##### Example2.1

In the state of Georgia, license plates consist of four digits followed by a space followed by three capital letters. The first digit cannot be a \(0\). How many license plates are possible?

Let \(n\) be a positive integer. Throughout this text, we will use the shorthand notation \([n]\) to denote the \(n\)-element set \(\{1,2,\dots,n\}\). Now let \(X\) be a set. Then a function \(s\colon[n]\rightarrow X\) is also called an *\(X\)-string of length \(n\)*. In discussions of \(X\)-strings, it is customary to refer to the elements of \(X\) as *characters*, while the element \(s(i)\) is the \(i^{\text{th} }\) character of \(s\). Whenever practical, we prefer to denote a string \(s\) by writing \(s=\)“\(x_1x_2x_3\dots x_n\)”, rather than the more cumbersome notation \(s(1)=x_1\), \(s(2)=x_2\), …, \(s(n)=x_n\).

There are a number of alternatives for the notation and terminology associated with strings. First, the characters in a string \(s\) are frequently written using subscripts as \(s_1,s_2,\dots,s_n\), so the \(i^{\text{th} }\)-term of \(s\) can be denoted \(s_i\) rather than \(s(i)\). Strings are also called *sequences*, especially when \(X\) is a set of numbers and the function \(s\) is defined by an algebraic rule. For example, the sequence of odd integers is defined by \(s_i=2i-1\).

Alternatively, strings are called *words*, the set \(X\) is called the *alphabet* and the elements of \(X\) are called *letters*. For example, \(aababbccabcbb\) is a \(13\)-letter word on the \(3\)-letter alphabet \(\{a,b,c\}\).

In many computing languages, strings are called *arrays*. Also, when the character \(s(i)\) is constrained to belong to a subset \(X_i\subseteq X\), a string can be considered as an element of the cartesian product \(X_1\times X_2\times \dots\times X_n\), which is normally viewed as \(n\)-tuples of the form \((x_1,x_2,\dots,x_n)\) such that \(x_i\in X_i\) for all \(i\in [n]\).

In the state of Georgia, license plates consist of four digits followed by a space followed by three capital letters. The first digit cannot be a \(0\). How many license plates are possible?

In the case that \(X=\{0,1\}\), an \(X\)-string is called a \(0\)–\(1\) string (also a *binary string* or *bit string.*). When \(X=\{0,1,2\}\), an \(X\)-string is also called a *ternary* string.

A machine instruction in a \(32\)-bit operating system is just a bit string of length \(32\). Thus, there are \(2\) options for each of \(32\) positions to fill, making the number of such strings \(2^{32} = 4\,294\,967\,296\). In general, the number of bit strings of length \(n\) is \(2^n\).

Suppose that a website allows its users to pick their own usernames for accounts, but imposes some restrictions. The first character must be an upper-case letter in the English alphabet. The second through sixth characters can be letters (both upper-case and lower-case allowed) in the English alphabet or decimal digits (\(0\)–\(9\)). The seventh position must be `@' or `.'. The eighth through twelfth positions allow lower-case English letters, `*', `%', and `#'. The thirteenth position must be a digit. How many users can the website accept registrations from?