# Source code for sympy.functions.special.gamma_functions

from __future__ import print_function, division

from sympy.core import Add, S, sympify, oo, pi, Dummy, expand_func
from sympy.core.function import Function, ArgumentIndexError
from sympy.core.numbers import Rational
from sympy.core.power import Pow
from sympy.core.compatibility import range
from .zeta_functions import zeta
from .error_functions import erf, erfc
from sympy.functions.elementary.exponential import exp, log
from sympy.functions.elementary.integers import ceiling, floor
from sympy.functions.elementary.miscellaneous import sqrt
from sympy.functions.elementary.trigonometric import sin, cos, cot
from sympy.functions.combinatorial.numbers import bernoulli, harmonic
from sympy.functions.combinatorial.factorials import factorial, rf, RisingFactorial

###############################################################################
############################ COMPLETE GAMMA FUNCTION ##########################
###############################################################################

[docs]class gamma(Function):
r"""
The gamma function

.. math::
\Gamma(x) := \int^{\infty}_{0} t^{x-1} e^{-t} \mathrm{d}t.

The gamma function implements the function which passes through the
values of the factorial function, i.e. \Gamma(n) = (n - 1)! when n is
an integer. More general, \Gamma(z) is defined in the whole complex
plane except at the negative integers where there are simple poles.

Examples
========

>>> from sympy import S, I, pi, oo, gamma
>>> from sympy.abc import x

Several special values are known:

>>> gamma(1)
1
>>> gamma(4)
6
>>> gamma(S(3)/2)
sqrt(pi)/2

The Gamma function obeys the mirror symmetry:

>>> from sympy import conjugate
>>> conjugate(gamma(x))
gamma(conjugate(x))

Differentiation with respect to x is supported:

>>> from sympy import diff
>>> diff(gamma(x), x)
gamma(x)*polygamma(0, x)

Series expansion is also supported:

>>> from sympy import series
>>> series(gamma(x), x, 0, 3)
1/x - EulerGamma + x*(EulerGamma**2/2 + pi**2/12) + x**2*(-EulerGamma*pi**2/12 + polygamma(2, 1)/6 - EulerGamma**3/6) + O(x**3)

We can numerically evaluate the gamma function to arbitrary precision
on the whole complex plane:

>>> gamma(pi).evalf(40)
2.288037795340032417959588909060233922890
>>> gamma(1+I).evalf(20)
0.49801566811835604271 - 0.15494982830181068512*I

========

lowergamma: Lower incomplete gamma function.
uppergamma: Upper incomplete gamma function.
polygamma: Polygamma function.
loggamma: Log Gamma function.
digamma: Digamma function.
trigamma: Trigamma function.
sympy.functions.special.beta_functions.beta: Euler Beta function.

References
==========

.. [1] http://en.wikipedia.org/wiki/Gamma_function
.. [2] http://dlmf.nist.gov/5
.. [3] http://mathworld.wolfram.com/GammaFunction.html
.. [4] http://functions.wolfram.com/GammaBetaErf/Gamma/
"""

unbranched = True

def fdiff(self, argindex=1):
if argindex == 1:
return self.func(self.args[0])*polygamma(0, 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.Infinity:
return S.Infinity
elif arg.is_Integer:
if arg.is_positive:
return factorial(arg - 1)
else:
return S.ComplexInfinity
elif arg.is_Rational:
if arg.q == 2:
n = abs(arg.p) // arg.q

if arg.is_positive:
k, coeff = n, S.One
else:
n = k = n + 1

if n & 1 == 0:
coeff = S.One
else:
coeff = S.NegativeOne

for i in range(3, 2*k, 2):
coeff *= i

if arg.is_positive:
return coeff*sqrt(S.Pi) / 2**n
else:
return 2**n*sqrt(S.Pi) / coeff

def _eval_expand_func(self, **hints):
arg = self.args[0]
if arg.is_Rational:
if abs(arg.p) > arg.q:
x = Dummy('x')
n = arg.p // arg.q
p = arg.p - n*arg.q
return self.func(x + n)._eval_expand_func().subs(x, Rational(p, arg.q))

if coeff and coeff.q != 1:
intpart = floor(coeff)
tail = (coeff - intpart,) + tail
coeff = intpart
tail = arg._new_rawargs(*tail, reeval=False)
return self.func(tail)*RisingFactorial(tail, coeff)

return self.func(*self.args)

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

def _eval_is_real(self):
x = self.args[0]
if x.is_positive or x.is_noninteger:
return True

def _eval_is_positive(self):
x = self.args[0]
if x.is_positive:
return True
elif x.is_noninteger:
return floor(x).is_even

def _eval_rewrite_as_tractable(self, z):
return exp(loggamma(z))

def _eval_rewrite_as_factorial(self, z):
return factorial(z - 1)

def _eval_nseries(self, x, n, logx):
x0 = self.args[0].limit(x, 0)
if not (x0.is_Integer and x0 <= 0):
return super(gamma, self)._eval_nseries(x, n, logx)
t = self.args[0] - x0
return (self.func(t + 1)/rf(self.args[0], -x0 + 1))._eval_nseries(x, n, logx)

###############################################################################
################## LOWER and UPPER INCOMPLETE GAMMA FUNCTIONS #################
###############################################################################

[docs]class lowergamma(Function):
r"""
The lower incomplete gamma function.

It can be defined as the meromorphic continuation of

.. math::
\gamma(s, x) := \int_0^x t^{s-1} e^{-t} \mathrm{d}t = \Gamma(s) - \Gamma(s, x).

This can be shown to be the same as

.. math::
\gamma(s, x) = \frac{x^s}{s} {}_1F_1\left({s \atop s+1} \middle| -x\right),

where :math:{}_1F_1 is the (confluent) hypergeometric function.

Examples
========

>>> from sympy import lowergamma, S
>>> from sympy.abc import s, x
>>> lowergamma(s, x)
lowergamma(s, x)
>>> lowergamma(3, x)
-2*(x**2/2 + x + 1)*exp(-x) + 2
>>> lowergamma(-S(1)/2, x)
-2*sqrt(pi)*erf(sqrt(x)) - 2*exp(-x)/sqrt(x)

========

gamma: Gamma function.
uppergamma: Upper incomplete gamma function.
polygamma: Polygamma function.
loggamma: Log Gamma function.
digamma: Digamma function.
trigamma: Trigamma function.
sympy.functions.special.beta_functions.beta: Euler Beta function.

References
==========

.. [1] http://en.wikipedia.org/wiki/Incomplete_gamma_function#Lower_incomplete_Gamma_function
.. [2] Abramowitz, Milton; Stegun, Irene A., eds. (1965), Chapter 6, Section 5,
Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables
.. [3] http://dlmf.nist.gov/8
.. [4] http://functions.wolfram.com/GammaBetaErf/Gamma2/
.. [5] http://functions.wolfram.com/GammaBetaErf/Gamma3/
"""

def fdiff(self, argindex=2):
from sympy import meijerg, unpolarify
if argindex == 2:
a, z = self.args
return exp(-unpolarify(z))*z**(a - 1)
elif argindex == 1:
a, z = self.args
return gamma(a)*digamma(a) - log(z)*uppergamma(a, z) \
- meijerg([], [1, 1], [0, 0, a], [], z)

else:
raise ArgumentIndexError(self, argindex)

@classmethod
def eval(cls, a, x):
# For lack of a better place, we use this one to extract branching
# information. The following can be
# found in the literature (c/f references given above), albeit scattered:
# 1) For fixed x != 0, lowergamma(s, x) is an entire function of s
# 2) For fixed positive integers s, lowergamma(s, x) is an entire
#    function of x.
# 3) For fixed non-positive integers s,
#    lowergamma(s, exp(I*2*pi*n)*x) =
#              2*pi*I*n*(-1)**(-s)/factorial(-s) + lowergamma(s, x)
#    (this follows from lowergamma(s, x).diff(x) = x**(s-1)*exp(-x)).
# 4) For fixed non-integral s,
#    lowergamma(s, x) = x**s*gamma(s)*lowergamma_unbranched(s, x),
#    where lowergamma_unbranched(s, x) is an entire function (in fact
#    of both s and x), i.e.
#    lowergamma(s, exp(2*I*pi*n)*x) = exp(2*pi*I*n*a)*lowergamma(a, x)
from sympy import unpolarify, I
if x == 0:
return S.Zero
nx, n = x.extract_branch_factor()
if a.is_integer and a.is_positive:
nx = unpolarify(x)
if nx != x:
return lowergamma(a, nx)
elif a.is_integer and a.is_nonpositive:
if n != 0:
return 2*pi*I*n*(-1)**(-a)/factorial(-a) + lowergamma(a, nx)
elif n != 0:
return exp(2*pi*I*n*a)*lowergamma(a, nx)

# Special values.
if a.is_Number:
if a is S.One:
return S.One - exp(-x)
elif a is S.Half:
return sqrt(pi)*erf(sqrt(x))
elif a.is_Integer or (2*a).is_Integer:
b = a - 1
if b.is_positive:
if a.is_integer:
return factorial(b) - exp(-x) * factorial(b) * Add(*[x ** k / factorial(k) for k in range(a)])
else:
return gamma(a) * (lowergamma(S.Half, x)/sqrt(pi) - exp(-x) * Add(*[x**(k-S.Half) / gamma(S.Half+k) for k in range(1, a+S.Half)]))

if not a.is_Integer:
return (-1)**(S.Half - a) * pi*erf(sqrt(x)) / gamma(1-a) + exp(-x) * Add(*[x**(k+a-1)*gamma(a) / gamma(a+k) for k in range(1, S(3)/2-a)])

def _eval_evalf(self, prec):
from mpmath import mp, workprec
from sympy import Expr
if all(x.is_number for x in self.args):
a = self.args[0]._to_mpmath(prec)
z = self.args[1]._to_mpmath(prec)
with workprec(prec):
res = mp.gammainc(a, 0, z)
return Expr._from_mpmath(res, prec)
else:
return self

def _eval_conjugate(self):
z = self.args[1]
if not z in (S.Zero, S.NegativeInfinity):
return self.func(self.args[0].conjugate(), z.conjugate())

def _eval_rewrite_as_uppergamma(self, s, x):
return gamma(s) - uppergamma(s, x)

def _eval_rewrite_as_expint(self, s, x):
from sympy import expint
if s.is_integer and s.is_nonpositive:
return self
return self.rewrite(uppergamma).rewrite(expint)

[docs]class uppergamma(Function):
r"""
The upper incomplete gamma function.

It can be defined as the meromorphic continuation of

.. math::
\Gamma(s, x) := \int_x^\infty t^{s-1} e^{-t} \mathrm{d}t = \Gamma(s) - \gamma(s, x).

where \gamma(s, x) is the lower incomplete gamma function,
:class:lowergamma. This can be shown to be the same as

.. math::
\Gamma(s, x) = \Gamma(s) - \frac{x^s}{s} {}_1F_1\left({s \atop s+1} \middle| -x\right),

where :math:{}_1F_1 is the (confluent) hypergeometric function.

The upper incomplete gamma function is also essentially equivalent to the
generalized exponential integral:

.. math::
\operatorname{E}_{n}(x) = \int_{1}^{\infty}{\frac{e^{-xt}}{t^n} \, dt} = x^{n-1}\Gamma(1-n,x).

Examples
========

>>> from sympy import uppergamma, S
>>> from sympy.abc import s, x
>>> uppergamma(s, x)
uppergamma(s, x)
>>> uppergamma(3, x)
2*(x**2/2 + x + 1)*exp(-x)
>>> uppergamma(-S(1)/2, x)
-2*sqrt(pi)*erfc(sqrt(x)) + 2*exp(-x)/sqrt(x)
>>> uppergamma(-2, x)
expint(3, x)/x**2

========

gamma: Gamma function.
lowergamma: Lower incomplete gamma function.
polygamma: Polygamma function.
loggamma: Log Gamma function.
digamma: Digamma function.
trigamma: Trigamma function.
sympy.functions.special.beta_functions.beta: Euler Beta function.

References
==========

.. [1] http://en.wikipedia.org/wiki/Incomplete_gamma_function#Upper_incomplete_Gamma_function
.. [2] Abramowitz, Milton; Stegun, Irene A., eds. (1965), Chapter 6, Section 5,
Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables
.. [3] http://dlmf.nist.gov/8
.. [4] http://functions.wolfram.com/GammaBetaErf/Gamma2/
.. [5] http://functions.wolfram.com/GammaBetaErf/Gamma3/
.. [6] http://en.wikipedia.org/wiki/Exponential_integral#Relation_with_other_functions
"""

def fdiff(self, argindex=2):
from sympy import meijerg, unpolarify
if argindex == 2:
a, z = self.args
return -exp(-unpolarify(z))*z**(a - 1)
elif argindex == 1:
a, z = self.args
return uppergamma(a, z)*log(z) + meijerg([], [1, 1], [0, 0, a], [], z)
else:
raise ArgumentIndexError(self, argindex)

def _eval_evalf(self, prec):
from mpmath import mp, workprec
from sympy import Expr
if all(x.is_number for x in self.args):
a = self.args[0]._to_mpmath(prec)
z = self.args[1]._to_mpmath(prec)
with workprec(prec):
res = mp.gammainc(a, z, mp.inf)
return Expr._from_mpmath(res, prec)
return self

@classmethod
def eval(cls, a, z):
from sympy import unpolarify, I, expint
if z.is_Number:
if z is S.NaN:
return S.NaN
elif z is S.Infinity:
return S.Zero
elif z is S.Zero:
# TODO: Holds only for Re(a) > 0:
return gamma(a)

# We extract branching information here. C/f lowergamma.
nx, n = z.extract_branch_factor()
if a.is_integer and (a > 0) == True:
nx = unpolarify(z)
if z != nx:
return uppergamma(a, nx)
elif a.is_integer and (a <= 0) == True:
if n != 0:
return -2*pi*I*n*(-1)**(-a)/factorial(-a) + uppergamma(a, nx)
elif n != 0:
return gamma(a)*(1 - exp(2*pi*I*n*a)) + exp(2*pi*I*n*a)*uppergamma(a, nx)

# Special values.
if a.is_Number:
if a is S.One:
return exp(-z)
elif a is S.Half:
return sqrt(pi)*erfc(sqrt(z))
elif a.is_Integer or (2*a).is_Integer:
b = a - 1
if b.is_positive:
if a.is_integer:
return exp(-z) * factorial(b) * Add(*[z**k / factorial(k) for k in range(a)])
else:
return gamma(a) * erfc(sqrt(z)) + (-1)**(a - S(3)/2) * exp(-z) * sqrt(z) * Add(*[gamma(-S.Half - k) * (-z)**k / gamma(1-a) for k in range(a - S.Half)])
elif b.is_Integer:
return expint(-b, z)*unpolarify(z)**(b + 1)

if not a.is_Integer:
return (-1)**(S.Half - a) * pi*erfc(sqrt(z))/gamma(1-a) - z**a * exp(-z) * Add(*[z**k * gamma(a) / gamma(a+k+1) for k in range(S.Half - a)])

def _eval_conjugate(self):
z = self.args[1]
if not z in (S.Zero, S.NegativeInfinity):
return self.func(self.args[0].conjugate(), z.conjugate())

def _eval_rewrite_as_lowergamma(self, s, x):
return gamma(s) - lowergamma(s, x)

def _eval_rewrite_as_expint(self, s, x):
from sympy import expint
return expint(1 - s, x)*x**s

###############################################################################
###################### POLYGAMMA and LOGGAMMA FUNCTIONS #######################
###############################################################################

[docs]class polygamma(Function):
r"""
The function polygamma(n, z) returns log(gamma(z)).diff(n + 1).

It is a meromorphic function on \mathbb{C} and defined as the (n+1)-th
derivative of the logarithm of the gamma function:

.. math::
\psi^{(n)} (z) := \frac{\mathrm{d}^{n+1}}{\mathrm{d} z^{n+1}} \log\Gamma(z).

Examples
========

Several special values are known:

>>> from sympy import S, polygamma
>>> polygamma(0, 1)
-EulerGamma
>>> polygamma(0, 1/S(2))
-2*log(2) - EulerGamma
>>> polygamma(0, 1/S(3))
-log(3) - sqrt(3)*pi/6 - EulerGamma - log(sqrt(3))
>>> polygamma(0, 1/S(4))
-pi/2 - log(4) - log(2) - EulerGamma
>>> polygamma(0, 2)
-EulerGamma + 1
>>> polygamma(0, 23)
-EulerGamma + 19093197/5173168

>>> from sympy import oo, I
>>> polygamma(0, oo)
oo
>>> polygamma(0, -oo)
oo
>>> polygamma(0, I*oo)
oo
>>> polygamma(0, -I*oo)
oo

Differentiation with respect to x is supported:

>>> from sympy import Symbol, diff
>>> x = Symbol("x")
>>> diff(polygamma(0, x), x)
polygamma(1, x)
>>> diff(polygamma(0, x), x, 2)
polygamma(2, x)
>>> diff(polygamma(0, x), x, 3)
polygamma(3, x)
>>> diff(polygamma(1, x), x)
polygamma(2, x)
>>> diff(polygamma(1, x), x, 2)
polygamma(3, x)
>>> diff(polygamma(2, x), x)
polygamma(3, x)
>>> diff(polygamma(2, x), x, 2)
polygamma(4, x)

>>> n = Symbol("n")
>>> diff(polygamma(n, x), x)
polygamma(n + 1, x)
>>> diff(polygamma(n, x), x, 2)
polygamma(n + 2, x)

We can rewrite polygamma functions in terms of harmonic numbers:

>>> from sympy import harmonic
>>> polygamma(0, x).rewrite(harmonic)
harmonic(x - 1) - EulerGamma
>>> polygamma(2, x).rewrite(harmonic)
2*harmonic(x - 1, 3) - 2*zeta(3)
>>> ni = Symbol("n", integer=True)
>>> polygamma(ni, x).rewrite(harmonic)
(-1)**(n + 1)*(-harmonic(x - 1, n + 1) + zeta(n + 1))*factorial(n)

========

gamma: Gamma function.
lowergamma: Lower incomplete gamma function.
uppergamma: Upper incomplete gamma function.
loggamma: Log Gamma function.
digamma: Digamma function.
trigamma: Trigamma function.
sympy.functions.special.beta_functions.beta: Euler Beta function.

References
==========

.. [1] http://en.wikipedia.org/wiki/Polygamma_function
.. [2] http://mathworld.wolfram.com/PolygammaFunction.html
.. [3] http://functions.wolfram.com/GammaBetaErf/PolyGamma/
.. [4] http://functions.wolfram.com/GammaBetaErf/PolyGamma2/
"""

def fdiff(self, argindex=2):
if argindex == 2:
n, z = self.args[:2]
return polygamma(n + 1, z)
else:
raise ArgumentIndexError(self, argindex)

def _eval_is_positive(self):
if self.args[1].is_positive and (self.args[0] > 0) == True:
return self.args[0].is_odd

def _eval_is_negative(self):
if self.args[1].is_positive and (self.args[0] > 0) == True:
return self.args[0].is_even

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

def _eval_aseries(self, n, args0, x, logx):
from sympy import Order
if args0[1] != oo or not \
(self.args[0].is_Integer and self.args[0].is_nonnegative):
return super(polygamma, self)._eval_aseries(n, args0, x, logx)
z = self.args[1]
N = self.args[0]

if N == 0:
# digamma function series
# Abramowitz & Stegun, p. 259, 6.3.18
r = log(z) - 1/(2*z)
o = None
if n < 2:
o = Order(1/z, x)
else:
m = ceiling((n + 1)//2)
l = [bernoulli(2*k) / (2*k*z**(2*k)) for k in range(1, m)]
o = Order(1/z**(2*m), x)
return r._eval_nseries(x, n, logx) + o
else:
# proper polygamma function
# Abramowitz & Stegun, p. 260, 6.4.10
# We return terms to order higher than O(x**n) on purpose
# -- otherwise we would not be able to return any terms for
#    quite a long time!
fac = gamma(N)
e0 = fac + N*fac/(2*z)
m = ceiling((n + 1)//2)
for k in range(1, m):
fac = fac*(2*k + N - 1)*(2*k + N - 2) / ((2*k)*(2*k - 1))
e0 += bernoulli(2*k)*fac/z**(2*k)
o = Order(1/z**(2*m), x)
if n == 0:
o = Order(1/z, x)
elif n == 1:
o = Order(1/z**2, x)
r = e0._eval_nseries(z, n, logx) + o
return (-1 * (-1/z)**N * r)._eval_nseries(x, n, logx)

@classmethod
def eval(cls, n, z):
n, z = list(map(sympify, (n, z)))
from sympy import unpolarify

if n.is_integer:
if n.is_nonnegative:
nz = unpolarify(z)
if z != nz:
return polygamma(n, nz)

if n == -1:
return loggamma(z)
else:
if z.is_Number:
if z is S.NaN:
return S.NaN
elif z is S.Infinity:
if n.is_Number:
if n is S.Zero:
return S.Infinity
else:
return S.Zero
elif z.is_Integer:
if z.is_nonpositive:
return S.ComplexInfinity
else:
if n is S.Zero:
return -S.EulerGamma + harmonic(z - 1, 1)
elif n.is_odd:
return (-1)**(n + 1)*factorial(n)*zeta(n + 1, z)

if n == 0:
if z is S.NaN:
return S.NaN
elif z.is_Rational:

p, q = z.as_numer_denom()

# only expand for small denominators to avoid creating long expressions
if q <= 5:
return expand_func(polygamma(n, z, evaluate=False))

elif z in (S.Infinity, S.NegativeInfinity):
return S.Infinity
else:
t = z.extract_multiplicatively(S.ImaginaryUnit)
if t in (S.Infinity, S.NegativeInfinity):
return S.Infinity

# TODO n == 1 also can do some rational z

def _eval_expand_func(self, **hints):
n, z = self.args

if n.is_Integer and n.is_nonnegative:
coeff = z.args[0]
if coeff.is_Integer:
e = -(n + 1)
if coeff > 0:
z - i, e) for i in range(1, int(coeff) + 1)])
else:
z + i, e) for i in range(0, int(-coeff))])
return polygamma(n, z - coeff) + (-1)**n*factorial(n)*tail

elif z.is_Mul:
coeff, z = z.as_two_terms()
if coeff.is_Integer and coeff.is_positive:
tail = [ polygamma(n, z + Rational(
i, coeff)) for i in range(0, int(coeff)) ]
if n == 0:
else:
z *= coeff

if n == 0 and z.is_Rational:
p, q = z.as_numer_denom()

# Reference:
#   Values of the polygamma functions at rational arguments, J. Choi, 2007
part_1 = -S.EulerGamma - pi * cot(p * pi / q) / 2 - log(q) + Add(
*[cos(2 * k * pi * p / q) * log(2 * sin(k * pi / q)) for k in range(1, q)])

if z > 0:
n = floor(z)
z0 = z - n
return part_1 + Add(*[1 / (z0 + k) for k in range(n)])
elif z < 0:
n = floor(1 - z)
z0 = z + n
return part_1 - Add(*[1 / (z0 - 1 - k) for k in range(n)])

return polygamma(n, z)

def _eval_rewrite_as_zeta(self, n, z):
if n >= S.One:
return (-1)**(n + 1)*factorial(n)*zeta(n + 1, z)
else:
return self

def _eval_rewrite_as_harmonic(self, n, z):
if n.is_integer:
if n == S.Zero:
return harmonic(z - 1) - S.EulerGamma
else:
return S.NegativeOne**(n+1) * factorial(n) * (zeta(n+1) - harmonic(z-1, n+1))

from sympy import Order
n, z = [a.as_leading_term(x) for a in self.args]
o = Order(z, x)
if n == 0 and o.contains(1/x):
return o.getn() * log(x)
else:
return self.func(n, z)

[docs]class loggamma(Function):
r"""
The loggamma function implements the logarithm of the
gamma function i.e, \log\Gamma(x).

Examples
========

Several special values are known. For numerical integral
arguments we have:

>>> from sympy import loggamma
>>> loggamma(-2)
oo
>>> loggamma(0)
oo
>>> loggamma(1)
0
>>> loggamma(2)
0
>>> loggamma(3)
log(2)

and for symbolic values:

>>> from sympy import Symbol
>>> n = Symbol("n", integer=True, positive=True)
>>> loggamma(n)
log(gamma(n))
>>> loggamma(-n)
oo

for half-integral values:

>>> from sympy import S, pi
>>> loggamma(S(5)/2)
log(3*sqrt(pi)/4)
>>> loggamma(n/2)
log(2**(-n + 1)*sqrt(pi)*gamma(n)/gamma(n/2 + 1/2))

and general rational arguments:

>>> from sympy import expand_func
>>> L = loggamma(S(16)/3)
>>> expand_func(L).doit()
-5*log(3) + loggamma(1/3) + log(4) + log(7) + log(10) + log(13)
>>> L = loggamma(S(19)/4)
>>> expand_func(L).doit()
-4*log(4) + loggamma(3/4) + log(3) + log(7) + log(11) + log(15)
>>> L = loggamma(S(23)/7)
>>> expand_func(L).doit()
-3*log(7) + log(2) + loggamma(2/7) + log(9) + log(16)

The loggamma function has the following limits towards infinity:

>>> from sympy import oo
>>> loggamma(oo)
oo
>>> loggamma(-oo)
zoo

The loggamma function obeys the mirror symmetry
if x \in \mathbb{C} \setminus \{-\infty, 0\}:

>>> from sympy.abc import x
>>> from sympy import conjugate
>>> conjugate(loggamma(x))
loggamma(conjugate(x))

Differentiation with respect to x is supported:

>>> from sympy import diff
>>> diff(loggamma(x), x)
polygamma(0, x)

Series expansion is also supported:

>>> from sympy import series
>>> series(loggamma(x), x, 0, 4)
-log(x) - EulerGamma*x + pi**2*x**2/12 + x**3*polygamma(2, 1)/6 + O(x**4)

We can numerically evaluate the gamma function to arbitrary precision
on the whole complex plane:

>>> from sympy import I
>>> loggamma(5).evalf(30)
3.17805383034794561964694160130
>>> loggamma(I).evalf(20)
-0.65092319930185633889 - 1.8724366472624298171*I

========

gamma: Gamma function.
lowergamma: Lower incomplete gamma function.
uppergamma: Upper incomplete gamma function.
polygamma: Polygamma function.
digamma: Digamma function.
trigamma: Trigamma function.
sympy.functions.special.beta_functions.beta: Euler Beta function.

References
==========

.. [1] http://en.wikipedia.org/wiki/Gamma_function
.. [2] http://dlmf.nist.gov/5
.. [3] http://mathworld.wolfram.com/LogGammaFunction.html
.. [4] http://functions.wolfram.com/GammaBetaErf/LogGamma/
"""
@classmethod
def eval(cls, z):
z = sympify(z)

if z.is_integer:
if z.is_nonpositive:
return S.Infinity
elif z.is_positive:
return log(gamma(z))
elif z.is_rational:
p, q = z.as_numer_denom()
# Half-integral values:
if p.is_positive and q == 2:
return log(sqrt(S.Pi) * 2**(1 - p) * gamma(p) / gamma((p + 1)*S.Half))

if z is S.Infinity:
return S.Infinity
elif abs(z) is S.Infinity:
return S.ComplexInfinity
if z is S.NaN:
return S.NaN

def _eval_expand_func(self, **hints):
from sympy import Sum
z = self.args[0]

if z.is_Rational:
p, q = z.as_numer_denom()
# General rational arguments (u + p/q)
# Split z as n + p/q with p < q
n = p // q
p = p - n*q
if p.is_positive and q.is_positive and p < q:
k = Dummy("k")
if n.is_positive:
return loggamma(p / q) - n*log(q) + Sum(log((k - 1)*q + p), (k, 1, n))
elif n.is_negative:
return loggamma(p / q) - n*log(q) + S.Pi*S.ImaginaryUnit*n - Sum(log(k*q - p), (k, 1, -n))
elif n.is_zero:
return loggamma(p / q)

return self

def _eval_nseries(self, x, n, logx=None):
x0 = self.args[0].limit(x, 0)
if x0 is S.Zero:
f = self._eval_rewrite_as_intractable(*self.args)
return f._eval_nseries(x, n, logx)
return super(loggamma, self)._eval_nseries(x, n, logx)

def _eval_aseries(self, n, args0, x, logx):
from sympy import Order
if args0[0] != oo:
return super(loggamma, self)._eval_aseries(n, args0, x, logx)
z = self.args[0]
m = min(n, ceiling((n + S(1))/2))
r = log(z)*(z - S(1)/2) - z + log(2*pi)/2
l = [bernoulli(2*k) / (2*k*(2*k - 1)*z**(2*k - 1)) for k in range(1, m)]
o = None
if m == 0:
o = Order(1, x)
else:
o = Order(1/z**(2*m - 1), x)
# It is very inefficient to first add the order and then do the nseries
return (r + Add(*l))._eval_nseries(x, n, logx) + o

def _eval_rewrite_as_intractable(self, z):
return log(gamma(z))

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

def _eval_conjugate(self):
z = self.args[0]
if not z in (S.Zero, S.NegativeInfinity):
return self.func(z.conjugate())

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

def _sage_(self):
import sage.all as sage
return sage.log_gamma(self.args[0]._sage_())

[docs]def digamma(x):
r"""
The digamma function is the first derivative of the loggamma function i.e,

.. math::
\psi(x) := \frac{\mathrm{d}}{\mathrm{d} z} \log\Gamma(z)
= \frac{\Gamma'(z)}{\Gamma(z) }

In this case, digamma(z) = polygamma(0, z).

========

gamma: Gamma function.
lowergamma: Lower incomplete gamma function.
uppergamma: Upper incomplete gamma function.
polygamma: Polygamma function.
loggamma: Log Gamma function.
trigamma: Trigamma function.
sympy.functions.special.beta_functions.beta: Euler Beta function.

References
==========

.. [1] http://en.wikipedia.org/wiki/Digamma_function
.. [2] http://mathworld.wolfram.com/DigammaFunction.html
.. [3] http://functions.wolfram.com/GammaBetaErf/PolyGamma2/
"""
return polygamma(0, x)

[docs]def trigamma(x):
r"""
The trigamma function is the second derivative of the loggamma function i.e,

.. math::
\psi^{(1)}(z) := \frac{\mathrm{d}^{2}}{\mathrm{d} z^{2}} \log\Gamma(z).

In this case, trigamma(z) = polygamma(1, z).

========

gamma: Gamma function.
lowergamma: Lower incomplete gamma function.
uppergamma: Upper incomplete gamma function.
polygamma: Polygamma function.
loggamma: Log Gamma function.
digamma: Digamma function.
sympy.functions.special.beta_functions.beta: Euler Beta function.

References
==========

.. [1] http://en.wikipedia.org/wiki/Trigamma_function
.. [2] http://mathworld.wolfram.com/TrigammaFunction.html
.. [3] http://functions.wolfram.com/GammaBetaErf/PolyGamma2/
"""
return polygamma(1, x)