The product of all the integers from 1 up to some non-negative integer n
that have the same parity as n. Denoted by n!!
n‼︎ = n(n - 2)(n - 4) ・・・
For even n:
n/2
n‼︎ = ∏ (2k) = n(n - 2) ・・・ 2
k=1
For odd n:
(n+1)/2
n‼︎ = ∏ (2k - 1) = n(n - 2) ・・・ 1
k=1
0‼︎ = 1
public static doubleFactorial ( integer $n ) : integer | ||
$n | integer | |
리턴 | integer |