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Factorials

Factorials are products of positive integers.

The value " $n$ factorial ", written $n!$ , for a whole number $n$ is the product of all the whole numbers from $1$ to $n$ .

$n!=1\times 2\times \mathrm{...}\times \left(n-2\right)\times \left(n-1\right)\times n$

For example, $5!$ is equal to $120$ :

$5!=1\times 2\times 3\times 4\times 5=120$

(For various reasons, $0!$ is defined to be equal to $1$ , not $0$ .)