Let be an alphabet, we can construct the set consisting of all finite string of elements ofΣ: (1)Basic step: AE2*. (2)Recursive step: If xe*and aE_ then xa∈ (3)Closed step: There are no other elements in the set *except those constructed using steps(1) and (2)Let Σ be an alphabet, we can construct the set Σ* consisting of all finite string of elements of Σ: (1)Basic step: Σ* . (2)Recursive step: If xΣ* and aΣ then xaΣ* . (3)Closed step: There are no other elements in the set Σ* except those constructed using steps (1) and (2)