Factorial

A factorial number is a number that equals k! (factorial of k) for some non-negative integer k: 0! = 1, 1! = 1, 2! = 2, 3! = 6, 4! = 24, 5! = 120, etc.

Key features

• Only finitely many factorial numbers fit in a fixed integer range (e.g. 0! through 20! in 64-bit).
• They grow very quickly.

Examples:

1, 2, 6, 24, 120, 720, 5040, 40320

Further reading

• Factorial on Wikipedia
• Factorial on OEIS