## Ordering Elements of a Set

Often it is the case that the order of things is not important. That is where permutations come in. It enables us to show how many different arrangements can be had of a given set of elements. Of course, this is not true of something like an event stream but we will discuss those later.

#### 2.2 Permutations

##### 2.2.1 Ordering Things

A number of applications of the rule of products are of a specific type, and because of their frequent appearance they are given their own designation, permutations. Consider the following examples.

Example 2.2.1: Ordering the elements of a set. How many different ways can we order the three different elements of the set A = {a, b, c}? Since we have three choices for position one, two choices for position two, and one choice for the third position, we have, by the rule of products, 3 · 2 · 1 = 6 different ways of ordering the three letters. We illustrate through a tree diagram.

Figure 2.2.2 A tree to enumerate permutations of a three element set.

Each of the six orderings is called a permutation of the set A.

Example 2.2.3: Ordering a schedule. A student is taking five courses in the fall semester. How many different ways can the five courses be listed?

There are 5 · 4 · 3 · 2 · 1 = 120 different permutations of the set of courses.

In each of the above examples of the rule of products we observe that:

(a) We are asked to order or arrange elements from a single set.

(b) Each element is listed exactly once in each list (permutation). So if there are n choices for position one in a list, there are n 1 choices for position two, n 2 choices for position three, etc.

Example 2.2.4: Some orderings of a baseball team. The alphabetical ordering of the players of a baseball team is one permutation of the set of players. Other orderings of the players’ names might be done by batting average, age, or height. The information that determines the ordering is called the key. We would expect that each key would give a different permutation of the names.

If there are twenty-five players on the team, there are 25 · 24 · 23 · · · · · 3 · 2 · different permutations of the players.

This number of permutations is huge. In fact, it is 15511210043330985984000000, but writing it like this isn’t all that instructive, while leaving it as a product as we originally had makes it easier to see where the number comes from. We just need to find a more compact way of writing these products.

We now develop notation that will be useful for permutation problems.

Definition 2.2.5: Factorial. If n is a positive integer then n factorial is the product of the first n positive integers and is denoted n!. Additionally, we define zero factorial, 0!, to be 1.

The first few factorials are

$\begin{matrix} n & 0 & 1 & 2 & 3 & 4 & 5 & 6 & 7\\ n! & 1 & 1 & 2 & 6 & 24 & 120 & 720 & 5040 \end{matrix}$

Note that 4! is 4 times 3!, or 24, and 5! is 5 times 4!, or 120. In addition, note that as n grows in size, n! grows extremely quickly. For example, 11! = 39916800. If the answer to a problem happens to be 25!, as in the previous example, you would never be expected to write that number out completely. However, a problem with an answer of $\frac{25!}{23!}$ can be reduced to 25 · 24, or 600.

If |A| = n, there are n! ways of permuting all n elements of A. We next consider the more general situation where we would like to permute k elements out of a set of n objects, where k n.

Example 2.2.6: Choosing Club Officers. A club of twenty-five members will hold an election for president, secretary, and treasurer in that order. Assume a person can hold only one position. How many ways are there of choosing these three officers? By the rule of products, there are 25 · 24 · 23 ways of making a selection.

Definition 2.2.7: Permutation. An ordered arrangement of k elements selected from a set of n elements, 0 k n, where no two elements of the arrangement are the same, is called a permutation of n objects taken k at a time. The total number of such permutations is denoted by P (n, k).

Theorem 2.2.8: Permutation Counting Formula. The number of possible permutations of k elements taken from a set of n elements is

$P(n,k) = n \cdot (n-1) \cdot (n-2) \cdot ... \cdot (n-k+1) = \prod_{j=0}^{k-1}(n-j) = \frac{n!}{n-k)!}$

Proof. Case I: If k = n we have $P(n,n) = n! = \frac{n!}{(n-n)!}$

Case II: If 0 k < n,then we have k positions to fill using n elements and

(a) Position 1 can be filled by any one of n 0 = n elements

(b) Position 2 can be filled by any one of n 1 elements

(c) · · ·

(d) Position k can be filled by any one of n (k 1) = n k + 1 elements

Hence, by the rule of products,

$P(n,k) = n \cdot (n-1) \cdot (n-2) \cdot ... \cdot (n-k+1) = \frac{n!}{(n-k)!}$

It is important to note that the derivation of the permutation formula given above was done solely through the rule of products. This serves to reiterate our introductory remarks in this section that permutation problems are really rule-of-products problems. We close this section with several examples.

Example 2.2.9: Another example of choosing officers. A club has eight members eligible to serve as president, vice-president, and treasurer. How many ways are there of choosing these officers?

Solution 1: Using the rule of products. There are eight possible choices for the presidency, seven for the vice-presidency, and six for the office of treasurer.

By the rule of products, there are 8 · 7 · 6 = 336 ways of choosing these officers.

Solution 2: Using the permutation formula. We want the total number of permutations of eight objects taken three at a time:

$P(8,3) = \frac{8!}{(8-3)!} = 8 \cdot 7 \cdot 6 = 336$

Example 2.2.10: Course ordering revisited. To count the number of ways to order five courses, we can use the permutation formula. We want the number of permutations of five courses taken five at a time:

$P(5,5) = \frac{5!}{(5-5)!} = 5! = 120$

Example 2.2.11: Ordering of digits under different conditions. Consider only the digits 1, 2, 3, 4, and 5.

(a) How many three-digit numbers can be formed if no repetition of digits can occur?

(b) How many three-digit numbers can be formed if repetition of digits is allowed?

(c) How many three-digit numbers can be formed if only non-consecutive repetition of digits are allowed?

Solutions to (a): Solution 1: Using the rule of products. We have any one of five choices for digit one, any one of four choices for digit two, and three

choices for digit three. Hence, 5 · 4 · 3 = 60 different three-digit numbers can be formed.

Solution 2; Using the permutation formula. We want the total number of permutations of five digits taken three at a time:

$P(5,3) = \frac{5!}{(5-3)!} = 5 \cdot 4 \cdot 3 = 60$

Solution to (b): The definition of permutation indicates “...no two elements in each list are the same.” Hence the permutation formula cannot be used. However, the rule of products still applies. We have any one of five choices for the first digit, five choices for the second, and five for the third. So there are 5 · 5 · 5 = 125 possible different three-digit numbers if repetition is allowed.

Solution to (c): Again, the rule of products applies here. We have any

one of five choices for the first digit, but then for the next two digits we have four choices since we are not allowed to repeat the previous digit So there are 5 · 4 · 4 = 80 possible different three-digit numbers if only non-consecutive repetitions are allowed.