# 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