ABSTRACT
An alphabet Σ is a finite nonempty set of symbols (or letters). A string (or word) is a finite sequence of symbols from Σ. We generally use u, v,w, x, y, and z to denote strings, and a, b, and c to denote symbols. For example, a and b are symbols and u = abbaa is a string. The length of a stringw is denoted by |w|. For example, abbaa has length 5. The empty string, denoted by , is the string consisting of zero symbols and thus || = 0. For a string w, wR denotes the string obtained from w by reversing the order of the symbols. For example, if w = abcde, then wR = edcba.
