Source code for sympy.functions.special.error_functions
""" This module contains various functions that are special cases
of incomplete gamma functions. It should probably be renamed. """
from __future__ import print_function, division
from sympy.core import Add, S, C, sympify, cacheit, pi, I
from sympy.core.function import Function, ArgumentIndexError
from sympy.functions.elementary.miscellaneous import sqrt, root
from sympy.functions.elementary.exponential import exp, log
from sympy.functions.elementary.complexes import polar_lift
from sympy.functions.special.hyper import hyper, meijerg
from sympy.core.compatibility import xrange
# TODO series expansions
# TODO see the "Note:" in Ei
###############################################################################
################################ ERROR FUNCTION ###############################
###############################################################################
[docs]class erf(Function):
r"""
The Gauss error function. This function is defined as:
.. math ::
\mathrm{erf}(x) = \frac{2}{\sqrt{\pi}} \int_0^x e^{-t^2} \mathrm{d}t.
Examples
========
>>> from sympy import I, oo, erf
>>> from sympy.abc import z
Several special values are known:
>>> erf(0)
0
>>> erf(oo)
1
>>> erf(-oo)
-1
>>> erf(I*oo)
oo*I
>>> erf(-I*oo)
-oo*I
In general one can pull out factors of -1 and I from the argument:
>>> erf(-z)
-erf(z)
The error function obeys the mirror symmetry:
>>> from sympy import conjugate
>>> conjugate(erf(z))
erf(conjugate(z))
Differentiation with respect to z is supported:
>>> from sympy import diff
>>> diff(erf(z), z)
2*exp(-z**2)/sqrt(pi)
We can numerically evaluate the error function to arbitrary precision
on the whole complex plane:
>>> erf(4).evalf(30)
0.999999984582742099719981147840
>>> erf(-4*I).evalf(30)
-1296959.73071763923152794095062*I
See Also
========
erfc: Complementary error function.
erfi: Imaginary error function.
erf2: Two-argument error function.
erfinv: Inverse error function.
erfcinv: Inverse Complementary error function.
erf2inv: Inverse two-argument error function.
References
==========
.. [1] http://en.wikipedia.org/wiki/Error_function
.. [2] http://dlmf.nist.gov/7
.. [3] http://mathworld.wolfram.com/Erf.html
.. [4] http://functions.wolfram.com/GammaBetaErf/Erf
"""
unbranched = True
def fdiff(self, argindex=1):
if argindex == 1:
return 2*C.exp(-self.args[0]**2)/sqrt(S.Pi)
else:
raise ArgumentIndexError(self, argindex)
def inverse(self, argindex=1):
"""
Returns the inverse of this function.
"""
return erfinv
@classmethod
def eval(cls, arg):
if arg.is_Number:
if arg is S.NaN:
return S.NaN
elif arg is S.Infinity:
return S.One
elif arg is S.NegativeInfinity:
return S.NegativeOne
elif arg is S.Zero:
return S.Zero
if arg.func is erfinv:
return arg.args[0]
if arg.func is erfcinv:
return S.One - arg.args[0]
if arg.func is erf2inv and arg.args[0] is S.Zero:
return arg.args[1]
# Try to pull out factors of I
t = arg.extract_multiplicatively(S.ImaginaryUnit)
if t is S.Infinity or t is S.NegativeInfinity:
return arg
# Try to pull out factors of -1
if arg.could_extract_minus_sign():
return -cls(-arg)
@staticmethod
@cacheit
def taylor_term(n, x, *previous_terms):
if n < 0 or n % 2 == 0:
return S.Zero
else:
x = sympify(x)
k = C.floor((n - 1)/S(2))
if len(previous_terms) > 2:
return -previous_terms[-2] * x**2 * (n - 2)/(n*k)
else:
return 2*(-1)**k * x**n/(n*C.factorial(k)*sqrt(S.Pi))
def _eval_conjugate(self):
return self.func(self.args[0].conjugate())
def _eval_is_real(self):
return self.args[0].is_real
def _eval_rewrite_as_uppergamma(self, z):
return sqrt(z**2)/z*(S.One - C.uppergamma(S.Half, z**2)/sqrt(S.Pi))
def _eval_rewrite_as_fresnels(self, z):
arg = (S.One - S.ImaginaryUnit)*z/sqrt(pi)
return (S.One + S.ImaginaryUnit)*(fresnelc(arg) - I*fresnels(arg))
def _eval_rewrite_as_fresnelc(self, z):
arg = (S.One - S.ImaginaryUnit)*z/sqrt(pi)
return (S.One + S.ImaginaryUnit)*(fresnelc(arg) - I*fresnels(arg))
def _eval_rewrite_as_meijerg(self, z):
return z/sqrt(pi)*meijerg([S.Half], [], [0], [-S.Half], z**2)
def _eval_rewrite_as_hyper(self, z):
return 2*z/sqrt(pi)*hyper([S.Half], [3*S.Half], -z**2)
def _eval_rewrite_as_expint(self, z):
return sqrt(z**2)/z - z*expint(S.Half, z**2)/sqrt(S.Pi)
def _eval_rewrite_as_tractable(self, z):
return S.One - _erfs(z)*C.exp(-z**2)
def _eval_rewrite_as_erfc(self, z):
return S.One - erfc(z)
def _eval_rewrite_as_erfi(self, z):
return -I*erfi(I*z)
def _eval_as_leading_term(self, x):
arg = self.args[0].as_leading_term(x)
if x in arg.free_symbols and C.Order(1, x).contains(arg):
return 2*x/sqrt(pi)
else:
return self.func(arg)
def as_real_imag(self, deep=True, **hints):
if self.args[0].is_real:
if deep:
hints['complex'] = False
return (self.expand(deep, **hints), S.Zero)
else:
return (self, S.Zero)
if deep:
x, y = self.args[0].expand(deep, **hints).as_real_imag()
else:
x, y = self.args[0].as_real_imag()
sq = -y**2/x**2
re = S.Half*(self.func(x + x*sqrt(sq)) + self.func(x - x*sqrt(sq)))
im = x/(2*y) * sqrt(sq) * (self.func(x - x*sqrt(sq)) -
self.func(x + x*sqrt(sq)))
return (re, im)
[docs]class erfc(Function):
r"""
Complementary Error Function. The function is defined as:
.. math ::
\mathrm{erfc}(x) = \frac{2}{\sqrt{\pi}} \int_x^\infty e^{-t^2} \mathrm{d}t
Examples
========
>>> from sympy import I, oo, erfc
>>> from sympy.abc import z
Several special values are known:
>>> erfc(0)
1
>>> erfc(oo)
0
>>> erfc(-oo)
2
>>> erfc(I*oo)
-oo*I
>>> erfc(-I*oo)
oo*I
The error function obeys the mirror symmetry:
>>> from sympy import conjugate
>>> conjugate(erfc(z))
erfc(conjugate(z))
Differentiation with respect to z is supported:
>>> from sympy import diff
>>> diff(erfc(z), z)
-2*exp(-z**2)/sqrt(pi)
It also follows
>>> erfc(-z)
-erfc(z) + 2
We can numerically evaluate the complementary error function to arbitrary precision
on the whole complex plane:
>>> erfc(4).evalf(30)
0.0000000154172579002800188521596734869
>>> erfc(4*I).evalf(30)
1.0 - 1296959.73071763923152794095062*I
See Also
========
erf: Gaussian error function.
erfi: Imaginary error function.
erf2: Two-argument error function.
erfinv: Inverse error function.
erfcinv: Inverse Complementary error function.
erf2inv: Inverse two-argument error function.
References
==========
.. [1] http://en.wikipedia.org/wiki/Error_function
.. [2] http://dlmf.nist.gov/7
.. [3] http://mathworld.wolfram.com/Erfc.html
.. [4] http://functions.wolfram.com/GammaBetaErf/Erfc
"""
unbranched = True
def fdiff(self, argindex=1):
if argindex == 1:
return -2*C.exp(-self.args[0]**2)/sqrt(S.Pi)
else:
raise ArgumentIndexError(self, argindex)
def inverse(self, argindex=1):
"""
Returns the inverse of this function.
"""
return erfcinv
@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.Zero:
return S.One
if arg.func is erfinv:
return S.One - arg.args[0]
if arg.func is erfcinv:
return arg.args[0]
# Try to pull out factors of I
t = arg.extract_multiplicatively(S.ImaginaryUnit)
if t is S.Infinity or t is S.NegativeInfinity:
return -arg
# Try to pull out factors of -1
if arg.could_extract_minus_sign():
return S(2) - cls(-arg)
@staticmethod
@cacheit
def taylor_term(n, x, *previous_terms):
if n == 0:
return S.One
elif n < 0 or n % 2 == 0:
return S.Zero
else:
x = sympify(x)
k = C.floor((n - 1)/S(2))
if len(previous_terms) > 2:
return -previous_terms[-2] * x**2 * (n - 2)/(n*k)
else:
return -2*(-1)**k * x**n/(n*C.factorial(k)*sqrt(S.Pi))
def _eval_conjugate(self):
return self.func(self.args[0].conjugate())
def _eval_is_real(self):
return self.args[0].is_real
def _eval_rewrite_as_tractable(self, z):
return self.rewrite(erf).rewrite("tractable", deep=True)
def _eval_rewrite_as_erf(self, z):
return S.One - erf(z)
def _eval_rewrite_as_erfi(self, z):
return S.One + I*erfi(I*z)
def _eval_rewrite_as_fresnels(self, z):
arg = (S.One - S.ImaginaryUnit)*z/sqrt(pi)
return S.One - (S.One + S.ImaginaryUnit)*(fresnelc(arg) - I*fresnels(arg))
def _eval_rewrite_as_fresnelc(self, z):
arg = (S.One-S.ImaginaryUnit)*z/sqrt(pi)
return S.One - (S.One + S.ImaginaryUnit)*(fresnelc(arg) - I*fresnels(arg))
def _eval_rewrite_as_meijerg(self, z):
return S.One - z/sqrt(pi)*meijerg([S.Half], [], [0], [-S.Half], z**2)
def _eval_rewrite_as_hyper(self, z):
return S.One - 2*z/sqrt(pi)*hyper([S.Half], [3*S.Half], -z**2)
def _eval_rewrite_as_uppergamma(self, z):
return S.One - sqrt(z**2)/z*(S.One - C.uppergamma(S.Half, z**2)/sqrt(S.Pi))
def _eval_rewrite_as_expint(self, z):
return S.One - sqrt(z**2)/z + z*expint(S.Half, z**2)/sqrt(S.Pi)
def _eval_as_leading_term(self, x):
arg = self.args[0].as_leading_term(x)
if x in arg.free_symbols and C.Order(1, x).contains(arg):
return S.One
else:
return self.func(arg)
def as_real_imag(self, deep=True, **hints):
if self.args[0].is_real:
if deep:
hints['complex'] = False
return (self.expand(deep, **hints), S.Zero)
else:
return (self, S.Zero)
if deep:
x, y = self.args[0].expand(deep, **hints).as_real_imag()
else:
x, y = self.args[0].as_real_imag()
sq = -y**2/x**2
re = S.Half*(self.func(x + x*sqrt(sq)) + self.func(x - x*sqrt(sq)))
im = x/(2*y) * sqrt(sq) * (self.func(x - x*sqrt(sq)) -
self.func(x + x*sqrt(sq)))
return (re, im)
[docs]class erfi(Function):
r"""
Imaginary error function. The function erfi is defined as:
.. math ::
\mathrm{erfi}(x) = \frac{2}{\sqrt{\pi}} \int_0^x e^{t^2} \mathrm{d}t
Examples
========
>>> from sympy import I, oo, erfi
>>> from sympy.abc import z
Several special values are known:
>>> erfi(0)
0
>>> erfi(oo)
oo
>>> erfi(-oo)
-oo
>>> erfi(I*oo)
I
>>> erfi(-I*oo)
-I
In general one can pull out factors of -1 and I from the argument:
>>> erfi(-z)
-erfi(z)
>>> from sympy import conjugate
>>> conjugate(erfi(z))
erfi(conjugate(z))
Differentiation with respect to z is supported:
>>> from sympy import diff
>>> diff(erfi(z), z)
2*exp(z**2)/sqrt(pi)
We can numerically evaluate the imaginary error function to arbitrary precision
on the whole complex plane:
>>> erfi(2).evalf(30)
18.5648024145755525987042919132
>>> erfi(-2*I).evalf(30)
-0.995322265018952734162069256367*I
See Also
========
erf: Gaussian error function.
erfc: Complementary error function.
erf2: Two-argument error function.
erfinv: Inverse error function.
erfcinv: Inverse Complementary error function.
erf2inv: Inverse two-argument error function.
References
==========
.. [1] http://en.wikipedia.org/wiki/Error_function
.. [2] http://mathworld.wolfram.com/Erfi.html
.. [3] http://functions.wolfram.com/GammaBetaErf/Erfi
"""
unbranched = True
def fdiff(self, argindex=1):
if argindex == 1:
return 2*C.exp(self.args[0]**2)/sqrt(S.Pi)
else:
raise ArgumentIndexError(self, argindex)
@classmethod
def eval(cls, z):
if z.is_Number:
if z is S.NaN:
return S.NaN
elif z is S.Zero:
return S.Zero
elif z is S.Infinity:
return S.Infinity
# Try to pull out factors of -1
if z.could_extract_minus_sign():
return -cls(-z)
# Try to pull out factors of I
nz = z.extract_multiplicatively(I)
if nz is not None:
if nz is S.Infinity:
return I
if nz.func is erfinv:
return I*nz.args[0]
if nz.func is erfcinv:
return I*(S.One - nz.args[0])
if nz.func is erf2inv and nz.args[0] is S.Zero:
return I*nz.args[1]
@staticmethod
@cacheit
def taylor_term(n, x, *previous_terms):
if n < 0 or n % 2 == 0:
return S.Zero
else:
x = sympify(x)
k = C.floor((n - 1)/S(2))
if len(previous_terms) > 2:
return previous_terms[-2] * x**2 * (n - 2)/(n*k)
else:
return 2 * x**n/(n*C.factorial(k)*sqrt(S.Pi))
def _eval_conjugate(self):
return self.func(self.args[0].conjugate())
def _eval_is_real(self):
return self.args[0].is_real
def _eval_rewrite_as_tractable(self, z):
return self.rewrite(erf).rewrite("tractable", deep=True)
def _eval_rewrite_as_erf(self, z):
return -I*erf(I*z)
def _eval_rewrite_as_erfc(self, z):
return I*erfc(I*z) - I
def _eval_rewrite_as_fresnels(self, z):
arg = (S.One + S.ImaginaryUnit)*z/sqrt(pi)
return (S.One - S.ImaginaryUnit)*(fresnelc(arg) - I*fresnels(arg))
def _eval_rewrite_as_fresnelc(self, z):
arg = (S.One + S.ImaginaryUnit)*z/sqrt(pi)
return (S.One - S.ImaginaryUnit)*(fresnelc(arg) - I*fresnels(arg))
def _eval_rewrite_as_meijerg(self, z):
return z/sqrt(pi)*meijerg([S.Half], [], [0], [-S.Half], -z**2)
def _eval_rewrite_as_hyper(self, z):
return 2*z/sqrt(pi)*hyper([S.Half], [3*S.Half], z**2)
def _eval_rewrite_as_uppergamma(self, z):
return sqrt(-z**2)/z*(C.uppergamma(S.Half, -z**2)/sqrt(S.Pi) - S.One)
def _eval_rewrite_as_expint(self, z):
return sqrt(-z**2)/z - z*expint(S.Half, -z**2)/sqrt(S.Pi)
def as_real_imag(self, deep=True, **hints):
if self.args[0].is_real:
if deep:
hints['complex'] = False
return (self.expand(deep, **hints), S.Zero)
else:
return (self, S.Zero)
if deep:
x, y = self.args[0].expand(deep, **hints).as_real_imag()
else:
x, y = self.args[0].as_real_imag()
sq = -y**2/x**2
re = S.Half*(self.func(x + x*sqrt(sq)) + self.func(x - x*sqrt(sq)))
im = x/(2*y) * sqrt(sq) * (self.func(x - x*sqrt(sq)) -
self.func(x + x*sqrt(sq)))
return (re, im)
[docs]class erf2(Function):
r"""
Two-argument error function. This function is defined as:
.. math ::
\mathrm{erf2}(x, y) = \frac{2}{\sqrt{\pi}} \int_x^y e^{-t^2} \mathrm{d}t
Examples
========
>>> from sympy import I, oo, erf2
>>> from sympy.abc import x, y
Several special values are known:
>>> erf2(0, 0)
0
>>> erf2(x, x)
0
>>> erf2(x, oo)
-erf(x) + 1
>>> erf2(x, -oo)
-erf(x) - 1
>>> erf2(oo, y)
erf(y) - 1
>>> erf2(-oo, y)
erf(y) + 1
In general one can pull out factors of -1:
>>> erf2(-x, -y)
-erf2(x, y)
The error function obeys the mirror symmetry:
>>> from sympy import conjugate
>>> conjugate(erf2(x, y))
erf2(conjugate(x), conjugate(y))
Differentiation with respect to x, y is supported:
>>> from sympy import diff
>>> diff(erf2(x, y), x)
-2*exp(-x**2)/sqrt(pi)
>>> diff(erf2(x, y), y)
2*exp(-y**2)/sqrt(pi)
See Also
========
erf: Gaussian error function.
erfc: Complementary error function.
erfi: Imaginary error function.
erfinv: Inverse error function.
erfcinv: Inverse Complementary error function.
erf2inv: Inverse two-argument error function.
References
==========
.. [1] http://functions.wolfram.com/GammaBetaErf/Erf2/
"""
def fdiff(self, argindex):
x, y = self.args
if argindex == 1:
return -2*C.exp(-x**2)/sqrt(S.Pi)
elif argindex == 2:
return 2*C.exp(-y**2)/sqrt(S.Pi)
else:
raise ArgumentIndexError(self, argindex)
@classmethod
def eval(cls, x, y):
I = S.Infinity
N = S.NegativeInfinity
O = S.Zero
if x is S.NaN or y is S.NaN:
return S.NaN
elif x == y:
return S.Zero
elif (x is I or x is N or x is O) or (y is I or y is N or y is O):
return erf(y) - erf(x)
if y.func is erf2inv and y.args[0] == x:
return y.args[1]
#Try to pull out -1 factor
sign_x = x.could_extract_minus_sign()
sign_y = y.could_extract_minus_sign()
if (sign_x and sign_y):
return -cls(-x, -y)
elif (sign_x or sign_y):
return erf(y)-erf(x)
def _eval_conjugate(self):
return self.func(self.args[0].conjugate(), self.args[1].conjugate())
def _eval_is_real(self):
return self.args[0].is_real and self.args[1].is_real
def _eval_rewrite_as_erf(self, x, y):
return erf(y) - erf(x)
def _eval_rewrite_as_erfc(self, x, y):
return erfc(x) - erfc(y)
def _eval_rewrite_as_erfi(self, x, y):
return I*(erfi(I*x)-erfi(I*y))
def _eval_rewrite_as_fresnels(self, x, y):
return erf(y).rewrite(fresnels) - erf(x).rewrite(fresnels)
def _eval_rewrite_as_fresnelc(self, x, y):
return erf(y).rewrite(fresnelc) - erf(x).rewrite(fresnelc)
def _eval_rewrite_as_meijerg(self, x, y):
return erf(y).rewrite(meijerg) - erf(x).rewrite(meijerg)
def _eval_rewrite_as_hyper(self, x, y):
return erf(y).rewrite(hyper) - erf(x).rewrite(hyper)
def _eval_rewrite_as_uppergamma(self, x, y):
return (sqrt(y**2)/y*(S.One - C.uppergamma(S.Half, y**2)/sqrt(S.Pi)) -
sqrt(x**2)/x*(S.One - C.uppergamma(S.Half, x**2)/sqrt(S.Pi)))
def _eval_rewrite_as_expint(self, x, y):
return erf(y).rewrite(expint) - erf(x).rewrite(expint)
[docs]class erfinv(Function):
r"""
Inverse Error Function. The erfinv function is defined as:
.. math ::
\mathrm{erf}(x) = y \quad \Rightarrow \quad \mathrm{erfinv}(y) = x
Examples
========
>>> from sympy import I, oo, erfinv
>>> from sympy.abc import x
Several special values are known:
>>> erfinv(0)
0
>>> erfinv(1)
oo
Differentiation with respect to x is supported:
>>> from sympy import diff
>>> diff(erfinv(x), x)
sqrt(pi)*exp(erfinv(x)**2)/2
We can numerically evaluate the inverse error function to arbitrary precision
on [-1, 1]:
>>> erfinv(0.2).evalf(30)
0.179143454621291692285822705344
See Also
========
erf: Gaussian error function.
erfc: Complementary error function.
erfi: Imaginary error function.
erf2: Two-argument error function.
erfcinv: Inverse Complementary error function.
erf2inv: Inverse two-argument error function.
References
==========
.. [1] http://en.wikipedia.org/wiki/Error_function#Inverse_functions
.. [2] http://functions.wolfram.com/GammaBetaErf/InverseErf/
"""
def fdiff(self, argindex =1):
if argindex == 1:
return sqrt(S.Pi)*C.exp(self.func(self.args[0])**2)*S.Half
else :
raise ArgumentIndexError(self, argindex)
def inverse(self, argindex=1):
"""
Returns the inverse of this function.
"""
return erf
@classmethod
def eval(cls, z):
if z is S.NaN:
return S.NaN
elif z is S.NegativeOne:
return S.NegativeInfinity
elif z is S.Zero:
return S.Zero
elif z is S.One:
return S.Infinity
if (z.func is erf) and z.args[0].is_real:
return z.args[0]
# Try to pull out factors of -1
nz = z.extract_multiplicatively(-1)
if nz is not None and ((nz.func is erf) and (nz.args[0]).is_real):
return -nz.args[0]
def _eval_rewrite_as_erfcinv(self, z):
return erfcinv(1-z)
[docs]class erfcinv (Function):
r"""
Inverse Complementary Error Function. The erfcinv function is defined as:
.. math ::
\mathrm{erfc}(x) = y \quad \Rightarrow \quad \mathrm{erfcinv}(y) = x
Examples
========
>>> from sympy import I, oo, erfcinv
>>> from sympy.abc import x
Several special values are known:
>>> erfcinv(1)
0
>>> erfcinv(0)
oo
Differentiation with respect to x is supported:
>>> from sympy import diff
>>> diff(erfcinv(x), x)
-sqrt(pi)*exp(erfcinv(x)**2)/2
See Also
========
erf: Gaussian error function.
erfc: Complementary error function.
erfi: Imaginary error function.
erf2: Two-argument error function.
erfinv: Inverse error function.
erf2inv: Inverse two-argument error function.
References
==========
.. [1] http://en.wikipedia.org/wiki/Error_function#Inverse_functions
.. [2] http://functions.wolfram.com/GammaBetaErf/InverseErfc/
"""
def fdiff(self, argindex =1):
if argindex == 1:
return -sqrt(S.Pi)*C.exp(self.func(self.args[0])**2)*S.Half
else:
raise ArgumentIndexError(self, argindex)
def inverse(self, argindex=1):
"""
Returns the inverse of this function.
"""
return erfc
@classmethod
def eval(cls, z):
if z is S.NaN:
return S.NaN
elif z is S.Zero:
return S.Infinity
elif z is S.One:
return S.Zero
elif z == 2:
return S.NegativeInfinity
def _eval_rewrite_as_erfinv(self, z):
return erfinv(1-z)
[docs]class erf2inv(Function):
r"""
Two-argument Inverse error function. The erf2inv function is defined as:
.. math ::
\mathrm{erf2}(x, w) = y \quad \Rightarrow \quad \mathrm{erf2inv}(x, y) = w
Examples
========
>>> from sympy import I, oo, erf2inv, erfinv, erfcinv
>>> from sympy.abc import x, y
Several special values are known:
>>> erf2inv(0, 0)
0
>>> erf2inv(1, 0)
1
>>> erf2inv(0, 1)
oo
>>> erf2inv(0, y)
erfinv(y)
>>> erf2inv(oo, y)
erfcinv(-y)
Differentiation with respect to x and y is supported:
>>> from sympy import diff
>>> diff(erf2inv(x, y), x)
exp(-x**2 + erf2inv(x, y)**2)
>>> diff(erf2inv(x, y), y)
sqrt(pi)*exp(erf2inv(x, y)**2)/2
See Also
========
erf: Gaussian error function.
erfc: Complementary error function.
erfi: Imaginary error function.
erf2: Two-argument error function.
erfinv: Inverse error function.
erfcinv: Inverse complementary error function.
References
==========
.. [1] http://functions.wolfram.com/GammaBetaErf/InverseErf2/
"""
def fdiff(self, argindex):
x, y = self.args
if argindex == 1:
return C.exp(self.func(x,y)**2-x**2)
elif argindex == 2:
return sqrt(S.Pi)*S.Half*C.exp(self.func(x,y)**2)
else:
raise ArgumentIndexError(self, argindex)
@classmethod
def eval(cls, x, y):
if x is S.NaN or y is S.NaN:
return S.NaN
elif x is S.Zero and y is S.Zero:
return S.Zero
elif x is S.Zero and y is S.One:
return S.Infinity
elif x is S.One and y is S.Zero:
return S.One
elif x is S.Zero:
return erfinv(y)
elif x is S.Infinity:
return erfcinv(-y)
elif y is S.Zero:
return x
elif y is S.Infinity:
return erfinv(x)
###############################################################################
#################### EXPONENTIAL INTEGRALS ####################################
###############################################################################
[docs]class Ei(Function):
r"""
The classical exponential integral.
For use in SymPy, this function is defined as
.. math:: \operatorname{Ei}(x) = \sum_{n=1}^\infty \frac{x^n}{n\, n!}
+ \log(x) + \gamma,
where `\gamma` is the Euler-Mascheroni constant.
If `x` is a polar number, this defines an analytic function on the
riemann surface of the logarithm. Otherwise this defines an analytic
function in the cut plane `\mathbb{C} \setminus (-\infty, 0]`.
**Background**
The name *exponential integral* comes from the following statement:
.. math:: \operatorname{Ei}(x) = \int_{-\infty}^x \frac{e^t}{t} \mathrm{d}t
If the integral is interpreted as a Cauchy principal value, this statement
holds for `x > 0` and `\operatorname{Ei}(x)` as defined above.
Note that we carefully avoided defining `\operatorname{Ei}(x)` for
negative real `x`. This is because above integral formula does not hold for
any polar lift of such `x`, indeed all branches of
`\operatorname{Ei}(x)` above the negative reals are imaginary.
However, the following statement holds for all `x \in \mathbb{R}^*`:
.. math:: \int_{-\infty}^x \frac{e^t}{t} \mathrm{d}t =
\frac{\operatorname{Ei}\left(|x|e^{i \arg(x)}\right) +
\operatorname{Ei}\left(|x|e^{- i \arg(x)}\right)}{2},
where the integral is again understood to be a principal value if
`x > 0`, and `|x|e^{i \arg(x)}`,
`|x|e^{- i \arg(x)}` denote two conjugate polar lifts of `x`.
Examples
========
>>> from sympy import Ei, polar_lift, exp_polar, I, pi
>>> from sympy.abc import x
The exponential integral in SymPy is strictly undefined for negative values
of the argument. For convenience, exponential integrals with negative
arguments are immediately converted into an expression that agrees with
the classical integral definition:
>>> Ei(-1)
-I*pi + Ei(exp_polar(I*pi))
This yields a real value:
>>> Ei(-1).n(chop=True)
-0.219383934395520
On the other hand the analytic continuation is not real:
>>> Ei(polar_lift(-1)).n(chop=True)
-0.21938393439552 + 3.14159265358979*I
The exponential integral has a logarithmic branch point at the origin:
>>> Ei(x*exp_polar(2*I*pi))
Ei(x) + 2*I*pi
Differentiation is supported:
>>> Ei(x).diff(x)
exp(x)/x
The exponential integral is related to many other special functions.
For example:
>>> from sympy import uppergamma, expint, Shi
>>> Ei(x).rewrite(expint)
-expint(1, x*exp_polar(I*pi)) - I*pi
>>> Ei(x).rewrite(Shi)
Chi(x) + Shi(x)
See Also
========
expint: Generalised exponential integral.
E1: Special case of the generalised exponential integral.
li: Logarithmic integral.
Li: Offset logarithmic integral.
Si: Sine integral.
Ci: Cosine integral.
Shi: Hyperbolic sine integral.
Chi: Hyperbolic cosine integral.
sympy.functions.special.gamma_functions.uppergamma: Upper incomplete gamma function.
References
==========
.. [1] http://dlmf.nist.gov/6.6
.. [2] http://en.wikipedia.org/wiki/Exponential_integral
.. [3] Abramowitz & Stegun, section 5: http://www.math.sfu.ca/~cbm/aands/page_228.htm
"""
@classmethod
def eval(cls, z):
if not z.is_polar and z.is_negative:
# Note: is this a good idea?
return Ei(polar_lift(z)) - pi*I
nz, n = z.extract_branch_factor()
if n:
return Ei(nz) + 2*I*pi*n
def fdiff(self, argindex=1):
from sympy import unpolarify
arg = unpolarify(self.args[0])
if argindex == 1:
return C.exp(arg)/arg
else:
raise ArgumentIndexError(self, argindex)
def _eval_evalf(self, prec):
if (self.args[0]/polar_lift(-1)).is_positive:
return Function._eval_evalf(self, prec) + (I*pi)._eval_evalf(prec)
return Function._eval_evalf(self, prec)
def _eval_rewrite_as_uppergamma(self, z):
from sympy import uppergamma
# XXX this does not currently work usefully because uppergamma
# immediately turns into expint
return -uppergamma(0, polar_lift(-1)*z) - I*pi
def _eval_rewrite_as_expint(self, z):
return -expint(1, polar_lift(-1)*z) - I*pi
def _eval_rewrite_as_li(self, z):
if isinstance(z, log):
return li(z.args[0])
# TODO:
# Actually it only holds that:
# Ei(z) = li(exp(z))
# for -pi < imag(z) <= pi
return li(exp(z))
def _eval_rewrite_as_Si(self, z):
return Shi(z) + Chi(z)
_eval_rewrite_as_Ci = _eval_rewrite_as_Si
_eval_rewrite_as_Chi = _eval_rewrite_as_Si
_eval_rewrite_as_Shi = _eval_rewrite_as_Si
def _eval_rewrite_as_tractable(self, z):
return C.exp(z) * _eis(z)
def _eval_nseries(self, x, n, logx):
x0 = self.args[0].limit(x, 0)
if x0 is S.Zero:
f = self._eval_rewrite_as_Si(*self.args)
return f._eval_nseries(x, n, logx)
return super(Ei, self)._eval_nseries(x, n, logx)
[docs]class expint(Function):
r"""
Generalized exponential integral.
This function is defined as
.. math:: \operatorname{E}_\nu(z) = z^{\nu - 1} \Gamma(1 - \nu, z),
where `\Gamma(1 - \nu, z)` is the upper incomplete gamma function
(``uppergamma``).
Hence for :math:`z` with positive real part we have
.. math:: \operatorname{E}_\nu(z)
= \int_1^\infty \frac{e^{-zt}}{z^\nu} \mathrm{d}t,
which explains the name.
The representation as an incomplete gamma function provides an analytic
continuation for :math:`\operatorname{E}_\nu(z)`. If :math:`\nu` is a
non-positive integer the exponential integral is thus an unbranched
function of :math:`z`, otherwise there is a branch point at the origin.
Refer to the incomplete gamma function documentation for details of the
branching behavior.
Examples
========
>>> from sympy import expint, S
>>> from sympy.abc import nu, z
Differentiation is supported. Differentiation with respect to z explains
further the name: for integral orders, the exponential integral is an
iterated integral of the exponential function.
>>> expint(nu, z).diff(z)
-expint(nu - 1, z)
Differentiation with respect to nu has no classical expression:
>>> expint(nu, z).diff(nu)
-z**(nu - 1)*meijerg(((), (1, 1)), ((0, 0, -nu + 1), ()), z)
At non-postive integer orders, the exponential integral reduces to the
exponential function:
>>> expint(0, z)
exp(-z)/z
>>> expint(-1, z)
exp(-z)/z + exp(-z)/z**2
At half-integers it reduces to error functions:
>>> expint(S(1)/2, z)
-sqrt(pi)*erf(sqrt(z))/sqrt(z) + sqrt(pi)/sqrt(z)
At positive integer orders it can be rewritten in terms of exponentials
and expint(1, z). Use expand_func() to do this:
>>> from sympy import expand_func
>>> expand_func(expint(5, z))
z**4*expint(1, z)/24 + (-z**3 + z**2 - 2*z + 6)*exp(-z)/24
The generalised exponential integral is essentially equivalent to the
incomplete gamma function:
>>> from sympy import uppergamma
>>> expint(nu, z).rewrite(uppergamma)
z**(nu - 1)*uppergamma(-nu + 1, z)
As such it is branched at the origin:
>>> from sympy import exp_polar, pi, I
>>> expint(4, z*exp_polar(2*pi*I))
I*pi*z**3/3 + expint(4, z)
>>> expint(nu, z*exp_polar(2*pi*I))
z**(nu - 1)*(exp(2*I*pi*nu) - 1)*gamma(-nu + 1) + expint(nu, z)
See Also
========
Ei: Another related function called exponential integral.
E1: The classical case, returns expint(1, z).
li: Logarithmic integral.
Li: Offset logarithmic integral.
Si: Sine integral.
Ci: Cosine integral.
Shi: Hyperbolic sine integral.
Chi: Hyperbolic cosine integral.
sympy.functions.special.gamma_functions.uppergamma
References
==========
.. [1] http://dlmf.nist.gov/8.19
.. [2] http://functions.wolfram.com/GammaBetaErf/ExpIntegralE/
.. [3] http://en.wikipedia.org/wiki/Exponential_integral
"""
@classmethod
def eval(cls, nu, z):
from sympy import (unpolarify, expand_mul, uppergamma, exp, gamma,
factorial)
nu2 = unpolarify(nu)
if nu != nu2:
return expint(nu2, z)
if nu.is_Integer and nu <= 0 or (not nu.is_Integer and (2*nu).is_Integer):
return unpolarify(expand_mul(z**(nu - 1)*uppergamma(1 - nu, z)))
# Extract branching information. This can be deduced from what is
# explained in lowergamma.eval().
z, n = z.extract_branch_factor()
if n == 0:
return
if nu.is_integer:
if (nu > 0) is not True:
return
return expint(nu, z) \
- 2*pi*I*n*(-1)**(nu - 1)/factorial(nu - 1)*unpolarify(z)**(nu - 1)
else:
return (exp(2*I*pi*nu*n) - 1)*z**(nu - 1)*gamma(1 - nu) + expint(nu, z)
def fdiff(self, argindex):
from sympy import meijerg
nu, z = self.args
if argindex == 1:
return -z**(nu - 1)*meijerg([], [1, 1], [0, 0, 1 - nu], [], z)
elif argindex == 2:
return -expint(nu - 1, z)
else:
raise ArgumentIndexError(self, argindex)
def _eval_rewrite_as_uppergamma(self, nu, z):
from sympy import uppergamma
return z**(nu - 1)*uppergamma(1 - nu, z)
def _eval_rewrite_as_Ei(self, nu, z):
from sympy import exp_polar, unpolarify, exp, factorial
if nu == 1:
return -Ei(z*exp_polar(-I*pi)) - I*pi
elif nu.is_Integer and nu > 1:
# DLMF, 8.19.7
x = -unpolarify(z)
return x**(nu - 1)/factorial(nu - 1)*E1(z).rewrite(Ei) + \
exp(x)/factorial(nu - 1) * \
Add(*[factorial(nu - k - 2)*x**k for k in range(nu - 1)])
else:
return self
def _eval_expand_func(self, **hints):
return self.rewrite(Ei).rewrite(expint, **hints)
def _eval_rewrite_as_Si(self, nu, z):
if nu != 1:
return self
return Shi(z) - Chi(z)
_eval_rewrite_as_Ci = _eval_rewrite_as_Si
_eval_rewrite_as_Chi = _eval_rewrite_as_Si
_eval_rewrite_as_Shi = _eval_rewrite_as_Si
def _eval_nseries(self, x, n, logx):
if not self.args[0].has(x):
nu = self.args[0]
if nu == 1:
f = self._eval_rewrite_as_Si(*self.args)
return f._eval_nseries(x, n, logx)
elif nu.is_Integer and nu > 1:
f = self._eval_rewrite_as_Ei(*self.args)
return f._eval_nseries(x, n, logx)
return super(expint, self)._eval_nseries(x, n, logx)
[docs]def E1(z):
"""
Classical case of the generalized exponential integral.
This is equivalent to ``expint(1, z)``.
See Also
========
Ei: Exponential integral.
expint: Generalised exponential integral.
li: Logarithmic integral.
Li: Offset logarithmic integral.
Si: Sine integral.
Ci: Cosine integral.
Shi: Hyperbolic sine integral.
Chi: Hyperbolic cosine integral.
"""
return expint(1, z)
[docs]class li(Function):
r"""
The classical logarithmic integral.
For the use in SymPy, this function is defined as
.. math:: \operatorname{li}(x) = \int_0^x \frac{1}{\log(t)} \mathrm{d}t \,.
Examples
========
>>> from sympy import I, oo, li
>>> from sympy.abc import z
Several special values are known:
>>> li(0)
0
>>> li(1)
-oo
>>> li(oo)
oo
Differentiation with respect to z is supported:
>>> from sympy import diff
>>> diff(li(z), z)
1/log(z)
Defining the `li` function via an integral:
The logarithmic integral can also be defined in terms of Ei:
>>> from sympy import Ei
>>> li(z).rewrite(Ei)
Ei(log(z))
>>> diff(li(z).rewrite(Ei), z)
1/log(z)
We can numerically evaluate the logarithmic integral to arbitrary precision
on the whole complex plane (except the singular points):
>>> li(2).evalf(30)
1.04516378011749278484458888919
>>> li(2*I).evalf(30)
1.0652795784357498247001125598 + 3.08346052231061726610939702133*I
We can even compute Soldner's constant by the help of mpmath:
>>> from sympy.mpmath import findroot
>>> findroot(li, 2)
1.45136923488338
Further transformations include rewriting `li` in terms of
the trigonometric integrals `Si`, `Ci`, `Shi` and `Chi`:
>>> from sympy import Si, Ci, Shi, Chi
>>> li(z).rewrite(Si)
-log(I*log(z)) - log(1/log(z))/2 + log(log(z))/2 + Ci(I*log(z)) + Shi(log(z))
>>> li(z).rewrite(Ci)
-log(I*log(z)) - log(1/log(z))/2 + log(log(z))/2 + Ci(I*log(z)) + Shi(log(z))
>>> li(z).rewrite(Shi)
-log(1/log(z))/2 + log(log(z))/2 + Chi(log(z)) - Shi(log(z))
>>> li(z).rewrite(Chi)
-log(1/log(z))/2 + log(log(z))/2 + Chi(log(z)) - Shi(log(z))
See Also
========
Li: Offset logarithmic integral.
Ei: Exponential integral.
expint: Generalised exponential integral.
E1: Special case of the generalised exponential integral.
Si: Sine integral.
Ci: Cosine integral.
Shi: Hyperbolic sine integral.
Chi: Hyperbolic cosine integral.
References
==========
.. [1] http://en.wikipedia.org/wiki/Logarithmic_integral
.. [2] http://mathworld.wolfram.com/LogarithmicIntegral.html
.. [3] http://dlmf.nist.gov/6
.. [4] http://mathworld.wolfram.com/SoldnersConstant.html
"""
@classmethod
def eval(cls, z):
if z is S.Zero:
return S.Zero
elif z is S.One:
return S.NegativeInfinity
elif z is S.Infinity:
return S.Infinity
def fdiff(self, argindex=1):
arg = self.args[0]
if argindex == 1:
return S.One / C.log(arg)
else:
raise ArgumentIndexError(self, argindex)
def _eval_conjugate(self):
z = self.args[0]
# Exclude values on the branch cut (-oo, 0)
if not (z.is_real and z.is_negative):
return self.func(z.conjugate())
def _eval_rewrite_as_Li(self, z):
return Li(z) + li(2)
def _eval_rewrite_as_Ei(self, z):
return Ei(C.log(z))
def _eval_rewrite_as_uppergamma(self, z):
from sympy import uppergamma
return (-uppergamma(0, -C.log(z)) +
S.Half*(C.log(C.log(z)) - C.log(S.One/C.log(z))) - C.log(-C.log(z)))
def _eval_rewrite_as_Si(self, z):
return (Ci(I*C.log(z)) - I*Si(I*C.log(z)) -
S.Half*(C.log(S.One/C.log(z)) - C.log(C.log(z))) - C.log(I*C.log(z)))
_eval_rewrite_as_Ci = _eval_rewrite_as_Si
def _eval_rewrite_as_Shi(self, z):
return (Chi(C.log(z)) - Shi(C.log(z)) - S.Half*(C.log(S.One/C.log(z)) - C.log(C.log(z))))
_eval_rewrite_as_Chi = _eval_rewrite_as_Shi
def _eval_rewrite_as_hyper(self, z):
return (C.log(z)*hyper((1, 1), (2, 2), C.log(z)) +
S.Half*(C.log(C.log(z)) - C.log(S.One/C.log(z))) + S.EulerGamma)
def _eval_rewrite_as_meijerg(self, z):
return (-C.log(-C.log(z)) - S.Half*(C.log(S.One/C.log(z)) - C.log(C.log(z)))
- meijerg(((), (1,)), ((0, 0), ()), -C.log(z)))
def _eval_rewrite_as_tractable(self, z):
return z * _eis(C.log(z))
[docs]class Li(Function):
r"""
The offset logarithmic integral.
For the use in SymPy, this function is defined as
.. math:: \operatorname{Li}(x) = \operatorname{li}(x) - \operatorname{li}(2)
Examples
========
>>> from sympy import I, oo, Li
>>> from sympy.abc import z
The following special value is known:
>>> Li(2)
0
Differentiation with respect to z is supported:
>>> from sympy import diff
>>> diff(Li(z), z)
1/log(z)
The shifted logarithmic integral can be written in terms of `li(z)`:
>>> from sympy import li
>>> Li(z).rewrite(li)
li(z) - li(2)
We can numerically evaluate the logarithmic integral to arbitrary precision
on the whole complex plane (except the singular points):
>>> Li(2).evalf(30)
0
>>> Li(4).evalf(30)
1.92242131492155809316615998938
See Also
========
li: Logarithmic integral.
Ei: Exponential integral.
expint: Generalised exponential integral.
E1: Special case of the generalised exponential integral.
Si: Sine integral.
Ci: Cosine integral.
Shi: Hyperbolic sine integral.
Chi: Hyperbolic cosine integral.
References
==========
.. [1] http://en.wikipedia.org/wiki/Logarithmic_integral
.. [2] http://mathworld.wolfram.com/LogarithmicIntegral.html
.. [3] http://dlmf.nist.gov/6
"""
@classmethod
def eval(cls, z):
if z is S.Infinity:
return S.Infinity
elif z is 2*S.One:
return S.Zero
def fdiff(self, argindex=1):
arg = self.args[0]
if argindex == 1:
return S.One / C.log(arg)
else:
raise ArgumentIndexError(self, argindex)
def _eval_evalf(self, prec):
return self.rewrite(li).evalf(prec)
def _eval_rewrite_as_li(self, z):
return li(z) - li(2)
def _eval_rewrite_as_tractable(self, z):
return self.rewrite(li).rewrite("tractable", deep=True)
###############################################################################
#################### TRIGONOMETRIC INTEGRALS ##################################
###############################################################################
class TrigonometricIntegral(Function):
""" Base class for trigonometric integrals. """
@classmethod
def eval(cls, z):
if z == 0:
return cls._atzero
elif z is S.Infinity:
return cls._atinf
elif z is S.NegativeInfinity:
return cls._atneginf
nz = z.extract_multiplicatively(polar_lift(I))
if nz is None and cls._trigfunc(0) == 0:
nz = z.extract_multiplicatively(I)
if nz is not None:
return cls._Ifactor(nz, 1)
nz = z.extract_multiplicatively(polar_lift(-I))
if nz is not None:
return cls._Ifactor(nz, -1)
nz = z.extract_multiplicatively(polar_lift(-1))
if nz is None and cls._trigfunc(0) == 0:
nz = z.extract_multiplicatively(-1)
if nz is not None:
return cls._minusfactor(nz)
nz, n = z.extract_branch_factor()
if n == 0 and nz == z:
return
return 2*pi*I*n*cls._trigfunc(0) + cls(nz)
def fdiff(self, argindex=1):
from sympy import unpolarify
arg = unpolarify(self.args[0])
if argindex == 1:
return self._trigfunc(arg)/arg
def _eval_rewrite_as_Ei(self, z):
return self._eval_rewrite_as_expint(z).rewrite(Ei)
def _eval_rewrite_as_uppergamma(self, z):
from sympy import uppergamma
return self._eval_rewrite_as_expint(z).rewrite(uppergamma)
def _eval_nseries(self, x, n, logx):
# NOTE this is fairly inefficient
from sympy import log, EulerGamma, Pow
n += 1
if self.args[0].subs(x, 0) != 0:
return super(TrigonometricIntegral, self)._eval_nseries(x, n, logx)
baseseries = self._trigfunc(x)._eval_nseries(x, n, logx)
if self._trigfunc(0) != 0:
baseseries -= 1
baseseries = baseseries.replace(Pow, lambda t, n: t**n/n, simultaneous=False)
if self._trigfunc(0) != 0:
baseseries += EulerGamma + log(x)
return baseseries.subs(x, self.args[0])._eval_nseries(x, n, logx)
[docs]class Si(TrigonometricIntegral):
r"""
Sine integral.
This function is defined by
.. math:: \operatorname{Si}(z) = \int_0^z \frac{\sin{t}}{t} \mathrm{d}t.
It is an entire function.
Examples
========
>>> from sympy import Si
>>> from sympy.abc import z
The sine integral is an antiderivative of sin(z)/z:
>>> Si(z).diff(z)
sin(z)/z
It is unbranched:
>>> from sympy import exp_polar, I, pi
>>> Si(z*exp_polar(2*I*pi))
Si(z)
Sine integral behaves much like ordinary sine under multiplication by ``I``:
>>> Si(I*z)
I*Shi(z)
>>> Si(-z)
-Si(z)
It can also be expressed in terms of exponential integrals, but beware
that the latter is branched:
>>> from sympy import expint
>>> Si(z).rewrite(expint)
-I*(-expint(1, z*exp_polar(-I*pi/2))/2 +
expint(1, z*exp_polar(I*pi/2))/2) + pi/2
See Also
========
Ci: Cosine integral.
Shi: Hyperbolic sine integral.
Chi: Hyperbolic cosine integral.
Ei: Exponential integral.
expint: Generalised exponential integral.
E1: Special case of the generalised exponential integral.
li: Logarithmic integral.
Li: Offset logarithmic integral.
References
==========
.. [1] http://en.wikipedia.org/wiki/Trigonometric_integral
"""
_trigfunc = C.sin
_atzero = S(0)
_atinf = pi*S.Half
_atneginf = -pi*S.Half
@classmethod
def _minusfactor(cls, z):
return -Si(z)
@classmethod
def _Ifactor(cls, z, sign):
return I*Shi(z)*sign
def _eval_rewrite_as_expint(self, z):
# XXX should we polarify z?
return pi/2 + (E1(polar_lift(I)*z) - E1(polar_lift(-I)*z))/2/I
[docs]class Ci(TrigonometricIntegral):
r"""
Cosine integral.
This function is defined for positive `x` by
.. math:: \operatorname{Ci}(x) = \gamma + \log{x}
+ \int_0^x \frac{\cos{t} - 1}{t} \mathrm{d}t
= -\int_x^\infty \frac{\cos{t}}{t} \mathrm{d}t,
where `\gamma` is the Euler-Mascheroni constant.
We have
.. math:: \operatorname{Ci}(z) =
-\frac{\operatorname{E}_1\left(e^{i\pi/2} z\right)
+ \operatorname{E}_1\left(e^{-i \pi/2} z\right)}{2}
which holds for all polar `z` and thus provides an analytic
continuation to the Riemann surface of the logarithm.
The formula also holds as stated
for `z \in \mathbb{C}` with `\Re(z) > 0`.
By lifting to the principal branch we obtain an analytic function on the
cut complex plane.
Examples
========
>>> from sympy import Ci
>>> from sympy.abc import z
The cosine integral is a primitive of `\cos(z)/z`:
>>> Ci(z).diff(z)
cos(z)/z
It has a logarithmic branch point at the origin:
>>> from sympy import exp_polar, I, pi
>>> Ci(z*exp_polar(2*I*pi))
Ci(z) + 2*I*pi
The cosine integral behaves somewhat like ordinary `\cos` under multiplication by `i`:
>>> from sympy import polar_lift
>>> Ci(polar_lift(I)*z)
Chi(z) + I*pi/2
>>> Ci(polar_lift(-1)*z)
Ci(z) + I*pi
It can also be expressed in terms of exponential integrals:
>>> from sympy import expint
>>> Ci(z).rewrite(expint)
-expint(1, z*exp_polar(-I*pi/2))/2 - expint(1, z*exp_polar(I*pi/2))/2
See Also
========
Si: Sine integral.
Shi: Hyperbolic sine integral.
Chi: Hyperbolic cosine integral.
Ei: Exponential integral.
expint: Generalised exponential integral.
E1: Special case of the generalised exponential integral.
li: Logarithmic integral.
Li: Offset logarithmic integral.
References
==========
.. [1] http://en.wikipedia.org/wiki/Trigonometric_integral
"""
_trigfunc = C.cos
_atzero = S.ComplexInfinity
_atinf = S.Zero
_atneginf = I*pi
@classmethod
def _minusfactor(cls, z):
return Ci(z) + I*pi
@classmethod
def _Ifactor(cls, z, sign):
return Chi(z) + I*pi/2*sign
def _eval_rewrite_as_expint(self, z):
return -(E1(polar_lift(I)*z) + E1(polar_lift(-I)*z))/2
[docs]class Shi(TrigonometricIntegral):
r"""
Sinh integral.
This function is defined by
.. math:: \operatorname{Shi}(z) = \int_0^z \frac{\sinh{t}}{t} \mathrm{d}t.
It is an entire function.
Examples
========
>>> from sympy import Shi
>>> from sympy.abc import z
The Sinh integral is a primitive of `\sinh(z)/z`:
>>> Shi(z).diff(z)
sinh(z)/z
It is unbranched:
>>> from sympy import exp_polar, I, pi
>>> Shi(z*exp_polar(2*I*pi))
Shi(z)
The `\sinh` integral behaves much like ordinary `\sinh` under multiplication by `i`:
>>> Shi(I*z)
I*Si(z)
>>> Shi(-z)
-Shi(z)
It can also be expressed in terms of exponential integrals, but beware
that the latter is branched:
>>> from sympy import expint
>>> Shi(z).rewrite(expint)
expint(1, z)/2 - expint(1, z*exp_polar(I*pi))/2 - I*pi/2
See Also
========
Si: Sine integral.
Ci: Cosine integral.
Chi: Hyperbolic cosine integral.
Ei: Exponential integral.
expint: Generalised exponential integral.
E1: Special case of the generalised exponential integral.
li: Logarithmic integral.
Li: Offset logarithmic integral.
References
==========
.. [1] http://en.wikipedia.org/wiki/Trigonometric_integral
"""
_trigfunc = C.sinh
_atzero = S(0)
_atinf = S.Infinity
_atneginf = S.NegativeInfinity
@classmethod
def _minusfactor(cls, z):
return -Shi(z)
@classmethod
def _Ifactor(cls, z, sign):
return I*Si(z)*sign
def _eval_rewrite_as_expint(self, z):
from sympy import exp_polar
# XXX should we polarify z?
return (E1(z) - E1(exp_polar(I*pi)*z))/2 - I*pi/2
[docs]class Chi(TrigonometricIntegral):
r"""
Cosh integral.
This function is defined for positive :math:`x` by
.. math:: \operatorname{Chi}(x) = \gamma + \log{x}
+ \int_0^x \frac{\cosh{t} - 1}{t} \mathrm{d}t,
where :math:`\gamma` is the Euler-Mascheroni constant.
We have
.. math:: \operatorname{Chi}(z) = \operatorname{Ci}\left(e^{i \pi/2}z\right)
- i\frac{\pi}{2},
which holds for all polar :math:`z` and thus provides an analytic
continuation to the Riemann surface of the logarithm.
By lifting to the principal branch we obtain an analytic function on the
cut complex plane.
Examples
========
>>> from sympy import Chi
>>> from sympy.abc import z
The `\cosh` integral is a primitive of `\cosh(z)/z`:
>>> Chi(z).diff(z)
cosh(z)/z
It has a logarithmic branch point at the origin:
>>> from sympy import exp_polar, I, pi
>>> Chi(z*exp_polar(2*I*pi))
Chi(z) + 2*I*pi
The `\cosh` integral behaves somewhat like ordinary `\cosh` under multiplication by `i`:
>>> from sympy import polar_lift
>>> Chi(polar_lift(I)*z)
Ci(z) + I*pi/2
>>> Chi(polar_lift(-1)*z)
Chi(z) + I*pi
It can also be expressed in terms of exponential integrals:
>>> from sympy import expint
>>> Chi(z).rewrite(expint)
-expint(1, z)/2 - expint(1, z*exp_polar(I*pi))/2 - I*pi/2
See Also
========
Si: Sine integral.
Ci: Cosine integral.
Shi: Hyperbolic sine integral.
Ei: Exponential integral.
expint: Generalised exponential integral.
E1: Special case of the generalised exponential integral.
li: Logarithmic integral.
Li: Offset logarithmic integral.
References
==========
.. [1] http://en.wikipedia.org/wiki/Trigonometric_integral
"""
_trigfunc = C.cosh
_atzero = S.ComplexInfinity
_atinf = S.Infinity
_atneginf = S.Infinity
@classmethod
def _minusfactor(cls, z):
return Chi(z) + I*pi
@classmethod
def _Ifactor(cls, z, sign):
return Ci(z) + I*pi/2*sign
def _eval_rewrite_as_expint(self, z):
from sympy import exp_polar
return -I*pi/2 - (E1(z) + E1(exp_polar(I*pi)*z))/2
def _latex(self, printer, exp=None):
if len(self.args) != 1:
raise ValueError("Arg length should be 1")
if exp:
return r'\operatorname{Chi}^{%s}{\left (%s \right )}' \
% (printer._print(exp), printer._print(self.args[0]))
else:
return r'\operatorname{Chi}{\left (%s \right )}' \
% printer._print(self.args[0])
@staticmethod
def _latex_no_arg(printer):
return r'\operatorname{Chi}'
###############################################################################
#################### FRESNEL INTEGRALS ########################################
###############################################################################
[docs]class FresnelIntegral(Function):
""" Base class for the Fresnel integrals."""
unbranched = True
@classmethod
def eval(cls, z):
# Value at zero
if z is S.Zero:
return S(0)
# Try to pull out factors of -1 and I
prefact = S.One
newarg = z
changed = False
nz = newarg.extract_multiplicatively(-1)
if nz is not None:
prefact = -prefact
newarg = nz
changed = True
nz = newarg.extract_multiplicatively(I)
if nz is not None:
prefact = cls._sign*I*prefact
newarg = nz
changed = True
if changed:
return prefact*cls(newarg)
# Values at positive infinities signs
# if any were extracted automatically
if z is S.Infinity:
return S.Half
elif z is I*S.Infinity:
return cls._sign*I*S.Half
def fdiff(self, argindex=1):
if argindex == 1:
return self._trigfunc(S.Half*pi*self.args[0]**2)
else:
raise ArgumentIndexError(self, argindex)
def _eval_is_real(self):
return self.args[0].is_real
def _eval_conjugate(self):
return self.func(self.args[0].conjugate())
def _as_real_imag(self, deep=True, **hints):
if self.args[0].is_real:
if deep:
hints['complex'] = False
return (self.expand(deep, **hints), S.Zero)
else:
return (self, 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 as_real_imag(self, deep=True, **hints):
# Fresnel S
# http://functions.wolfram.com/06.32.19.0003.01
# http://functions.wolfram.com/06.32.19.0006.01
# Fresnel C
# http://functions.wolfram.com/06.33.19.0003.01
# http://functions.wolfram.com/06.33.19.0006.01
x, y = self._as_real_imag(deep=deep, **hints)
sq = -y**2/x**2
re = S.Half*(self.func(x + x*sqrt(sq)) + self.func(x - x*sqrt(sq)))
im = x/(2*y) * sqrt(sq) * (self.func(x - x*sqrt(sq)) -
self.func(x + x*sqrt(sq)))
return (re, im)
[docs]class fresnels(FresnelIntegral):
r"""
Fresnel integral S.
This function is defined by
.. math:: \operatorname{S}(z) = \int_0^z \sin{\frac{\pi}{2} t^2} \mathrm{d}t.
It is an entire function.
Examples
========
>>> from sympy import I, oo, fresnels
>>> from sympy.abc import z
Several special values are known:
>>> fresnels(0)
0
>>> fresnels(oo)
1/2
>>> fresnels(-oo)
-1/2
>>> fresnels(I*oo)
-I/2
>>> fresnels(-I*oo)
I/2
In general one can pull out factors of -1 and `i` from the argument:
>>> fresnels(-z)
-fresnels(z)
>>> fresnels(I*z)
-I*fresnels(z)
The Fresnel S integral obeys the mirror symmetry
`\overline{S(z)} = S(\bar{z})`:
>>> from sympy import conjugate
>>> conjugate(fresnels(z))
fresnels(conjugate(z))
Differentiation with respect to `z` is supported:
>>> from sympy import diff
>>> diff(fresnels(z), z)
sin(pi*z**2/2)
Defining the Fresnel functions via an integral
>>> from sympy import integrate, pi, sin, gamma, expand_func
>>> integrate(sin(pi*z**2/2), z)
3*fresnels(z)*gamma(3/4)/(4*gamma(7/4))
>>> expand_func(integrate(sin(pi*z**2/2), z))
fresnels(z)
We can numerically evaluate the Fresnel integral to arbitrary precision
on the whole complex plane:
>>> fresnels(2).evalf(30)
0.343415678363698242195300815958
>>> fresnels(-2*I).evalf(30)
0.343415678363698242195300815958*I
See Also
========
fresnelc: Fresnel cosine integral.
References
==========
.. [1] http://en.wikipedia.org/wiki/Fresnel_integral
.. [2] http://dlmf.nist.gov/7
.. [3] http://mathworld.wolfram.com/FresnelIntegrals.html
.. [4] http://functions.wolfram.com/GammaBetaErf/FresnelS
"""
_trigfunc = C.sin
_sign = -S.One
@staticmethod
@cacheit
def taylor_term(n, x, *previous_terms):
if n < 0:
return S.Zero
else:
x = sympify(x)
if len(previous_terms) > 1:
p = previous_terms[-1]
return (-pi**2*x**4*(4*n - 1)/(8*n*(2*n + 1)*(4*n + 3))) * p
else:
return x**3 * (-x**4)**n * (S(2)**(-2*n - 1)*pi**(2*n + 1)) / ((4*n + 3)*C.factorial(2*n + 1))
def _eval_rewrite_as_erf(self, z):
return (S.One + I)/4 * (erf((S.One + I)/2*sqrt(pi)*z) - I*erf((S.One - I)/2*sqrt(pi)*z))
def _eval_rewrite_as_hyper(self, z):
return pi*z**3/6 * hyper([S(3)/4], [S(3)/2, S(7)/4], -pi**2*z**4/16)
def _eval_rewrite_as_meijerg(self, z):
return (pi*z**(S(9)/4) / (sqrt(2)*(z**2)**(S(3)/4)*(-z)**(S(3)/4))
* meijerg([], [1], [S(3)/4], [S(1)/4, 0], -pi**2*z**4/16))
[docs]class fresnelc(FresnelIntegral):
r"""
Fresnel integral C.
This function is defined by
.. math:: \operatorname{C}(z) = \int_0^z \cos{\frac{\pi}{2} t^2} \mathrm{d}t.
It is an entire function.
Examples
========
>>> from sympy import I, oo, fresnelc
>>> from sympy.abc import z
Several special values are known:
>>> fresnelc(0)
0
>>> fresnelc(oo)
1/2
>>> fresnelc(-oo)
-1/2
>>> fresnelc(I*oo)
I/2
>>> fresnelc(-I*oo)
-I/2
In general one can pull out factors of -1 and `i` from the argument:
>>> fresnelc(-z)
-fresnelc(z)
>>> fresnelc(I*z)
I*fresnelc(z)
The Fresnel C integral obeys the mirror symmetry
`\overline{C(z)} = C(\bar{z})`:
>>> from sympy import conjugate
>>> conjugate(fresnelc(z))
fresnelc(conjugate(z))
Differentiation with respect to `z` is supported:
>>> from sympy import diff
>>> diff(fresnelc(z), z)
cos(pi*z**2/2)
Defining the Fresnel functions via an integral
>>> from sympy import integrate, pi, cos, gamma, expand_func
>>> integrate(cos(pi*z**2/2), z)
fresnelc(z)*gamma(1/4)/(4*gamma(5/4))
>>> expand_func(integrate(cos(pi*z**2/2), z))
fresnelc(z)
We can numerically evaluate the Fresnel integral to arbitrary precision
on the whole complex plane:
>>> fresnelc(2).evalf(30)
0.488253406075340754500223503357
>>> fresnelc(-2*I).evalf(30)
-0.488253406075340754500223503357*I
See Also
========
fresnels: Fresnel sine integral.
References
==========
.. [1] http://en.wikipedia.org/wiki/Fresnel_integral
.. [2] http://dlmf.nist.gov/7
.. [3] http://mathworld.wolfram.com/FresnelIntegrals.html
.. [4] http://functions.wolfram.com/GammaBetaErf/FresnelC
"""
_trigfunc = C.cos
_sign = S.One
@staticmethod
@cacheit
def taylor_term(n, x, *previous_terms):
if n < 0:
return S.Zero
else:
x = sympify(x)
if len(previous_terms) > 1:
p = previous_terms[-1]
return (-pi**2*x**4*(4*n - 3)/(8*n*(2*n - 1)*(4*n + 1))) * p
else:
return x * (-x**4)**n * (S(2)**(-2*n)*pi**(2*n)) / ((4*n + 1)*C.factorial(2*n))
def _eval_rewrite_as_erf(self, z):
return (S.One - I)/4 * (erf((S.One + I)/2*sqrt(pi)*z) + I*erf((S.One - I)/2*sqrt(pi)*z))
def _eval_rewrite_as_hyper(self, z):
return z * hyper([S.One/4], [S.One/2, S(5)/4], -pi**2*z**4/16)
def _eval_rewrite_as_meijerg(self, z):
return (pi*z**(S(3)/4) / (sqrt(2)*root(z**2, 4)*root(-z, 4))
* meijerg([], [1], [S(1)/4], [S(3)/4, 0], -pi**2*z**4/16))
###############################################################################
#################### HELPER FUNCTIONS #########################################
###############################################################################
class _erfs(Function):
"""
Helper function to make the `\\mathrm{erf}(z)` function
tractable for the Gruntz algorithm.
"""
def _eval_aseries(self, n, args0, x, logx):
point = args0[0]
# Expansion at oo
if point is S.Infinity:
z = self.args[0]
l = [ 1/sqrt(S.Pi) * C.factorial(2*k)*(-S(
4))**(-k)/C.factorial(k) * (1/z)**(2*k + 1) for k in xrange(0, n) ]
o = C.Order(1/z**(2*n + 1), x)
# It is very inefficient to first add the order and then do the nseries
return (Add(*l))._eval_nseries(x, n, logx) + o
# Expansion at I*oo
t = point.extract_multiplicatively(S.ImaginaryUnit)
if t is S.Infinity:
z = self.args[0]
# TODO: is the series really correct?
l = [ 1/sqrt(S.Pi) * C.factorial(2*k)*(-S(
4))**(-k)/C.factorial(k) * (1/z)**(2*k + 1) for k in xrange(0, n) ]
o = C.Order(1/z**(2*n + 1), x)
# It is very inefficient to first add the order and then do the nseries
return (Add(*l))._eval_nseries(x, n, logx) + o
# All other points are not handled
return super(_erfs, self)._eval_aseries(n, args0, x, logx)
def fdiff(self, argindex=1):
if argindex == 1:
z = self.args[0]
return -2/sqrt(S.Pi) + 2*z*_erfs(z)
else:
raise ArgumentIndexError(self, argindex)
def _eval_rewrite_as_intractable(self, z):
return (S.One - erf(z))*C.exp(z**2)
class _eis(Function):
"""
Helper function to make the `\\mathrm{Ei}(z)` and `\\mathrm{li}(z)` functions
tractable for the Gruntz algorithm.
"""
def _eval_aseries(self, n, args0, x, logx):
if args0[0] != S.Infinity:
return super(_erfs, self)._eval_aseries(n, args0, x, logx)
z = self.args[0]
l = [ C.factorial(k) * (1/z)**(k + 1) for k in xrange(0, n) ]
o = C.Order(1/z**(n + 1), x)
# It is very inefficient to first add the order and then do the nseries
return (Add(*l))._eval_nseries(x, n, logx) + o
def fdiff(self, argindex=1):
if argindex == 1:
z = self.args[0]
return S.One / z - _eis(z)
else:
raise ArgumentIndexError(self, argindex)
def _eval_rewrite_as_intractable(self, z):
return C.exp(-z)*Ei(z)
def _eval_nseries(self, x, n, logx):
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(_eis, self)._eval_nseries(x, n, logx)
