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

.

and

.

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:

.

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

**Some Factorial Facts and Fun**

The definition above translates that

or the product of the non-negative integer and all the positive integers below it. From this definition, it is easy to see the following basic facts.

1.)

because and .

2.) which directly follows from (1)

3.) A prime number that is 1 less or 1 more than a value of a factorial is called a factorial prime. Therefore, 7 is a factorial prime since .

4.) Factorials are also used in Calculus particularly in Taylor’s Theorem.

5.) You can use factorials to remember time!

There are 4! hours in a day.

There are 8! minutes in four weeks.

There are 10! seconds in six weeks.

Below is a video about the basics of factorial from Numberphile including some explanations of why ! = 1 which is also discussed in this post.

Enjoy learning!