The Factorial Function

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Lesson Summary: 
The Factorial Function is denoted by $n!$ and means we multiply the numbers $1$ through $n$ together.
Lesson Inputs: 
Functions
Lesson Outputs: 
The Binomial Theorem

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Lesson: 

The Factorial Function is a function of positive integers that is denoted by $n!$ and means we multiply the numbers $1$ through $n$ together. So

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For example,

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Furthermore, we define the $0!$ as being equal to $1$. And just so we are clear, $1!=1$.

Notice that

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and

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Finally, the language we use is to say "factorial." For example, when referring to $5!$ we would say "five factorial."