scipy.special.

factorial2#

scipy.special.factorial2(n, exact=False)[source]#

Double factorial.

This is the factorial with every second value skipped. E.g., 7!! = 7 * 5 * 3 * 1. It can be approximated numerically as:

n!! = 2 ** (n / 2) * gamma(n / 2 + 1) * sqrt(2 / pi)  n odd
    = 2 ** (n / 2) * gamma(n / 2 + 1)                 n even
    = 2 ** (n / 2) * (n / 2)!                         n even

Parameters:

nint or array_like: Calculate n!!. If n < 0, the return value is 0.
exactbool, optional: The result can be approximated rapidly using the gamma-formula above (default). If exact is set to True, calculate the answer exactly using integer arithmetic.

Returns:

nfffloat or int: Double factorial of n, as an int or a float depending on exact.

Examples

>>> from scipy.special import factorial2
>>> factorial2(7, exact=False)
array(105.00000000000001)
>>> factorial2(7, exact=True)
105