Factorial2

n!! or Factorial2[n] gives the double factorial of n (product of every other number from 1 or 2 to n).

Examples

Double factorial of odd numbers:

7!!
(* 105 - equals 7×5×3×1 *)

Double factorial of even numbers:

8!!
(* 384 - equals 8×6×4×2 *)

Compare with regular factorial:

{5!, 5!!}
(* {120, 15} *)

Please visit the official Wolfram Language Reference for more details.