Factorial function

The factorial function (symbol: !) says to multiply all whole numbers from our chosen number down to 1.

Examples:

• $5!=5\times 4\times 3\times 2\times 1=120$
• $7!=7\times 6\times 5\times 4\times 3\times 2\times 1=5040$

Special case 0

It is generally agreed that $0!=1$.

It can be explained following this logic:

$n!=n\times (n-1)!$

$1!=1\times (1-1)!$
$1!=1\times 0!$

Or also:
${\displaystyle 0!=\frac{1!}{1}=\frac{1}{1}=1}$

References

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