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# Source code for sympy.functions.combinatorial.factorials

from sympy.core import S, C, sympify
from sympy.core.function import Function, ArgumentIndexError
from sympy.ntheory import sieve
from math import sqrt as _sqrt

from sympy.core.compatibility import reduce

class CombinatorialFunction(Function):
"""Base class for combinatorial functions. """

###############################################################################
######################## FACTORIAL and MULTI-FACTORIAL ########################
###############################################################################

[docs]class factorial(CombinatorialFunction):
"""Implementation of factorial function over nonnegative integers.
For the sake of convenience and simplicity of procedures using
this function it is defined for negative integers and returns
zero in this case.

The factorial is very important in combinatorics where it gives
the number of ways in which 'n' objects can be permuted. It also
arises in calculus, probability, number theory etc.

There is strict relation of factorial with gamma function. In
fact n! = gamma(n+1) for nonnegative integers. Rewrite of this
kind is very useful in case of combinatorial simplification.

Computation of the factorial is done using two algorithms. For
small arguments naive product is evaluated. However for bigger
input algorithm Prime-Swing is used. It is the fastest algorithm
known and computes n! via prime factorization of special class
of numbers, called here the 'Swing Numbers'.

Examples
========

>>> from sympy import Symbol, factorial
>>> n = Symbol('n', integer=True)

>>> factorial(-2)
0

>>> factorial(0)
1

>>> factorial(7)
5040

>>> factorial(n)
n!

>>> factorial(2*n)
(2*n)!

========

factorial2, RisingFactorial, FallingFactorial
"""

nargs = 1

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

_small_swing = [
1,1,1,3,3,15,5,35,35,315,63,693,231,3003,429,6435,6435,109395,
12155,230945,46189,969969,88179,2028117,676039,16900975,1300075,
35102025,5014575,145422675,9694845,300540195,300540195
]

@classmethod
def _swing(cls, n):
if n < 33:
return cls._small_swing[n]
else:
N, primes = int(_sqrt(n)), []

for prime in sieve.primerange(3, N+1):
p, q = 1, n

while True:
q //= prime

if q > 0:
if q & 1 == 1:
p *= prime
else:
break

if p > 1:
primes.append(p)

for prime in sieve.primerange(N+1, n//3 + 1):
if (n // prime) & 1 == 1:
primes.append(prime)

L_product = R_product = 1

for prime in sieve.primerange(n//2 + 1, n+1):
L_product *= prime

for prime in primes:
R_product *= prime

return L_product*R_product

@classmethod
def _recursive(cls, n):
if n < 2:
return 1
else:
return (cls._recursive(n//2)**2)*cls._swing(n)

@classmethod
def eval(cls, n):
n = sympify(n)

if n.is_Number:
if n is S.Zero:
return S.One
elif n.is_Integer:
if n.is_negative:
return S.Zero
else:
n, result = n.p, 1

if n < 20:
for i in range(2, n+1):
result *= i
else:
N, bits = n, 0

while N != 0:
if N & 1 == 1:
bits += 1

N = N >> 1

result = cls._recursive(n)*2**(n-bits)

return C.Integer(result)

if n.is_negative:
return S.Zero

def _eval_rewrite_as_gamma(self, n):
return C.gamma(n + 1)

def _eval_is_integer(self):
return self.args[0].is_integer

[docs]class MultiFactorial(CombinatorialFunction):
pass

[docs]class factorial2(CombinatorialFunction):
"""The double factorial n!!, not to be confused with (n!)!

The double factorial is defined for integers >= -1 as::

,
|  n*(n - 2)*(n - 4)* ... * 1    for n odd
n!! =  -|  n*(n - 2)*(n - 4)* ... * 2    for n even
|  1                             for n = 0, -1
'

Examples
========

>>> from sympy import factorial2, var
>>> var('n')
n
>>> factorial2(n + 1)
(n + 1)!!
>>> factorial2(5)
15
>>> factorial2(-1)
1

========

factorial, RisingFactorial, FallingFactorial
"""
nargs = 1

@classmethod
def eval(cls, arg):
if arg.is_Number:
if arg == S.Zero or arg == S.NegativeOne:
return S.One
return factorial2(arg - 2)*arg

def _sympystr(self, p):
if self.args[0].is_Atom:
return "%s!!" % p.doprint(self.args[0])
else:
return "(%s)!!" % p.doprint(self.args[0])

###############################################################################
######################## RISING and FALLING FACTORIALS ########################
###############################################################################

[docs]class RisingFactorial(CombinatorialFunction):
"""Rising factorial (also called Pochhammer symbol) is a double valued
function arising in concrete mathematics, hypergeometric functions
and series expansions. It is defined by:

rf(x, k) = x * (x+1) * ... * (x + k-1)

where 'x' can be arbitrary expression and 'k' is an integer. For
or visit http://mathworld.wolfram.com/RisingFactorial.html page.

Examples
========

>>> from sympy import rf
>>> from sympy.abc import x

>>> rf(x, 0)
1

>>> rf(1, 5)
120

>>> rf(x, 5) == x*(1 + x)*(2 + x)*(3 + x)*(4 + x)
True

========

factorial, factorial2, FallingFactorial
"""

nargs = 2

@classmethod
def eval(cls, x, k):
x = sympify(x)
k = sympify(k)

if x is S.NaN:
return S.NaN
elif x is S.One:
return factorial(k)
elif k.is_Integer:
if k is S.NaN:
return S.NaN
elif k is S.Zero:
return S.One
else:
if k.is_positive:
if x is S.Infinity:
return S.Infinity
elif x is S.NegativeInfinity:
if k.is_odd:
return S.NegativeInfinity
else:
return S.Infinity
else:
return reduce(lambda r, i: r*(x+i), xrange(0, int(k)), 1)
else:
if x is S.Infinity:
return S.Infinity
elif x is S.NegativeInfinity:
return S.Infinity
else:
return 1/reduce(lambda r, i: r*(x-i), xrange(1, abs(int(k))+1), 1)

def _eval_rewrite_as_gamma(self, x, k):
return C.gamma(x + k) / C.gamma(x)

[docs]class FallingFactorial(CombinatorialFunction):
"""Falling factorial (related to rising factorial) is a double valued
function arising in concrete mathematics, hypergeometric functions
and series expansions. It is defined by

ff(x, k) = x * (x-1) * ... * (x - k+1)

where 'x' can be arbitrary expression and 'k' is an integer. For
or visit http://mathworld.wolfram.com/FallingFactorial.html page.

>>> from sympy import ff
>>> from sympy.abc import x

>>> ff(x, 0)
1

>>> ff(5, 5)
120

>>> ff(x, 5) == x*(x-1)*(x-2)*(x-3)*(x-4)
True

========

factorial, factorial2, RisingFactorial
"""

nargs = 2

@classmethod
def eval(cls, x, k):
x = sympify(x)
k = sympify(k)

if x is S.NaN:
return S.NaN
elif k.is_Integer:
if k is S.NaN:
return S.NaN
elif k is S.Zero:
return S.One
else:
if k.is_positive:
if x is S.Infinity:
return S.Infinity
elif x is S.NegativeInfinity:
if k.is_odd:
return S.NegativeInfinity
else:
return S.Infinity
else:
return reduce(lambda r, i: r*(x-i), xrange(0, int(k)), 1)
else:
if x is S.Infinity:
return S.Infinity
elif x is S.NegativeInfinity:
return S.Infinity
else:
return 1/reduce(lambda r, i: r*(x+i), xrange(1, abs(int(k))+1), 1)

def _eval_rewrite_as_gamma(self, x, k):
return (-1)**k * C.gamma(-x + k) / C.gamma(-x)

rf = RisingFactorial
ff = FallingFactorial

###############################################################################
########################### BINOMIAL COEFFICIENTS #############################
###############################################################################

[docs]class binomial(CombinatorialFunction):
"""Implementation of the binomial coefficient. It can be defined
in two ways depending on its desired interpretation:

C(n,k) = n!/(k!(n-k)!)   or   C(n, k) = ff(n, k)/k!

First, in a strict combinatorial sense it defines the
number of ways we can choose 'k' elements from a set of
'n' elements. In this case both arguments are nonnegative
integers and binomial is computed using an efficient
algorithm based on prime factorization.

The other definition is generalization for arbitrary 'n',
however 'k' must also be nonnegative. This case is very
useful when evaluating summations.

For the sake of convenience for negative 'k' this function
will return zero no matter what valued is the other argument.

Examples
========

>>> from sympy import Symbol, Rational, binomial
>>> n = Symbol('n', integer=True)

>>> binomial(15, 8)
6435

>>> binomial(n, -1)
0

>>> [ binomial(0, i) for i in range(1)]
[1]
>>> [ binomial(1, i) for i in range(2)]
[1, 1]
>>> [ binomial(2, i) for i in range(3)]
[1, 2, 1]
>>> [ binomial(3, i) for i in range(4)]
[1, 3, 3, 1]
>>> [ binomial(4, i) for i in range(5)]
[1, 4, 6, 4, 1]

>>> binomial(Rational(5,4), 3)
-5/128

>>> binomial(n, 3)
n*(n - 2)*(n - 1)/6

"""

nargs = 2

def fdiff(self, argindex=1):
if argindex == 1:
# http://functions.wolfram.com/GammaBetaErf/Binomial/20/01/01/
n, k = self.args
return binomial(n, k)*(C.polygamma(0, n + 1) - C.polygamma(0, n - k + 1))
elif argindex == 2:
# http://functions.wolfram.com/GammaBetaErf/Binomial/20/01/02/
n, k = self.args
return binomial(n, k)*(C.polygamma(0, n - k + 1) - C.polygamma(0, k + 1))
else:
raise ArgumentIndexError(self, argindex)

@classmethod
def eval(cls, n, k):
n, k = map(sympify, (n, k))

if k.is_Number:
if k.is_Integer:
if k < 0:
return S.Zero
elif k == 0 or n == k:
return S.One
elif n.is_Integer and n >= 0:
n, k = int(n), int(k)

if k > n:
return S.Zero
elif k > n // 2:
k = n - k

M, result = int(_sqrt(n)), 1

for prime in sieve.primerange(2, n+1):
if prime > n - k:
result *= prime
elif prime > n // 2:
continue
elif prime > M:
if n % prime < k % prime:
result *= prime
else:
N, K = n, k
exp = a = 0

while N > 0:
a = int((N % prime) < (K % prime + a))
N, K = N // prime, K // prime
exp = a + exp

if exp > 0:
result *= prime**exp

return C.Integer(result)
else:
result = n - k + 1

for i in xrange(2, k+1):
result *= n-k+i
result /= i

return result
elif k.is_negative:
return S.Zero
elif (n - k).simplify().is_negative:
return S.Zero
else:
d = n - k

if d.is_Integer:
return cls.eval(n, d)

def _eval_rewrite_as_factorial(self, n, k):
return C.factorial(n)/(C.factorial(k)*C.factorial(n - k))

def _eval_rewrite_as_gamma(self, n, k):
return C.gamma(n + 1)/(C.gamma(k + 1)*C.gamma(n - k + 1))

def _eval_is_integer(self):
return self.args[0].is_integer and self.args[1].is_integer