Factorial

The factorial of a whole number n, denoted n!, is the product of all the positive integers equal to or less than n: n!=1×2×3×...×n. The first values are:
1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
6! = 720
By convention, we set 0! = 1.

Stirling Formula: the factorial is a very rapidly increasing series, equivalent at infinity to

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