How to Use the Factorial Notation

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 n is the product of all the positive numbers less than n. For example, the

4! = 4 \times 3 \times 2 \times 1 = 24.

and

5! = 5 \times 4 \times 3 \times 2 \times 1 = 120.

In Introduction to Permutations, we have discussed that there are n! ways to arrange n distinct objects into a sequence. For instance, if we have 3 objects namely A, B, and C, then they can be arranged in 3! = 3 \times 2 \times 1 = 6 ways. The arrangement are as follows:

ABC, ACB, BAC, BCA, CAB, CBA.

 We have also learned some reasons why mathematicians chose the definition 0! = 1.

Some Factorial Facts and Fun

The definition above translates that

n! = n(n-1)(n-2) \cdots (3)(2)(1)

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

1.) n! = n(n - 1)!

because n! = n(n-1)(n-2) \cdots (3)(2)(1) and (n-1)! = (n - 1)(n-2) \cdots (3)(2)(1).

2.) \frac{n!}{(n-1)!} = n 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 7 = 3! + 1.

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!

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