Function doubleFactorial
Computes the double factorial of n: n * (n - 2) * (n - 4) * ... * 1
ulong doubleFactorial
(
ulong n
);
Example
import std .array: array;
auto dfacs = iota(1, 21) .map!(n => doubleFactorial(n)) .array;
assert(dfacs == [1, 2, 3, 8, 15, 48, 105, 384, 945, 3840,
10395, 46080, 135135, 645120, 2027025,
10321920, 34459425, 185794560, 654729075,
3715891200]);