Let 2 be an alphabet, we can construct the set 2+ consisting of all finite nonempty string of elements of 2: ()Basic step: If, then aE2t. ()Recursive step: If a and XEZt, then ax∈+ (3)Closed step: There are no other elements in the set 2+ except those constructed using steps(1) and (2). + element or string: infinite Length of string: finite, 1,2,3 9Let Σ be an alphabet, we can construct the set Σ+ consisting of all finite nonempty string of elements of Σ: (1)Basic step: If aΣ, then aΣ+ . (2)Recursive step: If a and xΣ+ , then axΣ+ . (3)Closed step: There are no other elements in the set Σ+ except those constructed using steps (1) and (2). Σ+ element or string: infinite Length of string: finite, 1,2,3,…