Why is 0 Factorial Equal to 1?

The factorial of k! is defined as the product of all positive integers from 1 to k. This means that k! can also be written as the following:

k! = k*(k - 1)!

Here is an example:

k = 2

2*(2-1)! = 2

By using this rule, if we set k = 1, then the formula results in the following:

k = 1

1*(1-1)! = 1

1*(0)! = 1

0! = 1

Therefore, 0! is equal to 1.

(Source: http://www.zero-factorial.com/whatis.html)