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
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: