Factorial
Factorial
The factorial of 4, which is written as , is . Similarly, . The number grows very quickly as increases. The double factorial of a number , which is written as , is an extension to the normal factorial. When is odd, . When is even, .
4!
1×2×3×4=24
n!=1×2×3×4×...×n
n!
n
n
n!!
n
n!!=n×(n-2)×(n-4)×...×5×3×1
n
n!!=n×(n-2)×(n-4)×...×6×4×2