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Section2.6Principles of counting

It is often useful in both pure and applied mathematics to count the sizes of finite sets. In this section we prove some basic theorems of this sort. If \(A\) is a set, let \(\abs{A}\) denote the number of elements in \(A\text{.}\) Note that \(\emptyset\) is a finite set with \(\abs{\emptyset} = 0\text{.}\)

Obvious, but we omit the technical proof until we have a proper discussion of the definition of the number of elements in a set, which won't occur until Chapter 5.

Obvious when you look at a Venn diagram, but we omit the technical proof for similar reasons.

Definition2.6.5Permutation (combinatorics)

A permutation of a finite set is an arrangement of the elements in a particular order.

For the next theorem, recall that the factorial of a positive integer \(n\) is defined inductively by

\begin{gather*} 0! = 1\\ n! = n(n-1)! \end{gather*}

Equivalently, \(n! = n(n-1)(n-2)\cdots 1\text{.}\)

By induction on the number of elements in the set.

Definition2.6.8Combination

For \(n \in \mathbb{N}\) and \(r \in \mathbb{Z}\) with \(0 \le r \le n\text{,}\) a combination of \(r\) elements from a set of size \(n\) is just a subset of size \(r\text{.}\)

The number of combinations is called the binomial coefficient and is denoted \(\binom{n}{r}\text{.}\) We read this symbol as “\(n\) choose \(r\text{.}\)”

Note that the number of permutations of any \(r\) distinct objects from a set of \(n\) objects is necessarily the number of subsets of size \(r\) (i.e., the number of combinations, the binomial coefficient) times the number of ways to arrange those \(r\) elements (i.e., permutations of a set of size \(r\)). Therefore, by Theorem 2.6.6 and Theorem 2.6.7, we have

\begin{equation*} \binom{n}{r} r! = \frac{n!}{(n-r)!} \end{equation*}

thereby proving the result.

Note: the above theorem guarantees \(\binom{n}{r} = \binom{n}{n-r}\text{.}\)

By induction and using the fact that when \(r \ge 1\text{,}\)

\begin{equation*} \binom{n}{r} = \binom{n-1}{r} + \binom{n-1}{r-1}, \end{equation*}

which you should prove. This displayed equation essentially says that Pascal's triangle generates the binomial coefficients.