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 Description  
 Factorial:  The expression n! is a symbol to indicate the product of all counting numbers beginning with n and counting backwards to 1.


The factorial function is given by:



Factorials are defined to use the natural numbers (0, 1, 2, 3, ...) as shown. John Wallis (1616-1703) is credited with being the first person to consider n! for values of n other than the natural numbers. He found that



and furthered the development of something called the gamma function. Search on the web to see if you can learn more about John Wallis and the gamma function.