A factorial (denoted with an exclamation point N!) is a quantity calculated by taking the product of the first N integers of an integer N. 0! is defined to be 1. 

University of Texas at Austin

Given a set of N objects, N! (N factorial) is defined to be the product of the first N integers. The number of different permutations of N objects (that is, the number of different ways of ordering them) is N!. In this scenario, the number of objects in the set N is equivalent to the number of trials n. 

Permutations of N Objects

Morin, D. (2016). Probability for the Enthusiastic Beginner. Las Vegas, Nevada: CreateSpace Independent Publishing Platform.

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