We have had several discussions about the factorial notation, so I think this introduction is a bit late. However, it is important that you know these basic facts in order to perform calculations and understand better in later discussions.

The factorial of a non-negative integer is the product of all the positive numbers less than . For example, the

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and

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In **Introduction to Permutations**, we have discussed that there are ways to arrange distinct objects into a sequence. For instance, if we have 3 objects namely *A*, *B*, and C, then they can be arranged in ways. The arrangement are as follows:

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We have also learned **some reasons **why mathematicians chose the definition . Continue reading