Discrete Mathematics with Applications (Epp) p.13
Given a finite set \(A\), a string of length \(n\) over \(A\) is an ordered n-tuple of elements of \(A\) written without parentheses or commas.
The elements of \(A\) are called the characters of the string.
A null string over \(A\) is defined to be a string with no characters.
A null string is denoted as \(\lambda\) and is said to have length 0.
A string over \(A = \{0, 1\}\) is called a bit string.
For example, all strings of length 3 over \(A\) with at least two of the same characters:
$$aab, aba, baa, aaa, bba, bab, abb, bbb$$