Consider the homomorphism unpair : ∆∗ → Σ∗ where unpair((a, b)) = ab. Now, unpair(L3) = perfect shuffle(A, B), and so regular languages are closed under the perfect shuffle operation.
Reversal. Statement: Under reversal, the set of regular languages is closed. Proof: Let M be a deterministic finite automaton that accepts L; we will create M' from M so that M and M' states are the same. Make the final state of M the accepting state of M' and the final state of M the beginning state of M'.
What are closure properties of regular languages? Regular languages are closed under complement, union, intersection, concatenation, Kleene star, reversal, homomorphism, and substitution.
Closure under Union For any regular languages L and M, then L ∪ M is regular. Proof: Since L and M are regular, they have regular expressions, say: Let L = L(E) and M = L(F). Then L ∪ M = L(E + F) by the definition of the + operator.
Languages in P are also closed under reversal, intersection, union, concatenation, Kleene closure, inverse homomorphism, and complementation.
The set of regular languages is closed under complementation. The complement of language L, written L, is all strings not in L but with the same alphabet. The statement says that if L is a regular lan- guage, then so is L. To see this fact, take deterministic FA for L and interchange the accept and reject states.
Regular Languages are closed under intersection, i.e., if L1 and L2 are regular then L1 ∩ L2 is also regular. L1 and L2 are regular • L1 ∪ L2 is regular • Hence, L1 ∩ L2 = L1 ∪ L2 is regular.
Regular Languages are closed under intersection, i.e., if L1 and L2 are regular then L1 ∩ L2 is also regular. L1 and L2 are regular • L1 ∪ L2 is regular • Hence, L1 ∩ L2 = L1 ∪ L2 is regular.
The closure property formula for multiplication for a given set S is: ∀ a, b ∈ S ⇒ a × b ∈ S. Here are some examples of sets that are closed under multiplication: Natural Numbers (ℕ): ∀ a, b ∈ ℕ ⇒ a × b ∈ ℕ Whole Numbers (W): ∀ a, b ∈ W ⇒ a × b ∈ W.