factorial2(n, exact=False)

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

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

Parameters

n : int or array_like

Calculate n!! . Arrays are only supported with :None:None:`exact` set to False. If n < 0 , the return value is 0.

exact : bool, optional

The result can be approximated rapidly using the gamma-formula above (default). If :None:None:`exact` is set to True, calculate the answer exactly using integer arithmetic.

Returns

nff : float or int

Double factorial of n, as an int or a float depending on :None:None:`exact`.

Double factorial.

Examples

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

>>> factorial2(7, exact=True)
105

See :

Back References

The following pages refer to to this document either explicitly or contain code examples using this.

scipy.special._basic.factorial2

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