Decidable language is closed under union, intersection, concatenation, and star.
Consider these functions from languages over {0,1} to languages over {0,1}:
K(A,B) = {w Γ’ΛΛ {0,1}* | (w Γ’ΛΛ A Γ’ΛΒ§ w Γ’ΛΛ A Γ’ΛΛ B) or w Γ’ΛΛ B}
N(A,B) = {w Γ’ΛΛ {0,1}* | (w Γ’ΛΛ A Γ’ΛΒ§ w Γ’ΛΛ A Γ’ΛΛ B) or w Γ’ΛΛ A}
[Γ’ΛΛ is the concatenation operator.]
i. Prove that the set of Turing-decidable languages is closed under both K and N. That is, if A and B are Turing-decidable, so are K(A,B) and N(A,B).