ABSTRACT
An alphabet X is any finite set. The elements of X are called letters or symbols. A word or string over the alphabet X is a finite sequence of letters from X. A word (xi, X2, • •, %n) can be written also as x\x2 • • • xn. The length of the word w = x\x2 • • ■ xn, where Xi 6 X, is the number n of letters in w. It is denoted by |w|.
