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Traktor/myenv/Lib/site-packages/sympy/functions/special/zeta_functions.py

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2024-05-23 01:57:24 +02:00
""" Riemann zeta and related function. """
from sympy.core.add import Add
from sympy.core.cache import cacheit
from sympy.core.function import ArgumentIndexError, expand_mul, Function
from sympy.core.numbers import pi, I, Integer
from sympy.core.relational import Eq
from sympy.core.singleton import S
from sympy.core.symbol import Dummy
from sympy.core.sympify import sympify
from sympy.functions.combinatorial.numbers import bernoulli, factorial, genocchi, harmonic
from sympy.functions.elementary.complexes import re, unpolarify, Abs, polar_lift
from sympy.functions.elementary.exponential import log, exp_polar, exp
from sympy.functions.elementary.integers import ceiling, floor
from sympy.functions.elementary.miscellaneous import sqrt
from sympy.functions.elementary.piecewise import Piecewise
from sympy.polys.polytools import Poly
###############################################################################
###################### LERCH TRANSCENDENT #####################################
###############################################################################
class lerchphi(Function):
r"""
Lerch transcendent (Lerch phi function).
Explanation
===========
For $\operatorname{Re}(a) > 0$, $|z| < 1$ and $s \in \mathbb{C}$, the
Lerch transcendent is defined as
.. math :: \Phi(z, s, a) = \sum_{n=0}^\infty \frac{z^n}{(n + a)^s},
where the standard branch of the argument is used for $n + a$,
and by analytic continuation for other values of the parameters.
A commonly used related function is the Lerch zeta function, defined by
.. math:: L(q, s, a) = \Phi(e^{2\pi i q}, s, a).
**Analytic Continuation and Branching Behavior**
It can be shown that
.. math:: \Phi(z, s, a) = z\Phi(z, s, a+1) + a^{-s}.
This provides the analytic continuation to $\operatorname{Re}(a) \le 0$.
Assume now $\operatorname{Re}(a) > 0$. The integral representation
.. math:: \Phi_0(z, s, a) = \int_0^\infty \frac{t^{s-1} e^{-at}}{1 - ze^{-t}}
\frac{\mathrm{d}t}{\Gamma(s)}
provides an analytic continuation to $\mathbb{C} - [1, \infty)$.
Finally, for $x \in (1, \infty)$ we find
.. math:: \lim_{\epsilon \to 0^+} \Phi_0(x + i\epsilon, s, a)
-\lim_{\epsilon \to 0^+} \Phi_0(x - i\epsilon, s, a)
= \frac{2\pi i \log^{s-1}{x}}{x^a \Gamma(s)},
using the standard branch for both $\log{x}$ and
$\log{\log{x}}$ (a branch of $\log{\log{x}}$ is needed to
evaluate $\log{x}^{s-1}$).
This concludes the analytic continuation. The Lerch transcendent is thus
branched at $z \in \{0, 1, \infty\}$ and
$a \in \mathbb{Z}_{\le 0}$. For fixed $z, a$ outside these
branch points, it is an entire function of $s$.
Examples
========
The Lerch transcendent is a fairly general function, for this reason it does
not automatically evaluate to simpler functions. Use ``expand_func()`` to
achieve this.
If $z=1$, the Lerch transcendent reduces to the Hurwitz zeta function:
>>> from sympy import lerchphi, expand_func
>>> from sympy.abc import z, s, a
>>> expand_func(lerchphi(1, s, a))
zeta(s, a)
More generally, if $z$ is a root of unity, the Lerch transcendent
reduces to a sum of Hurwitz zeta functions:
>>> expand_func(lerchphi(-1, s, a))
zeta(s, a/2)/2**s - zeta(s, a/2 + 1/2)/2**s
If $a=1$, the Lerch transcendent reduces to the polylogarithm:
>>> expand_func(lerchphi(z, s, 1))
polylog(s, z)/z
More generally, if $a$ is rational, the Lerch transcendent reduces
to a sum of polylogarithms:
>>> from sympy import S
>>> expand_func(lerchphi(z, s, S(1)/2))
2**(s - 1)*(polylog(s, sqrt(z))/sqrt(z) -
polylog(s, sqrt(z)*exp_polar(I*pi))/sqrt(z))
>>> expand_func(lerchphi(z, s, S(3)/2))
-2**s/z + 2**(s - 1)*(polylog(s, sqrt(z))/sqrt(z) -
polylog(s, sqrt(z)*exp_polar(I*pi))/sqrt(z))/z
The derivatives with respect to $z$ and $a$ can be computed in
closed form:
>>> lerchphi(z, s, a).diff(z)
(-a*lerchphi(z, s, a) + lerchphi(z, s - 1, a))/z
>>> lerchphi(z, s, a).diff(a)
-s*lerchphi(z, s + 1, a)
See Also
========
polylog, zeta
References
==========
.. [1] Bateman, H.; Erdelyi, A. (1953), Higher Transcendental Functions,
Vol. I, New York: McGraw-Hill. Section 1.11.
.. [2] https://dlmf.nist.gov/25.14
.. [3] https://en.wikipedia.org/wiki/Lerch_transcendent
"""
def _eval_expand_func(self, **hints):
z, s, a = self.args
if z == 1:
return zeta(s, a)
if s.is_Integer and s <= 0:
t = Dummy('t')
p = Poly((t + a)**(-s), t)
start = 1/(1 - t)
res = S.Zero
for c in reversed(p.all_coeffs()):
res += c*start
start = t*start.diff(t)
return res.subs(t, z)
if a.is_Rational:
# See section 18 of
# Kelly B. Roach. Hypergeometric Function Representations.
# In: Proceedings of the 1997 International Symposium on Symbolic and
# Algebraic Computation, pages 205-211, New York, 1997. ACM.
# TODO should something be polarified here?
add = S.Zero
mul = S.One
# First reduce a to the interaval (0, 1]
if a > 1:
n = floor(a)
if n == a:
n -= 1
a -= n
mul = z**(-n)
add = Add(*[-z**(k - n)/(a + k)**s for k in range(n)])
elif a <= 0:
n = floor(-a) + 1
a += n
mul = z**n
add = Add(*[z**(n - 1 - k)/(a - k - 1)**s for k in range(n)])
m, n = S([a.p, a.q])
zet = exp_polar(2*pi*I/n)
root = z**(1/n)
up_zet = unpolarify(zet)
addargs = []
for k in range(n):
p = polylog(s, zet**k*root)
if isinstance(p, polylog):
p = p._eval_expand_func(**hints)
addargs.append(p/(up_zet**k*root)**m)
return add + mul*n**(s - 1)*Add(*addargs)
# TODO use minpoly instead of ad-hoc methods when issue 5888 is fixed
if isinstance(z, exp) and (z.args[0]/(pi*I)).is_Rational or z in [-1, I, -I]:
# TODO reference?
if z == -1:
p, q = S([1, 2])
elif z == I:
p, q = S([1, 4])
elif z == -I:
p, q = S([-1, 4])
else:
arg = z.args[0]/(2*pi*I)
p, q = S([arg.p, arg.q])
return Add(*[exp(2*pi*I*k*p/q)/q**s*zeta(s, (k + a)/q)
for k in range(q)])
return lerchphi(z, s, a)
def fdiff(self, argindex=1):
z, s, a = self.args
if argindex == 3:
return -s*lerchphi(z, s + 1, a)
elif argindex == 1:
return (lerchphi(z, s - 1, a) - a*lerchphi(z, s, a))/z
else:
raise ArgumentIndexError
def _eval_rewrite_helper(self, target):
res = self._eval_expand_func()
if res.has(target):
return res
else:
return self
def _eval_rewrite_as_zeta(self, z, s, a, **kwargs):
return self._eval_rewrite_helper(zeta)
def _eval_rewrite_as_polylog(self, z, s, a, **kwargs):
return self._eval_rewrite_helper(polylog)
###############################################################################
###################### POLYLOGARITHM ##########################################
###############################################################################
class polylog(Function):
r"""
Polylogarithm function.
Explanation
===========
For $|z| < 1$ and $s \in \mathbb{C}$, the polylogarithm is
defined by
.. math:: \operatorname{Li}_s(z) = \sum_{n=1}^\infty \frac{z^n}{n^s},
where the standard branch of the argument is used for $n$. It admits
an analytic continuation which is branched at $z=1$ (notably not on the
sheet of initial definition), $z=0$ and $z=\infty$.
The name polylogarithm comes from the fact that for $s=1$, the
polylogarithm is related to the ordinary logarithm (see examples), and that
.. math:: \operatorname{Li}_{s+1}(z) =
\int_0^z \frac{\operatorname{Li}_s(t)}{t} \mathrm{d}t.
The polylogarithm is a special case of the Lerch transcendent:
.. math:: \operatorname{Li}_{s}(z) = z \Phi(z, s, 1).
Examples
========
For $z \in \{0, 1, -1\}$, the polylogarithm is automatically expressed
using other functions:
>>> from sympy import polylog
>>> from sympy.abc import s
>>> polylog(s, 0)
0
>>> polylog(s, 1)
zeta(s)
>>> polylog(s, -1)
-dirichlet_eta(s)
If $s$ is a negative integer, $0$ or $1$, the polylogarithm can be
expressed using elementary functions. This can be done using
``expand_func()``:
>>> from sympy import expand_func
>>> from sympy.abc import z
>>> expand_func(polylog(1, z))
-log(1 - z)
>>> expand_func(polylog(0, z))
z/(1 - z)
The derivative with respect to $z$ can be computed in closed form:
>>> polylog(s, z).diff(z)
polylog(s - 1, z)/z
The polylogarithm can be expressed in terms of the lerch transcendent:
>>> from sympy import lerchphi
>>> polylog(s, z).rewrite(lerchphi)
z*lerchphi(z, s, 1)
See Also
========
zeta, lerchphi
"""
@classmethod
def eval(cls, s, z):
if z.is_number:
if z is S.One:
return zeta(s)
elif z is S.NegativeOne:
return -dirichlet_eta(s)
elif z is S.Zero:
return S.Zero
elif s == 2:
dilogtable = _dilogtable()
if z in dilogtable:
return dilogtable[z]
if z.is_zero:
return S.Zero
# Make an effort to determine if z is 1 to avoid replacing into
# expression with singularity
zone = z.equals(S.One)
if zone:
return zeta(s)
elif zone is False:
# For s = 0 or -1 use explicit formulas to evaluate, but
# automatically expanding polylog(1, z) to -log(1-z) seems
# undesirable for summation methods based on hypergeometric
# functions
if s is S.Zero:
return z/(1 - z)
elif s is S.NegativeOne:
return z/(1 - z)**2
if s.is_zero:
return z/(1 - z)
# polylog is branched, but not over the unit disk
if z.has(exp_polar, polar_lift) and (zone or (Abs(z) <= S.One) == True):
return cls(s, unpolarify(z))
def fdiff(self, argindex=1):
s, z = self.args
if argindex == 2:
return polylog(s - 1, z)/z
raise ArgumentIndexError
def _eval_rewrite_as_lerchphi(self, s, z, **kwargs):
return z*lerchphi(z, s, 1)
def _eval_expand_func(self, **hints):
s, z = self.args
if s == 1:
return -log(1 - z)
if s.is_Integer and s <= 0:
u = Dummy('u')
start = u/(1 - u)
for _ in range(-s):
start = u*start.diff(u)
return expand_mul(start).subs(u, z)
return polylog(s, z)
def _eval_is_zero(self):
z = self.args[1]
if z.is_zero:
return True
def _eval_nseries(self, x, n, logx, cdir=0):
from sympy.series.order import Order
nu, z = self.args
z0 = z.subs(x, 0)
if z0 is S.NaN:
z0 = z.limit(x, 0, dir='-' if re(cdir).is_negative else '+')
if z0.is_zero:
# In case of powers less than 1, number of terms need to be computed
# separately to avoid repeated callings of _eval_nseries with wrong n
try:
_, exp = z.leadterm(x)
except (ValueError, NotImplementedError):
return self
if exp.is_positive:
newn = ceiling(n/exp)
o = Order(x**n, x)
r = z._eval_nseries(x, n, logx, cdir).removeO()
if r is S.Zero:
return o
term = r
s = [term]
for k in range(2, newn):
term *= r
s.append(term/k**nu)
return Add(*s) + o
return super(polylog, self)._eval_nseries(x, n, logx, cdir)
###############################################################################
###################### HURWITZ GENERALIZED ZETA FUNCTION ######################
###############################################################################
class zeta(Function):
r"""
Hurwitz zeta function (or Riemann zeta function).
Explanation
===========
For $\operatorname{Re}(a) > 0$ and $\operatorname{Re}(s) > 1$, this
function is defined as
.. math:: \zeta(s, a) = \sum_{n=0}^\infty \frac{1}{(n + a)^s},
where the standard choice of argument for $n + a$ is used. For fixed
$a$ not a nonpositive integer the Hurwitz zeta function admits a
meromorphic continuation to all of $\mathbb{C}$; it is an unbranched
function with a simple pole at $s = 1$.
The Hurwitz zeta function is a special case of the Lerch transcendent:
.. math:: \zeta(s, a) = \Phi(1, s, a).
This formula defines an analytic continuation for all possible values of
$s$ and $a$ (also $\operatorname{Re}(a) < 0$), see the documentation of
:class:`lerchphi` for a description of the branching behavior.
If no value is passed for $a$ a default value of $a = 1$ is assumed,
yielding the Riemann zeta function.
Examples
========
For $a = 1$ the Hurwitz zeta function reduces to the famous Riemann
zeta function:
.. math:: \zeta(s, 1) = \zeta(s) = \sum_{n=1}^\infty \frac{1}{n^s}.
>>> from sympy import zeta
>>> from sympy.abc import s
>>> zeta(s, 1)
zeta(s)
>>> zeta(s)
zeta(s)
The Riemann zeta function can also be expressed using the Dirichlet eta
function:
>>> from sympy import dirichlet_eta
>>> zeta(s).rewrite(dirichlet_eta)
dirichlet_eta(s)/(1 - 2**(1 - s))
The Riemann zeta function at nonnegative even and negative integer
values is related to the Bernoulli numbers and polynomials:
>>> zeta(2)
pi**2/6
>>> zeta(4)
pi**4/90
>>> zeta(0)
-1/2
>>> zeta(-1)
-1/12
>>> zeta(-4)
0
The specific formulae are:
.. math:: \zeta(2n) = -\frac{(2\pi i)^{2n} B_{2n}}{2(2n)!}
.. math:: \zeta(-n,a) = -\frac{B_{n+1}(a)}{n+1}
No closed-form expressions are known at positive odd integers, but
numerical evaluation is possible:
>>> zeta(3).n()
1.20205690315959
The derivative of $\zeta(s, a)$ with respect to $a$ can be computed:
>>> from sympy.abc import a
>>> zeta(s, a).diff(a)
-s*zeta(s + 1, a)
However the derivative with respect to $s$ has no useful closed form
expression:
>>> zeta(s, a).diff(s)
Derivative(zeta(s, a), s)
The Hurwitz zeta function can be expressed in terms of the Lerch
transcendent, :class:`~.lerchphi`:
>>> from sympy import lerchphi
>>> zeta(s, a).rewrite(lerchphi)
lerchphi(1, s, a)
See Also
========
dirichlet_eta, lerchphi, polylog
References
==========
.. [1] https://dlmf.nist.gov/25.11
.. [2] https://en.wikipedia.org/wiki/Hurwitz_zeta_function
"""
@classmethod
def eval(cls, s, a=None):
if a is S.One:
return cls(s)
elif s is S.NaN or a is S.NaN:
return S.NaN
elif s is S.One:
return S.ComplexInfinity
elif s is S.Infinity:
return S.One
elif a is S.Infinity:
return S.Zero
sint = s.is_Integer
if a is None:
a = S.One
if sint and s.is_nonpositive:
return bernoulli(1-s, a) / (s-1)
elif a is S.One:
if sint and s.is_even:
return -(2*pi*I)**s * bernoulli(s) / (2*factorial(s))
elif sint and a.is_Integer and a.is_positive:
return cls(s) - harmonic(a-1, s)
elif a.is_Integer and a.is_nonpositive and \
(s.is_integer is False or s.is_nonpositive is False):
return S.NaN
def _eval_rewrite_as_bernoulli(self, s, a=1, **kwargs):
if a == 1 and s.is_integer and s.is_nonnegative and s.is_even:
return -(2*pi*I)**s * bernoulli(s) / (2*factorial(s))
return bernoulli(1-s, a) / (s-1)
def _eval_rewrite_as_dirichlet_eta(self, s, a=1, **kwargs):
if a != 1:
return self
s = self.args[0]
return dirichlet_eta(s)/(1 - 2**(1 - s))
def _eval_rewrite_as_lerchphi(self, s, a=1, **kwargs):
return lerchphi(1, s, a)
def _eval_is_finite(self):
arg_is_one = (self.args[0] - 1).is_zero
if arg_is_one is not None:
return not arg_is_one
def _eval_expand_func(self, **hints):
s = self.args[0]
a = self.args[1] if len(self.args) > 1 else S.One
if a.is_integer:
if a.is_positive:
return zeta(s) - harmonic(a-1, s)
if a.is_nonpositive and (s.is_integer is False or
s.is_nonpositive is False):
return S.NaN
return self
def fdiff(self, argindex=1):
if len(self.args) == 2:
s, a = self.args
else:
s, a = self.args + (1,)
if argindex == 2:
return -s*zeta(s + 1, a)
else:
raise ArgumentIndexError
def _eval_as_leading_term(self, x, logx=None, cdir=0):
if len(self.args) == 2:
s, a = self.args
else:
s, a = self.args + (S.One,)
try:
c, e = a.leadterm(x)
except NotImplementedError:
return self
if e.is_negative and not s.is_positive:
raise NotImplementedError
return super(zeta, self)._eval_as_leading_term(x, logx, cdir)
class dirichlet_eta(Function):
r"""
Dirichlet eta function.
Explanation
===========
For $\operatorname{Re}(s) > 0$ and $0 < x \le 1$, this function is defined as
.. math:: \eta(s, a) = \sum_{n=0}^\infty \frac{(-1)^n}{(n+a)^s}.
It admits a unique analytic continuation to all of $\mathbb{C}$ for any
fixed $a$ not a nonpositive integer. It is an entire, unbranched function.
It can be expressed using the Hurwitz zeta function as
.. math:: \eta(s, a) = \zeta(s,a) - 2^{1-s} \zeta\left(s, \frac{a+1}{2}\right)
and using the generalized Genocchi function as
.. math:: \eta(s, a) = \frac{G(1-s, a)}{2(s-1)}.
In both cases the limiting value of $\log2 - \psi(a) + \psi\left(\frac{a+1}{2}\right)$
is used when $s = 1$.
Examples
========
>>> from sympy import dirichlet_eta, zeta
>>> from sympy.abc import s
>>> dirichlet_eta(s).rewrite(zeta)
Piecewise((log(2), Eq(s, 1)), ((1 - 2**(1 - s))*zeta(s), True))
See Also
========
zeta
References
==========
.. [1] https://en.wikipedia.org/wiki/Dirichlet_eta_function
.. [2] Peter Luschny, "An introduction to the Bernoulli function",
https://arxiv.org/abs/2009.06743
"""
@classmethod
def eval(cls, s, a=None):
if a is S.One:
return cls(s)
if a is None:
if s == 1:
return log(2)
z = zeta(s)
if not z.has(zeta):
return (1 - 2**(1-s)) * z
return
elif s == 1:
from sympy.functions.special.gamma_functions import digamma
return log(2) - digamma(a) + digamma((a+1)/2)
z1 = zeta(s, a)
z2 = zeta(s, (a+1)/2)
if not z1.has(zeta) and not z2.has(zeta):
return z1 - 2**(1-s) * z2
def _eval_rewrite_as_zeta(self, s, a=1, **kwargs):
from sympy.functions.special.gamma_functions import digamma
if a == 1:
return Piecewise((log(2), Eq(s, 1)), ((1 - 2**(1-s)) * zeta(s), True))
return Piecewise((log(2) - digamma(a) + digamma((a+1)/2), Eq(s, 1)),
(zeta(s, a) - 2**(1-s) * zeta(s, (a+1)/2), True))
def _eval_rewrite_as_genocchi(self, s, a=S.One, **kwargs):
from sympy.functions.special.gamma_functions import digamma
return Piecewise((log(2) - digamma(a) + digamma((a+1)/2), Eq(s, 1)),
(genocchi(1-s, a) / (2 * (s-1)), True))
def _eval_evalf(self, prec):
if all(i.is_number for i in self.args):
return self.rewrite(zeta)._eval_evalf(prec)
class riemann_xi(Function):
r"""
Riemann Xi function.
Examples
========
The Riemann Xi function is closely related to the Riemann zeta function.
The zeros of Riemann Xi function are precisely the non-trivial zeros
of the zeta function.
>>> from sympy import riemann_xi, zeta
>>> from sympy.abc import s
>>> riemann_xi(s).rewrite(zeta)
s*(s - 1)*gamma(s/2)*zeta(s)/(2*pi**(s/2))
References
==========
.. [1] https://en.wikipedia.org/wiki/Riemann_Xi_function
"""
@classmethod
def eval(cls, s):
from sympy.functions.special.gamma_functions import gamma
z = zeta(s)
if s in (S.Zero, S.One):
return S.Half
if not isinstance(z, zeta):
return s*(s - 1)*gamma(s/2)*z/(2*pi**(s/2))
def _eval_rewrite_as_zeta(self, s, **kwargs):
from sympy.functions.special.gamma_functions import gamma
return s*(s - 1)*gamma(s/2)*zeta(s)/(2*pi**(s/2))
class stieltjes(Function):
r"""
Represents Stieltjes constants, $\gamma_{k}$ that occur in
Laurent Series expansion of the Riemann zeta function.
Examples
========
>>> from sympy import stieltjes
>>> from sympy.abc import n, m
>>> stieltjes(n)
stieltjes(n)
The zero'th stieltjes constant:
>>> stieltjes(0)
EulerGamma
>>> stieltjes(0, 1)
EulerGamma
For generalized stieltjes constants:
>>> stieltjes(n, m)
stieltjes(n, m)
Constants are only defined for integers >= 0:
>>> stieltjes(-1)
zoo
References
==========
.. [1] https://en.wikipedia.org/wiki/Stieltjes_constants
"""
@classmethod
def eval(cls, n, a=None):
if a is not None:
a = sympify(a)
if a is S.NaN:
return S.NaN
if a.is_Integer and a.is_nonpositive:
return S.ComplexInfinity
if n.is_Number:
if n is S.NaN:
return S.NaN
elif n < 0:
return S.ComplexInfinity
elif not n.is_Integer:
return S.ComplexInfinity
elif n is S.Zero and a in [None, 1]:
return S.EulerGamma
if n.is_extended_negative:
return S.ComplexInfinity
if n.is_zero and a in [None, 1]:
return S.EulerGamma
if n.is_integer == False:
return S.ComplexInfinity
@cacheit
def _dilogtable():
return {
S.Half: pi**2/12 - log(2)**2/2,
Integer(2) : pi**2/4 - I*pi*log(2),
-(sqrt(5) - 1)/2 : -pi**2/15 + log((sqrt(5)-1)/2)**2/2,
-(sqrt(5) + 1)/2 : -pi**2/10 - log((sqrt(5)+1)/2)**2,
(3 - sqrt(5))/2 : pi**2/15 - log((sqrt(5)-1)/2)**2,
(sqrt(5) - 1)/2 : pi**2/10 - log((sqrt(5)-1)/2)**2,
I : I*S.Catalan - pi**2/48,
-I : -I*S.Catalan - pi**2/48,
1 - I : pi**2/16 - I*S.Catalan - pi*I/4*log(2),
1 + I : pi**2/16 + I*S.Catalan + pi*I/4*log(2),
(1 - I)/2 : -log(2)**2/8 + pi*I*log(2)/8 + 5*pi**2/96 - I*S.Catalan
}
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