Let a:, sequences of the form aa,...a. are often in computer science. These finite sequences are also called strings. The length of the string S is the number of terms in this string. The empty string, denoted by A, is the string that has no terms. The empty string has length zero. If x=a., and y=b.. bm are strings, where a m be(1≤in,jm), we define the catenation of and as the string1a2anb1b2.bm n The catenation of x and y is written as xy, and is another string from 2, i.e. xy= a2..a b b2...bm' Note xA=x and Ax=.Let aiΣ, sequences of the form a1 a2…an are often in computer science. These finite sequences are also called strings. The length of the string S is the number of terms in this string. The empty string, denoted by , is the string that has no terms. The empty string has length zero. If x=a1 a2…an , and y=b1b2…bm are strings, where ai , bjΣ(1≦i≦n,1≦j≦m), we define the catenation of x and y as the string a1 a2…an b1b2…bm . The catenation of x and y is written as xy, and is another string from Σ, i.e. xy=a1 a2…an b1b2…bm. Note x=x and x=x