as_log(self, x):
        return -S.ImaginaryUnit*log(S.ImaginaryUnit*x + sqrt(1 - x**2))

    def _eval_rewrite_as_acot(self, arg):
        return 2*acot((1 + sqrt(1 - arg**2))/arg)

    def _eval_rewrite_as_asec(self, arg):
        return S.Pi/2 - asec(1/arg)

    def _eval_rewrite_as_acsc(self, arg):
        return acsc(1/arg)

    def _eval_is_real(self):
        x = self.args[0]
        return x.is_real and (1 - abs(x)).is_nonnegative

    def inverse(self, argindex=1):
        """
        Returns the inverse of this function.
        """
        return sin


class acos(InverseTrigonometricFunction):
    """
    The inverse cosine function.

    Returns the arc cosine of x (measured in radians).

    Notes
    =====

    acos(x) will evaluate automatically in the cases
    oo, -oo, 0, 1, -1.

    Examples
    ========

    >>> from sympy import acos, oo, pi
    >>> acos(1)
    0
    >>> acos(0)
    pi/2
    >>> acos(oo)
    oo*I

    See Also
    ========

    sin, csc, cos, sec, tan, cot
    asin, acsc, asec, atan, acot, atan2

    References
    ==========

    .. [1] http://en.wikipedia.org/wiki/Inverse_trigonometric_functions
    .. [2] http://dlmf.nist.gov/4.23
    .. [3] http://functions.wolfram.com/ElementaryFunctions/ArcCos
    """

    def fdiff(self, argindex=1):
        if argindex == 1:
            return -1/sqrt(1 - self.args[0]**2)
        else:
            raise ArgumentIndexError(self, argindex)

    def _eval_is_rational(self):
        s = self.func(*self.args)
        if s.func == self.func:
            if s.args[0].is_rational:
                return False
        else:
            return s.is_rational

    def _eval_is_positive(self):
        x = self.args[0]
        return (1 - abs(x)).is_nonnegative

    @classmethod
    def eval(cls, arg):
        if arg.is_Number:
            if arg is S.NaN:
                return S.NaN
            elif arg is S.Infinity:
                return S.Infinity * S.ImaginaryUnit
            elif arg is S.NegativeInfinity:
                return S.NegativeInfinity * S.ImaginaryUnit
            elif arg is S.Zero:
                return S.Pi / 2
            elif arg is S.One:
                return S.Zero
            elif arg is S.NegativeOne:
                return S.Pi

        if arg.is_number:
            cst_table = {
                S.Half: S.Pi/3,
                -S.Half: 2*S.Pi/3,
                sqrt(2)/2: S.Pi/4,
                -sqrt(2)/2: 3*S.Pi/4,
                1/sqrt(2): S.Pi/4,
                -1/sqrt(2): 3*S.Pi/4,
                sqrt(3)/2: S.Pi/6,
                -sqrt(3)/2: 5*S.Pi/6,
            }

            if arg in cst_table:
                return cst_table[arg]

    @staticmethod
    @cacheit
    def taylor_term(n, x, *previous_terms):
        if n == 0:
            return S.Pi / 2
        elif n < 0 or n % 2 == 0:
            return S.Zero
        else:
            x = sympify(x)
            if len(previous_terms) >= 2 and n > 2:
                p = previous_terms[-2]
                return p * (n - 2)**2/(n*(n - 1)) * x**2
            else:
                k = (n - 1) // 2
                R = RisingFactorial(S.Half, k)
                F = factorial(k)
                return -R / F * x**n / n

    def _eval_as_leading_term(self, x):
        from sympy import Order
        arg = self.args[0].as_leading_term(x)

        if x in arg.free_symbols and Order(1, x).contains(arg):
            return arg
        else:
            return self.func(arg)

    def _eval_is_real(self):
        x = self.args[0]
        return x.is_real and (1 - abs(x)).is_nonnegative

    def _eval_rewrite_as_log(self, x):
        return S.Pi/2 + S.ImaginaryUnit * \
            log(S.ImaginaryUnit * x + sqrt(1 - x**2))

    def _eval_rewrite_as_asin(self, x):
        return S.Pi/2 - asin(x)

    def _eval_rewrite_as_atan(self, x):
        return atan(sqrt(1 - x**2)/x) + (S.Pi/2)*(1 - x*sqrt(1/x**2))

    def inverse(self, argindex=1):
        """
        Returns the inverse of this function.
        """
        return cos

    def _eval_rewrite_as_acot(self, arg):
        return S.Pi/2 - 2*acot((1 + sqrt(1 - arg**2))/arg)

    def _eval_rewrite_as_asec(self, arg):
        return asec(1/arg)

    def _eval_rewrite_as_acsc(self, arg):
        return S.Pi/2 - acsc(1/arg)

    def _eval_conjugate(self):
        z = self.args[0]
        r = self.func(self.args[0].conjugate())
        if z.is_real is False:
            return r
        elif z.is_real and (z + 1).is_nonnegative and (z - 1).is_nonpositive:
            return r


class atan(InverseTrigonometricFunction):
    """
    The inverse tangent function.

    Returns the arc tangent of x (measured in radians).

    Notes
    =====

    atan(x) will evaluate automatically in the cases
    oo, -oo, 0, 1, -1.

    Examples
    ========

    >>> from sympy import atan, oo, pi
    >>> atan(0)
    0
    >>> atan(1)
    pi/4
    >>> atan(oo)
    pi/2

    See Also
    ========

    sin, csc, cos, sec, tan, cot
    asin, acsc, acos, asec, acot, atan2

    References
    ==========

    .. [1] http://en.wikipedia.org/wiki/Inverse_trigonometric_functions
    .. [2] http://dlmf.nist.gov/4.23
    .. [3] http://functions.wolfram.com/ElementaryFunctions/ArcTan
    """

    def fdiff(self, argindex=1):
        if argindex == 1:
            return 1/(1 + self.args[0]**2)
        else:
            raise ArgumentIndexError(self, argindex)

    def _eval_is_rational(self):
        s = self.func(*self.args)
        if s.func == self.func:
            if s.args[0].is_rational:
                return False
        else:
            return s.is_rational

    def _eval_is_positive(self):
        return self.args[0].is_positive

    def _eval_is_nonnegative(self):
        return self.args[0].is_nonnegative

    @classmethod
    def eval(cls, arg):
        if arg.is_Number:
            if arg is S.NaN:
                return S.NaN
            elif arg is S.Infinity:
                return S.Pi / 2
            elif arg is S.NegativeInfinity:
                return -S.Pi / 2
            elif arg is S.Zero:
                return S.Zero
            elif arg is S.One:
                return S.Pi / 4
            elif arg is S.NegativeOne:
                return -S.Pi / 4
        if arg.could_extract_minus_sign():
            return -cls(-arg)

        if arg.is_number:
            cst_table = {
                sqrt(3)/3: 6,
                -sqrt(3)/3: -6,
                1/sqrt(3): 6,
                -1/sqrt(3): -6,
                sqrt(3): 3,
                -sqrt(3): -3,
                (1 + sqrt(2)): S(8)/3,
                -(1 + sqrt(2)): S(8)/3,
                (sqrt(2) - 1): 8,
                (1 - sqrt(2)): -8,
                sqrt((5 + 2*sqrt(5))): S(5)/2,
                -sqrt((5 + 2*sqrt(5))): -S(5)/2,
                (2 - sqrt(3)): 12,
                -(2 - sqrt(3)): -12
            }

            if arg in cst_table:
                return S.Pi / cst_table[arg]

        i_coeff = arg.as_coefficient(S.ImaginaryUnit)
        if i_coeff is not None:
            return S.ImaginaryUnit * atanh(i_coeff)

    @staticmethod
    @cacheit
    def taylor_term(n, x, *previous_terms):
        if n < 0 or n % 2 == 0:
            return S.Zero
        else:
            x = sympify(x)
            return (-1)**((n - 1)//2) * x**n / n

    def _eval_as_leading_term(self, x):
        from sympy import Order
        arg = self.args[0].as_leading_term(x)

        if x in arg.free_symbols and Order(1, x).contains(arg):
            return arg
        else:
            return self.func(arg)

    def _eval_is_real(self):
        return self.args[0].is_real

    def _eval_rewrite_as_log(self, x):
        return S.ImaginaryUnit/2 * (log(
            (S(1) - S.ImaginaryUnit * x)/(S(1) + S.ImaginaryUnit * x)))

    def _eval_aseries(self, n, args0, x, logx):
        if args0[0] == S.Infinity:
            return (S.Pi/2 - atan(1/self.args[0]))._eval_nseries(x, n, logx)
        elif args0[0] == S.NegativeInfinity:
            return (-S.Pi/2 - atan(1/self.args[0]))._eval_nseries(x, n, logx)
        else:
            return super(atan, self)._eval_aseries(n, args0, x, logx)

    def inverse(self, argindex=1):
        """
        Returns the inverse of this function.
        """
        return tan

    def _eval_rewrite_as_asin(self, arg):
        return sqrt(arg**2)/arg*(S.Pi/2 - asin(1/sqrt(1 + arg**2)))

    def _eval_rewrite_as_acos(self, arg):
        return sqrt(arg**2)/arg*acos(1/sqrt(1 + arg**2))

    def _eval_rewrite_as_acot(self, arg):
        return acot(1/arg)

    def _eval_rewrite_as_asec(self, arg):
        return sqrt(arg**2)/arg*asec(sqrt(1 + arg**2))

    def _eval_rewrite_as_acsc(self, arg):
        return sqrt(arg**2)/arg*(S.Pi/2 - acsc(sqrt(1 + arg**2)))


class acot(InverseTrigonometricFunction):
    """
    The inverse cotangent function.

    Returns the arc cotangent of x (measured in radians).

    See Also
    ========

    sin, csc, cos, sec, tan, cot
    asin, acsc, acos, asec, atan, atan2

    References
    ==========

    .. [1] http://en.wikipedia.org/wiki/Inverse_trigonometric_functions
    .. [2] http://dlmf.nist.gov/4.23
    .. [3] http://functions.wolfram.com/ElementaryFunctions/ArcCot
    """

    def fdiff(self, argindex=1):
        if argindex == 1:
            return -1 / (1 + self.args[0]**2)
        else:
            raise ArgumentIndexError(self, argindex)

    def _eval_is_rational(self):
        s = self.func(*self.args)
        if s.func == self.func:
            if s.args[0].is_rational:
                return False
        else:
            return s.is_rational

    def _eval_is_positive(self):
        return self.args[0].is_real

    @classmethod
    def eval(cls, arg):
        if arg.is_Number:
            if arg is S.NaN:
                return S.NaN
            elif arg is S.Infinity:
                return S.Zero
            elif arg is S.NegativeInfinity:
                return S.Zero
            elif arg is S.Zero:
                return S.Pi/ 2
            elif arg is S.One:
                return S.Pi / 4
            elif arg is S.NegativeOne:
                return -S.Pi / 4

        if arg.could_extract_minus_sign():
            return -cls(-arg)

        if arg.is_number:
            cst_table = {
                sqrt(3)/3: 3,
                -sqrt(3)/3: -3,
                1/sqrt(3): 3,
                -1/sqrt(3): -3,
                sqrt(3): 6,
                -sqrt(3): -6,
                (1 + sqrt(2)): 8,
                -(1 + sqrt(2)): -8,
                (1 - sqrt(2)): -S(8)/3,
                (sqrt(2) - 1): S(8)/3,
                sqrt(5 + 2*sqrt(5)): 10,
                -sqrt(5 + 2*sqrt(5)): -10,
                (2 + sqrt(3)): 12,
                -(2 + sqrt(3)): -12,
                (2 - sqrt(3)): S(12)/5,
                -(2 - sqrt(3)): -S(12)/5,
            }

            if arg in cst_table:
                return S.Pi / cst_table[arg]

        i_coeff = arg.as_coefficient(S.ImaginaryUnit)
        if i_coeff is not None:
            return -S.ImaginaryUnit * acoth(i_coeff)

    @staticmethod
    @cacheit
    def taylor_term(n, x, *previous_terms):
        if n == 0:
            return S.Pi / 2  # FIX THIS
        elif n < 0 or n % 2 == 0:
            return S.Zero
        else:
            x = sympify(x)
            return (-1)**((n + 1)//2) * x**n / n

    def _eval_as_leading_term(self, x):
        from sympy import Order
        arg = self.args[0].as_leading_term(x)

        if x in arg.free_symbols and Order(1, x).contains(arg):
            return arg
        else:
            return self.func(arg)

    def _eval_is_real(self):
        return self.args[0].is_real

    def _eval_aseries(self, n, args0, x, logx):
        if args0[0] == S.Infinity:
            return (S.Pi/2 - acot(1/self.args[0]))._eval_nseries(x, n, logx)
        elif args0[0] == S.NegativeInfinity:
            return (3*S.Pi/2 - acot(1/self.args[0]))._eval_nseries(x, n, logx)
        else:
            return super(atan, self)._eval_aseries(n, args0, x, logx)

    def _eval_rewrite_as_log(self, x):
        return S.ImaginaryUnit/2 * \
            (log((x - S.ImaginaryUnit)/(x + S.ImaginaryUnit)))

    def inverse(self, argindex=1):
        """
        Returns the inverse of this function.
        """
        return cot

    def _eval_rewrite_as_asin(self, arg):
        return (arg*sqrt(1/arg**2)*
                (S.Pi/2 - asin(sqrt(-arg**2)/sqrt(-arg**2 - 1))))

    def _eval_rewrite_as_acos(self, arg):
        return arg*sqrt(1/arg**2)*acos(sqrt(-arg**2)/sqrt(-arg**2 - 1))

    def _eval_rewrite_as_atan(self, arg):
        return atan(1/arg)

    def _eval_rewrite_as_asec(self, arg):
        return arg*sqrt(1/arg**2)*asec(sqrt((1 + arg**2)/arg**2))

    def _eval_rewrite_as_acsc(self, arg):
        return arg*sqrt(1/arg**2)*(S.Pi/2 - acsc(sqrt((1 + arg**2)/arg**2)))


class asec(InverseTrigonometricFunction):
    """
    The inverse secant function.

    Returns the arc secant of x (measured in radians).

    Notes
    =====

    asec(x) will evaluate automatically in the cases
    oo, -oo, 0, 1, -1.

    Examples
    ========

    >>> from sympy import asec, oo, pi
    >>> asec(1)
    0
    >>> asec(-1)
    pi

    See Also
    ========

    sin, csc, cos, sec, tan, cot
    asin, acsc, acos, atan, acot, atan2

    References
    ==========

    .. [1] http://en.wikipedia.org/wiki/Inverse_trigonometric_functions
    .. [2] http://dlmf.nist.gov/4.23
    .. [3] http://functions.wolfram.com/ElementaryFunctions/ArcSec
    """

    @classmethod
    def eval(cls, arg):
        if arg.is_Number:
            if arg is S.NaN:
                return S.NaN
            elif arg is S.One:
                return S.Zero
            elif arg is S.NegativeOne:
                return S.Pi
        if arg in [S.Infinity, S.NegativeInfinity, S.ComplexInfinity]:
            return S.Pi/2

    def fdiff(self, argindex=1):
        if argindex == 1:
            return 1/(self.args[0]**2*sqrt(1 - 1/self.args[0]**2))
        else:
            raise ArgumentIndexError(self, argindex)

    def inverse(self, argindex=1):
        """
        Returns the inverse of this function.
        """
        return sec

    def _eval_as_leading_term(self, x):
        from sympy import Order
        arg = self.args[0].as_leading_term(x)
        if Order(1,x).contains(arg):
            return log(arg)
        else:
            return self.func(arg)

    def _eval_rewrite_as_log(self, arg):
        return S.Pi/2 + S.ImaginaryUnit*log(S.ImaginaryUnit/arg + sqrt(1 - 1/arg**2))

    def _eval_rewrite_as_asin(self, arg):
        return S.Pi/2 - asin(1/arg)

    def _eval_rewrite_as_acos(self, arg):
        return acos(1/arg)

    def _eval_rewrite_as_atan(self, arg):
        return sqrt(arg**2)/arg*(-S.Pi/2 + 2*atan(arg + sqrt(arg**2 - 1)))

    def _eval_rewrite_as_acot(self, arg):
        return sqrt(arg**2)/arg*(-S.Pi/2 + 2*acot(arg - sqrt(arg**2 - 1)))

    def _eval_rewrite_as_acsc(self, arg):
        return S.Pi/2 - acsc(arg)


class acsc(InverseTrigonometricFunction):
    """
    The inverse cosecant function.

    Returns the arc cosecant of x (measured in radians).

    Notes
    =====

    acsc(x) will evaluate automatically in the cases
    oo, -oo, 0, 1, -1.

    Examples
    ========

    >>> from sympy import acsc, oo, pi
    >>> acsc(1)
    pi/2
    >>> acsc(-1)
    -pi/2

    See Also
    ========

    sin, csc, cos, sec, tan, cot
    asin, acos, asec, atan, acot, atan2

    References
    ==========

    .. [1] http://en.wikipedia.org/wiki/Inverse_trigonometric_functions
    .. [2] http://dlmf.nist.gov/4.23
    .. [3] http://functions.wolfram.com/ElementaryFunctions/ArcCsc
    """

    @classmethod
    def eval(cls, arg):
        if arg.is_Number:
            if arg is S.NaN:
                return S.NaN
            elif arg is S.One:
                return S.Pi/2
            elif arg is S.NegativeOne:
                return -S.Pi/2
        if arg in [S.Infinity, S.NegativeInfinity, S.ComplexInfinity]:
            return S.Zero

    def fdiff(self, argindex=1):
        if argindex == 1:
            return -1/(self.args[0]**2*sqrt(1 - 1/self.args[0]**2))
        else:
            raise ArgumentIndexError(self, argindex)

    def inverse(self, argindex=1):
        """
        Returns the inverse of this function.
        """
        return csc

    def _eval_as_leading_term(self, x):
        from sympy import Order
        arg = self.args[0].as_leading_term(x)
        if Order(1,x).contains(arg):
            return log(arg)
        else:
            return self.func(arg)

    def _eval_rewrite_as_log(self, arg):
        return -S.ImaginaryUnit*log(S.ImaginaryUnit/arg + sqrt(1 - 1/arg**2))

    def _eval_rewrite_as_asin(self, arg):
        return asin(1/arg)

    def _eval_rewrite_as_acos(self, arg):
        return S.Pi/2 - acos(1/arg)

    def _eval_rewrite_as_atan(self, arg):
        return sqrt(arg**2)/arg*(S.Pi/2 - atan(sqrt(arg**2 - 1)))

    def _eval_rewrite_as_acot(self, arg):
        return sqrt(arg**2)/arg*(S.Pi/2 - acot(1/sqrt(arg**2 - 1)))

    def _eval_rewrite_as_asec(self, arg):
        return S.Pi/2 - asec(arg)


class atan2(InverseTrigonometricFunction):
    r"""
    The function ``atan2(y, x)`` computes `\operatorname{atan}(y/x)` taking
    two arguments `y` and `x`.  Signs of both `y` and `x` are considered to
    determine the appropriate quadrant of `\operatorname{atan}(y/x)`.
    The range is `(-\pi, \pi]`. The complete definition reads as follows:

    .. math::

        \operatorname{atan2}(y, x) =
        \begin{cases}
          \arctan\left(\frac y x\right) & \qquad x > 0 \\
          \arctan\left(\frac y x\right) + \pi& \qquad y \ge 0 , x < 0 \\
          \arctan\left(\frac y x\right) - \pi& \qquad y < 0 , x < 0 \\
          +\frac{\pi}{2} & \qquad y > 0 , x = 0 \\
          -\frac{\pi}{2} & \qquad y < 0 , x = 0 \\
          \text{undefined} & \qquad y = 0, x = 0
        \end{cases}

    Attention: Note the role reversal of both arguments. The `y`-coordinate
    is the first argument and the `x`-coordinate the second.

    Examples
    ========

    Going counter-clock wise around the origin we find the
    following angles:

    >>> from sympy import atan2
    >>> atan2(0, 1)
    0
    >>> atan2(1, 1)
    pi/4
    >>> atan2(1, 0)
    pi/2
    >>> atan2(1, -1)
    3*pi/4
    >>> atan2(0, -1)
    pi
    >>> atan2(-1, -1)
    -3*pi/4
    >>> atan2(-1, 0)
    -pi/2
    >>> atan2(-1, 1)
    -pi/4

    which are all correct. Compare this to the results of the ordinary
    `\operatorname{atan}` function for the point `(x, y) = (-1, 1)`

    >>> from sympy import atan, S
    >>> atan(S(1) / -1)
    -pi/4
    >>> atan2(1, -1)
    3*pi/4

    where only the `\operatorname{atan2}` function reurns what we expect.
    We can differentiate the function with respect to both arguments:

    >>> from sympy import diff
    >>> from sympy.abc import x, y
    >>> diff(atan2(y, x), x)
    -y/(x**2 + y**2)

    >>> diff(atan2(y, x), y)
    x/(x**2 + y**2)

    We can express the `\operatorname{atan2}` function in terms of
    complex logarithms:

    >>> from sympy import log
    >>> atan2(y, x).rewrite(log)
    -I*log((x + I*y)/sqrt(x**2 + y**2))

    and in terms of `\operatorname(atan)`:

    >>> from sympy import atan
    >>> atan2(y, x).rewrite(atan)
    2*atan(y/(x + sqrt(x**2 + y**2)))

    but note that this form is undefined on the negative real axis.

    See Also
    ========

    sin, csc, cos, sec, tan, cot
    asin, acsc, acos, asec, atan, acot

    References
    ==========

    .. [1] http://en.wikipedia.org/wiki/Inverse_trigonometric_functions
    .. [2] http://en.wikipedia.org/wiki/Atan2
    .. [3] http://functions.wolfram.com/ElementaryFunctions/ArcTan2
    """

    @classmethod
    def eval(cls, y, x):
        from sympy import Heaviside, im, re
        if x is S.NegativeInfinity:
            if y.is_zero:
                # Special case y = 0 because we define Heaviside(0) = 1/2
                return S.Pi
            return 2*S.Pi*(Heaviside(re(y))) - S.Pi
        elif x is S.Infinity:
            return S.Zero
        elif x.is_imaginary and y.is_imaginary and x.is_number and y.is_number:
            x = im(x)
            y = im(y)

        if x.is_real and y.is_real:
            if x.is_positive:
                return atan(y / x)
            elif x.is_negative:
                if y.is_negative:
                    return atan(y / x) - S.Pi
                elif y.is_nonnegative:
                    return atan(y / x) + S.Pi
            elif x.is_zero:
                if y.is_positive:
                    return S.Pi/2
                elif y.is_negative:
                    return -S.Pi/2
                elif y.is_zero:
                    return S.NaN
        if y.is_zero and x.is_real and x.is_nonzero:
            return S.Pi * (S.One - Heaviside(x))
        if x.is_number and y.is_number:
            return -S.ImaginaryUnit*log(
                (x + S.ImaginaryUnit*y)/sqrt(x**2 + y**2))

    def _eval_rewrite_as_log(self, y, x):
        return -S.ImaginaryUnit*log((x + S.ImaginaryUnit*y) / sqrt(x**2 + y**2))

    def _eval_rewrite_as_atan(self, y, x):
        return 2*atan(y / (sqrt(x**2 + y**2) + x))

    def _eval_rewrite_as_arg(self, y, x):
        from sympy import arg
        if x.is_real and y.is_real:
            return arg(x + y*S.ImaginaryUnit)
        I = S.ImaginaryUnit
        n = x + I*y
        d = x**2 + y**2
        return arg(n/sqrt(d)) - I*log(abs(n)/sqrt(abs(d)))

    def _eval_is_real(self):
        return self.args[0].is_real and self.args[1].is_real

    def _eval_conjugate(self):
        return self.func(self.args[0].conjugate(), self.args[1].conjugate())

    def fdiff(self, argindex):
        y, x = self.args
        if argindex == 1:
            # Diff wrt y
            return x/(x**2 + y**2)
        elif argindex == 2:
            # Diff wrt x
            return -y/(x**2 + y**2)
        else:
            raise ArgumentIndexError(self, argindex)

    def _eval_evalf(self, prec):
        y, x = self.args
        if x.is_real and y.is_real:
            super(atan2, self)._eval_evalf(prec)
